Reduced transit-time sensitivity in noninvasive magnetic resonance imaging of human cerebral blood flow

Measuring resting cerebral haemodynamics using MRI arterial spin labelling and transcranial Doppler ultrasound: Comparison in younger and older adults

INTRODUCTION

Resting cerebral blood flow (CBF) and perfusion measures have been used to determine brain health. Studies showing variation in resting CBF with age and fitness level using different imaging approaches have produced mixed findings. We assess the degree to which resting CBF measures through transcranial Doppler (TCD) and arterial spin labeling (ASL) MRI provide complementary information in older and younger, fit and unfit cohorts.

METHODS

Thirty-five healthy volunteers (20 younger: 24 ± 7y; 15 older: 66 ± 7y) completed two experimental sessions (TCD/MRI). Aging and fitness effects within and between imaging modalities were assessed.

RESULTS

Middle cerebral artery blood velocity (MCAv, TCD) was lower and transit time (MRI) slower in older compared with younger participants (p < .05). The younger group had higher gray matter cerebral perfusion (MRI) than the older group, albeit not significantly (p = .13). Surprisingly, fitness effects in the younger group (decrease/increase in MCAv/transit time with fitness, respectively) opposed the older group (increase/decrease in MCAv/transit time). Whole cohort transit times correlated with MCAv (r=-0.63; p < .05), whereas tissue perfusion did not correlate with TCD measures.

CONCLUSION

TCD and MRI modalities provide complementary resting CBF measures, with similar effects across the whole cohort and between subgroups (age/fitness) if metrics are comparable (e.g., velocity [TCD] versus transit time [MRI]).

MRI-Based Investigation of Association Between Cerebrovascular Structural Alteration and White Matter Hyperintensity Induced by High Blood Pressure

BACKGROUND

High blood pressure (BP) is a common risk factor for cerebral small vessel disease including white matter hyperintensity (WMH). Whether increased BP exacerbates WMH by impacting cerebral vascular morphologies remains poorly studied.

PURPOSE

To determine the relationships among high BP, cerebrovascular morphologies, and WMH in elderly individuals.

STUDY TYPE

Cohort.

SUBJECTS

Eight hundred sixty-three participants (54.2% female) from the Taizhou Imaging Study without clinical evidence of neurologic disorders were included in the analyses.

FIELD STRENGTH/SEQUENCE

3.0 T; time-of-flight magnetic resonance angiography (TOF MRA); T2 fluid-attenuated inversion recovery (FLAIR); T1 magnetization-prepared rapid gradient-echo; gradient echo T2*-weighted; diffusion tensor imaging; pulsed arterial spin labeling.

ASSESSMENT

Cerebrovascular morphologic measurements were quantified based on the TOF MRA images, including vessel density, radius, tortuosity, and branch number. WMH lesion volumes (WMHV) and WMH lesion counts (WMHC) were calculated automatically based on the T2 FLAIR images.

STATISTICAL TESTS

Multivariable linear regression analysis and path analysis with a linear single-mediator model were employed. A P value <0.05 was considered statistically significant.

RESULTS

Higher BP, especially diastolic BP, was significantly correlated with lower cerebrovascular density (β = -104) and lower branch numbers (β = -0.02). Although decreased tortuosity (β = -1.25) and increased radius (β = 93.8) were correlated with BP, no significant relationship of tortuosity (β = -4.6 × 10^-4 , P = 0.58) or radius (β = 0.03, P = 0.08) with BP in small vessels was found. The proportion of small vessels decreased as BP increased (SBP: β = -6.6 × 10^-4 ; DBP: β = -9.0 × 10^-4 ). Similarly, increased WMHV and WMHC were associated with decreased vessel density (volumes: β = -24, counts: β = -127), decreased tortuosity (volumes: β = -0.08, counts: β = -0.53), and increased radius (volumes: β = 12.6, counts: β = 86.6). Path analyses suggested an association between high BP and WMHs that were mediated by cerebrovascular morphologic changes.

DATA CONCLUSION

Structural alterations of cerebral vessels induced by high BP are correlated with WMH. This result suggested that elevated BP might be one of the pathophysiological mechanisms involving in the co-occurrence of cerebrovascular alteration and small vessel disease.

LEVEL OF EVIDENCE

1 Technical Efficacy Stage: 1.

Cardiopulmonary values and organ blood flows before and during heat stress: data in nine subjects at rest in the upright position

Physiological changes associated with thermoregulation can influence the kinetics of chemicals in the human body, such as alveolar ventilation (VA) and redistribution of blood flow to organs. In this study, the influence of heat stress on various physiological parameters was evaluated in nine male volunteers during sessions of exposure to wet blub globe temperatures (WBGT) of 21, 25 and 30°C for four hours. Skin and core temperatures and more than twenty cardiopulmonary parameters were measured. Liver, kidneys, brain, skin and muscles blood flows were also determined based on published measurements. Results show that most subjects (8 out of 9) have been affected by the inhalation of hot and dry air at the WBGT of 30°C. High respiratory rates, superficial tidal volumes and low VA values were notably observed. The skin blood flow has increased by 2.16-fold, whereas the renal blood flow and liver blood flow have decreased by about by 11 and 18% respectively. A complete set of key cardiopulmonary parameters in healthy male adults before and during heat stress was generated for use in PBPK modeling. A toxicokinetic studies are ongoing to evaluate the impact of heat stress on the absorption, biotransformation and excretion rates of volatile xenobiotics.

Diagnostic Accuracy of Screening Arterial Spin-Labeling MRI Using Hadamard Encoding for the Detection of Reduced CBF in Adult Patients with Ischemic Moyamoya Disease

BACKGROUND AND PURPOSE

Adult patients with ischemic Moyamoya disease are advised to undergo selective revascularization surgery based on cerebral hemodynamics. The purpose of this study was to determine the diagnostic accuracy of arterial spin-labeling MR imaging using Hadamard-encoded multiple postlabeling delays for the detection of reduced CBF in such patients.

MATERIALS AND METHODS

Thirty-seven patients underwent brain perfusion SPECT and pseudocontinuous arterial spin-labeling MR imaging using standard postlabeling delay (1525 ms) and Hadamard-encoded multiple postlabeling delays. For Hadamard-encoded multiple postlabeling delays, based on data obtained from the 7 sub-boluses with combinations of different labeling durations and postlabeling delays, CBF corrected by the arterial transit time was calculated on a voxel-by-voxel basis. Using a 3D stereotaxic template, we automatically placed ROIs in the ipsilateral cerebellar hemisphere and 5 MCA territories in the symptomatic cerebral hemisphere; then, the ratio of the MCA to cerebellar ROI was calculated.

RESULTS

The area under the receiver operating characteristic curve for detecting reduced SPECT-CBF ratios (<0.686) was significantly greater for the Hadamard-encoded multiple postlabeling delays-CBF ratios (0.885) than for the standard postlabeling delay-CBF ratios (0.786) (P = .001). The sensitivity and negative predictive value for the Hadamard-encoded multiple postlabeling delays-CBF ratios were 100% (95% confidence interval, 100%-100%) and significantly higher than the sensitivity (95% CI, 44%-80%) and negative predictive value (95% CI, 88%-97%) for the standard postlabeling delay-CBF ratio, respectively.

CONCLUSIONS

ASL MR imaging using Hadamard-encoded multiple postlabeling delays may be applicable as a screening tool because it can detect reduced CBF on brain perfusion SPECT with 100% sensitivity and a 100% negative predictive value in adult patients with ischemic Moyamoya disease.

Long-term microstructure and cerebral blood flow changes in patients recovered from COVID-19 without neurological manifestations

BACKGROUNDThe coronavirus disease 2019 (COVID-19) rapidly progressed to a global pandemic. Although some patients totally recover from COVID-19 pneumonia, the disease's long-term effects on the brain still need to be explored.METHODSWe recruited 51 patients with 2 subtypes of COVID-19 (19 mild and 32 severe) with no specific neurological manifestations at the acute stage and no obvious lesions on the conventional MRI 3 months after discharge. Changes in gray matter morphometry, cerebral blood flow (CBF), and white matter (WM) microstructure were investigated using MRI. The relationship between brain imaging measurements and inflammation markers was further analyzed.RESULTSCompared with healthy controls, the decrease in cortical thickness/CBF and the changes in WM microstructure were more severe in patients with severe disease than in those with mild disease, especially in the frontal and limbic systems. Furthermore, changes in brain microstructure, CBF, and tract parameters were significantly correlated (P < 0.05) with the inflammatory markers C-reactive protein, procalcitonin, and interleukin 6.CONCLUSIONIndirect injury related to inflammatory storm may damage the brain, altering cerebral volume, CBF, and WM tracts. COVID-19-related hypoxemia and dysfunction of vascular endothelium may also contribute to neurological changes. The abnormalities in these brain areas need to be monitored during recovery, which could help clinicians understand the potential neurological sequelae of COVID-19.FUNDINGNatural Science Foundation of China.

Altered cerebral perfusion in bipolar disorder: A pCASL MRI study

BACKGROUND

Neurovascular abnormalities are relevant to the pathophysiology of bipolar disorder (BD), which can be assessed using cerebral blood flow (CBF) imaging. CBF alterations have been identified in BD, but studies to date have been small and inconclusive. We aimed to determine cortical gray matter CBF (GM-CBF) differences between BD and healthy controls (HC) and to identify relationships between CBF and clinical or cognitive measures.

METHODS

Cortical GM-CBF maps were generated using Pseudo-Continuous Arterial Spin Labeling (pCASL) for 109 participants (BD, n = 61; HC, n = 48). We used SnPM13 to perform non-parametric voxel-wise two-sample t-tests comparing CBF between groups. We performed multiple linear regression to relate GM-CBF with clinical and cognitive measures. Analysis was adjusted for multiple comparisons with 10,000 permutations. Significance was set at a voxel level threshold of P < .001 followed by AlphaSim cluster-wise correction of P < .05.

RESULTS

Compared to HCs, BD patients had greater GM-CBF in the left lateral occipital cortex, superior division and lower CBF in the right lateral occipital, angular and middle temporal gyrus. Greater GM-CBF in the left lateral occipital cortex correlated with worse working memory, verbal memory, attention and speed of processing. We found using voxel-wise regression that decreased gray matter CBF in the bilateral thalamus and cerebellum, and increased right fronto-limbic CBF were associated with worse working memory. No clusters were associated with clinical variables after FDR correction.

CONCLUSIONS

Cortical GM-CBF alterations are seen in BD and may be related to cognitive function, which suggest neurovascular unit dysfunction as a possible pathophysiologic mechanism.

Can 3D Pseudo-Continuous Territorial Arterial Spin Labeling Effectively Diagnose Patients With Recanalization of Unilateral Middle Cerebral Artery Stenosis?

BACKGROUND

Unilateral middle cerebral artery (MCA) stenosis, as an independent risk factor for stroke, requires an intervention operation for vessel recanalization. Accurate perfusion measurement is thus essential after the operation.

PURPOSE

To explore the feasibility of three-dimensional (3D) pseudo-continuous territorial arterial-spin-labeling (tASL) in evaluating MCA recanalization.

STUDY TYPE

Prospective and longitudinal.

SUBJECTS

Forty-seven patients with unilateral MCA stenosis or occlusion.

FIELD STRENGTH/SEQUENCE

A 3.0 T, 3D time-of-flight fast-field-echo magnetic resonance (MR) angiography sequence, spin-echo echo-planar diffusion-weighted imaging sequence, 3D fast-spin-echo pseudo-continuous ASL (pcASL) and tASL sequences.

ASSESSMENT

All patients underwent MR examination before and after MCA recanalization and scored using the National Institutes of Health Stroke Scale (NIHSS) and modified Rankin Scale (mRS) at admission and discharge. An mRS score <2 was defined as a good prognosis. 3D-pcASL and tASL cerebral blood flow (CBF) maps were obtained, and the corresponding Alberta Stroke Program Early CT Score (ASPECTS)-based scores were evaluated.

STATISTICAL TESTS

The Kolmogorov-Smirnov test, intra-class correlation coefficient, paired t-test, receiver operating characteristic (ROC) curve, and multivariable logistic regression analysis.

RESULTS

After recanalization, tASL derived absolute CBFs between the affected and contralateral sides were significantly higher than before the operation (mean: 34.3 ± 8.5 mL/100 g/min vs. 40.6 ± 9.2 mL/100 g/min, 42.6 ± 9.8 mL/100 g/min vs. 43.5 ± 9.9 mL/100 g/min, both P < 0.05). In ROC analysis, tASL provided good prognosis (area under ROC curve [AUC] = 0.829; 95% CI: 0.651-1.000, P < 0.05), while pcASL had lower prognostic value (AUC = 0.760; 95% CI: 0.574-0.946, P < 0.05). The NIHSS score before recanalization, pcASL, and tASL-based ASPECTS scores were significantly associated with good clinical outcome (P < 0.05). Multivariable analysis revealed that ASPECTS-based scores of pcASL and tASL before and after surgery were independent predictors of good clinical outcome (all P < 0.05). DATA CONCLUSION: tASL can determine hypoperfusion in the responsible vascular perfusion area and predict clinical outcome.

EVIDENCE LEVEL

4 TECHNICAL EFFICACY: Stage 2.

Spatial dependency and the role of local susceptibility for velocity selective arterial spin labeling (VS-ASL) relative tagging efficiency using accelerated 3D radial sampling with a BIR-8 preparation

PURPOSE

Velocity selective arterial spin labeling (VS-ASL) is a promising approach for non-contrast perfusion imaging that provides robustness to vascular geometry and transit times; however, VS-ASL assumes spatially uniform tagging efficiency. This work presents a mapping approach to investigate VS-ASL relative tagging efficiency including the impact of local susceptibility effects on a BIR-8 preparation.

METHODS

Numerical simulations of tagging efficiency were performed to evaluate sensitivity to regionally varying local susceptibility gradients and blood velocity. Tagging efficiency mapping was performed in susceptibility phantoms and healthy human subjects (N = 7) using a VS-ASL preparation module followed by a short, high spatial resolution 3D radial-based image acquisition. Tagging efficiency maps were compared to 4D-flow, B₁ , and B₀ maps acquired in the same imaging session for six of the seven subjects.

RESULTS

Numerical simulations were found to predict reduced tagging efficiency with the combination of high blood velocity and local gradient fields. Phantom experiments corroborated numerical results. Relative efficiency mapping in normal volunteers showed unique efficiency patterns depending on individual subject anatomy and physiology. Uniform tagging efficiency was generally observed in vivo, but reduced efficiency was noted in regions of high blood velocity and local susceptibility gradients.

CONCLUSION

We demonstrate an approach to map the relative tagging efficiency and show application of this methodology to a novel BIR-8 preparation recently proposed in the literature. We present results showing rapid flow in the presence of local susceptibility gradients can lead to complicated signal modulations in both tag and control images and reduced tagging efficiency.

The value of a shorter-delay arterial spin labeling protocol for detecting cerebrovascular impairment

Background

The aim of this study was to determine the relationship between blood oxygen level dependent (BOLD) cerebrovascular reactivity (CVR) and cerebral blood flow (CBF) obtained from arterial spin labeling (ASL) using different post labeling delays (PLD).

Methods

Forty-two patients with steno-occlusive diseases and impaired CVR were divided into two groups, one scanned with a 1.5-second (1.5-s) and the other with a 2.5-second (2.5-s) PLD ASL protocol. For all patients, a region of interest (ROI) was drawn around the CVR impairment. This affected ROI was then left-right flipped across the brain midline to obtain the control ROI. For both groups, the difference in grey matter CVR between affected and control ROI was first tested to confirm significance. The average grey matter CBF of affected and control ROIs were then compared. The same analysis method was used to compare affected and control hemispheres.

Results

In both groups of 1.5-s and 2.5-s PLD, CVR values in the affected ROI (-0.049±0.055 and -0.042±0.074%/mmHg, respectively) were significantly lower compared to that in the control ROI (0.152±0.054 and 0.152±0.053%/mmHg, respectively, P<0.0001). In the group with the 1.5-s PLD, CBF in the affected ROI (37.62±11.37 mL/100 g/min) was significantly lower compared to CBF in the control ROI (44.13±11.58 mL/100 g/min, P<0.05). However, in the group with the 2.5-s PLD, no significant differences could be seen between CBF in the affected ROI (40.50±14.82 mL/100 g/min) and CBF in the control ROI (39.68±12.49 mL/100 g/min, P=0.73). In the hemisphere-based analysis, CBF was significantly lower in the affected side than in the control side for the group with the 1.5-s PLD (P<0.05) when CVR was impaired (P<0.0001), but not for the group with the 2.5-s PLD (P=0.49).

Conclusions

In conclusion, our study reveals and highlights the value of a shorter-PLD ASL protocol, which is able to reflect CVR impairment. At the same time, we offer a better understanding of the relationship between BOLD CVR and CBF obtained from ASL.

Arterial Spin Labeling Applications in Pediatric and Adult Neurologic Disorders

Arterial spin labeling (ASL) is a powerful noncontrast magnetic resonance imaging (MRI) technique that enables quantitative evaluation of brain perfusion. To optimize the clinical and research utilization of ASL, radiologists and physicists must understand the technical considerations and age-related variations in normal and disease states. We discuss advanced applications of ASL across the lifespan, with example cases from children and adults covering a wide variety of pathologies. Through literature review and illustrated clinical cases, we highlight the subtleties as well as pitfalls of ASL interpretation. First, we review basic physical principles, techniques, and artifacts. This is followed by a discussion of normal perfusion variants based on age and physiology. The three major categories of perfusion abnormalities-hypoperfusion, hyperperfusion, and mixed patterns-are covered with an emphasis on clinical interpretation and relationship to the disease process. Major etiologies of hypoperfusion include large artery, small artery, and venous disease; other vascular conditions; global hypoxic-ischemic injury; and neurodegeneration. Hyperperfusion is characteristic of vascular malformations and tumors. Mixed perfusion patterns can be seen with epilepsy, migraine, trauma, infection/inflammation, and toxic-metabolic encephalopathy. LEVEL OF EVIDENCE: 4 TECHNICAL EFFICACY STAGE: 3.

The costs and benefits of estimating T1 of tissue alongside cerebral blood flow and arterial transit time in pseudo-continuous arterial spin labeling

Multi-post-labeling-delay pseudo-continuous arterial spin labeling (multi-PLD PCASL) allows for absolute quantification of the cerebral blood flow (CBF) as well as the arterial transit time (ATT). Estimating these perfusion parameters from multi-PLD PCASL data is a non-linear inverse problem, which is commonly tackled by fitting the single-compartment model (SCM) for PCASL, with CBF and ATT as free parameters. The longitudinal relaxation time of tissue T_1t is an important parameter in this model, as it governs the decay of the perfusion signal entirely upon entry in the imaging voxel. Conventionally, T_1t is fixed to a population average. This approach can cause CBF quantification errors, as T_1t can vary significantly inter- and intra-subject. This study compares the impact on CBF quantification, in terms of accuracy and precision, of either fixing T_1t , the conventional approach, or estimating it alongside CBF and ATT. It is shown that the conventional approach can cause a significant bias in CBF. Indeed, simulation experiments reveal that if T_1t is fixed to a value that is 10% off its true value, this may already result in a bias of 15% in CBF. On the other hand, as is shown by both simulation and real data experiments, estimating T_1t along with CBF and ATT results in a loss of CBF precision of the same order, even if the experiment design is optimized for the latter estimation problem. Simulation experiments suggest that an optimal balance between accuracy and precision of CBF estimation from multi-PLD PCASL data can be expected when using the two-parameter estimator with a fixed T_1t value between population averages of T_1t and the longitudinal relaxation time of blood T_1b .

Can Arterial Spin Labeling Perfusion Imaging be Used to Differentiate Nasopharyngeal Carcinoma From Nasopharyngeal Lymphoma?

BACKGROUND

Differentiating nasopharyngeal carcinoma (NPC) from nasopharyngeal lymphoma (NPL) is useful for deciding the appropriate treatment. However, the diagnostic accuracy of current imaging methods is low.

PURPOSE

To explore the feasibility of arterial spin labeling (ASL) perfusion imaging in the qualitative and quantitative differentiation between NPC and NPL to improve the diagnosis of malignancies in the nasopharynx.

STUDY TYPE

Retrospective.

POPULATION

Ninety seven patients: NPC (65 cases) and NPL (32 cases), histologically confirmed.

FIELD STRENGTH/SEQUENCE

3T/3D fast spin echo pseudo-continuous ASL imaging with spiral readout scheme, 3D inverse recovery_- fast spoiled gradient recalled echo brain volume (BRAVO) imaging.

ASSESSMENT

Cerebral blood flow (CBF) images from ASL perfusion imaging were assessed by three radiologists. Each tumor was visually scored based on CBF images. Intratumoral CBF and intramuscular CBF values were obtained from intratumoral and lateral pterygoid muscle areas, respectively. Through dividing intratumoral CBF by intramuscular CBF, normalized CBF (nCBF) was further calculated.

STATISTICAL TESTS

Fleiss's kappa and intraclass correlation coefficients (ICCs) were used to assess interobserver agreement among the three readers. The Mann-Whitney U-test was used to compare visual scoring, and an unpaired t-test was performed to compare CBF value between the NPC and NPL groups. The area under the curve (AUC) value was used to quantify the diagnostic ability of each parameter.

RESULTS

Good interobserver agreements were validated by high Fleiss's kappa and ICC values (all >0.80). NPCs showed significantly higher visual scores than NPLs (P < 0.05). Both intratumoral CBF and nCBF in NPC were significantly higher than those in NPL (both P < 0.05). Intratumoral CBF showed the highest AUC of 0.861 (P < 0.05) in differentiating NPC (n = 65) from NPL (n = 32), while the AUCs of nCBF and visual scoring were 0.847 and 0.753, respectively.

DATA CONCLUSION

For the diagnosis of distinguishing NPC from NPL, ASL perfusion imaging demonstrated high diagnostic efficiency.

LEVEL OF EVIDENCE

3 TECHNICAL EFFICACY STAGE: 2.

Reproducibility and Variability of Quantitative Cerebral Blood Flow Measured by Multi-delay 3D Arterial Spin Labeling According to Sex and Menstrual Cycle

Purpose : To determine the reproducibility of corrected quantitative cerebral blood flow (qCBF) through measurement of transit flow time using multi-delay three-dimensional pseudo-continuous arterial spin labeling (pCASL) in healthy men and women and to evaluate the differences in qCBF between not only men and women, but also the follicular and luteal phases of the women's menstrual cycle. Methods : The participants were 16 healthy volunteers (8 men and 8 women ; mean age, 25.3 years). Two MRI were conducted for all participants ; female participants were conducted in the follicular and luteal phases. The reproducibility of qCBF values was evaluated by the intraclass correlation coefficient (ICC) and differences between the two groups were estimated by voxel-based morphometry (VBM) analysis. Results : The qCBF values were lower in men than in women, and those in females were significantly different between the follicular and luteal phases (P < 0.05). In VBM analysis, the qCBF values of the lower frontal lobes were significantly higher in women than in men (P < 0.05). The qCBF values of the frontal pole were significantly higher in the follicular phase than in the luteal phase (P < 0.01). Conclusion : Multi-delay pCASL can reveal physiological and sex differences in cerebral perfusion. J. Med. Invest. 67 : 321-327, August, 2020.

Predicting 15O-Water PET cerebral blood flow maps from multi-contrast MRI using a deep convolutional neural network with evaluation of training cohort bias

To improve the quality of MRI-based cerebral blood flow (CBF) measurements, a deep convolutional neural network (dCNN) was trained to combine single- and multi-delay arterial spin labeling (ASL) and structural images to predict gold-standard ¹⁵O-water PET CBF images obtained on a simultaneous PET/MRI scanner. The dCNN was trained and tested on 64 scans in 16 healthy controls (HC) and 16 cerebrovascular disease patients (PT) with 4-fold cross-validation. Fidelity to the PET CBF images and the effects of bias due to training on different cohorts were examined. The dCNN significantly improved CBF image quality compared with ASL alone (mean ± standard deviation): structural similarity index (0.854 ± 0.036 vs. 0.743 ± 0.045 [single-delay] and 0.732 ± 0.041 [multi-delay], P < 0.0001); normalized root mean squared error (0.209 ± 0.039 vs. 0.326 ± 0.050 [single-delay] and 0.344 ± 0.055 [multi-delay], P < 0.0001). The dCNN also yielded mean CBF with reduced estimation error in both HC and PT (P < 0.001), and demonstrated better correlation with PET. The dCNN trained with the mixed HC and PT cohort performed the best. The results also suggested that models should be trained on cases representative of the target population.

Comparison of velocity-selective arterial spin labeling schemes

PURPOSE

In velocity-selective (VS) arterial spin labeling, strategies using multiple saturation modules or using VS inversion (VSI) pulse can provide improved SNR efficiency compared to the original labeling scheme using one VS saturation (VSS) module. Their performance improvement, however, has not been directly compared.

METHODS

Different VS labeling schemes were evaluated by Bloch simulation for their SNR efficiency, eddy current sensitivity, and robustness against B₁ and B₀ variation. These schemes included dual-module double-refocused hyperbolic secant and symmetric 8-segment B₁ -insensitive rotation (sBIR8-) VSS pulses, the original and modified Fourier transform-based VSI pulses. A subset of the labeling schemes was examined further in phantom and in vivo experiments for their eddy current sensitivity and SNR performance. An additional sBIR8-VSS with a built-in inversion (sBIR8-VSS-inversion) was evaluated for the effects of partial background suppression to allow a fairer comparison to VSI.

RESULTS

According to the simulations, the sBIR8-VSS was the most robust against field imperfections and had similarly high SNR efficiency (dual-module, dual-sBIR8-VSS) compared with the best VSI pulse (sinc-modulated, sinc-VSI). These were confirmed by the phantom and in vivo data. Without additional background suppression, the sinc-VSI pulses had the highest temporal SNR, closely followed by the sBIR8-VSS-inversion pulse, both benefited from partial background suppression effects.

CONCLUSION

Dual-sBIR8-VSS and sinc-VSI measured the highest SNR efficiency among the VS labeling schemes. Dual-sBIR8-VSS was the most robust against field imperfections, whereas sinc-VSI may provide a higher SNR efficiency if its immunity to field imperfections can be improved.

Development of fast multi-slice apparent T1 mapping for improved arterial spin labeling MRI measurement of cerebral blood flow

PURPOSE

To develop fast multi-slice apparent T₁ (T_1app ) mapping for accurate cerebral blood flow (CBF) quantification with arterial spin labeling (ASL) MRI.

METHODS

Fast multi-slice T_1app was measured using a modified inversion recovery echo planar imaging (EPI) sequence with simultaneous application of ASL tagging radiofrequency (RF) and gradient pulses. The fast multi-slice T_1app measurement was compared with the single-slice T_1app imaging approach, repeated per slice. CBF was assessed in healthy adult Wistar rats (N = 5) and rats with acute stroke 24 hours after a transient middle cerebral artery occlusion (N = 5).

RESULTS

The fast multi-slice T_1app measurement was in good agreement with that of a single-slice T_1app imaging approach (Lin's concordance correlation coefficient = 0.92). CBF calculated using T_1app reasonably accounted for the finite labeling RF duration, whereas the routine T₁ -normalized ASL MRI underestimated the CBF, particularly at short labeling durations. In acute stroke rats, the labeling time and the CBF difference (ΔCBF) between the contralateral normal area and the ischemic lesion were significantly correlated when using T₁ -normalized perfusion calculation (R = 0.844, P = .035). In comparison, T_1app -normalized ΔCBF had little labeling time dependence based on the linear regression equation of ΔCBF = -0.0247*τ + 1.579 mL/g/min (R = -0.352, P = .494).

CONCLUSIONS

Our study found fast multi-slice T_1app imaging improves the accuracy and reproducibility of CBF measurement.

Rotated spiral RARE for high spatial and temporal resolution volumetric arterial spin labeling acquisition

Background:

Arterial Spin Labeling (ASL) MRI can provide quantitative images that are sensitive to both time averaged blood flow and its temporal fluctuations. 3D image acquisitions for ASL are desirable because they are more readily compatible with background suppression to reduce noise, can reduce signal loss and distortion, and provide uniform flow sensitivity across the brain. However, single-shot 3D acquisition for maximal temporal resolution typically involves degradation of image quality through blurring or noise amplification by parallel imaging. Here, we report a new approach to accelerate a common stack of spirals 3D image acquisition by pseudo golden-angle rotation and compressed sensing reconstruction without any degradation of time averaged blood flow images.

Methods:

28 healthy volunteers were imaged at 3T with background-suppressed unbalanced pseudo-continuous ASL combined with a pseudo golden-angle Stack-of-Spirals 3D RARE readout. A fully-sampled perfusion-weighted volume was reconstructed by 3D non-uniform Fast Fourier Transform (nuFFT) followed by sum-of-squares combination of the 32 individual channels. Coil sensitivities were estimated followed by reconstruction of the 39 single-shot volumes using an L₁-wavelet Compressed-Sensing reconstruction. Finally, brain connectivity analyses were performed in regions where BOLD signal suffers from low signal-to-noise ratio and susceptibility artifacts.

Results:

Image quality, assessed with a non-reference 3D blurring metric, of full time averaged blood flow was comparable to a conventional interleaved acquisition. The temporal resolution provided by the acceleration enabled identification and quantification of resting-state networks even in inferior regions such as the amygdala and inferior frontal lobes, where susceptibility artifacts can degrade conventional resting-state fMRI acquisitions.

Conclusion:

This approach can provide measures of blood flow modulations and resting-state networks for free within any research or clinical protocol employing ASL for resting blood flow.

Increased blood-brain barrier permeability to water in the aging brain detected using noninvasive multi-TE ASL MRI

Purpose

A fundamental goal in the drive to understand and find better treatments for dementia is the identification of the factors that render the aging brain vulnerable to neurodegenerative disease. Recent evidence indicates the integrity of the blood–brain barrier (BBB) to be an important component of functional failure underlying age‐related cognitive decline. Practical and sensitive measurement is necessary, therefore, to support diagnostic and therapeutic strategies targeted at maintaining BBB integrity in aging patients. Here, we investigated changes in BBB permeability to endogenous blood water in the aging brain.

Methods

A multiple‐echo‐time arterial spin‐labeling MRI technique, implemented on a 9.4T Bruker imaging system, was applied to 7‐ and 27‐month‐old mice to measure changes in water permeability across the BBB with aging.

Results

We observed that BBB water permeability was 32% faster in aged mice. This occurred along with a 2.1‐fold increase in mRNA expression of aquaporin‐4 water channels and a 7.1‐fold decrease in mRNA expression of α‐syntrophin protein, which anchors aquaporin‐4 to the BBB.

Conclusion

Age‐related changes to water permeability across the BBB can be captured using noninvasive noncontrast MRI techniques.

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Designing and comparing optimized pseudo-continuous Arterial Spin Labeling protocols for measurement of cerebral blood flow

Arterial Spin Labeling (ASL) is a non-invasive, non-contrast, perfusion imaging technique which is inherently SNR limited. It is, therefore, important to carefully design scan protocols to ensure accurate measurements. Many pseudo-continuous ASL (PCASL) protocol designs have been proposed for measuring cerebral blood flow (CBF), but it has not yet been demonstrated which design offers the most accurate and repeatable CBF measurements. In this study, a wide range of literature PCASL protocols were first optimized for CBF accuracy and then compared using Monte Carlo simulations and in vivo experiments. The protocols included single-delay, sequential and time-encoded multi-timepoint protocols, and several novel protocol designs, which are hybrids of time-encoded and sequential multi-timepoint protocols. It was found that several multi-timepoint protocols produced more confident, accurate, and repeatable CBF estimates than the single-delay protocol, while also generating maps of arterial transit time. Of the literature protocols, the time-encoded protocol with T₁-adjusted label durations gave the most confident and accurate CBF estimates in vivo (16% and 40% better than single-delay), while the sequential multi-timepoint protocol was the most repeatable (20% more repeatable than single-delay). One of the novel hybrid protocols, Hybrid_T1-adj, was found to produce the most confident, accurate and repeatable CBF estimates out of all the protocols tested in both simulations and in vivo (24%, 47%, and 28% more confident, accurate, and repeatable than single-delay in vivo). The Hybrid_T1-adj protocol makes use of the best aspects of both time-encoded and sequential multi-timepoint protocols and should be a useful tool for accurately and efficiently measuring CBF.

Robust arterial transit time and cerebral blood flow estimation using combined acquisition of Hadamard-encoded multi-delay and long-labeled long-delay pseudo-continuous arterial spin labeling: a simulation and in vivo study

Arterial transit time (ATT) prolongation causes an error of cerebral blood flow (CBF) measurement during arterial spin labeling (ASL). To improve the accuracy of ATT and CBF in patients with prolonged ATT, we propose a robust ATT and CBF estimation method for clinical practice. The proposed method consists of a three-delay Hadamard-encoded pseudo-continuous ASL (H-pCASL) with an additional-encoding and single-delay with long-labeled long-delay (1dLLLD) acquisition. The additional-encoding allows for the reconstruction of a single-delay image with long-labeled short-delay (1dLLSD) in addition to the normal Hadamard sub-bolus images. Five different images (normal Hadamard 3 delay, 1dLLSD, 1dLLLD) were reconstructed to calculate ATT and CBF. A Monte Carlo simulation and an in vivo study were performed to access the accuracy of the proposed method in comparison to normal 7-delay (7d) H-pCASL with equally divided sub-bolus labeling duration (LD). The simulation showed that the accuracy of CBF is strongly affected by ATT. It was also demonstrated that underestimation of ATT and CBF by 7d H-pCASL was higher with longer ATT than with the proposed method. Consistent with the simulation, the 7d H-pCASL significantly underestimated the ATT compared to that of the proposed method. This underestimation was evident in the distal anterior cerebral artery (ACA; P = 0.0394) and the distal posterior cerebral artery (PCA; 2 P = 0.0255). Similar to the ATT, the CBF was underestimated with 7d H-pCASL in the distal ACA (P = 0.0099), distal middle cerebral artery (P = 0.0109), and distal PCA (P = 0.0319) compared to the proposed method. Improving the SNR of each delay image (even though the number of delays is small) is crucial for ATT estimation. This is opposed to acquiring many delays with short LD. The proposed method confers accurate ATT and CBF estimation within a practical acquisition time in a clinical setting.

Quantitative Cerebrovascular Reactivity in Normal Aging: Comparison Between Phase-Contrast and Arterial Spin Labeling MRI

Purpose: Cerebrovascular reactivity (CVR) is an index of the dilatory function of cerebral blood vessels and has shown great promise in the diagnosis of risk factors in cerebrovascular disease. Aging is one such risk factor; thus, it is important to characterize age-related differences in CVR. CVR can be measured by BOLD MRI but few studies have measured quantitative cerebral blood flow (CBF)-based CVR in the context of aging. This study aims to determine the age effect on CVR using two quantitative CBF techniques, phase-contrast (PC), and arterial spin labeling (ASL) MRI.

Methods: In 49 participants (32 younger and 17 older), CVR was measured with PC, ASL, and BOLD MRI. These CVR methods were compared across young and older groups to determine their dependence on age. PC and ASL CVR were also studied for inter-correlation and mean differences. Gray and white matter CVR values were also studied.

Results: PC CVR was higher in younger participants than older participants (by 17%, p = 0.046). However, there were no age differences in ASL or BOLD CVR. ASL CVR was significantly correlated with PC CVR (p = 0.042) and BOLD CVR (p = 0.016), but its values were underestimated compared to PC CVR (p = 0.045). ASL CVR map revealed no difference between gray matter and white matter tissue types, whereas gray matter was significantly higher than white matter in the BOLD CVR map.

Conclusion: This study compared two quantitative CVR techniques in the context of brain aging and revealed that PC CVR is a more sensitive method for detection of age differences, despite the absence of spatial information. The ASL method showed a significant correlation with PC and BOLD, but it tends to underestimate CVR due to confounding factors associated with this technique. Importantly, our data suggest that there is not a difference in CBF-based CVR between the gray and white matter, in contrast to previous observation using BOLD MRI.

Imaging Pulmonary Blood Flow Using Pseudocontinuous Arterial Spin Labeling (PCASL) With Balanced Steady-State Free-Precession (bSSFP) Readout at 1.5T

BACKGROUND

Quantitative assessment of pulmonary blood flow and visualization of its temporal and spatial distribution without contrast media is of clinical significance.

PURPOSE

To assess the potential of electrocardiogram (ECG)-triggered pseudocontinuous arterial spin labeling (PCASL) imaging with balanced steady-state free-precession (bSSFP) readout to measure lung perfusion under free-breathing (FB) conditions and to study temporal and spatial characteristics of pulmonary blood flow.

STUDY TYPE

Prospective, observational.

SUBJECTS

Fourteen volunteers; three patients with pulmonary embolism.

FIELD STRENGTH/SEQUENCES

1.5T, PCASL-bSSFP.

ASSESSMENT

The pulmonary trunk was labeled during systole. The following examinations were performed: 1) FB and timed breath-hold (TBH) examinations with a postlabeling delay (PLD) of 1000 msec, and 2) TBH examinations with multiple PLDs (100-1500 msec). Scan-rescan measurements were performed in four volunteers and one patient. Images were registered and the perfusion was evaluated in large vessels, small vessels, and parenchyma. Mean structural similarity indices (MSSIM) was computed and time-to-peak (TTP) of parenchymal perfusion in multiple PLDs was evaluated. Image quality reading was performed with three independent blinded readers.

STATISTICAL TESTS

Wilcoxon test to compare MSSIM, perfusion, and Likert scores. Spearman's correlation to correlate TTP and cardiac cycle duration. The repeatability coefficient (RC) and within-subject coefficient of variation (wCV) for scan-rescan measurements. Intraclass correlation coefficient (ICC) for interreader agreement.

RESULTS

Image registration resulted in a significant (P < 0.05) increase of MSSIM. FB perfusion values were 6% higher than TBH (3.28 ± 1.09 vs. 3.10 ± 0.99 mL/min/mL). TTP was highly correlated with individuals' cardiac cycle duration (Spearman = 0.89, P < 0.001). RC and wCV were better for TBH than FB (0.13-0.19 vs. 0.47-1.54 mL/min/mL; 6-7 vs. 19-60%). Image quality was rated very good, with ICCs 0.71-0.89.

DATA CONCLUSION

ECG-triggered PCASL-bSSFP imaging of the lung at 1.5T can provide very good image quality and quantitative perfusion maps even under FB. The course of labeled blood through the lung shows a strong dependence on the individuals' cardiac cycle duration.

LEVEL OF EVIDENCE

2 TECHNICAL EFFICACY STAGE: 2 J. MAGN. RESON. IMAGING 2020;52:1767-1782.

Neuroimaging of Cerebral Blood Flow and Sodium in Women with Lipedema

OBJECTIVE

Lipedema is characterized by pain, fatigue, and excessive adipose tissue and sodium accumulation of the lower extremities. This case-control study aims to determine whether sodium or vascular dysfunction is present in the central nervous system.

METHODS

Brain magnetic resonance imaging was performed at 3 T in patients with lipedema (n = 15) and control (n = 18) participants matched for sex, age, race, and BMI. Standard anatomical imaging and intracranial angiography were applied to evaluate brain volume and vasculopathy, respectively; arterial spin labeling and sodium magnetic resonance imaging were applied to quantify cerebral blood flow (CBF) (milliliters per 100 grams of tissue/minute) and brain tissue sodium content (millimoles per liter), respectively. A Mann-Whitney U test (significance criteria P < 0.05) was applied to evaluate group differences.

RESULTS

No differences in tissue volume, white matter hyperintensities, intracranial vasculopathy, or tissue sodium content were observed between groups. Gray matter CBF was elevated (P = 0.03) in patients with lipedema (57.2 ± 9.6 mL per 100 g/min) versus control participants (49.8 ± 9.1 mL per 100 g/min).

CONCLUSIONS

Findings provide evidence that brain sodium and tissue fractions are similar between patients with lipedema and control participants and that patients with lipedema do not exhibit abnormal radiological indicators of intracranial vasculopathy or ischemic injury. Potential explanations for elevated CBF are discussed in the context of the growing literature on lipedema symptomatology and vascular dysfunction.

ExploreASL: An image processing pipeline for multi-center ASL perfusion MRI studies

Arterial spin labeling (ASL) has undergone significant development since its inception, with a focus on improving standardization and reproducibility of its acquisition and quantification. In a community-wide effort towards robust and reproducible clinical ASL image processing, we developed the software package ExploreASL, allowing standardized analyses across centers and scanners. The procedures used in ExploreASL capitalize on published image processing advancements and address the challenges of multi-center datasets with scanner-specific processing and artifact reduction to limit patient exclusion. ExploreASL is self-contained, written in MATLAB and based on Statistical Parameter Mapping (SPM) and runs on multiple operating systems. To facilitate collaboration and data-exchange, the toolbox follows several standards and recommendations for data structure, provenance, and best analysis practice. ExploreASL was iteratively refined and tested in the analysis of >10,000 ASL scans using different pulse-sequences in a variety of clinical populations, resulting in four processing modules: Import, Structural, ASL, and Population that perform tasks, respectively, for data curation, structural and ASL image processing and quality control, and finally preparing the results for statistical analyses on both single-subject and group level. We illustrate ExploreASL processing results from three cohorts: perinatally HIV-infected children, healthy adults, and elderly at risk for neurodegenerative disease. We show the reproducibility for each cohort when processed at different centers with different operating systems and MATLAB versions, and its effects on the quantification of gray matter cerebral blood flow. ExploreASL facilitates the standardization of image processing and quality control, allowing the pooling of cohorts which may increase statistical power and discover between-group perfusion differences. Ultimately, this workflow may advance ASL for wider adoption in clinical studies, trials, and practice.

Usefulness of Post-labeling Delay for the Assessment of Bright Vessel Appearance by Arterial Spin Labeling

Objective

This study was performed to clarify the differences in blood flow strength, blood vessel diameter, and post-labeling delay (PLD) by physical experiments, and to examine whether bright vessel appearance (BVA) can be observed by arterial spin labeling (ASL).

Methods

We introduced simulated blood flow (25 cm/sec, 12.5 cm/sec) using a specially made phantom of fixed tubes in a plastic container. At each speed, we scanned at several points of PLD using ASL imaging. We measured the signal in the tube to obtain a signal intensity (SI). We revised the T1 level from the SI and obtained SI_blood. We used SI_tissue with normal perfusion measured from obtained clinical images by ASL and compared it with SI_blood.

Results

In tubes with a narrow inner diameter, the signal slightly decreased. SI also decreased under slow flow compared with fast flow. At each flow rate, SI_blood significantly exceeded SI_tissue.

Conclusion

PLD distinguishes spin in brain tissue from 1525 msec to 2525 msec, and it can be observed. As spin signal decreases when the flow rate is slow, attention is necessary for observation. Assessment at PLD1525-2525 msec where normal perfusion was obtained suggested that BVA can be observed.

Abnormal perfusion fluctuation and perfusion connectivity in bipolar disorder measured by dynamic arterial spin labeling

OBJECTIVES

We sought to evaluate whether dynamic Arterial Spin Labeling (dASL), a novel quantitative technique robust to artifacts and noise that especially arise in inferior brain regions, could characterize neural substrates of BD pathology and symptoms.

METHODS

Forty-five subjects (19 BD patients, 26 controls) were imaged using a dASL sequence. Maps of average perfusion, perfusion fluctuation, and perfusion connectivity with anterior cingulate cortex (ACC) were derived. Patient symptoms were quantified along four symptom dimensions determined using factor analysis of the subjects from the Bipolar and Schizophrenia Network on Intermediate Phenotypes (BSNIP) study. Maps of the perfusion measures were compared between BD patients and controls and correlated with the symptom dimensions in the BD patients only by voxel-level and region-level analyses.

RESULTS

BD patients exhibited (i) significantly increased perfusion fluctuations in the left fusiform and inferior temporal regions (P = .020, voxel-level corrected) and marginally increased perfusion fluctuations in the right temporal pole and inferior temporal regions (P = .063, cluster-level corrected), (ii) significantly increased perfusion connectivity between ACC and the occipitoparietal cortex (P = .050, cluster-level corrected). In BD patients, positive symptoms were negatively associated with ACC perfusion connectivity to the right orbitofrontal and superior frontal regions (P = .002, cluster-level corrected) and right orbitofrontal and inferior frontal regions (P = .023, cluster-level corrected).

CONCLUSION

The abnormal perfusion fluctuations and connectivity alterations may underlie the mood fluctuations and cognitive and emotional dysregulation that characterize BD.

Optimizing MRF-ASL scan design for precise quantification of brain hemodynamics using neural network regression

PURPOSE

Arterial Spin Labeling (ASL) is a quantitative, non-invasive alternative for perfusion imaging that does not use contrast agents. The magnetic resonance fingerprinting (MRF) framework can be adapted to ASL to estimate multiple physiological parameters simultaneously. In this work, we introduce an optimization scheme to increase the sensitivity of the ASL fingerprint. We also propose a regression based estimation framework for MRF-ASL.

METHODS

To improve the sensitivity of MRF-ASL signals to underlying parameters, we optimized ASL labeling durations using the Cramer-Rao Lower Bound (CRLB). This paper also proposes a neural network regression based estimation framework trained using noisy synthetic signals generated from our ASL signal model. We tested our methods in silico and in vivo, and compared with multiple post labeling delay (multi-PLD) ASL and unoptimized MRF-ASL. We present comparisons of estimated maps for the six parameters of our signal model.

RESULTS

The scan design process facilitated precise estimates of multiple hemodynamic parameters and tissue properties from a single scan, in regions of normal gray and white matter, as well as regions with anomalous perfusion activity in the brain. In particular, there was a 86.7% correlation of perfusion estimates with the ground truth in silico, using our proposed techniques. In vivo, there was roughly a 7 fold improvement in the Coefficient of Variation (CoV) for white matter perfusion, and 2 fold improvement in gray matter perfusion CoV in comparison to a reference Multi PLD method. The regression based estimation approach provided perfusion estimates rapidly, with estimation times of around 1s per map.

CONCLUSIONS

Scan design optimization, coupled with regression-based estimation is a powerful tool for improving precision in MRF-ASL.

Improved velocity-selective-inversion arterial spin labeling for cerebral blood flow mapping with 3D acquisition

PURPOSE

To further optimize the velocity-selective arterial spin labeling (VSASL) sequence utilizing a Fourier-transform based velocity-selective inversion (FT-VSI) pulse train, and to evaluate its utility for 3D mapping of cerebral blood flow (CBF) with a gradient- and spin-echo (GRASE) readout.

METHODS

First, numerical simulations and phantom experiments were done to test the susceptibility to eddy currents and B₁ field inhomogeneities for FT-VSI pulse trains with block and composite refocusing pulses. Second, the choices of the post-labeling delay (PLD) for FT-VSI prepared 3D VSASL were evaluated for the sensitivity to perfusion signal. The study was conducted among a young-age and a middle-age group at 3T. Both signal-to-noise ratio (SNR) and CBF were quantitatively compared with pseudo-continuous ASL (PCASL). The optimized 3D VSI-ASL was also qualitatively compared with PCASL in a whole-brain coverage among two healthy volunteers and a brain tumor patient.

RESULTS

The simulations and phantom test showed that composite refocusing pulses are more robust to both eddy-currents and B₁ field inhomogeneities than block pulses. 3D VSASL images with FT-VSI preparation were acquired over a range of PLDs and PLD = 1.2 s was selected for its higher perfusion signal. FT-VSI labeling produced quantitative CBF maps with 27% higher SNR in gray matter compared to PCASL. 3D whole-brain CBF mapping using VSI-ASL were comparable to the corresponding PCASL results.

CONCLUSION

FT-VSI with 3D-GRASE readout was successfully implemented and showed higher sensitivity to perfusion signal than PCASL for both young and middle-aged healthy volunteers.

Cerebral Perfusion Does Not Increase after Shunt Surgery for Normal Pressure Hydrocephalus

BACKGROUND AND PURPOSE

Cerebral blood flow (CBF) has been reported to increase after shunt surgery in patients with idiopathic normal pressure hydrocephalus (iNPH). The aims of this study were to investigate if CBF, measured using the noninvasive perfusion MRI method arterial spin labeling (ASL), increased after shunt surgery, if postoperative change in CBF correlated with improvement in symptoms, and if baseline CBF data correlated with postoperative outcome.

METHODS

Twenty-three patients with iNPH were prospectively included and examined with MRI of the brain and clinical tests of symptoms at baseline. Eighteen of the patients were treated with shunt implantation and were reexamined with clinical tests and MRI 3 months postoperatively. The MRI protocol included a pseudo-continuous ASL sequence for perfusion imaging. The perfusion was measured in 12 manually drawn regions of interest (ROIs).

RESULTS

In the whole sample, CBF did not increase after shunting in any ROI. Preoperative CBF in medial frontal cortex correlated with an improvement in urinary incontinence after shunt surgery, r = .53, P = .022. There were no correlations between change in CBF and change in clinical symptoms postoperatively.

CONCLUSIONS

The clinical value of ASL in the work-up of patients with iNPH is uncertain. In this study, ASL could not predict outcome after shunt surgery and there were no correlations between change in CBF and change in clinical symptoms after shunt surgery.

Numerical approximation to the general kinetic model for ASL quantification

PURPOSE

To develop a numerical approximation to the general kinetic model for arterial spin labeling (ASL) quantification that will enable greater flexibility in ASL acquisition methods.

THEORY

The Bloch-McConnell equations are extended to include the effects of single-compartment inflow and outflow on both the transverse and longitudinal magnetization. These can be solved using an extension of Jaynes' matrix formalism with piecewise constant approximation of incoming labeled arterial flow and a clearance operator for outgoing venous flow.

METHODS

The proposed numerical approximation is compared with the general kinetic model using simulations of pulsed labeling and pseudo-continuous labeling and a broad range of transit time and bolus duration for tissue blood flow of 0.6 mL/g/min. Accuracy of the approximation is studied as a function of the timestep using Monte-Carlo simulations. Three additional scenarios are demonstrated: (1) steady-pulsed ASL, (2) MR fingerprinting ASL, and (3) balanced SSFP and spoiled gradient-echo sequences.

RESULTS

The proposed approximation was found to be arbitrarily accurate for pulsed labeling and pseudo-continuous labeling. The pulsed labeling/pseudo-continuous labeling approximation error compared with the general kinetic model was less than 0.002% (<0.002%) and less than 0.05% (<0.05%) for timesteps of 3 ms and 35 ms, respectively. The proposed approximation matched well with customized signal expressions of steady-pulsed ASL and MR fingerprinting ASL. The simulations of simultaneous modeling of flow, T₂ , and magnetization transfer showed an increase in steady-state balanced SSFP and spoiled gradient signals.

CONCLUSION

We demonstrate a numerical approximation of the "Bloch-McConnell flow" equations that enables arbitrarily accurate modeling of pulsed ASL and pseudo-continuous labeling signals comparable to the general kinetic model. This enables increased flexibility in the experiment design for quantitative ASL.

Non-Invasive MRI of Blood-Cerebrospinal Fluid Barrier Function

The blood–cerebrospinal fluid barrier (BCSFB) is a highly dynamic transport interface that serves brain homeostasis. To date, however, understanding of its role in brain development and pathology has been hindered by the absence of a non-invasive technique for functional assessment. Here we describe a method for non-invasive measurement of BSCFB function by using tracer-free MRI to quantify rates of water delivery from arterial blood to ventricular cerebrospinal fluid. Using this method, we record a 36% decrease in BCSFB function in aged mice, compared to a 13% decrease in parenchymal blood flow, itself a leading candidate biomarker of early neurodegenerative processes. We then apply the method to explore the relationship between BCSFB function and ventricular morphology. Finally, we provide proof of application to the human brain. Our findings position the BCSFB as a promising new diagnostic and therapeutic target, the function of which can now be safely quantified using non-invasive MRI.

The blood–cerebrospinal fluid barrier (BCSFB) is an important interface for brain homeostasis. Here the authors describe a non-invasive MRI technique for the quantitative assessment of BCSFB function.

Calibration of arterial spin labeling data-potential pitfalls in post-processing

PURPOSE

To assess the impact of the different post-processing options in the calibration of arterial spin labeling (ASL) data on perfusion quantification and its reproducibility.

THEORY AND METHODS

Absolute quantification of perfusion measurements is one of the promises of ASL techniques. However, it is highly dependent on a calibration procedure that involves a complex processing pipeline for which no standardized procedure has been fully established. In this work, we systematically compare the main ASL calibration methods as well as various post-processing calibration options, using 2 data sets acquired with the most common sequences, pulsed ASL and pseudo-continuous ASL.

RESULTS

Significant and sometimes large discrepancies in ASL perfusion quantification were obtained when using different post-processing calibration options. Nevertheless, when using a set of theoretically based and carefully chosen options, only small differences were observed for both reference tissue and voxelwise methods. The voxelwise and white matter reference tissue methods were less sensitive to post-processing options than the cerebrospinal fluid reference tissue method. However, white matter reference tissue calibration also produced poorer reproducibility results. Moreover, it may also not be an appropriate reference in case of white matter pathology.

CONCLUSION

Poor post-processing calibration options can lead to large errors in perfusion quantification, and a complete description of the calibration procedure should therefore be reported in ASL studies. Overall, our results further support the voxelwise calibration method proposed by the ASL white paper, particularly given the advantage of being relatively simple to implement and intrinsically correcting for the coil sensitivity profile.

Multimodal analysis using [11C]PiB-PET/MRI for functional evaluation of patients with Alzheimer's disease

BACKGROUND

Multimodal PET/MRI image data simultaneously obtained from patients with early-stage of Alzheimer's disease (eAD) were assessed in order to observe pathophysiologic and functional changes, as well as alterations of morphology and connectivity in the brain. Fifty-eight patients with mild cognitive impairment and early dementia (29 males, 69 ± 12 years) underwent [¹¹C]Pittsburgh compound-B (PiB) PET/MRI with 70-min PET and MRI scans. Sixteen age-matched healthy controls (CTL) (9 males, 68 ± 11 years) were also studied with the same scanning protocol. Cerebral blood flow (CBF) was calculated from the early phase PET images using the image-derived input function method. A standardized uptake value ratio (SUVr) was calculated from 50 to 70 min PET data with a reference region of the cerebellar cortex. MR images such as 3D-T1WI, resting-state functional MRI (RS-fMRI), diffusion tensor image (DTI), and perfusion MRI acquired during the dynamic PET scan were also analyzed to evaluate various brain functions on MRI.

RESULTS

Twenty-seven of the 58 patients were determined as eAD based on the results of PiB-PET and clinical findings, and a total of 43 subjects' data including CTL were analyzed in this study. PiB SUVr values in all cortical regions of eAD were significantly greater than those of CTL. The PiB accumulation intensity was negatively correlated with cognitive scores. The regional PET-CBF values of eAD were significantly lower in the bilateral parietal lobes and right temporal lobe compared with CTL, but not in MRI perfusion; however, SPM showed regional differences on both PET- and MRI-CBF. SPM analysis of RS-fMRI delineated regional differences between the groups in the anterior cingulate cortex and the left precuneus. VBM analysis showed atrophic changes in the AD group in a part of the bilateral hippocampus; however, analysis of fractional anisotropy calculated from DTI data did not show differences between the two groups.

CONCLUSION

Multimodal analysis conducted with various image data from PiB-PET/MRI scans showed differences in regional CBF, cortical volume, and neuronal networks in different regions, indicating that pathophysiologic and functional changes in the AD brain can be observed from various aspects of neurophysiologic parameters. Application of multimodal brain images using PET/MRI would be ideal for investigating pathophysiologic changes in patients with dementia and other neurodegenerative diseases.

Spatial coefficient of variation of arterial spin labeling MRI as a cerebrovascular correlate of carotid occlusive disease

Clinical interpretation of arterial spin labeling (ASL) perfusion MRI in cerebrovascular disease remains challenging mainly because of the method's sensitivity to concomitant contributions from both intravascular and tissue compartments. While acquisition of multi-delay images can differentiate between the two contributions, the prolonged acquisition is prone to artifacts and not practical for clinical applications. Here, the utility of the spatial coefficient of variation (sCoV) of a single-delay ASL image as a marker of the intravascular contribution was evaluated by testing the hypothesis that sCoV can detect the effects of differences in label arrival times between ipsi- and contra-lateral hemispheres even in the absence of a hemispheric difference in CBF. Hemispheric lateralization values for sCoV and CBF were computed from ASL images acquired on 28 patients (age 73.9 ± 10.2 years, 8 women) with asymptomatic unilateral carotid occlusion. The results showed that sCoV lateralization predicted the occluded side with 96.4% sensitivity, missing only 1 patient. In contrast, the sensitivity of the CBF lateralization was 71.4%, with 8 patients showing no difference in CBF between hemispheres. The findings demonstrate the potential clinical utility of sCoV as a cerebrovascular correlate of large vessel disease. Using sCoV in tandem with CBF, vascular information can be obtained in image processing without the need for additional scan-time.

Measuring water exchange across the blood-brain barrier using MRI

The blood-brain barrier (BBB) regulates the transfer of solutes and essential nutrients into the brain. Growing evidence supports BBB dysfunction in a range of acute and chronic brain diseases, justifying the need for novel research and clinical tools that can non-invasively detect, characterize, and quantify BBB dysfunction in-vivo. Many approaches already exist for measuring BBB dysfunction in man using positron emission tomography and magnetic resonance imaging (e.g. dynamic contrast-enhanced MRI measurements of gadolinium leakage). This review paper focusses on MRI measurements of water exchange across the BBB, which occurs through a wide range of pathways, and is likely to be a highly sensitive marker of BBB dysfunction. Key mathematical models and acquisition methods are discussed for the two main approaches: those that utilize contrast agents to enhance relaxation rate differences between the intravascular and extravascular compartments and so enhance the sensitivity of MRI signals to BBB water exchange, and those that utilize the dynamic properties of arterial spin labelling to first isolate signal from intravascular spins and then estimate the impact of water exchange on the evolving signal. Data from studies in healthy and pathological brain tissue are discussed, in addition to validation studies in rodents.

Discovery of Balovaptan, a Vasopressin 1a Receptor Antagonist for the Treatment of Autism Spectrum Disorder

We recently reported the discovery of a potent, selective, and brain-penetrant V1a receptor antagonist, which was not suitable for full development. Nevertheless, this compound was found to improve surrogates of social behavior in adults with autism spectrum disorder in an exploratory proof-of-mechanism study. Here we describe scaffold hopping that gave rise to triazolobenzodiazepines with improved pharmacokinetic properties. The key to balancing potency and selectivity while minimizing P-gp mediated efflux was fine-tuning of hydrogen bond acceptor basicity. Ascertaining a V1a antagonist specific brain activity pattern by pharmacological magnetic resonance imaging in the rat played a seminal role in guiding optimization efforts, culminating in the discovery of balovaptan (RG7314, RO5285119) 1. In a 12-week clinical phase 2 study in adults with autism spectrum disorder balovaptan demonstrated improvements in Vineland-II Adaptive Behavior Scales, a secondary end point comprising communication, socialization, and daily living skills. Balovaptan entered phase 3 clinical development in August 2018.

Cerebral Blood Flow Alterations in High Myopia: An Arterial Spin Labeling Study

Objective

The aim of this study was to explore cerebral blood flow (CBF) alterations in subjects with high myopia (HM) using three-dimensional pseudocontinuous arterial spin labeling (3D-pcASL).

Methods

A total of sixteen patients with bilateral HM and sixteen age- and sex-matched healthy controls (HCs) were recruited. All subjects were right-handed. Image data preprocessing was performed using SPM8 and the DPABI toolbox. Clinical parameters were acquired in the HM group. Two-sample t-tests and Pearson correlation analysis were applied in this study.

Results

Compared to HCs, patients with HM exhibited significantly increased CBF in the bilateral cerebellum, and no decreases in CBF were detected in the brain. However, no relationship was found between the mean CBF values in the different brain areas and the disease duration (P > 0.05).

Conclusions

Using ASL analysis, we detected aberrant blood perfusion in the cerebellum in HM patients, contributing to a better understanding of brain abnormalities and brain plasticity through a different perspective.

Detection of hemodynamic impairment on 15O gas PET using visual assessment of arterial spin-labeling MR imaging in patients with moyamoya disease

It is unclear whether the visual assessment of noninvasive arterial spin labeling magnetic resonance imaging (ASL) can identify instances of hemodynamic compromise including an elevated oxygen extraction fraction (OEF) measured by ¹⁵O-gas positron emission tomography (PET). Here we evaluated the relationship between a four-point visual assessment system referred to as 'ASL scores' using ASL with two postlabeling delays (PLDs; 1525 ms and 2525 ms) and some quantitative hemodynamic parameters measured by PET. We retrospectively evaluated the cases of 18 Japanese patients with moyamoya disease who underwent ASL and PET. We compared the patients' regional ASL scores on two ASL images to the regional values of PET parameters, and we observed a significant trend in accord with the presumed clinical severity among all PET parameters and ASL scores (p < .003). The ASL score of the long PLD (2525 ms) showed the highest specificity (98.5%) for elevated OEF. Our results suggest that hemodynamic impairment (including elevated OEF) in patients with moyamoya disease may be grossly assessed by a visual assessment of noninvasive ASL images, which can be easily obtained in clinical settings.

Consensus-based technical recommendations for clinical translation of renal ASL MRI

OBJECTIVES

This study aimed at developing technical recommendations for the acquisition, processing and analysis of renal ASL data in the human kidney at 1.5 T and 3 T field strengths that can promote standardization of renal perfusion measurements and facilitate the comparability of results across scanners and in multi-centre clinical studies.

METHODS

An international panel of 23 renal ASL experts followed a modified Delphi process, including on-line surveys and two in-person meetings, to formulate a series of consensus statements regarding patient preparation, hardware, acquisition protocol, analysis steps and data reporting.

RESULTS

Fifty-nine statements achieved consensus, while agreement could not be reached on two statements related to patient preparation. As a default protocol, the panel recommends pseudo-continuous (PCASL) or flow-sensitive alternating inversion recovery (FAIR) labelling with a single-slice spin-echo EPI readout with background suppression and a simple but robust quantification model.

DISCUSSION

This approach is considered robust and reproducible and can provide renal perfusion images of adequate quality and SNR for most applications. If extended kidney coverage is desirable, a 2D multislice readout is recommended. These recommendations are based on current available evidence and expert opinion. Nonetheless they are expected to be updated as more data become available, since the renal ASL literature is rapidly expanding.

Consensus-based technical recommendations for clinical translation of renal ASL MRI

Objectives

This study aimed at developing technical recommendations for the acquisition, processing and analysis of renal ASL data in the human kidney at 1.5 T and 3 T field strengths that can promote standardization of renal perfusion measurements and facilitate the comparability of results across scanners and in multi-centre clinical studies.

Methods

An international panel of 23 renal ASL experts followed a modified Delphi process, including on-line surveys and two in-person meetings, to formulate a series of consensus statements regarding patient preparation, hardware, acquisition protocol, analysis steps and data reporting.

Results

Fifty-nine statements achieved consensus, while agreement could not be reached on two statements related to patient preparation. As a default protocol, the panel recommends pseudo-continuous (PCASL) or flow-sensitive alternating inversion recovery (FAIR) labelling with a single-slice spin-echo EPI readout with background suppression and a simple but robust quantification model.

Discussion

This approach is considered robust and reproducible and can provide renal perfusion images of adequate quality and SNR for most applications. If extended kidney coverage is desirable, a 2D multislice readout is recommended. These recommendations are based on current available evidence and expert opinion. Nonetheless they are expected to be updated as more data become available, since the renal ASL literature is rapidly expanding.

Electronic supplementary material

The online version of this article (10.1007/s10334-019-00800-z) contains supplementary material, which is available to authorized users.

Associations between cerebral blood flow and structural and functional brain imaging measures in individuals with neuropsychologically defined mild cognitive impairment

Reduced cerebral blood flow (CBF), an indicator of neurovascular processes and metabolic demands, is a common finding in Alzheimer's disease. However, little is known about what contributes to CBF deficits in individuals with mild cognitive impairment (MCI). We examine regional CBF differences in 17 MCI compared with 21 age-matched cognitively healthy older adults. Next, we examined associations between CBF, white matter lesion (WML) volume, amplitude of low-frequency fluctuations, and cortical thickness to better understand whether altered CBF was detectable before other markers and the potential mechanistic underpinnings of CBF deficits in MCI. MCI had significantly reduced CBF, whereas cortical thickness and amplitude of low-frequency fluctuation were not affected. Reduced CBF was associated with the WML volume but not associated with other measures. Given the presumed vascular etiology of WML and relative worsening of vascular health in MCI, it may suggest CBF deficits result from early vascular as opposed to metabolic deficits in MCI. These findings may support vascular mechanisms as an underlying component of cognitive impairment.

Using variable-rate selective excitation (VERSE) radiofrequency pulses to reduce power deposition in pulsed arterial spin labeling sequence at 7 Tesla

PURPOSE

Arterial spin labeling (ASL) is an established noninvasive MRI technique used for cerebral blood flow measurement, which generally suffers from a low signal-to-noise ratio (SNR). The use of ultra-high fields to enhance sensitivity inevitably results in an increase in TR because of specific absorption rate (SAR) constraints, causing inadequate sampling of hemodynamic response in functional MRI, and adversely affecting concurrent measurement such as blood oxygen level dependent. To address this problem, variable-rate selective excitation (VERSE) radiofrequency (RF) pulses were used.

METHODS

The conventional (sinc) selective RF pulses of the Q2TIPS block in the PICORE pulsed ASL (PASL) sequence used for blood saturation were replaced by their equivalent VERSE RF waveforms. Nine healthy volunteers were scanned using the conventional and VERSE PASL sequences on a head-only 7T scanner operating in parallel transmit mode.

RESULTS

VERSE PASL sequence provides perfusion images similar to the conventional version, with comparable perfusion SNR (conventional, 3.33 ± 0.48; VERSE, 3.26 ± 0.55) and temporal SNR (conventional, 1.02 ± 0.20; VERSE, 1.05 ± 0.12) for TR = 3.5 seconds and 70 measurements. With shorter acquisition time (TR = 2.5 seconds), VERSE PASL sequence still provides similar results to those acquired using the conventional PASL sequence with longer TR = 3.5 seconds.

CONCLUSION

The use of VERSE RF pulses in the Q2TIPS block of a PASL sequence allowed for the reduction of RF power deposition and, consequently, an increase in the temporal resolution and/or perfusion SNR.

Identifying cardiovascular risk factors that impact cerebrovascular reactivity: An ASL MRI study

BACKGROUND

To maintain cerebral blood flow (CBF), cerebral blood vessels dilate and contract in response to blood supply through cerebrovascular reactivity (CR).

PURPOSE

Cardiovascular (CV) disease is associated with increased stroke risk, but which risk factors specifically impact CR is unknown.

STUDY TYPE

Prospective longitudinal.

SUBJECTS

Fifty-three subjects undergoing carotid endarterectomy or stenting.

FIELD STRENGTH/SEQUENCE

3T, 3D pseudo-continuous arterial spin labeling (PCASL) ASL, and T₁ 3D fast spoiled gradient echo (FSPGR).

ASSESSMENT

We evaluated group differences in CBF changes for multiple cardiovascular risk factors in patients undergoing carotid revascularization surgery.

STATISTICAL TESTS

PRE (baseline), POST (48-hour postop), and 6MO (6 months postop) whole-brain CBF measurements, as 129 CBF maps from 53 subjects were modeled as within-subject analysis of variance (ANOVA). To identify CV risk factors associated with CBF change, the CBF change from PRE to POST, POST to 6MO, and PRE to 6MO were modeled as multiple linear regression with each CV risk factor as an independent variable. Statistical models were performed controlling for age on a voxel-by-voxel basis using SPM8. Significant clusters were reported if familywise error (FWE)-corrected cluster-level was P < 0.05, while the voxel-level significance threshold was set for P < 0.001.

RESULTS

The entire group showed significant (cluster-level P < 0.001) CBF increase from PRE to POST, decrease from POST to 6MO, and no significant difference (all voxels with P > 0.001) from PRE to 6MO. Of multiple CV risk factors evaluated, only elevated systolic blood pressure (SBP, P = 0.001), chronic renal insufficiency (CRI, P = 0.026), and history of prior stroke (CVA, P < 0.001) predicted lower increases in CBF PRE to POST. Over POST to 6MO, obesity predicted lower (P > 0.001) and cholesterol greater CBF decrease (P > 0.001).

DATA CONCLUSION

The CV risk factors of higher SBP, CRI, CVA, BMI, and cholesterol may indicate altered CR, and may warrant different stroke risk mitigation and special consideration for CBF change evaluation.

LEVEL OF EVIDENCE

1 Technical Efficacy: Stage 5 J. Magn. Reson. Imaging 2020;51:734-747.

Self-directed down-regulation of auditory cortex activity mediated by real-time fMRI neurofeedback augments attentional processes, resting cerebral perfusion, and auditory activation

In this work, we investigated the use of real-time functional magnetic resonance imaging (fMRI) with neurofeedback training (NFT) to teach volitional down-regulation of the auditory cortex (AC) using directed attention strategies as there is a growing interest in the application of fMRI-NFT to treat neurologic disorders. Healthy participants were separated into two groups: the experimental group received real feedback regarding activity in the AC; the control group was supplied sham feedback yoked from a random participant in the experimental group and matched for fMRI-NFT experience. Each participant underwent five fMRI-NFT sessions. Each session contained 2 neurofeedback runs where participants completed alternating blocks of "rest" and "lower" conditions while viewing a continuously-updated bar representing AC activation and listening to continuous noise. Average AC deactivation was extracted from each closed-loop neuromodulation run and used to quantify the control over AC (AC control), which was found to significantly increase across training in the experimental group. Additionally, behavioral testing was completed outside of the MRI on sessions 1 and 5 consisting of a subjective questionnaire to assess attentional control and two quantitative tests of attention. No significant changes in behavior were observed; however, there was a significant correlation between changes in AC control and attentional control. Also, in a neural assessment before and after fMRI-NFT, AC activity in response to continuous noise stimulation was found to significantly decrease across training while changes in AC resting perfusion were found to be significantly greater in the experimental group. These results may be useful in formulating effective therapies outside of the MRI, specifically for chronic tinnitus which is often characterized by hyperactivity of the primary auditory cortex and altered attentional processes. Furthermore, the modulation of attention may be useful in developing therapies for other disorders such as chronic pain.

A general framework for optimizing arterial spin labeling MRI experiments

PURPOSE

Arterial spin labeling (ASL) MRI is a non-invasive perfusion imaging technique that is inherently SNR limited, so scan protocols ideally need to be rigorously optimized to provide the most accurate measurements. A general framework is presented for optimizing ASL experiments to achieve optimal accuracy for perfusion estimates and, if required, other hemodynamic parameters, within a fixed scan time. The effectiveness of this framework is then demonstrated by optimizing the post-labeling delays (PLDs) of a multi-PLD pseudo-continuous ASL experiment and validating the improvement using simulations and in vivo data.

THEORY AND METHODS

A simple framework is proposed based on the use of the Cramér-Rao lower bound to find the protocol design which minimizes the predicted parameter estimation errors. Protocols were optimized for cerebral blood flow (CBF) accuracy or both CBF and arterial transit time (ATT) accuracy and compared to a conventional multi-PLD protocol, with evenly spaced PLDs, and a single-PLD protocol, using simulations and in vivo experiments in healthy volunteers.

RESULTS

Simulations and in vivo data agreed extremely well with the predicted performance of all protocols. For the in vivo experiments, optimizing for just CBF resulted in a 48% and 15% decrease in CBF errors, relative to the reference multi-PLD and single-PLD protocols, respectively. Optimizing for both CBF and ATT reduced CBF errors by 37%, without a reduction in ATT accuracy, relative to the reference multi-PLD protocol.

CONCLUSION

The presented framework can effectively design ASL experiments to minimize measurement errors based on the requirements of the scan.

The value of arterial spin labelling in adults glioma grading: systematic review and meta-analysis

This study aimed to evaluate the diagnostic performance of arterial spin labelling (ASL) in grading of adult gliomas. Eighteen studies matched the inclusion criteria and were included after systematic searches through EMBASE and MEDLINE databases. The quality of the included studies was assessed utilizing Quality Assessment of Diagnostic Accuracy Studies-2 (QUADAS-2). The quantitative values were extracted and a meta-analysis was subsequently based on a random-effect model with forest plot and joint sensitivity and specificity modelling. Hierarchical summary receiver operating characteristic (HROC) curve analysis was also conducted. The absolute tumour blood flow (TBF) values can differentiate high-grade gliomas (HGGs) from low-grade gliomas (LGGs) and grade II from grade IV tumours. However, it lacked the capacity to differentiate grade II from grade III tumours and grade III from grade IV tumours. In contrast, the relative TBF (rTBF) is effective in differentiating HGG from LGG and in glioma grading. The maximum rTBF (rTBFmax) demonstrated the best results in glioma grading. These results were also reflected in the sensitivity/specificity analysis in which the rTBFmax showed the highest discrimination performance in glioma grading. The estimated effect size for the rTBF was approximately similar between HGGs and LGGs, and grade II and grade III tumours, (-1.46 (-2.00, -0.91), p-value < 0.001), (-1.39 (-1.89, -0.89), p-value < 0.001), respectively; while it exhibited smaller effect size between grade III and grade IV (-1.05 (-1.82, -0.27)), p < 0.05). Sensitivity and specificity analysis replicate these results as well. This meta-analysis suggests that ASL is useful for glioma grading, especially when considering the rTBFmax parameter.

Recent progress in ASL

This article aims to provide the reader with an overview of recent developments in Arterial Spin Labeling (ASL) MRI techniques. A great deal of progress has been made in recent years in terms of the SNR and acquisition speed. New strategies have been introduced to improve labeling efficiency, reduce artefacts, and estimate other relevant physiological parameters besides perfusion. As a result, ASL techniques has become a reliable workhorse for researchers as well as clinicians. After a brief overview of the technique's fundamentals, this article will review new trends and variants in ASL including vascular territory mapping and velocity selective ASL, as well as arterial blood volume imaging techniques. This article will also review recent processing techniques to reduce partial volume effects and physiological noise. Next the article will examine how ASL techniques can be leveraged to calculate additional physiological parameters beyond perfusion and finally, it will review a few recent applications of ASL in the literature.

Partial volume correction for arterial spin labeling using the inherent perfusion information of multiple measurements

Background

Arterial spin labeling (ASL) provides a noninvasive way to measure cerebral blood flow (CBF). The CBF estimation from ASL is heavily contaminated by noise and the partial volume (PV) effect. The multiple measurements of perfusion signals in the ASL sequence are generally acquired and were averaged to suppress the noise. To correct the PV effect, several methods were proposed, but they were all performed directly on the averaged image, thereby ignoring the inherent perfusion information of mixed tissues that are embedded in multiple measurements. The aim of the present study is to correct the PV effect of ASL sequence using the inherent perfusion information in the multiple measurements.

Methods

In this study, we first proposed a statistical perfusion model of mixed tissues based on the distribution of multiple measurements. Based on the tissue mixture that was obtained from the high-resolution structural image, a structure-based expectation maximization (sEM) scheme was developed to estimate the perfusion contributions of different tissues in a mixed voxel from its multiple measurements. Finally, the performance of the proposed method was evaluated using both computer simulations and in vivo data.

Results

Compared to the widely used linear regression (LR) method, the proposed sEM-based method performs better on edge preservation, noise suppression, and lesion detection, and demonstrates a potential to estimate the CBF within a shorter scanning time. For in vivo data, the corrected CBF values of gray matter (GM) were independent of the GM probability, thereby indicating the effectiveness of the sEM-based method for the PV correction of the ASL sequence.

Conclusions

This study validates the proposed sEM scheme for the statistical perfusion model of mixed tissues and demonstrates the effectiveness of using inherent perfusion information in the multiple measurements for PV correction of the ASL sequence.

Determining the optimal postlabeling delay for arterial spin labeling using subject-specific estimates of blood velocity in the carotid artery

BACKGROUND

Arterial spin labeling with 3D acquisition requires determining a single postlabeling delay (PLD) value. PLD affects the signal-to-noise ratio (SNR) per unit time as well as quantitative cerebral blood flow (CBF) values due to its bearing on the presence of a vascular signal.

PURPOSE

To search for an optimal PLD for pseudocontinuous arterial spin labeling (pCASL) using patient-specific carotid artery blood velocity measurements.

STUDY TYPE

Prospective.

SUBJECTS

A control group of 11 volunteers with no known pathology. Corroboration was through a separate group of six volunteers and a noncontrol group of five sickle cell disease (SCD) patients.

FIELD STRENGTH/SEQUENCE

Pseudocontinuous arterial spin labeling with 3D nonsegmented echo planar imaging acquisition at 3T.

ASSESSMENT

A perfusion-based measure was determined over a range of PLDs for each of 11 volunteers. A third-order polynomial was used to find the optimal PLD where the defined measure was maximum. This was plotted against the corresponding carotid artery velocity to determine a relationship between the perfusion measure and velocity. Corroboration was done using a group of six volunteers and a noncontrol group of five patients with SCD. PLD was determined from the carotid artery velocity and derived relationship and compared with optimal PLD obtained from measured perfusion over a range of PLD values. Error between the perfusion measure at predicted and measured optimal PLD was determined.

STATISTICAL TESTS

Chi-squared goodness of fit; Pearson correlation; Bland-Altman.

RESULTS

Carotid artery velocity was 63.8 ± 6.6 cm/s (53.1 ≤ v ≤ 72.3 cm/s) while optimal PLD was 1374 ± 226.5 msec (1102 ≤ PLD ≤ 1787 msec) across the 11 volunteers. PLD as a function of carotid velocity was determined to be PLD = -31.94. v + 3410 msec (Pearson correlation -0.93). In six volunteers, mean error between the perfusion measure at predicted and measured optimal PLD was 1.35%. Pearson correlation between the perfusion measure at the predicted PLD and the measure obtained experimentally was r = 0.96 (P < 0.001). Bland-Altman revealed a slight bias of 1.3%. For the test case of five SCD patients, the mean error was 1.3%.

DATA CONCLUSION

Carotid artery velocity was used to determine optimal PLD for pCASL with 3D acquisition. The derived relationship was used to predict optimal PLD and the associated perfusion measure, which was found to be accurate when compared with its measured counterpart.

LEVEL OF EVIDENCE

2 Technical Efficacy: Stage 1 J. Magn. Reson. Imaging 2019;50:951-960.