Velocity-Selective Arterial Spin Labeling Perfusion in Monitoring High Grade Gliomas Following Therapy: Clinical Feasibility at 1.5T and Comparison with Dynamic Susceptibility Contrast Perfusion

MR perfusion imaging is important in the clinical evaluation of primary brain tumors, particularly in differentiating between true progression and treatment-induced change. The utility of velocity-selective ASL (VSASL) compared to the more commonly utilized DSC perfusion technique was assessed in routine clinical surveillance MR exams of 28 patients with high-grade gliomas at 1.5T. Using RANO criteria, patients were assigned to two groups, one with detectable residual/recurrent tumor ("RT", n = 9), and the other with no detectable residual/recurrent tumor ("NRT", n = 19). An ROI was drawn to encompass the largest dimension of the lesion with measures normalized against normal gray matter to yield rCBF and tSNR from VSASL, as well as rCBF and leakage-corrected relative CBV (lc-rCBV) from DSC. VSASL (rCBF and tSNR) and DSC (rCBF and lc-rCBV) metrics were significantly higher in the RT group than the NRT group allowing adequate discrimination (p < 0.05, Mann-Whitney test). Lin's concordance analyses showed moderate to excellent concordance between the two methods, with a stronger, moderate correlation between VSASL rCBF and DSC lc-rCBV (r = 0.57, p = 0.002; Pearson's correlation). These results suggest that VSASL is clinically feasible at 1.5T and has the potential to offer a noninvasive alternative to DSC perfusion in monitoring high-grade gliomas following therapy.

The correlation between changes in gray matter microstructure and cerebral blood flow in Alzheimer's disease

Objective

To investigate the relationship between changes in cerebral blood flow (CBF) and gray matter (GM) microstructure in Alzheimer's disease (AD) and mild cognitive impairment (MCI).

Methods

A recruited cohort of 23 AD patients, 40 MCI patients, and 37 normal controls (NCs) underwent diffusional kurtosis imaging (DKI) for microstructure evaluation and pseudo-continuous arterial spin labeling (pCASL) for CBF assessment. We investigated the differences in diffusion- and perfusion-related parameters across the three groups, including CBF, mean diffusivity (MD), mean kurtosis (MK), and fractional anisotropy (FA). These quantitative parameters were compared using volume-based analyses for the deep GM and surface-based analyses for the cortical GM. The correlation between CBF, diffusion parameters, and cognitive scores was assessed using Spearman coefficients, respectively. The diagnostic performance of different parameters was investigated with k-nearest neighbor (KNN) analysis, using fivefold cross-validation to generate the mean accuracy (mAcc), mean precision (mPre), and mean area under the curve (mAuc).

Results

In the cortical GM, CBF reduction primarily occurred in the parietal and temporal lobes. Microstructural abnormalities were predominantly noted in the parietal, temporal, and frontal lobes. In the deep GM, more regions showed DKI and CBF parametric changes at the MCI stage. MD showed most of the significant abnormalities among all the DKI metrics. The MD, FA, MK, and CBF values of many GM regions were significantly correlated with cognitive scores. In the whole sample, the MD, FA, and MK were associated with CBF in most evaluated regions, with lower CBF values associated with higher MD, lower FA, or lower MK values in the left occipital lobe, left frontal lobe, and right parietal lobe. CBF values performed best (mAuc = 0.876) for distinguishing the MCI from the NC group. Last, MD values performed best (mAuc = 0.939) for distinguishing the AD from the NC group.

Conclusion

Gray matter microstructure and CBF are closely related in AD. Increased MD, decreased FA, and MK are accompanied by decreased blood perfusion throughout the AD course. Furthermore, CBF values are valuable for the predictive diagnosis of MCI and AD. GM microstructural changes are promising as novel neuroimaging biomarkers of AD.

Update on state-of-the-art for arterial spin labeling (ASL) human perfusion imaging outside of the brain

This review article provides an overview of developments for arterial spin labeling (ASL) perfusion imaging in the body (i.e., outside of the brain). It is part of a series of review/recommendation papers from the International Society for Magnetic Resonance in Medicine (ISMRM) Perfusion Study Group. In this review, we focus on specific challenges and developments tailored for ASL in a variety of body locations. After presenting common challenges, organ-specific reviews of challenges and developments are presented, including kidneys, lungs, heart (myocardium), placenta, eye (retina), liver, pancreas, and muscle, which are regions that have seen the most developments outside of the brain. Summaries and recommendations of acquisition parameters (when appropriate) are provided for each organ. We then explore the possibilities for wider adoption of body ASL based on large standardization efforts, as well as the potential opportunities based on recent advances in high/low-field systems and machine-learning. This review seeks to provide an overview of the current state-of-the-art of ASL for applications in the body, highlighting ongoing challenges and solutions that aim to enable more widespread use of the technique in clinical practice.

Cerebral blood flow patterns in preterm and term neonates assessed with pseudo-continuous arterial spin labeling perfusion MRI

In preterm (PT) infants, regional cerebral blood flow (CBF) disturbances may predispose to abnormal brain maturation even without overt brain injury. Therefore, it would be informative to determine the spatial distribution of grey matter (GM) CBF in PT and full-term (FT) newborns at term-equivalent age (TEA) and to assess the relationship between the features of the CBF pattern and both prematurity and prematurity-related brain lesions. In this prospective study, we obtained measures of CBF in 66 PT (51 without and 15 with prematurity-related brain lesions) and 38 FT newborns through pseudo-continuous arterial spin labeling (pCASL) MRI acquired at TEA. The pattern of GM CBF was characterized by combining an atlas-based automated segmentation of structural MRI with spatial normalization and hierarchical clustering. The effects of gestational age (GA) at birth and brain injury on the CBF pattern were investigated. We identified 4 physiologically-derived clusters of brain regions that were labeled Fronto-Temporal, Parieto-Occipital, Insular-Deep GM (DGM) and Sensorimotor, from the least to the most perfused. We demonstrated that GM perfusion was associated with GA at birth in the Fronto-Temporal and Sensorimotor clusters, positively and negatively, respectively. Moreover, the presence of periventricular leukomalacia was associated with significantly increased Fronto-Temporal GM perfusion and decreased Insular-DGM perfusion, while the presence of germinal matrix hemorrhage appeared to mildly decrease the Insular-DGM perfusion. Prematurity and prematurity-related brain injury heterogeneously affect brain perfusion. ASL MRI may, therefore, have strong potential as a noninvasive tool for the accurate stratification of individuals at risk of domain-specific impairment.

Multiparametric MRI of Solid Renal Masses: Principles and Applications of Advanced Quantitative and Functional Methods for Tumor Diagnosis and Characterization

Solid renal masses (SRMs) are increasingly detected and encompass both benign and malignant masses, with renal cell carcinoma (RCC) being the most common malignant SRM. Most patients with SRMs will undergo management without a priori pathologic confirmation. There is an unmet need to noninvasively diagnose and characterize RCCs, as significant variability in clinical behavior is observed and a wide range of differing management options exist. Cross-sectional imaging modalities, including magnetic resonance imaging (MRI), are increasingly used for SRM characterization. Multiparametric (mp) MRI techniques can provide insight into tumor biology by probing different physiologic/pathophysiologic processes noninvasively. These include sequences that probe tissue microstructure, including intravoxel incoherent motion diffusion-weighted imaging (IVIM-DWI) and T₁ relaxometry; oxygen metabolism (blood oxygen level dependent [BOLD-MRI]); as well as vascular flow and perfusion (dynamic contrast-enhanced MRI [DCE-MRI] and arterial spin labeling [ASL]). In this review, we will discuss each mpMRI method in terms of its principles, roles, and discuss the results of human studies for SRM assessment. Future validation of these methods may help to enable a personalized management approach for patients with SRM in the emerging era of precision medicine. EVIDENCE LEVEL: 5. TECHNICAL EFFICACY: 2.

Imaging effective oxygen diffusivity in the human brain with multiparametric magnetic resonance imaging

Cerebrovascular diseases can impair blood circulation and oxygen extraction from the blood. The effective oxygen diffusivity (EOD) of the capillary bed is a potential biomarker of microvascular function that has gained increasing interest, both for clinical diagnosis and for elucidating oxygen transport mechanisms. Models of capillary oxygen transport link EOD to measurable oxygen extraction fraction (OEF) and cerebral blood flow (CBF). In this work, we confirm that two well established mathematical models of oxygen transport yield nearly equivalent EOD maps. Furthermore, we propose an easy-to-implement and clinically applicable multiparametric magnetic resonance imaging (MRI) protocol for quantitative EOD mapping. Our approach is based on imaging OEF and CBF with multiparametric quantitative blood oxygenation level dependent (mq-BOLD) MRI and pseudo-continuous arterial spin labeling (pCASL), respectively. We evaluated the imaging protocol by comparing MRI-EOD maps of 12 young healthy volunteers to PET data from a published study in different individuals. Our results show comparably good correlation between MRI- and PET-derived cortical EOD, OEF and CBF. Importantly, absolute values of MRI and PET showed high accordance for all three parameters. In conclusion, our data indicates feasibility of the proposed MRI protocol for EOD mapping, rendering the method promising for future clinical evaluation of patients with cerebrovascular diseases.

Analgesia Effect of Verum and Sham Acupuncture Treatments in Primary Dysmenorrhea: A MRI Pilot Study

Acupuncture is an alternative treatment for primary dysmenorrhea (PDM). However, mechanisms by which acupuncture exerts its analgesic properties are still unclear. This study aimed to explore the cerebral blood flow (CBF) response to verum and sham acupuncture treatments, and further investigate whether pre-treatment CBF is capable of assessing symptom changes after interventions. A total of 11 PDM patients in the verum group and 12 patients in the sham group participated in this study. Pain rating index (PRI), CBF, and gonadal hormone levels were acquired before and after 8-week treatments. Both verum and sham acupuncture treatments exert its analgesic effect on PDM after intervention as PRI reduced (p < 0.05). Blood gonadal levels were not significantly different after acupuncture in both groups (all p > 0.05). In the verum group, intervention-related decreases in CBF were observed in the right dorsal anterior cingulate cortex. In the sham group, regions identified as showing reductions in CBF after acupuncture included the left ventromedial prefrontal cortex, left caudate, and left insula. Patients with higher baseline CBF in the left precuneus and right hippocampus were accompanied with worse treatment response to acupuncture intervention. Mechanisms of verum and sham acupuncture treatments are dissimilar as manifested by different brain responses.

Arterial spin labeling in neonatal magnetic resonance imaging - first experience and new observations

Purpose

Arterial spin labeling (ASL) is a noninvasive non-contrast technique of perfusion imaging that uses endogenous water from the blood as the perfusion tracer, with very scant data on its use in neonates. The authors present the added value of ASL in the examined babies in their own material and discuss it in the light of the existing literature.

Material and methods

During the first 10 months after the purchase of a new magnetic resonance imaging (MRI) scanner, 123 neonates were examined using it in an MR-compatible incubator, 117 of them had brain MRI, and in 104 ASL was incorporated in the routine protocol, which resulted in prolongation of the study time by approximately 4 minutes. 3D ASL sequence uses Pulsed Continuous Arterial Spin Labeling (PCASL; aka pseudo continuous) technique.

Results

The quality of the cerebral blood flow (CBF) maps was good in all cases but 2 because all the babies were sedated. Apart from the knowledge about normal perfusion patterns in the preterm and term neonatal brains, the use of ASL sequence provided important additional information in 11 cases (10.8%): increased CBF correlating with electroencephalographic seizure localization in otherwise normal MRI (n = 1), increased CBF in the cortex without clinical information about seizures and with posthaemorrhagic changes (n = 1), increased CBF in the brain stem and decreased in the upper parts of the brain (n = 2), probably reflecting the homeostatic mechanism which allows preferential perfusion of the vital structures of the brain stem, hypoperfusion (n = 1) or hypoperfusion with peripheral hyperperfusion (n = 1) in the area of stroke, hypoperfusion of the posterior areas of the brain in the presence of subarachnoid or epidural haemorrhage (n = 3), significantly increased CBF in the presumed nidus of arteriovenous malformation causing haemorrhage (n = 1), and lack of perfusion in the supratentorial compartment in a case of suspected brain death (n = 1).

Conclusions

Our short experience but relatively large volume of material encourages the use of ASL in routine neonatal MRI as a useful and non-time-consuming tool providing additional important clinical information in a significant percentage of cases.

Cerebral Hemodynamic Correlates of Transcutaneous Auricular Vagal Nerve Stimulation in Consciousness Restoration: An Open-Label Pilot Study

This study aimed to preliminarily illustrate the cerebral hemodynamic correlates of transcutaneous auricular vagal nerve stimulation (taVNS) in consciousness restoration. Arterial spin labeling (ASL) was adopted with functional magnetic resonance imaging (fMRI) to measure cerebral blood flow (CBF) changes before and after taVNS in 10 qualified patients with disorders of consciousness (DOC). Before taVNS, five patients responded to auditory stimuli (RtAS), and five did not respond to auditory stimuli (nRtAS). The RtAS DOC patients obtained favorable prognoses after the 4-week taVNS treatment, whereas the nRtAS ones did not. Simultaneously, taVNS increased CBF of multiple brain regions in the RtAS DOC patients, but hardly in the nRtAS ones. In conclusion, the preserved auditory function might be the prior key factor of the taVNS responders in DOC patients, and taVNS might alleviate RtAS DOC by activating the salience network, the limbic system, and the interoceptive system.

Optimization of adiabatic pulses for pulsed arterial spin labeling at 7 tesla: Comparison with pseudo-continuous arterial spin labeling

PURPOSE

To optimize and evaluate adiabatic pulses for pulsed arterial spin labeling at ultrahigh field 7 tesla.

METHODS

Four common adiabatic inversion pulses, including hyperbolic secant, wideband uniform rate smooth truncation, frequency offset corrected inversion, and time-resampled frequency offset corrected inversion pulses, were optimized based on a custom-defined loss function that included labeling efficiency and inversion band uniformity. The optimized pulses were implemented in flow-sensitive alternating inversion recovery sequences and tested on phantom and 11 healthy volunteers with 2 constraints: 1) specific absorption rate normalized; and 2) equal peak RF amplitude, respectively. A pseudo-continuous arterial spin labeling sequence was implemented for comparison. Quantitative metrics such as perfusion and relative labeling efficiency versus residual tissue signal were calculated.

RESULTS

Among the 4 pulses, the wideband uniform rate smooth truncation pulse yielded the lowest loss in simulation and achieved a good balance between labeling efficiency and residual tissue signal from both phantom and in vivo experiments. Wideband uniform rate smooth truncation-pulsed arterial spin labeling showed significantly higher relative labeling efficiency compared to the other sequences (P < .01), whereas the perfusion signal was increased by 40% when the highest B 1 + amplitude was used. The 4 pulsed arterial spin labeling sequences yielded comparable perfusion signals compared to pseudo-continuous arterial spin labeling but with less than half the specific absorption rate.

CONCLUSION

Optimized wideband uniform rate smooth truncation pulse with the highest B 1 + amplitude allowed was recommended for 7 tesla pulsed arterial spin labeling.

Comparison Between Blood-Brain Barrier Water Exchange Rate and Permeability to Gadolinium-Based Contrast Agent in an Elderly Cohort

Background: Dynamic contrast-enhanced (DCE) MRI using intravenous injection of gadolinium-based contrast agents (GBCAs) is commonly used for imaging blood-brain barrier (BBB) permeability. Water is an alternative endogenous tracer with limited exchange rate across the BBB. A direct comparison between BBB water exchange rate and BBB permeability to GBCA is missing. The purpose of this study was to directly compare BBB permeability to GBCA (Ktrans and k_Gad = Ktrans/Vp) and water exchange rate (kw) in a cohort of elderly subjects at risk of cerebral small vessel disease (cSVD).

Methods: Ktrans/k_Gad and kw were measured by DCE-MRI and diffusion prepared pseudo-continuous arterial spin labeling (DP-pCASL), respectively, at 3 Tesla in 16 elderly subjects (3 male, age = 67.9 ± 3.0 yrs) at risk of cSVD. The test-retest reproducibility of kw measurements was evaluated with repeated scans ~6 weeks apart. Mixed effects linear regression was performed in the whole brain, gray matter (GM), white matter (WM), and 6 subcortical brain regions to investigate associations between Ktrans/k_Gad and test-retest kw. In addition, kw and Ktrans/k_Gad were compared in normal appearing white matter (NAWM), white matter hyperintensity (WMH) lesions and penumbra.

Results: Significant correlation was found between kw and Ktrans only in WM (β = 6.7 × 10⁴, P = 0.036), caudate (β = 8.6 × 10⁴, P = 0.029), and middle cerebral artery (MCA) perforator territory (β = 6.9 × 10⁴, P = 0.009), but not in the whole brain, GM or rest 5 brain regions. Significant correlation was found between kw and k_Gad in MCA perforator territory (β = 1.5 × 10³, P = 0.049), medial-temporal lobe (β = 3.5 × 10³, P = 0.032), and hippocampus (β = 3.4 × 10³, P = 0.038), but not in the rest brain regions. Good reproducibility of kw measurements (ICC=0.75) was achieved. Ktrans was significantly lower inside WMH than WMH penumbra (16.2%, P = 0.026), and k_Gad was significantly lower in NAWM than in the WMH penumbra (20.8%, P < 0.001).

Conclusion: kw provides a measure of water exchange rate across the BBB with good test-retest reproducibility. The BBB mechanism underlying kw and Ktrans/k_Gad is likely to be different, as manifested by correlations in only three brain regions for each pair of comparison between kw and Ktrans or k_Gad.

Quantitative transport mapping (QTM) of the kidney with an approximate microvascular network

Purpose

Proof‐of‐concept study of mapping renal blood flow vector field according to the inverse solution to a mass transport model of time resolved tracer‐labeled MRI data.

Theory and Methods

To determine tissue perfusion according to the underlying physics of spatiotemporal tracer concentration variation, the mass transport equation is integrated over a voxel with an approximate microvascular network for fitting time‐resolved tracer imaging data. The inverse solution to the voxelized transport equation provides the blood flow vector field, which is referred to as quantitative transport mapping (QTM). A numerical microvascular network modeling the kidney with computational fluid dynamics reference was used to verify the accuracy of QTM and the current Kety’s method that uses a global arterial input function. Multiple post‐label delay arterial spin labeling (ASL) of the kidney on seven subjects was used to assess QTM in vivo feasibility.

Results

Against the ground truth in the numerical model, the error in flow estimated by QTM (18.6%) was smaller than that in Kety’s method (45.7%, 2.5‐fold reduction). The in vivo kidney perfusion quantification by QTM (cortex: 443 ± 58 mL/100 g/min and medulla: 190 ± 90 mL/100 g/min) was in the range of that by Kety’s method (482 ± 51 mL/100 g/min in the cortex and 242 ± 73 mL/100 g/min in the medulla), and QTM provided better flow homogeneity in the cortex region.

Conclusions

QTM flow velocity mapping is feasible from multi‐delay ASL MRI data based on inverting the transport equation. In a numerical simulation, QTM with deconvolution in space and time provided more accurate perfusion quantification than Kety’s method with deconvolution in time only.

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.

Reperfusion Into Severely Damaged Brain Tissue Is Associated With Occurrence of Parenchymal Hemorrhage for Acute Ischemic Stroke

Background and Purpose: This study aims to quantify the reperfusion status within severely damaged brain tissue and to evaluate its relationship with high grade of hemorrhagic transformation (HT).

Methods: Pseudo-continuous ASL was performed along with DWI in 102 patients within 24 h post-treatments. The infarction core was identified using ADC values <550 × 10⁻⁶ mm²/s. CBF within the infarction core and its contralateral counterpart were acquired. CBF at the 25th, median, and 75th percentiles of the contralateral counterpart were used as thresholds and the ASL reperfusion volume above the threshold was labeled as vol-25, -50, and -75, respectively. Recanalization was defined according to Thrombolysis in Myocardial Infarction (TIMI) criteria.

Results: Quantified reperfusion within the infarction core differed significantly in patients with complete and incomplete recanalization. In the ROC analysis for the prediction of parenchymal hematoma (PH), ASL reperfusion vol-25 had the highest area under the curve (AUC) when compared with ASL vol-50 and ASL vol-75. ASL reperfusion vol-25 had significantly higher AUC compared with ADC threshold volume in the prediction of PH (0.783 vs. 0.685, P = 0.0036) and PH-2 (0.844 vs. 0.754, P = 0.0035). In stepwise multivariate logistic regression analysis, only ASL reperfusion vol-25 emerged as an independent predictor of PH (OR = 3.51, 95% CI: 1.65–7.45, P < 0.001) and PH-2 (OR = 2.32, 95% CI: 1.13–4.76, P = 0.022).

Conclusions: Increased reperfusion volume within severely damaged brain tissue is associated with the occurrence of higher grade of HT.

Systematic evaluation of velocity-selective arterial spin labeling settings for placental perfusion measurement

Purpose

Placental function is key for successful human pregnancies. Perfusion may be a sensitive marker for the in vivo assessment of placental function. Arterial spin labeling (ASL) MRI enables noninvasive measurement of tissue perfusion and it was recently suggested that ASL with velocity‐selective (VS) labeling could be advantageous in the placenta. We systematically evaluated essential VS‐ASL sequence parameters to determine optimal settings for efficient placental perfusion measurements.

Methods

Eleven pregnant women were scanned at 3T using VS‐ASL with 2D multislice echo planar imaging (EPI)‐readout. One reference VS‐ASL scan was acquired in all subjects; within subgroups the following parameters were systematically varied: cutoff velocity, velocity encoding direction, and inflow time. Visual evaluation and region of interest analyses were performed to compare perfusion signal differences between acquisitions.

Results

In all subjects, a perfusion pattern with clear hyperintense focal regions was observed. Perfusion signal decreased with inflow time and cutoff velocity. Subject‐specific dependence on velocity encoding direction was observed. High temporal signal‐to‐noise ratios with high contrast on the perfusion images between the hyperintense regions and placental tissue were seen at ~1.6 cm/s cutoff velocity and ~1000 ms inflow time. Evaluation of measurements at multiple inflow times revealed differences in blood flow dynamics between placental regions.

Conclusion

Placental perfusion measurements are feasible at 3T using VS‐ASL with 2D multislice EPI‐readout. A clear dependence of perfusion signal on VS labeling parameters and inflow time was demonstrated. Whereas multiple parameter combinations may advance the interpretation of placental circulation dynamics, this study provides a basis to select an effective set of parameters for the observation of placenta perfusion natural history and its potential pathological changes.

Intracranial 3D and 4D MR Angiography Using Arterial Spin Labeling: Technical Considerations

In the 1980’s some of the earliest studies of arterial spin labeling (ASL) MRI have demonstrated its ability to generate MR angiography (MRA) images. Thanks to many technical improvements, ASL has been successfully moving its position from the realm of research into the clinical area, albeit more known as perfusion imaging than as MRA. For MRA imaging, other techniques such as time-of-flight, phase contrast MRA and contrast-enhanced (CE) MRA are more popular choices for clinical applications. In the last decade, however, ASL-MRA has been experiencing a remarkable revival, especially because of its non-invasive nature, i.e. the fact that it does not rely on the use of contrast agent. Very importantly, there are additional benefits of using ASL for MRA. For example, its higher flexibility to achieve both high spatial and temporal resolution than CE dynamic MRA, and the capability of vessel specific visualization, in which the vascular tree arising from a selected artery can be exclusively visualized. In this article, the implementation and recent developments of ASL-based MRA are discussed; not only focusing on the basic sequences based upon pulsed ASL or pseudo-continuous ASL, but also including more recent labeling approaches, such as vessel-selective labeling, velocity-selective ASL, vessel-encoded ASL and time-encoded ASL. Although these ASL techniques have been already utilized in perfusion imaging and their usefulness has been suggested by many studies, some additional considerations should be made when employing them for MRA, since there is something more than the difference of the spatial resolution of the readout sequence. Moreover, extensive discussion is included on what readout sequence to use, especially by highlighting how to achieve high spatial resolution while keeping scan-time reasonable such that the ASL-MRA sequence can easily be included into a clinical examination.

SAR comparison between CASL and pCASL at high magnetic field and evaluation of the benefit of a dedicated labeling coil

PURPOSE

To investigate the heating induced by (pseudo)-continuous arterial spin labeling ((p)CASL) sequences in vivo at 9.4T and to evaluate the benefit of a dedicated labeling coil.

METHODS

Temperature was measured continuously in the brain, neck, and rectum of 9 rats with fiber-optic temperature probes while running pCASL-EPI and CASL-EPI sequences, with labeling B₁ amplitudes (B_1ave ) of 3, 5, and 7 μT and using a dedicated labeling RF coil or a volume coil. From the temperature time courses, the corresponding specific absorption rate (SAR) was computed. A trade-off between SAR and labeling quality was determined based on measured inversion efficiencies.

RESULTS

ASL experiments with standard parameters (B_1ave = 5 µT, T_acq = 4 min, labeling with volume coil) lead to a brain temperature increase due to RF of 0.72 ± 0.46 K for pCASL and 0.25 ± 0.17 K for CASL. Using a dedicated labeling coil reduced the RF-induced SAR by a factor of 10 in the brain and a factor of 2 in the neck. Besides SAR due to RF, heat from the coil decoupling circuits produced significant temperature increases. When labeling with a dedicated coil, this mechanism was the dominant source of brain heating. At equivalent RF-SAR, CASL provided slightly superior label efficiency to pCASL and is therefore the preferred sequence when an ASL coil is available.

CONCLUSION

B_1ave = 4-5 µT provided a good compromise between label efficiency and SAR, both for pCASL and CASL. The sensitivity of animals to heating should be taken into account when optimizing preclinical ASL protocols and may require reducing scan duration or lowering B_1ave .

Quantification of intracranial arterial blood flow using noncontrast enhanced 4D dynamic MR angiography

PURPOSE

Noncontrast enhanced dynamic magnetic resonance angiography delineates the pattern of dynamic blood flow of the cerebral vasculature. A model-free solution was proposed to quantify arterial blood flow (aBF) by using the monotonic property of the residual function.

THEORY AND METHODS

Analytical simulations and in-vivo studies were performed to evaluate the performance of the proposed method by comparing the aBF values generated from the proposed and conventional singular value decomposition methods. The aBF values were compared with blood flow velocity measured by 2D phase contrast MRI, and compared between balanced steady-state free precession-based radial and spoiled GRE-based Cartesian acquisitions. Hemodynamic parametric maps were generated in 1 patient with arteriovenous malformation.

RESULTS

The proposed method generates reliable aBF measurement at different signal-to-noise ratio levels, whereas overestimation/underestimation of aBF was observed when a high/low threshold was applied in the singular value decomposition method. Average aBF in large vascular branches was 214.4 and 214.5 mL/mL/min with radial and Cartesian acquisitions, respectively. Significant correlations were found between aBF and blood flow velocity measured by phase contrast MRI (P = 0.0008), and between Cartesian and radial acquisitions (P < 0.0001). Altered hemodynamics were observed at the lesion site of the arteriovenous malformation patient.

CONCLUSION

A robust analytical solution was proposed for quantifying aBF. This model-free method is robust to noise, and its clinical value in the diagnosis of cerebrovascular disorders awaits further evaluation.

A framework for motion correction of background suppressed arterial spin labeling perfusion images acquired with simultaneous multi-slice EPI

PURPOSE

When using simultaneous multi-slice (SMS) EPI for background suppressed (BGS) arterial spin labeling (ASL), correction of through-plane motion could introduce artefacts, because the slices with most effective BGS are adjacent to slices with the least BGS. In this study, a new framework is presented to correct for such artefacts.

METHODS

The proposed framework consists of 3 steps: (1) homogenization of the static tissue signal over the different slices to eliminate most inter-slice differences because of different levels of BGS, (2) application of motion correction, and (3) extraction of a perfusion-weighted signal using a general linear model. The proposed framework was evaluated by simulations and a functional ASL study with intentional head motion.

RESULTS

Simulation studies demonstrated that the strong signal differences between slices with the most and least effective BGS caused sub-optimal estimation of motion parameters when through-plane motion was present. Although use of the M0 image as the reference for registration allowed 82% improvement of motion estimation for through-plane motion, it still led to residual subtraction errors caused by different static tissue signal between control and label because of different BGS levels. By using our proposed framework, those problems were minimized, and the accuracy of CBF estimation was improved. Moreover, the functional ASL study showed improved detection of visual and motor activation when applying the framework as compared to conventional motion correction, as well as when motion correction was completely omitted.

CONCLUSION

When combining BGS-ASL with SMS-EPI, particular attention is needed to avoid artefacts introduced by motion correction. With the proposed framework, these issues are minimized.

Precuneus Dysfunction in Parkinson's Disease With Mild Cognitive Impairment

Background: Mild cognitive impairment (MCI) frequently occurs in Parkinson’s disease (PD). Neurovascular changes interact with neurodegenerative processes in PD. However, the deficits of cerebral blood flow (CBF) perfusion and the associated functional connectivity (FC) in PD patients with MCI (PD-MCI) remain unclear.

Purpose: This study aimed to explore the specific neurovascular perfusion alterations in PD-MCI compared to PD with normal cognition (PD-NC) and healthy controls (HCs), and to further examine the resultant whole brain FC changes in the abnormal perfusion regions.

Methods: Relative CBF (rCBF) was calculated using arterial spin labeling (ASL) in 54 patients with PD (27 patients with PD-NC and 27 patients with PD-MCI) and 25 HCs matched for age and gender ratio, who also underwent the structural MRI, resting-state functional MRI (rs-fMRI) and neuropsychological examinations. The gray matter (GM) changes in PD patients were analyzed using voxel-based morphometry (VBM). The alterations in rCBF perfusion and FC among groups were then analyzed respectively. Additionally, correlations between these alterations and neuropsychological performances were further examined.

Results: Compared to HC, left caudate atrophy was detected in patients with PD. In comparison to both PD-NC and HC, patients with PD-MCI specifically exhibited hypoperfusion in the parietal memory network (PMN) in the precuneus (PCu) and decreased PCu-FC in the right striatum. Moreover, PCu perfusion and PCu-FC strengths in the right striatum were positively associated with memory performance in PD-MCI.

Conclusions: These findings suggest that the posterior PMN dysfunction underlies memory deficits in PD-MCI.

Mapping water exchange across the blood-brain barrier using 3D diffusion-prepared arterial spin labeled perfusion MRI

PURPOSE

To present a novel MR pulse sequence and modeling algorithm to quantify the water exchange rate (k_w ) across the blood-brain barrier (BBB) without contrast, and to evaluate its clinical utility in a cohort of elderly subjects at risk of cerebral small vessel disease (SVD).

METHODS

A diffusion preparation module with spoiling of non-Carr-Purcell-Meiboom-Gill signals was integrated with pseudo-continuous arterial spin labeling (pCASL) and 3D gradient and spin echo (GRASE) readout. The tissue/capillary fraction of the arterial spin labeling (ASL) signal was separated by appropriate diffusion weighting (b = 50 s/mm² ). k_w was quantified using a single-pass approximation (SPA) model with total generalized variation (TGV) regularization. Nineteen elderly subjects were recruited and underwent 2 MRIs to evaluate the reproducibility of the proposed technique. Correlation analysis was performed between k_w and vascular risk factors, Clinical Dementia Rating (CDR) scale, neurocognitive assessments, and white matter hyperintensity (WMH).

RESULTS

The capillary/tissue fraction of ASL signal can be reliably differentiated with the diffusion weighting of b = 50 s/mm² , given ~100-fold difference between the (pseudo-)diffusion coefficients of the 2 compartments. Good reproducibility of k_w measurements (intraclass correlation coefficient = 0.75) was achieved. Average k_w was 105.0 ± 20.6, 109.6 ± 18.9, and 94.1 ± 19.6 min^-1 for whole brain, gray and white matter. k_w was increased by 28.2%/19.5% in subjects with diabetes/hypercholesterolemia. Significant correlations between k_w and vascular risk factors, CDR, executive/memory function, and the Fazekas scale of WMH were observed.

CONCLUSION

A diffusion prepared 3D GRASE pCASL sequence with TGV regularized SPA modeling was proposed to measure BBB water permeability noninvasively with good reproducibility. k_w may serve as an imaging marker of cerebral SVD and associated cognitive impairment.

Robust kidney perfusion mapping in pediatric chronic kidney disease using single-shot 3D-GRASE ASL with optimized retrospective motion correction

PURPOSE

To develop a robust renal arterial spin labeling (ASL) acquisition and processing strategy for mapping renal blood flow (RBF) in a pediatric cohort with severe kidney disease.

METHODS

A single-shot background-suppressed 3D gradient and spin-echo (GRASE) flow-sensitive alternating inversion recovery (FAIR) ASL acquisition method was used to perform 2 studies. First, an evaluation of the feasibility of single-shot 3D-GRASE and retrospective noise reduction methods was performed in healthy volunteers. Second, a pediatric cohort with severe chronic kidney disease underwent single-shot 3D-GRASE FAIR ASL and RBF was quantified following several retrospective motion correction pipelines, including image registration and threshold-free weighted averaging. The effect of motion correction on the fit errors of saturation recovery (SR) images (required for RBF quantification) and on the perfusion-weighted image (PWI) temporal signal-to-noise ratio (tSNR) was evaluated, as well as the intra- and inter-session repeatability of renal longitudinal relaxation time (T₁ ) and RBF.

RESULTS

The mean cortical and/or functional renal parenchyma RBF in healthy volunteers and CKD patients was 295 ± 97 and 95 ± 47 mL/100 g/min, respectively. Motion-correction reduced image artefacts in both T₁ and RBF maps, significantly reduced SR fit errors, significantly increased the PWI tSNR and improved the improved the repeatability of T₁ and RBF in the pediatric patient cohort.

CONCLUSION

Single-shot 3D-GRASE ASL combined with retrospective motion correction enabled repeatable non-invasive RBF mapping in the first pediatric cohort with severe kidney disease undergoing ASL scans.

Intravoxel Incoherent Motion and Arterial Spin Labeling MRI Analysis of Reversible Unilateral Ureteral Obstruction in Rats

BACKGROUND

Renal fibrosis is a common consequence of chronic kidney disease (CKD) and is the mechanism by which various forms of CKD progress to endstage renal failure. Accurate assessment of renal fibrosis is important for treatment.

PURPOSE

To measure longitudinal changes of intravoxel incoherent motion (IVIM) and arterial spin labeling (ASL) before and after reversible unilateral ureteral obstruction in an animal model.

STUDY TYPE

Self-controlled animal study.

ANIMAL MODEL

Surgical obstruction of the ureters was performed and then removed after 5 days. Rats were scanned on Days 0, 1, 3, and 5 after creating the obstruction and on Days 4, 7, and 12 after releasing the obstruction.

FIELD STRENGTH/SEQUENCE

3.0T/IVIM/ASL.

ASSESSMENT

The apparent diffusion coefficient (ADC), pure molecular diffusion (D), perfusion fraction (f), pseudodiffusion (D*), and renal blood flow (RBF) obtained from the ASL were measured.

STATISTICAL TESTS

Using SPSS v. 20.0 software, P < 0.05 were considered statistically significant. The data from each timepoint were compared using one-way analysis of variance and correlation analysis was applied to various parameters.

RESULTS

The postobstruction kidneys showed renal tubule swelling and increased collagen fiber content. Renal tubule swelling was relieved after reversing the obstruction, but Masson staining and cell density analysis revealed progressive changes that were primarily localized to the medulla. In general, ADC, D, f, D*, and RBF decreased with time during the 5 days of obstruction, and increased after release of the obstruction. ADC positively correlated with D, f, D*, and RBF (r = 0.415, r = 0.634, r = 0.465 r = 0.586, P < 0.001, respectively) in the cortex in this study. Also, ADC showed a positive correlation with D, f, and D* (r = 0.724, r = 0.749, r = 0.151, P < 0.001, respectively) in the medulla.

DATA CONCLUSION

Kidney perfusion was the major factor affecting ADC. Functional imaging may be useful for following progression of CKD.

LEVEL OF EVIDENCE

1 Technical Efficacy: Stage 2 J. Magn. Reson. Imaging 2019;50:288-296.

[Effects of "Tongdu Tiaoshen" acupuncture on cerebral blood flow in patients with high risk of cerebral ischemic stroke based on ASL and PWI technique]

OBJECTIVE

To observe the effects of "Tongdu Tiaoshen" acupuncture on cerebral blood flow in high-risk patients of cerebral ischemic stroke based on arterial spin labeling (ASL) and perfusion-weighted imaging (PWI), and to evaluate the clinical efficacy.

METHODS

A total of 180 patients with transient ischemic attacks (TIA) / minor ischemic stroke (MIS) were randomly divided into an acupuncture A group, an acupuncture B group and a medication group, 60 cases in each group. The patients in the acupuncture A group were treated with "Tongdu Tiaoshen" acupuncture at Baihui (GV 20), Fengfu (GV 16), Yamen (GV 15), Dazhui (GV 14), Shenzhu (GV 12), Zhiyang (GV 9), Mingmen (GV 4), Yaoyangguan (GV 3) and Jingjiaji (EX-B 2), once a day; the patients in the acupuncture B group were treated with identical acupoints but was given once every other day; the patients in the medication group were treated with oral administration of nimodipine tablets, 30 mg, three times daily. All the three groups were treated for four weeks. ASL and PWI, including relative cerebral blood volume (rCBV), relative cerebral blood flow (rCBF), relative mean transit time (rMTT) and relative time to peak (rTTP), were conducted before and after treatment; the changes of the test indexes were compared before and after treatment. The clinical efficacy of the three groups was compared.

RESULTS

Compared before treatment, the numbers of ASL normal perfusion in the 3 groups were significantly increased after treatment (all P<0.01); the number of ASL normal perfusion in the acupuncture A group was higher than that in the acupuncture B group (P<0.05), but was not significantly different from that in the medication group (P>0.05). Compared before treatment, rCBV and rCBF in the 3 groups were significantly increased after treatment (all P<0.01), and rMTT and rTTP were significantly reduced (all P<0.01). After treatment, rCBV and rCBF in the acupuncture A group were higher than those in the acupuncture B group (all P<0.05); the rMTT and rTTP in the acupuncture A group were lower than those in the acupuncture B group (all P<0.05); the differences of PWI parameters after treatment were not statistically significant between the acupuncture A group and medication group (all P>0.05). The total effective rate was 88.3% (53/60) in the acupuncture A group, 73.3% (44/60) in the acupuncture B group and 90.0% (54/60) in the medication group; the total effective rate in the acupuncture A group was superior to that in the acupuncture B group (P<0.05), but was not significantly different from that in the medication group (P>0.05).

CONCLUSION

"Tongdu Tiaoshen" acupuncture could effectively improve the hypoperfusion of cerebral blood flow in patients with high risk of cerebral ischemic stroke, reduce the incidence of severe CIS; acupuncture for once a day is better than once every other day.

Thirst-Dependent Activity of the Insular Cortex Reflects its Emotion-Related Subdivision: A Cerebral Blood Flow Study

Recent studies investigating neural correlates of human thirst have identified various subcortical and telencephalic brain areas. The experience of thirst represents a homeostatic emotion and a state that slowly evolves over time. Therefore, the present study aims at systematically examining cerebral perfusion during the parametric progression of thirst. We measured subjective thirst ratings, serum parameters and cerebral blood flow in 20 healthy subjects across four different thirst stages: intense thirst, moderate thirst, subjective satiation and physiological satiation. Imaging data revealed dehydration-related perfusion differences in previously identified brain areas, such as the anterior cingulate cortex, the middle temporal gyrus and the insular cortex. However, significant differences across all four thirst stages (including the moderate thirst level), were exclusively found in the posterior insular cortex. The subjective thirst ratings over the different thirst stages, however, were associated with perfusion differences in the right anterior insula. These findings add to our understanding of the insular cortex as a key player in human thirst - both on the level of physiological dehydration and the level of the subjective thirst experience.

Controlling T2 blurring in 3D RARE arterial spin labeling acquisition through optimal combination of variable flip angles and k-space filtering

PURPOSE

To improve the SNR efficiency and reduce the T₂ blurring of 3D rapid acquisition with relaxation enhancement stack-of-spiral arterial spin labeling imaging by using variable refocusing flip angles and k-space filtering.

METHODS

An algorithm for determining the optimal combination of variable flip angles and filtering correction is proposed. The flip angles are designed using extended phase graph physical simulations in an analytical and global optimization framework, with an optional constraint on deposited power. Optimal designs for correcting to Hann and Fermi window functions were compared with conventional constant amplitude or variable flip angle only designs on 6 volunteers.

RESULTS

With the Fermi window correction, the proposed optimal designs provided 39.8 and 27.3% higher SNR (P < .05) than conventional constant amplitude and variable flip angle designs. Even when power deposition was limited to 50% of the constant amplitude design, the proposed method outperformed the SNR (P < .05) of these 2 conventional approaches by 32.5 and 20.7%. The sharpness and the contrast between gray and white matter were improved with the k-space filtering correction for all of the flip angle designs. The improvements were moderate for the Hann window correction.

CONCLUSION

This work demonstrates that variable flip angles can be derived as the output of an optimization problem. The combined design of variable flip angle and k-space filtering provided superior SNR to designs primarily emphasizing either approach singly.

Quantitative single breath-hold renal arterial spin labeling imaging at 7T

PURPOSE

To evaluate the feasibility of quantitative single breath-hold renal arterial spin labeling (ASL) imaging at 7T.

METHODS

A single-shot fast spin echo FAIR (flow-sensitive alternating inversion recovery) method was used to perform two studies. First, a multi-delay perfusion study was performed to estimate the spin labeling temporal bolus width achievable with a local transceiver array coil at 7T. Second, with a conservatively defined bolus width, a quantitative perfusion study was performed using the single subtraction approach. To address issues of B1+ inhomogeneity/efficiency and excessive short-term specific absorption rates, various strategies were used, such as dynamic radiofrequency shimming and optimization.

RESULTS

A conservative temporal bolus width of 600 ms determined from the multi-delay study was applied for single-subtraction imaging to measure the renal blood flow in the cortex and medulla: 303 ± 31.8 and 91.3 ± 15.2 (mL/100 g/min), respectively. The estimated spatial and temporal signal-to-noise ratios of renal perfusion measurements were 3.8 ± 0.7 and 2.4 ± 0.6 for the cortex, and 2.2 ± 0.6 and 1.4 ± 0.2 for the medulla.

CONCLUSION

With proper management of field strength specific challenges, quantitative renal ASL imaging can be achieved at 7T within a single breath-hold. Magn Reson Med 79:815-825, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Measuring human placental blood flow with multidelay 3D GRASE pseudocontinuous arterial spin labeling at 3T

BACKGROUND

Placenta influences the health of both a woman and her fetus during pregnancy. Maternal blood supply to placenta can be measured noninvasively using arterial spin labeling (ASL).

PURPOSE

To present a multidelay pseudocontinuous arterial spin labeling (pCASL) combined with a fast 3D inner-volume gradient- and spin-echo (GRASE) imaging technique to simultaneously measure placental blood flow (PBF) and arterial transit time (ATT), and to study PBF and ATT evolution with gestational age during the second trimester. The PBF values were compared with uterine arterial Doppler ultrasound to assess its potential clinical utility.

STUDY TYPE

This was a prospective study.

SUBJECTS

Thirty-four pregnant women.

FIELD STRENGTH/SEQUENCE

Multidelay 3D inner-volume GRASE pCASL sequence on 3T MR scanners.

ASSESSMENT

Subjects underwent two longitudinal MRI scans within the second trimester, conducted between 14-16 and 19-22 weeks of gestational age, respectively. Placental perfusion was measured using the free-breathing pCASL sequence at three postlabeling delays (PLDs), followed by offline motion correction and model fitting for estimation of PBF and ATT.

STATISTICAL TESTS

A paired t-test was conducted to evaluate the significance of PBF/ATT variations with placental development. A two-sample t-test was conducted to evaluate the significance of PBF difference in subjects with and without early diastolic notch.

RESULTS

The mean PBF and ATT for the second trimester were 111.4 ± 26.7 ml/100g/min and 1387.5 ± 88.0 msec, respectively. The average PBF increased by 10.4% (P < 0.05), while no significant change in ATT (P = 0.72) was found along gestational ages during the second trimester. PBF decreased 20.3% (P < 0.01) in subjects with early diastolic notches in ultrasound flow waveform patterns.

DATA CONCLUSION

Multidelay pCASL with inner-volume 3D GRASE is promising for noninvasive assessment of PBF during pregnancy. Its clinical use for the detection of aberrations in placental function and prediction of fetal developmental disorders awaits evaluation.

LEVEL OF EVIDENCE

1 Technical Efficacy: Stage 2 J. Magn. Reson. Imaging 2018;47:1667-1676.

Cerebral Blood Flow and Amyloid-β Interact to Affect Memory Performance in Cognitively Normal Older Adults

Cerebral blood flow (CBF) alterations and amyloid-β (Aβ) accumulation have been independently linked to cognitive deficits in older adults at risk for dementia. Less is known about how CBF and Aβ may interact to affect cognition in cognitively normal older adults. Therefore, we examined potential statistical interactions between CBF and Aβ status in regions typically affected in Alzheimer's disease (AD) within a sample of older adults from the Alzheimer's Disease Neuroimaging Initiative (ADNI) study. Sixty-two cognitively normal participants (mean age = 72 years) underwent neuroimaging and memory testing. Arterial spin labeling magnetic resonance imaging was used to quantify CBF and florbetapir PET amyloid imaging was used to measure Aβ deposition. Aβ status (i.e., positivity versus negativity) was determined based on established cutoffs (Landau et al., 2013). The Rey Auditory Verbal Learning Test was used to assess memory. Linear regression models adjusted for age, education, and sex, demonstrated significant interactions between CBF and Aβ status on memory performance. Among Aβ positive older adults, there were significant negative associations between higher CBF in hippocampus, posterior cingulate, and precuneus and poorer memory performance. In contrast, among Aβ negative older adults, there were no significant associations between CBF and cognition. Our findings extend previous CBF studies of dementia risk by reporting interactions between Aβ status and CBF on memory performance in a sample of well-characterized, cognitively normal older adults. Results suggest that differential CBF-cognition associations can be identified in healthy, asymptomatic Aβ positive older adults relative to Aβ negative individuals. Associations between higherCBF and poorer memory among Aβ positive older adults may reflect a cellular and/or vascular compensatory response to pathologic processes whereby higher CBF is needed to maintain normal memory abilities. Findings indicate that CBF and its associations with cognition may have utility as a reliable marker of brain function early in the AD process when interventions are likely to be beneficial.

Acceleration of ASL-based time-resolved MR angiography by acquisition of control and labeled images in the same shot (ACTRESS)

PURPOSE

Noncontrast 4D-MR-angiography (MRA) using arterial spin labeling (ASL) is beneficial because high spatial and temporal resolution can be achieved. However, ASL requires acquisition of labeled and control images for each phase. The purpose of this study is to present a new accelerated 4D-MRA approach that requires only a single control acquisition, achieving similar image quality in approximately half the scan time.

METHODS

In a multi-phase Look-Locker sequence, the first phase was used as the control image and the labeling pulse was applied before the second phase. By acquiring the control and labeled images within a single Look-Locker cycle, 4D-MRA was generated in nearly half the scan time of conventional ASL. However, this approach potentially could be more sensitive to off-resonance and magnetization transfer (MT) effects. To counter this, careful optimizations of the labeling pulse were performed by Bloch simulations. In in-vivo studies arterial visualization was compared between the new and conventional ASL approaches.

RESULTS

Optimization of the labeling pulse successfully minimized off-resonance effects. Qualitative assessment showed that residual MT effects did not degrade visualization of the peripheral arteries.

CONCLUSION

This study demonstrated that the proposed approach achieved similar image quality as conventional ASL-MRA approaches in just over half the scan time. Magn Reson Med 79:224-233, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

A constrained slice-dependent background suppression scheme for simultaneous multislice pseudo-continuous arterial spin labeling

PURPOSE

To present a constrained slice-dependent (CSD) background-suppression (BS) scheme in 2D arterial spin labeling (ASL) using simultaneous multislice acquisition with blipped-CAIPIRINHA (controlled aliasing in parallel imaging results in higher acceleration).

METHODS

Background suppression for 2D acquisition is challenging because of the multiple nulling points required for sequential slice readout. Constrained slice-dependent BS exploits the simultaneous multislice technique to reduce the readout duration, and uses slice-dependent premodulation pulses to achieve BS across slice groups. The proposed scheme was evaluated by in vivo brain experiments at 3 Tesla with multiband acceleration factors up to four. The utility of CSD BS was demonstrated through comparison against standard 2D acquisitions as well as 3D-BS pseudo-continuous ASL (pCASL).

RESULTS

An average of 95% background signal reduction was achieved with CSD BS. As a result, the temporal signal-to-noise ratio (SNR) increased 48.2/39.9/36.9/36.0% and spatial SNR increased 132.5/80.0/63.5/54.2 in CSD-BS MB-1/2/3/4 scans, respectively. Whole-brain coverage was achievable with CSD-BS pCASL with MB-4, which yielded comparable spatial SNR as 3D BS pCASL.

CONCLUSIONS

The proposed CSD-BS scheme for 2D-SMS pCASL offers a promising approach for effective suppression of background signals across a wide range of T₁ to achieve whole-brain perfusion imaging. Magn Reson Med 79:394-400, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Diagnostic value of renal perfusion in patients with chronic kidney disease using 3D arterial spin labeling

PURPOSE

To evaluate the diagnostic value of 3D arterial spin labeling (ASL) for noninvasive quantification of renal blood flow (RBF) in patients with chronic kidney disease (CKD).

MATERIALS AND METHODS

CKD patients (n = 27) and healthy volunteers (n = 36) underwent renal 3T ASL magnetic resonance imaging, with inversion times from 1200 to 2000 msec for volunteers in the preliminary test, and 1800 to 2000 msec for volunteers and CKD patients in the formal experiments. The cortical RBFs were compared, and a correlation between RBF and estimated glomerular filtration rate (eGFR) was evaluated.

RESULTS

For healthy volunteers, RBF values increased with TIs from 1200 to 1600 msec, but were almost constant at TIs from 1600 to 2000 msec. The cortical RBF values of CKD patients were lower than that of healthy volunteers at TIs from 1800 to 2000 msec. In addition, the CKD patients had lower cortical RBF values than the healthy volunteers (P < 0.01 for both), and their RBF values positively correlated with eGFR.

CONCLUSION

3D ASL is a potential noninvasive method for measuring renal perfusion that can provide valuable information for clinical CKD diagnosis.

LEVEL OF EVIDENCE

1 Technical Efficacy: Stage 3 J. MAGN. RESON. IMAGING 2017;46:589-594.

Comparison of long-labeled pseudo-continuous arterial spin labeling (ASL) features between young and elderly adults: special reference to parameter selection

BACKGROUND

The signal intensity obtained by arterial spin labeling (ASL) depends not only on perfusion signal, but also on arterial transit time (ATT). Although ATT has a more significant effect on accurate regional cerebral blood flow (CBF) calculations, the multiple post-labeling delay (PLD) approach is difficult to use in routine examinations.

PURPOSE

To optimize imaging parameters for labeling duration (LD) and PLD and to confirm their validity in long-labeled pseudo-continuous ASL.

MATERIAL AND METHODS

The perfusion signal was simulated in four LDs and theoretical signal-to-noise ratio efficiency (SNR_eff) was calculated. In vivo studies were performed on a 3.0 T magnetic resonance imaging (MRI) scanner and 15 volunteers were categorized into either the young or elderly adult groups. We compared the differences in CBF values with or without ATT correction.

RESULTS

Regarding signal simulation, perfusion signal increased with the length of LD. SNR_eff also improved with LD, but SNR_eff plateaued at an LD of 3.0 s. As for the in vivo study, SNR linearly increased along with the LD. The CBF differences with the correction of ATT were larger in the elderly adult group. This trend was most prominent in the longer ATT area in the occipital cortical region.

CONCLUSION

A combination of imaging settings of LD = 3.5 s and PLD = 2.0 s were suggested as optimal imaging parameters for allowing acceptable CBF quantification and sufficient SNR in both young and elderly individuals.

Noise Reduction in Arterial Spin Labeling Based Functional Connectivity Using Nuisance Variables

Arterial Spin Labeling (ASL) perfusion image series have recently been utilized for functional connectivity (FC) analysis in healthy volunteers and children with autism spectrum disorders (ASD). Noise reduction by using nuisance variables has been shown to be necessary to minimize potential confounding effects of head motion and physiological signals on BOLD based FC analysis. The purpose of the present study is to systematically evaluate the effectiveness of different noise reduction strategies (NRS) using nuisance variables to improve perfusion based FC analysis in two cohorts of healthy adults using state of the art 3D background-suppressed (BS) GRASE pseudo-continuous ASL (pCASL) and dual-echo 2D-EPI pCASL sequences. Five different NRS were performed in healthy volunteers to compare their performance. We then compared seed-based FC analysis using 3D BS GRASE pCASL in a cohort of 12 children with ASD (3f/9m, age 12.8 ± 1.3 years) and 13 typically developing (TD) children (1f/12m; age 13.9 ± 3 years) in conjunction with NRS. Regression of different combinations of nuisance variables affected FC analysis from a seed in the posterior cingulate cortex (PCC) to other areas of the default mode network (DMN) in both BOLD and pCASL data sets. Consistent with existing literature on BOLD-FC, we observed improved spatial specificity after physiological noise reduction and improved long-range connectivity using head movement related regressors. Furthermore, 3D BS GRASE pCASL shows much higher temporal SNR compared to dual-echo 2D-EPI pCASL and similar effects of noise reduction as those observed for BOLD. Seed-based FC analysis using 3D BS GRASE pCASL in children with ASD and TD children showed that noise reduction including physiological and motion related signals as nuisance variables is crucial for identifying altered long-range connectivity from PCC to frontal brain areas associated with ASD. This is the first study that systematically evaluated the effects of different NRS on ASL based FC analysis. 3D BS GRASE pCASL is the preferred ASL sequence for FC analysis due to its superior temporal SNR. Removing physiological noise and motion parameters is critical for detecting altered FC in neurodevelopmental disorders such as ASD.

Higher Brain Perfusion May Not Support Memory Functions in Cognitively Normal Carriers of the ApoE ε4 Allele Compared to Non-Carriers

Age-related changes in cerebral blood flow (CBF), which carries necessary nutrients to the brain, are associated with increased risk for mild cognitive impairment (MCI) and Alzheimer’s disease (AD). Whether the association between CBF and cognition is moderated by apolipoprotein E (ApoE) ε4 genotype, a known risk factor for AD, remains understudied, with most research focusing on exploring brain regions in which there are diagnostic group differences in CBF (i.e., cognitively normal vs. MCI vs. AD). This study measured resting CBF via arterial spin labeling (ASL) magnetic resonance imaging (MRI) and verbal memory functions using a composite score in 59 older adults with normal cognition (38 ε3; 21 ε4). Linear mixed effect models were employed to investigate if the voxel-wise relationship between verbal memory performance and resting CBF was modified by ApoE genotype. Results indicated that carriers of the ApoE ε4 allele display negative associations between verbal memory functions and CBF in medial frontal cortex, medial and lateral temporal cortex, parietal regions, insula, and the basal ganglia. Contrarily, ε3 carriers exhibited positive associations between verbal memory functions and CBF in medial frontal cortex, thalamus, insula, and basal ganglia. Findings suggest that higher CBF was associated with worse verbal memory functions in cognitively normal ε4 carriers, perhaps reflecting dysregulation within the neurovascular unit, which is no longer supportive of cognition. Results are discussed within the context of the vascular theory of AD risk.

Distinctive Patterns of Three-Dimensional Arterial Spin-Labeled Perfusion Magnetic Resonance Imaging in Subtypes of Acute Ischemic Stroke

BACKGROUND

Ischemic penumbra in acute ischemic stroke (AIS) can be evaluated using arterial spin-labeled (ASL) perfusion magnetic resonance imaging (MRI). We used three-dimensional ASL-MRI to examine patients with different stroke subtypes and the clinical utility of the method within 24 hours of AIS onset.

SUBJECTS AND METHODS

The 55 male and 48 female patients (mean age, 79.0 years) underwent diffusion-weighted imaging (DWI), fluid-attenuated inversion recovery imaging, magnetic resonance angiography, and pulsed continuous ASL perfusion imaging to determine stroke subtype, hypoperfused ASL area, and neurological deficit severity (National Institutes of Health Stroke Scale). Arterial transit artifacts, indicative of occlusive regions or collateral flow, and other stroke indices were compared.

RESULTS

ASL hypoperfusion was detected in 3 of 9 patients with transient ischemic attack (TIA), 2 of 27 patients with lacunar infarction (LI), 19 of 31 patients with atherothrombotic infarction (AT), and 30 of 36 patients with cardiogenic embolic infarction (CE). ASL abnormalities were significantly less frequent in LI than in AT and CE, and more frequent in CE than in TIA. ASL abnormalities were more prevalent in patients with medium-to-large DWI-assessed lesions than in those with small lesions on DWI. Patients with medium-sized lesions following AT and CE had a high frequency of diffusion-perfusion mismatch. In 4 of the 5 patients who underwent intravenous thrombolytic therapy, ASL hypoperfusion and diffusion-perfusion mismatch were improved and the occluded arteries were recanalized.

CONCLUSIONS

ASL perfusion studies may provide useful clinical information allowing diffusion-perfusion mismatch detection and treatment selection in AIS patients, depending on stroke subtype.

Magnetic Resonance Imaging of Cerebral Blood Flow in Animal Stroke Models

Perfusion could provide useful information on the metabolic status and functional status of tissues and organs. This review summarizes the most commonly used perfusion measurement methods: Dynamic susceptibility contrast (DSC) and arterial spin labeling (ASL) and their applications in experimental stroke. Some new developments of cerebral blood flow (CBF) techniques in animal models are also discussed.

Intravoxel incoherent motion diffusion-weighted imaging analysis of diffusion and microperfusion in grading gliomas and comparison with arterial spin labeling for evaluation of tumor perfusion

PURPOSE

To determine the utility of intravoxel incoherent motion (IVIM) imaging in grading gliomas and compare IVIM perfusion metrics with arterial spin labeling (ASL)-derived cerebral blood flow (CBF).

MATERIALS AND METHODS

Fifty-two patients with pathologically confirmed gliomas underwent IVIM and ASL imaging at 3.0T. IVIM perfusion-related diffusivity (D*), perfusion fraction (f), product of f and D*(f×D*), true diffusivity (D), and apparent diffusion coefficient (ADC) were obtained to distinguish glioma grades. The CBF derived from pseudocontinuous ASL within the solid tumor was compared and correlated with IVIM perfusion metrics for grading of gliomas. Values were also normalized to the contralateral normal-appearing white matter. Receiver-operating characteristic was performed to determine diagnostic efficiency. The reliability was estimated with intraclass coefficient, coefficient of variance, and Bland-Altman plots.

RESULTS

IVIM perfusion metrics and CBF were significantly higher in the high-grade than the low-grade gliomas (P < 0.001), ADC and D were significantly lower in the high-grade than the low-grade gliomas (P < 0.001). f×D* differed significantly between grades II through IV (P < 0.05 for all). The other metrics showed significant difference between grade II and grade III (P < 0.05 for all). Area under the curve (AUC) was largest for f×D* in distinguishing high-grade from low-grade gliomas (AUC = 0.979, P < 0.001) and between grade II and grade III (AUC = 0.957, P < 0.001). f×D* improved diagnostic performance of CBF in grading gliomas and showed strong correlation with CBF (r = 0.696, P < 0.001).

CONCLUSION

IVIM-derived metrics are promising biomarkers in preoperative grading gliomas. IVIM imaging may be an additive method to ASL and ADC for evaluating tumor perfusion and diffusion. J. Magn. Reson. Imaging 2016;44:620-632.

Impaired Cerebrovascular Function in Coronary Artery Disease Patients and Recovery Following Cardiac Rehabilitation

Coronary artery disease (CAD) poses a risk to the cerebrovascular function of older adults and has been linked to impaired cognitive abilities. Using magnetic resonance perfusion imaging, we investigated changes in resting cerebral blood flow (CBF) and cerebrovascular reactivity (CVR) to hypercapnia in 34 CAD patients and 21 age-matched controls. Gray matter volume (GMV) images were acquired and used as a confounding variable to separate changes in structure from function. Compared to healthy controls, CAD patients demonstrated reduced CBF in the superior frontal, anterior cingulate (AC), insular, pre- and post-central gyri, middle temporal, and superior temporal regions. Subsequent analysis of these regions demonstrated decreased CVR in the AC, insula, post-central and superior frontal regions. Except in the superior frontal and precentral regions, regional reductions in CBF and CVR were identified in brain areas where no detectable reductions in GMV were observed, demonstrating that these vascular changes were independent of brain atrophy. Because aerobic fitness training can improve brain function, potential changes in regional CBF were investigated in the CAD patients after completion of a 6-months exercise-based cardiac rehabilitation program. Increased CBF was observed in the bilateral AC, as well as recovery of CBF in the dorsal aspect of the right AC, where the magnitude of increased CBF was roughly equal to the reduction in CBF at baseline compared to controls. These exercise-related improvements in CBF in the AC is intriguing given the role of this area in cognitive processing and regulation of cardiovascular autonomic control.

Assessment of vessel permeability by combining dynamic contrast-enhanced and arterial spin labeling MRI

The forward volumetric transfer constant (K(trans)), a physiological parameter extracted from dynamic contrast-enhanced (DCE) MRI, is weighted by vessel permeability and tissue blood flow. The permeability × surface area product per unit mass of tissue (PS) in brain tumors was estimated in this study by combining the blood flow obtained through pseudo-continuous arterial spin labeling (PCASL) and K(trans) obtained through DCE MRI. An analytical analysis and a numerical simulation were conducted to understand how errors in the flow and K(trans) estimates would propagate to the resulting PS. Fourteen pediatric patients with brain tumors were scanned on a clinical 3-T MRI scanner. PCASL perfusion imaging was performed using a three-dimensional (3D) fast-spin-echo readout module to determine blood flow. DCE imaging was performed using a 3D spoiled gradient-echo sequence, and the K(trans) map was obtained with the extended Tofts model. The numerical analysis demonstrated that the uncertainty of PS was predominantly dependent on that of K(trans) and was relatively insensitive to the flow. The average PS values of the whole tumors ranged from 0.006 to 0.217 min(-1), with a mean of 0.050 min(-1) among the patients. The mean K(trans) value was 18% lower than the PS value, with a maximum discrepancy of 25%. When the parametric maps were compared on a voxel-by-voxel basis, the discrepancies between PS and K(trans) appeared to be heterogeneous within the tumors. The PS values could be more than two-fold higher than the K(trans) values for voxels with high K(trans) levels. This study proposes a method that is easy to implement in clinical practice and has the potential to improve the quantification of the microvascular properties of brain tumors.

Direct imaging of functional networks

In blood-oxygenation-level-dependent functional magnetic resonance imaging (fMRI), current methods typically acquire ∼ 500,000 imaging voxels at each time point, and then use computer algorithms to reduce this data to the coefficients of a few hundred parcels or networks. This suggests that the amount of relevant information present in the fMRI signal is relatively small, and presents an opportunity to greatly improve the speed and signal to noise ratio (SNR) of the fMRI process. In this work, a theoretical framework is presented for calculating the coefficients of functional networks directly from highly undersampled fMRI data. Using predefined functional parcellations or networks and a compact k-space trajectory that samples data at optimal spatial scales, the problem of estimating network coefficients is reformulated to allow for direct least squares estimation, without Fourier encoding. By simulation, this approach is shown to allow for acceleration of the imaging process under ideal circumstances by nearly three orders of magnitude.

Steady pulsed imaging and labeling scheme for noninvasive perfusion imaging

PURPOSE

A steady pulsed imaging and labeling (SPIL) scheme is proposed to obtain high-resolution multislice perfusion images of mice brain using standard preclinical MRI equipment.

THEORY AND METHODS

The SPIL scheme repeats a pulsed arterial spin labeling (PASL) module together with a short mixing time to extend the temporal duration of the generated PASL bolus to the total experimental time. Multislice image acquisition takes place during the mixing times. The mixing time is also used for magnetization recovery following image acquisition. The new scheme is able to yield multislice perfusion images rapidly. The perfusion kinetic curve can be measured by a multipulsed imaging and labeling (MPIL) scheme, i.e., acquiring single-slice ASL signals before reaching steady-state in the SPIL sequence.

RESULTS

When applying the SPIL method to normal mice, and to mice with unilateral ischemia, high-resolution multislice (five slices) CBF images could be obtained in 8 min. Perfusion data from ischemic mice showed clear CBF reductions in ischemic regions. The SPIL method was also applied to postmortem mice, showing that the method is free from magnetization transfer confounds.

CONCLUSION

The new SPIL scheme provides for robust measurement of CBF with multislice imaging capability in small animals.

A comparison of arterial spin labeling perfusion MRI and DCE-MRI in human prostate cancer

Perfusion MRI has the potential to provide pathophysiological biomarkers for the evaluating, staging and therapy monitoring of prostate cancer. The objective of this study was to explore the feasibility of noninvasive arterial spin labeling (ASL) to detect prostate cancer in the peripheral zone and to investigate the correlation between the blood flow (BF) measured by ASL and the pharmacokinetic parameters K(trans) (forward volume transfer constant), kep (reverse reflux rate constant between extracellular space and plasma) and ve (the fractional volume of extracellular space per unit volume of tissue) measured by dynamic contrast-enhanced (DCE) MRI in patients with prostate cancer. Forty-three consecutive patients (ages ranging from 49 to 86 years, with a median age of 74 years) with pathologically confirmed prostate cancer were recruited. An ASL scan with four different inversion times (TI = 1000, 1200, 1400 and 1600 ms) and a DCE-MRI scan were performed on a clinical 3.0 T GE scanner. BF, K(trans), kep and ve maps were calculated. In order to determine whether the BF values in the cancerous area were statistically different from those in the noncancerous area, an independent t-test was performed. Spearman's bivariate correlation was used to assess the relationship between BF and the pharmacokinetic parameters K(trans), kep and ve. The mean BF values in the cancerous areas (97.1 ± 30.7, 114.7 ± 28.7, 102.3 ± 22.5, 91.2 ± 24.2 ml/100 g/min, respectively, for TI = 1000, 1200, 1400, 1600 ms) were significantly higher (p < 0.01 for all cases) than those in the noncancerous regions (35.8 ± 12.5, 42.2 ± 13.7, 53.5 ± 19.1, 48.5 ± 13.5 ml/100 g/min, respectively). Significant positive correlations (p < 0.01 for all cases) between BF and the pharmacokinetic parameters K(trans), kep and ve were also observed for all four TI values (r = 0.671, 0.407, 0.666 for TI = 1000 ms; 0.713, 0.424, 0.698 for TI = 1200 ms; 0.604, 0.402, 0.595 for TI = 1400 ms; 0.605, 0.422, 0.548 for TI = 1600 ms). It can be seen that the quantitative ASL measurements show significant differences between cancerous and benign tissues, and exhibit strong to moderate correlations with the parameters obtained using DCE-MRI. These results show the promise of ASL as a noninvasive alternative to DCE-MRI.

Arterial spin labeling perfusion-weighted MRI for long-term follow-up of a cerebral arteriovenous malformation after stereotactic radiosurgery

We present a longitudinal series of arterial spin-labeling magnetic resonance imaging (ASL-MRI) in a patient with cerebral arteriovenous malformations (AVMs) treated by stereotactic radiosurgery (SRS). Pretreatment ASL-MRI showed high signal intensity in both the nidus and draining veins, and the latter signal abnormality gradually moved proximally by 14 months after SRS. At 24 months, the signal abnormalities finally disappeared, indicating complete obliteration of the nidus. The hemodynamic changes in the AVM were clearly visualized in the longitudinal ASL-MRI series, thus this non-invasive MR method may be useful not only for detecting AVMs but also for assessment of their response after SRS.

Reliability of two-dimensional and three-dimensional pseudo-continuous arterial spin labeling perfusion MRI in elderly populations: comparison with 15O-water positron emission tomography

PURPOSE

To investigate the reliability and accuracy of two pseudo-continuous arterial spin labeling (pCASL) sequences, using two-dimensional (2D) gradient-echo echo planar imaging (EPI) and 3D gradient and spin echo (GRASE) as the readout, respectively.

MATERIALS AND METHODS

Each sequence was performed twice 4 weeks apart on six normal control subjects, six elderly subjects with mild cognitive impairment (MCI), and one participant with Alzheimer's disease (AD). Eight of these subjects also underwent H2 (15) O positron emission tomography (PET) scans on the same day or proximal to their second MRI scan. The longitudinal repeatability of EPI and GRASE pCASL were evaluated with the intraclass correlation coefficient (ICC) and within-subject coefficient of variation (wsCV).

RESULTS

The ICCs of global perfusion measurements were 0.697 and 0.413 for GRASE and EPI based pCASL respectively. GRASE pCASL also demonstrated a higher longitudinal repeatability for regional perfusion measurements across 24 regions-of-interests (ICC = 0.707; wsCV = 10.9%) compared with EPI pCASL (ICC = 0.362; wsCV = 15.3%). When compared with PET, EPI pCASL showed a higher degree of spatial correlation with PET than GRASE pCASL, although the difference was not statistically significant.

CONCLUSION

The 3D GRASE pCASL offers better repeatability than 2D EPI pCASL, thereby may provide a reliable imaging marker for the evaluation of disease progression and treatment effects in MCI and early AD subjects.

Comparison of three-dimensional pseudo-continuous arterial spin labeling perfusion imaging with gradient-echo and spin-echo dynamic susceptibility contrast MRI

PURPOSE

To compare the relative cerebral blood flow (CBF) obtained by pseudo-continuous arterial spin labeling sequence incorporated with volumetric fast spin-echo readout (3D-PCASL) with those by gradient-echo (GE) and spin-echo (SE) dynamic susceptibility contrast (DSC) MRI.

MATERIALS AND METHODS

Thirty patients with various neurological diseases participated in this study. In addition to 3D-PCASL, 15 patients received GE-DSC and the others received SE-DSC imaging on a 3 Tesla scanner. A cortical gray matter (GM) to white matter (WM) and a thalamus (TM) to WM CBF ratio were determined from each perfusion scan. In addition, histograms of relative CBF distributions were obtained from each method for comparison.

RESULTS

Significant correlations of CBF ratios were found between 3D-PCASL and the two DSC methods (P < 0.05). The 3D-PCASL resulted in GM/WM CBF ratios similar to SE-DSC but significantly smaller than GE-DSC (P = 2.3 × 10(-7) ). TM/WM CBF ratio obtained by 3D-PCASL was significantly smaller than those by GE- and SE-DSC (P = 4.1 × 10(-7) and 1.2 × 10(-6) , respectively). The histogram of relative CBF maps obtained from SE-DSC, after applied spatial smoothing, agreed well with that from 3D-PCASL.

CONCLUSION

This study suggested that perfusion images obtained from 3D-PCASL exhibited significant correlations with DSC-MRI, with greater microvascular weighting like SE-DSC.

Estimation of single-kidney glomerular filtration rate without exogenous contrast agent

PURPOSE

Measurement of single-kidney filtration fraction and glomerular filtration rate (GFR) without exogenous contrast is clinically important to assess renal function and pathophysiology, especially for patients with comprised renal function. The objective of this study is to develop a novel MR-based tool for noninvasive quantification of renal function using conventional MR arterial spin labeling water as endogenous tracer.

THEORY AND METHODS

The regional differentiation of the arterial spin labeling water between the glomerular capsular space and the renal parenchyma was characterized and measured according to their MR relaxation properties (T1ρ or T2 ), and applied to the estimation of filtration fraction and single-kidney GFR. The proposed approach was tested to quantify GFR in healthy volunteers at baseline and after a protein-loading challenge.

RESULTS

Biexponential decay of the cortical arterial spin labeling water MR signal was observed. The major component decays the same as parenchyma water; the minor component decays much slower as expected from glomerular ultra-filtrates. The mean single-kidney GFR was estimated to be 49 ± 9 mL/min at baseline and increased by 28% after a protein-loading challenge.

CONCLUSION

We developed an arterial spin labeling-based MR imaging method that allows us to estimate renal filtration fraction and singe-kidney GFR without use of exogenous contrast.

Gender difference in neural response to psychological stress

Gender is an important biological determinant of vulnerability to psychosocial stress. We used perfusion based functional magnetic resonance imaging (fMRI) to measure cerebral blood flow (CBF) responses to mild to moderate stress in 32 healthy people (16 males and 16 females). Psychological stress was elicited using mental arithmetic tasks under varying pressure. Stress in men was associated with CBF increase in the right prefrontal cortex (RPFC) and CBF reduction in the left orbitofrontal cortex (LOrF), a robust response that persisted beyond the stress task period. In contrast, stress in women primarily activated the limbic system, including the ventral striatum, putamen, insula and cingulate cortex. The asymmetric prefrontal activity in males was associated with a physiological index of stress responses-salivary cortisol, whereas the female limbic activation showed a lower degree of correlations with cortisol. Conjunction analyses indicated only a small degree of overlap between the stress networks in men and women at the threshold level of P < 0.01. Increased overlap of stress networks between the two genders was revealed when the threshold for conjunction analyses was relaxed to P < 0.05. Further, machine classification was used to differentiate the central stress responses between the two genders with over 94% accuracy. Our study may represent an initial step in uncovering the neurobiological basis underlying the contrasting health consequences of psychosocial stress in men and women.