Estimation of the onset time of cerebral ischemia using T1rho and T2 MRI in rats

Detection of tissue pH with quantitative chemical exchange saturation transfer magnetic resonance imaging

Chemical exchange saturation transfer (CEST) magnetic resonance imaging (MRI) has emerged as a novel means for sensitive detection of dilute labile protons and chemical exchange rates. By sensitizing to pH-dependent chemical exchange, CEST MRI has shown promising results in monitoring tissue statuses such as pH changes in disorders like acute stroke, tumor, and acute kidney injury. This article briefly reviews the basic principles for CEST imaging and quantitative measures, from the simplistic asymmetry analysis to multipool Lorentzian decoupling and quasi-steady-state reconstruction. In particular, the advantages and limitations of commonly used quantitative approaches for CEST applications are discussed.

Three-dimensional multi-parameter brain mapping using MR fingerprinting

The purpose of this study was to develop and test a 3D multi-parameter MR fingerprinting (MRF) method for brain imaging applications. The subject cohort included 5 healthy volunteers, repeatability tests done on 2 healthy volunteers and tested on two multiple sclerosis (MS) patients. A 3D-MRF imaging technique capable of quantifying T1, T2 and T1ρ was used. The imaging sequence was tested in standardized phantoms and 3D-MRF brain imaging with multiple shots (1, 2 and 4) in healthy human volunteers and MS patients. Quantitative parametric maps for T1, T2, T1ρ, were generated. Mean gray matter (GM) and white matter (WM) ROIs were compared for each mapping technique, Bland-Altman plots and intra-class correlation coefficient (ICC) were used to assess repeatability and Student T-tests were used to compare results in MS patients. Standardized phantom studies demonstrated excellent agreement with reference T1/T2/T1ρ mapping techniques. This study demonstrates that the 3D-MRF technique is able to simultaneously quantify T1, T2 and T1ρ for tissue property characterization in a clinically feasible scan time. This multi-parametric approach offers increased potential to detect and differentiate brain lesions and to better test imaging biomarker hypotheses for several neurological diseases, including MS.

Demonstration of accurate multi-pool chemical exchange saturation transfer MRI quantification - Quasi-steady-state reconstruction empowered quantitative CEST analysis

Chemical exchange saturation transfer (CEST) MRI is sensitive to dilute labile protons and microenvironment properties, yet CEST quantification has been challenging. This difficulty is because the CEST measurement depends not only on the underlying CEST system but also on the scan protocols, including RF saturation amplitude, duration, and repetition time. In addition, T1 normalization is not straightforward under non-equilibrium conditions. Recently, a quasi-steady-state (QUASS) algorithm was established to reconstruct the desired equilibrium state from experimental measurements. Our study aimed to determine the accuracy of spinlock-model-based multi-pool CEST quantification using numerical simulations and phantom experiments. In short, CEST Z-spectra were simulated for a representative 3-pool model, and CEST amplitudes were solved with spinlock model-based multi-pool fitting and assessed as a function of RF saturation time (Ts), repetition time (TR), and T1. Although the apparent CEST signals showed significant T1 dependence, such relationships were not observed following QUASS reconstruction. To test the accuracy of T1 correction, a multi-vial phantom of nicotinamide and creatine was doped with manganese chloride, resulting in T1 ranging from 1 s to beyond 2 s. The multi-labile signals determined from the routine measurements showed significant dependence on Ts, TR, and T1. In contrast, CEST signals from the QUASS reconstruction showed consistent quantification independent of such variables. To summarize, our study demonstrated that accurate CEST quantification is feasible in multi-pool CEST systems with the spinlock-model-based fitting of QUASS CEST MRI.

Consistent depiction of the acidic ischemic lesion with APT MRI-Dual RF power evaluation of pH-sensitive image in acute stroke

PURPOSE

Amide proton transfer-weighted (APTw) MRI provides a non-invasive pH-sensitive image, complementing perfusion and diffusion imaging for refined stratification of ischemic tissue. Although the commonly used magnetization transfer (MT) asymmetry (MTR_asym ) calculation reasonably corrects the direct RF saturation effect, it is susceptible to the concomitant semisolid macromolecular MT contribution. Therefore, this study aimed to compare the performance of MTR_asym and magnetization transfer and relaxation-normalized APT (MRAPT) analyses under 2 representative experimental conditions.

METHODS

Multiparametric MRI scans were performed in a rodent model of acute stroke, including relaxation, diffusion, and Z spectral images under 2 representative RF levels of 0.75 and 1.5 µT. Both MTR_asym and MRAPT values in the ischemic diffusion lesion and the contralateral normal areas were compared using correlation and Bland-Altman tests. In addition, the acidic lesion volumes were compared.

RESULTS

MRAPT measurements from the diffusion lesion under the 2 conditions were highly correlated (R² = 0.97) versus MTR_asym measures (R² = 0.58). The pH lesion sizes determined from MRAPT analysis were in good agreement (178 ± 43 mm³ vs. 186 ± 55 mm³ for B₁ of 0.75 and 1.5 µT, respectively).

CONCLUSIONS

The study demonstrated that MRAPT analysis could be generalized to moderately different RF amplitudes, providing a more consistent depiction of acidic lesions than the MTR_asym analysis.

T1ρ and T2 mapping detect acute ischemic injury in a piglet model of Legg-Calvé-Perthes disease

This study investigated the sensitivity of T1ρ and T2 relaxation time mapping to detect acute ischemic injury to the secondary ossification center (SOC) and epiphyseal cartilage of the femoral head in a piglet model of Legg-Calvé-Perthes disease. Six piglets underwent surgery to induce global right femoral head ischemia and were euthanized 48 h later. Fresh operated and contralateral-control femoral heads were imaged ex vivo with T1, T2, and T1ρ mapping using a 9.4T magnetic resonance imaging scanner. The specimens were imaged a second time after a freeze/thaw cycle and then processed for histology. T1, T2, and T1ρ measurements in the SOC, epiphyseal cartilage, articular cartilage, and metaphysis were compared between operated and control femoral heads using paired t tests. The effects of freeze/thaw, T1ρ spin-lock frequency, and fat saturation were also investigated. Five piglets with histologically confirmed ischemic injury were quantitatively analyzed. T1ρ was increased in the SOC (101 ± 15 vs. 73 ± 16 ms; p = 0.0026) and epiphyseal cartilage (84.9 ± 9.2 vs. 74.3 ± 3.6 ms; p = 0.031) of the operated versus control femoral heads. T2 was also increased in the SOC (28.7 ± 2.0 vs. 22.7 ± 1.7; p = 0.0037) and epiphyseal cartilage (57.4 ± 4.7 vs. 49.0 ± 2.7; p = 0.0041). No changes in T1 were detected. The sensitivities of T1ρ and T2 mapping in detecting ischemic injury were maintained after a freeze/thaw cycle, and T1ρ sensitivity was maintained after varying spin-lock frequency and applying fat saturation. In conclusion, T1ρ and T2 mapping are sensitive in detecting ischemic injury to the SOC and epiphyseal cartilage of the femoral head as early as 48 h after ischemia induction.

Performance Comparison of Compressed Sensing Algorithms for Accelerating T1ρ Mapping of Human Brain

BACKGROUND

3D-T_1ρ mapping is useful to quantify various neurologic disorders, but data are currently time-consuming to acquire.

PURPOSE

To compare the performance of five compressed sensing (CS) algorithms-spatiotemporal finite differences (STFD), exponential dictionary (EXP), 3D-wavelet transform (WAV), low-rank (LOW) and low-rank plus sparse model with spatial finite differences (L + S SFD)-for 3D-T_1ρ mapping of the human brain with acceleration factors (AFs) of 2, 5, and 10.

STUDY TYPE

Retrospective.

SUBJECTS

Eight healthy volunteers underwent T_1ρ imaging of the whole brain.

FIELD STRENGTH/SEQUENCE

The sequence was fully sampled 3D Cartesian ultrafast gradient echo sequence with a customized T_1ρ preparation module on a clinical 3T scanner.

ASSESSMENT

The fully sampled data was undersampled by factors of 2, 5, and 10 and reconstructed with the five CS algorithms. Image reconstruction quality was evaluated and compared to the SENSE reconstruction of the fully sampled data (reference) and T_1ρ estimation errors were assessed as a function of AF.

STATISTICAL TESTS

Normalized root mean squared errors (nRMSE) and median normalized absolute deviation (MNAD) errors were calculated to compare image reconstruction errors and T_1ρ estimation errors, respectively. Linear regression plots, Bland-Altman plots, and Pearson correlation coefficients (CC) are shown.

RESULTS

For image reconstruction quality, at AF = 2, EXP transforms had the lowest mRMSE (1.56%). At higher AF values, STFD performed better, with the smallest errors (3.16% at AF = 5, 4.32% at AF = 10). For whole-brain quantitative T_1ρ mapping, at AF = 2, EXP performed best (MNAD error = 1.62%). At higher AF values (AF = 5, 10), the STFD technique had the least errors (2.96% at AF = 5, 4.24% at AF = 10) and the smallest variance from the reference T_1ρ estimates.

DATA CONCLUSION

This study demonstrates the use of different CS algorithms that may be useful in reducing the scan time required to perform volumetric T_1ρ mapping of the brain.

LEVEL OF EVIDENCE

2.

TECHNICAL EFFICACY STAGE

1.

Three-dimensional whole-brain simultaneous T1, T2, and T1ρ quantification using MR Multitasking: Method and initial clinical experience in tissue characterization of multiple sclerosis

PURPOSE

To develop a 3D whole-brain simultaneous T1/T2/T1ρ quantification method with MR Multitasking that provides high quality, co-registered multiparametric maps in 9 min.

METHODS

MR Multitasking conceptualizes T1/T2/T1ρ relaxations as different time dimensions, simultaneously resolving all three dimensions with a low-rank tensor image model. The proposed method was validated on a phantom and in healthy volunteers, comparing quantitative measurements against corresponding reference methods and evaluating the scan-rescan repeatability. Initial clinical validation was performed in age-matched relapsing-remitting multiple sclerosis (RRMS) patients to examine the feasibility of quantitative tissue characterization and to compare with the healthy control cohort. The feasibility of synthesizing six contrast-weighted images was also examined.

RESULTS

Our framework produced high quality, co-registered T1/T2/T1ρ maps that closely resemble the reference maps. Multitasking T1/T2/T1ρ measurements showed substantial agreement with reference measurements on the phantom and in healthy controls. Bland-Altman analysis indicated good in vivo repeatability of all three parameters. In RRMS patients, lesions were conspicuously delineated on all three maps and on four synthetic weighted images (T2-weighted, T2-FLAIR, double inversion recovery, and a novel "T1ρ-FLAIR" contrast). T1 and T2 showed significant differences for normal appearing white matter between patients and controls, while T1ρ showed significant differences for normal appearing white matter, cortical gray matter, and deep gray matter. The combination of three parameters significantly improved the differentiation between RRMS patients and healthy controls, compared to using any single parameter alone.

CONCLUSION

MR Multitasking simultaneously quantifies whole-brain T1/T2/T1ρ and is clinically promising for quantitative tissue characterization of neurological diseases, such as MS.

Accelerating T2 mapping of the brain by integrating deep learning priors with low-rank and sparse modeling

PURPOSE

To accelerate T₂ mapping with highly sparse sampling by integrating deep learning image priors with low-rank and sparse modeling.

METHODS

The proposed method achieves high-speed T₂ mapping by highly sparsely sampling (k, TE)-space. Image reconstruction from the undersampled data was done by exploiting the low-rank structure and sparsity in the T₂ -weighted image sequence and image priors learned from training data. The image priors for a single TE were generated from the public Human Connectome Project data using a tissue-based deep learning method; the image priors were then transferred to other TEs using a generalized series-based method. With these image priors, the proposed reconstruction method used a low-rank model and a sparse model to capture subject-dependent novel features.

RESULTS

The proposed method was evaluated using experimental data obtained from both healthy subjects and tumor patients using a turbo spin-echo sequence. High-quality T₂ maps at the resolution of 0.9 × 0.9 × 3.0 mm³ were obtained successfully from highly undersampled data with an acceleration factor of 8. Compared with the existing compressed sensing-based methods, the proposed method produced significantly reduced reconstruction errors. Compared with the deep learning-based methods, the proposed method recovered novel features better.

CONCLUSION

This paper demonstrates the feasibility of learning T₂ -weighted image priors for multiple TEs using tissue-based deep learning and generalized series-based learning. A new method was proposed to effectively integrate these image priors with low-rank and sparse modeling to reconstruct high-quality images from highly undersampled data. The proposed method will supplement other acquisition-based methods to achieve high-speed T₂ mapping.

A Comparison of T2 Relaxation-Based MRI Stroke Timing Methods in Hyperacute Ischemic Stroke Patients: A Pilot Study

Background:

T₂ relaxation-based magnetic resonance imaging (MRI) signals may provide onset time for acute ischemic strokes with an unknown onset. The ability of visual and quantitative MRI-based methods in a cohort of hyperacute ischemic stroke patients was studied.

Methods:

A total of 35 patients underwent 3T (3 Tesla) MRI (<9-hour symptom onset). Diffusion-weighted (DWI), apparent diffusion coefficient (ADC), T₁-weighted (T₁w), T₂-weighted (T₂w), and T₂ relaxation time (T₂) images were acquired. T₂-weighted fluid attenuation inversion recovery (FLAIR) images were acquired for 17 of these patients. Image intensity ratios of the average intensities in ischemic and non-ischemic reference regions were calculated for ADC, DWI, T₂w, T₂ relaxation, and FLAIR images, and optimal image intensity ratio cut-offs were determined. DWI and FLAIR images were assessed visually for DWI/FLAIR mismatch.

Results:

The T₂ relaxation time image intensity ratio was the only parameter with significant correlation with stroke duration (r = 0.49, P = .003), an area under the receiver operating characteristic curve (AUC = 0.77, P < .0001), and an optimal cut-off (T₂ ratio = 1.072) that accurately identified patients within the 4.5-hour thrombolysis treatment window with sensitivity of 0.74 and specificity of 0.74. In the patients with the additional FLAIR, areas under the precision-recall-gain curve (AUPRG) and F₁ scores showed that the T₂ relaxation time ratio (AUPRG = 0.60, F₁ = 0.73) performed considerably better than the FLAIR ratio (AUPRG = 0.39, F₁ = 0.57) and the visual DWI/FLAIR mismatch (F₁ = 0.25).

Conclusions:

Quantitative T₂ relaxation time is the preferred MRI parameter in the assessment of patients with unknown onset for treatment stratification.

Three-Dimensional GRE T1ρ mapping of the brain using tailored variable flip-angle scheduling

PURPOSE

To introduce a new approach called tailored variable flip-angle (VFA) scheduling for SNR-efficient 3D T_1ρ mapping of the brain using a magnetization-prepared gradient-echo sequence.

METHODS

Simulations were used to assess the relative SNR efficiency, quantitative accuracy, and spatial blurring of tailored VFA scheduling for T_1ρ mapping of brain tissue compared with magnetization-prepared angle-modulated partitioned k-space spoiled gradient-echo snapshots (MAPSS), a state-of-the-art technique for accurate 3D gradient-echo T_1ρ mapping. Simulations were also used to calculate optimal imaging parameters for tailored VFA scheduling versus MAPSS, without and with nulling of CSF. Four participants were imaged at 3T MRI to demonstrate the feasibility of tailored VFA scheduling for T_1ρ mapping of the brain. Using MAPSS as a reference standard, in vivo data were used to validate the relative SNR efficiency and quantitative accuracy of the new approach.

RESULTS

Tailored VFA scheduling can provide a 2-fold to 4-fold gain in the SNR of the resulting T_1ρ map as compared with MAPSS when using identical sequence parameters while limiting T_1ρ quantification errors to 2% or less. In vivo whole-brain 3D T_1ρ maps acquired with tailored VFA scheduling had superior SNR efficiency than is achievable with MAPSS, and the SNR efficiency improved with a greater number of views per segment.

CONCLUSIONS

Tailored VFA scheduling is an SNR-efficient GRE technique for 3D T_1ρ mapping of the brain that provides increased flexibility in choice of imaging parameters compared with MAPSS, which may benefit a variety of applications.

R1ρ sensitivity to pH and other compounds at clinically accessible spin-lock fields in the presence of proteins

Numerous human diseases involve abnormal metabolism, and proton exchange is an effective source of magnetic resonance imaging (MRI) contrast for assessing metabolism. One MRI technique that capitalizes on proton exchange is R₁ relaxation in the rotating frame (R_1ρ ). Here, we investigated the sensitivity of R_1ρ to various proton-exchange mechanisms at spin-lock pulses within Food and Drug Administration (FDA) safety guidelines for radiofrequency-induced heating. We systematically varied pH known to change the rate of proton exchange as well as the glucose and lysine concentrations, thus changing the number of amide, hydroxyl and amine exchangeable sites in a series of egg-white albumin phantoms. The resulting effects on quantitative relaxation time measurements of R_1ρ , R₁ and R₂ were observed at 3 T. Using spin-lock amplitudes available for human imaging (less than 23.5 μT) at near physiologic temperatures, we found R_1ρ was more sensitive to physiologic changes in pH than to changes in glucose and lysine concentrations. In addition, R_1ρ was more sensitive to pH changes than R₁ and R₂ . Models of proton exchange fitted to the relaxation measurements suggest that amide groups were the primary source of pH sensitivity. Together, these experiments suggest an optimal spin-lock amplitude for measuring pH changes while not exceeding FDA-subject heating limitations.

Comparison of T1Rho MRI, Glucose Metabolism, and Amyloid Burden Across the Cognitive Spectrum: A Pilot Study

OBJECTIVE

The pathological cascades associated with the development of Alzheimer's disease (AD) have a common element: acidosis. T₁rho MRI is a pH-sensitive measure, with higher values associated with greater neuropathological burden. The authors investigated the relationship between T₁rho imaging and AD-associated pathologies as determined by available diagnostic imaging techniques.

METHODS

Twenty-seven participants (men, N=13, women, N=14; ages 55-90) across the cognitive spectrum (healthy control subjects [HCs] with normal cognition, N=17; participants with mild cognitive impairment [MCI], N=7; participants with mild AD, N=3) underwent neuropsychological testing, MRI (T₁-weighted and T₁rho [spin-lattice relaxation time in the rotating frame]), and positron emission tomography imaging ([¹¹C]Pittsburg compound B for amyloid burden [N=26] and [¹⁸F]fluorodeoxyglucose for cerebral glucose metabolism [N=12]). The relationships between global T₁rho values and neuropsychological, demographic, and imaging measures were explored.

RESULTS

Global mean and median T₁rho were positively associated with age. After controlling for age, higher global T₁rho was associated with poorer cognitive function, poorer memory function (immediate and delayed memory scores), higher amyloid burden, and more abnormal cerebral glucose metabolism. Regional T₁rho values, when controlling for age, significantly differed between HCs and participants with MCI or AD in select frontal, cingulate, and parietal regions.

CONCLUSIONS

Higher T₁rho values were associated with greater cognitive impairment and pathological burden. T₁rho, a biomarker that varies according to a feature common to each cascade rather than one that is unique to a particular pathology, has the potential to serve as a metric of neuropathology, theoretically providing a measure for assessing pathological status and for monitoring the neurodegeneration trajectory.

Determining T2 relaxation time and stroke onset relationship in ischaemic stroke within apparent diffusion coefficient-defined lesions. A user-independent method for quantifying the impact of stroke in the human brain

BACKGROUND AND OBJECTIVE

In hyperacute ischaemic stroke, T2 of cerebral water increases with time. Quantifying this change may be informative of the extent of tissue damage and onset time. Our objective was to develop a user-unbiased method to measure the effect of cerebral ischaemia on T2 to study stroke onset time-dependency in human acute stroke lesions.

METHODS

Six rats were subjected to permanent middle cerebral occlusion to induce focal ischaemia, and a consecutive cohort of acute stroke patients (n = 38) were recruited within 9 hours from symptom onset. T1-weighted structural, T2 relaxometry, and diffusion MRI for apparent diffusion coefficient (ADC) were acquired. Ischaemic lesions were defined as regions of lowered ADC. The median T2 difference (ΔT2) between lesion and contralateral non-ischaemic control region was determined by the newly-developed spherical reference method, and data compared to that obtained by the mirror reference method. Linear regressions and receiver operating characteristics (ROC) were compared between the two methods.

RESULTS

ΔT2 increases linearly in rat brain ischaemia by 1.9 ± 0.8 ms/h during the first 6 hours, as determined by the spherical reference method. In patients, ΔT2 linearly increases by 1.6 ± 1.4 and 1.9 ± 0.9 ms/h in the lesion, as determined by the mirror reference and spherical reference method, respectively. ROC analyses produced areas under the curve of 0.83 and 0.71 for the spherical and mirror reference methods, respectively.

CONCLUSIONS

Data from the spherical reference method showed that the median T2 increase in the ischaemic lesion is correlated with stroke onset time in a rat as well as in a human patient cohort, opening the possibility of using the approach as a timing tool in clinics.

Quantifying T 2 relaxation time changes within lesions defined by apparent diffusion coefficient in grey and white matter in acute stroke patients

The apparent diffusion coefficient (ADC) of cerebral water, as measured by diffusion MRI, rapidly decreases in ischaemia, highlighting a lesion in acute stroke patients. The MRI T ₂ relaxation time changes in ischaemic brain such that T ₂ in ADC lesions may be informative of the extent of tissue damage, potentially aiding in stratification for treatment. We have developed a novel user-unbiased method of determining the changes in T ₂ in ADC lesions as a function of clinical symptom duration based on voxel-wise referencing to a contralateral brain volume. The spherical reference method calculates the most probable pre-ischaemic T ₂ on a voxel-wise basis, making use of features of the contralateral hemisphere presumed to be largely unaffected. We studied whether T ₂ changes in the two main cerebral tissue types, i.e. in grey matter (GM) and white matter (WM), would differ in stroke. Thirty-eight acute stroke patients were accrued within 9 h of symptom onset and scanned at 3 T for 3D T ₁-weighted, multi b-value diffusion and multi-echo spin echo MRI for tissue type segmentation, quantitative ADC and absolute T ₂ images, respectively. T ₂ changes measured by the spherical reference method were 1.94  ±  0.61, 1.50  ±  0.52 and 1.40  ±  0.54 ms h^-1 in the whole, GM, and WM lesions, respectively. Thus, T ₂ time courses were comparable between GM and WM independent of brain tissue type involved. We demonstrate that T ₂ changes in ADC-delineated lesions can be quantified in the clinical setting in a user unbiased manner and that T ₂ change correlated with symptom onset time, opening the possibility of using the approach as a tool to assess severity of tissue damage in the clinical setting.

Quantitative MRI Helps to Detect Hip Ischemia: Preclinical Model of Legg-Calvé-Perthes Disease

Purpose To determine whether quantitative MRI relaxation time mapping techniques can help to detect ischemic injury to the developing femoral head. Materials and Methods For this prospective animal study conducted from November 2015 to February 2018, 10 male 6-week-old piglets underwent an operation to induce complete right femoral head ischemia. Animals were humanely killed at 48 hours (n = 2) or 4 weeks (n = 8) after the operation, and the operated and contralateral-control femoral heads were harvested and frozen. Thawed specimens were imaged at 9.4-T MRI by using T1, T2, T1 in the rotating frame (T1ρ), adiabatic T1ρ, relaxation along a fictitious field (RAFF), and T2* mapping and evaluated with histologic analysis. Paired relaxation time differences between the operated and control femoral heads were measured in the secondary ossification center (SOC), epiphyseal cartilage, articular cartilage, and metaphysis and were analyzed by using a paired t test. Results In the SOC, T1ρ and RAFF had the greatest percent increases in the operated versus control femoral heads at both 48 hours (112% and 72%, respectively) and 4 weeks (74% and 70%, respectively). In the epiphyseal and articular cartilage, T2, T1ρ, and RAFF were similarly increased at both points (range, 24%-49%). At 4 weeks, T2, T1ρ, adiabatic T1ρ, and RAFF were increased in the SOC (P = .004, .018, < .001, and .001, respectively), epiphyseal cartilage (P = .009, .008, .011, and .007, respectively), and articular cartilage (P = .005, .016, .033, and .018, respectively). Histologic assessment identified necrosis in SOC and deep layer of the epiphyseal cartilage at both points. Conclusion T2, T1 in the rotating frame, adiabatic T1 in the rotating frame, and relaxation along a fictitious field maps are sensitive in helping to detect ischemic injury to the developing femoral head. © RSNA, 2018 Online supplemental material is available for this article.

Spin-lock imaging of early tissue pH changes in ischemic rat brain

We have previously reported that the dispersion of spin-lattice relaxation rates in the rotating frame (R_1ρ ) of tissue water protons at high field can be dominated by chemical exchange contributions. Ischemia in brain causes changes in tissue pH, which in turn may affect proton exchange rates. Amide proton transfer (APT, a form of chemical exchange saturation transfer) has been shown to be sensitive to chemical exchange rates and able to detect pH changes non-invasively following ischemic stroke. However, the specificity of APT to pH changes is decreased because of the influence of several other factors that affect magnetization transfer. R_1ρ is less influenced by such confounding factors and thus may be more specific for detecting variations in pH. Here, we applied a spin-locking sequence to detect ischemic stroke in animal models. Although R_1ρ images acquired with a single spin-locking amplitude (ω₁ ) have previously been used to assess stroke, here we use ΔR_1ρ , which is the difference in R_1ρ values acquired with two different locking fields to emphasize selectively the contribution of chemical exchange effects. Numerical simulations with different exchange rates and measurements of tissue homogenates with different pH were performed to evaluate the specificity of ΔR_1ρ to detect tissue acidosis. Spin-lock and APT data were acquired on five rat brains after ischemic strokes induced via middle cerebral artery occlusions. Correlations between these data were analyzed at different time points after the onset of stroke. The results show that ΔR_1ρ (but not R_1ρ acquired with a single ω₁ ) was significantly correlated with APT metrics consistent with ΔR_1ρ varying with pH.

Acidity and Acid-Sensing Ion Channels in the Normal and Alzheimer's Disease Brain

Alzheimer's disease prevalence has reached epidemic proportion with very few treatment options, which are associated with a multitude of side effects. A potential avenue of research for new therapies are protons, and their associated receptor: acid-sensing ion channels (ASIC). Protons are often overlooked neurotransmitters, and proton-gated currents have been identified in the brain. Furthermore, ASICs have been determined to be crucial for proper brain function. While there is more work to be done, this review is intended to highlight protons as neurotransmitters and their role along with the role of ASICs within physiological functioning of the brain. We will also cover the pathophysiological associations between ASICs and modulators of ASICs. Finally, this review will sum up how the studies of protons, ASICs and their modulators may generate new therapeutic molecules for Alzheimer's disease and other neurodegenerative diseases.

Use of T1 relaxation time in rotating frame (T1 ρ) and apparent diffusion coefficient to estimate cerebral stroke evolution

BACKGROUND

The major factor for the appropriate treatment strategies for ischemia patients is its onset timing.

PURPOSE/HYPOTHESIS

To study to evaluate the diagnostic accuracy of T₁ relaxation time in a rotating frame (T₁ ρ) and apparent diffusion coefficient (ADC) from MRI to estimate ischemia stages.

STUDY TYPE

Prospective.

POPULATION/SUBJECTS/PHANTOM/SPECIMEN/ANIMAL MODEL

In all, 73 patients (49 males, aged 29-78 years and 24 females, aged 22-94 years) with ischemia were prospectively imaged with T₁ ρ and diffusion MRI during the postischemic period.

FIELD STRENGTH/SEQUENCE

3T/T₁ ρ and diffusion-weighted imaging (DWI).

ASSESSMENT

Ischemic parenchyma included tissue with elevated signal areas on DWI and correlative hypointense areas on ADC maps.

STATISTICAL TESTS

The sensitivity of variables to ischemia time was quantified by analyzing the respective correlations of these values with onset time.

RESULTS

ΔT₁ ρ (ipsilateral-contralateral differences in T₁ ρ) (R²  = 0.956) and T₁ ρ_ipsi (ipsilateral ischemia T₁ ρ values) (R²  = 0.941) were elevated in all ischemic lesions; these values increased linearly as a function of time, unlike ΔADC (ipsilateral-contralateral differences in ADC) (R²  = -0.410) and ADC_ipsi (ipsilateral ischemia ADC values) (R²  = 0.550). ΔT₁ ρ and T₁ ρ_ipsi were significantly different between all stages (P < 0.01), except the acute and hyperacute stages (P = 0.589 for ΔT₁ ρ, P = 0.290 for T₁ ρ_ipsi , respectively), but ΔADC and ADC_ipsi only between the late subacute and early subacute stages (P < 0.01) and the late subacute and chronic stages (P < 0.01).

DATA CONCLUSION

These data suggest that T₁ ρ can provide estimates for the ischemic time in patients. T₁ ρ has the potential to outperform diffusion for single-timepoint examination because the T₁ ρ change during strokes is positive and linear. If patients with suspected stroke are scanned by MRI within the appropriate timeframe, T₁ ρ may provide tools for evaluating stroke onset, potentially aiding in treatment strategies.

LEVEL OF EVIDENCE

4 Technical Efficacy: Stage 3 J. Magn. Reson. Imaging 2018;47:1247-1254.

Magnetic Resonance Imaging Protocol for Stroke Onset Time Estimation in Permanent Cerebral Ischemia

MRI provides a sensitive and specific imaging tool to detect acute ischemic stroke by means of a reduced diffusion coefficient of brain water. In a rat model of ischemic stroke, differences in quantitative T₁ and T₂ MRI relaxation times (qT₁ and qT₂) between the ischemic lesion (delineated by low diffusion) and the contralateral non-ischemic hemisphere increase with time from stroke onset. The time dependency of MRI relaxation time differences is heuristically described by a linear function and thus provides a simple estimate of stroke onset time. Additionally, the volumes of abnormal qT₁ and qT₂ within the ischemic lesion increase linearly with time providing a complementary method for stroke timing. A (semi)automated computer routine based on the quantified diffusion coefficient is presented to delineate acute ischemic stroke tissue in rat ischemia. This routine also determines hemispheric differences in qT₁ and qT₂ relaxation times and the location and volume of abnormal qT₁ and qT₂ voxels within the lesion. Uncertainties associated with onset time estimates of qT₁ and qT₂ MRI data vary from ± 25 min to ± 47 min for the first 5 hours of stroke. The most accurate onset time estimates can be obtained by quantifying the volume of overlapping abnormal qT₁ and qT₂ lesion volumes, termed 'V^overlap' (± 25 min) or by quantifying hemispheric differences in qT₂ relaxation times only (± 28 min). Overall, qT₂ derived parameters outperform those from qT₁. The current MRI protocol is tested in the hyperacute phase of a permanent focal ischemia model, which may not be applicable to transient focal brain ischemia.

Biologically aggressive regions within glioblastoma identified by spin-lock contrast T1 relaxation in the rotating frame (T1ρ) MRI

Spin-lattice relaxation in the rotating frame magnetic resonance imaging allows for the quantitative assessment of spin-lock contrast within tissues. We describe the utility of spin-lattice relaxation in the rotating frame metrics in characterizing glioblastoma biological heterogeneity. A 84-year-old man presented to our institution with a right frontal temporal mass. Prior tissue sampling from a peripheral nonenhancing lesion was nondiagnostic. Stereotactic image-guided tissue sampling of the nonenhancing T2-fluid-attenuated inversion recovery hyperintense region involving the anterior cingulate gyrus with elevated spin-lattice relaxation in the rotating frame metrics provided a pathologic diagnosis of glioblastoma. This case illustrates the utility of spin-lattice relaxation in the rotating frame magnetic resonance imaging in identifying biologically aggressive regions within glioblastoma.

Improving the detection sensitivity of pH-weighted amide proton transfer MRI in acute stroke patients using extrapolated semisolid magnetization transfer reference signals

PURPOSE

To quantify amide protein transfer (APT) effects in acidic ischemic lesions and assess the spatial-temporal relationship among diffusion, perfusion, and pH deficits in acute stroke patients.

METHODS

Thirty acute stroke patients were scanned at 3 T. Quantitative APT (APT^# ) effects in acidic ischemic lesions were measured using an extrapolated semisolid magnetization transfer reference signal technique and compared with commonly used MTR_asym (3.5ppm) or APT-weighted parameters.

RESULTS

The APT^# images showed clear pH deficits in the ischemic lesion, whereas the MTR_asym (3.5ppm) signals were slightly hypointense. The APT^# contrast between acidic ischemic lesions and normal tissue in acute stroke patients was more than three times larger than MTR_asym (3.5ppm) contrast (-1.45 ± 0.40% for APT^# versus -0.39 ± 0.52% for MTR_asym (3.5ppm), P < 4.6 × 10^-4 ). Hypoperfused and acidic areas without an apparent diffusion coefficient abnormality were observed and assigned to an ischemic acidosis penumbra. Hypoperfused areas at normal pH were also observed and assigned to benign oligemia. Hyperintense APT signals were observed in a hemorrhage area in one case.

CONCLUSIONS

The quantitative APT study using the extrapolated semisolid magnetization transfer reference signal approach enhances APT MRI sensitivity to pH compared with conventional APT-weighted MRI, allowing more reliable delineation of an ischemic acidosis in the penumbra. Magn Reson Med 78:871-880, 2017. © 2017 International Society for Magnetic Resonance in Medicine.

Stroke Onset Time Determination Using MRI Relaxation Times without Non-Ischaemic Reference in A Rat Stroke Model

BACKGROUND

Objective timing of stroke in emergency departments is expected to improve patient stratification. Magnetic resonance imaging (MRI) relaxations times, T₂ and T_1ρ , in abnormal diffusion delineated ischaemic tissue were used as proxies of stroke time in a rat model.

METHODS

Both 'non-ischaemic reference'-dependent and -independent estimators were generated. Apparent diffusion coefficient (ADC), T₂ and T_1ρ , were sequentially quantified for up to 6 hours of stroke in rats (n = 8) at 4.7T. The ischaemic lesion was identified as a contiguous collection of voxels with low ADC. T₂ and T_1ρ in the ischaemic lesion and in the contralateral non-ischaemic brain tissue were determined. Differences in mean MRI relaxation times between ischaemic and non-ischaemic volumes were used to create reference-dependent estimator. For the reference-independent procedure, only the parameters associated with log-logistic fits to the T₂ and T_1ρ distributions within the ADC-delineated lesions were used for the onset time estimation.

RESULT

The reference-independent estimators from T₂ and T_1ρ data provided stroke onset time with precisions of ±32 and ±27 minutes, respectively. The reference-dependent estimators yielded respective precisions of ±47 and ±54 minutes.

CONCLUSIONS

A 'non-ischaemic anatomical reference'-independent estimator for stroke onset time from relaxometric MRI data is shown to yield greater timing precision than previously obtained through reference-dependent procedures.

Differentiation of brain tumor-related edema based on 3D T1rho imaging

BACKGROUND AND PURPOSE

Cerebral edema associated with brain tumors is an important source of morbidity. Its type depends largely on the capillary ultra-structures of the histopathologic subtype of underlying brain tumor. The purpose of our study was to differentiate vasogenic edema associated with brain metastases and infiltrative edema related to diffuse gliomas using quantitative 3D T1 rho (T1ρ) imaging.

MATERIALS AND METHODS

Preoperative MR examination including whole brain 3D T1ρ imaging was performed in 23 patients with newly diagnosed brain tumors (9 with metastasis, 8 with lower grade glioma, LGG, 6 with glioblastoma, GBM). Mean T1ρ values were measured in regions of peritumoral non-enhancing T2 signal hyperintensity, excluding both enhancing and necrotic or cystic component, and normal-appearing white matter.

RESULTS

Mean T1ρ values were significantly elevated in the vasogenic edema surrounding intracranial metastases when compared to the infiltrative edema associated with either LGG or GBM (p=0.02 and <0.01, respectively). No significant difference was noted between T1ρ values of infiltrative edema between LGG and GBM (p=0.84 and 0.96, respectively).

CONCLUSION

Our study demonstrates the feasibility and potential diagnostic role of T1ρ in the quantitative differentiation between edema related to intracranial metastases and gliomas and as a potentially complementary tool to standard MR techniques in further characterizing pathophysiology of vasogenic and infiltrative edema.

Ultralow-field and spin-locking relaxation dispersion in postmortem pig brain

PURPOSE

To investigate tissue-specific differences, a quantitative comparison was made between relaxation dispersion in postmortem pig brain measured at ultralow fields (ULF) and spin locking at 7 tesla (T). The goal was to determine whether ULF-MRI has potential advantages for in vivo human brain imaging.

METHODS

Separate specimens of gray matter and white matter were investigated using an ULF-MRI system with superconducting quantum interference device (SQUID) signal detection to measure T1ULF at fields from 58.7 to 235.0 μT and using a commercial MRI scanner to measure T1ρ7T at spin-locking fields from 5.0 to 235.0 μT.

RESULTS

At matched field strengths, T1ρ7T is 50 to 100% longer than T1ULF. Furthermore, dispersion in T1ULF is close to linear between 58.7 and 235 µT, whereas dispersion in T1ρ7T is highly nonlinear over the same range. A subtle elbow in the T1ULF dispersion at approximately 140 µT is tentatively attributed to the local dipolar field of macromolecules. It is suggested that different relaxation mechanisms dominate each method and that ULF-MRI has a fundamentally different sensitivity to the macromolecular structure of neural tissue.

CONCLUSIONS

Ultralow-field MRI may offer distinct, quantitative advantages for human brain imaging, while simultaneously avoiding the severe heating limitation imposed on high-field spin locking. Magn Reson Med 78:2342-2351, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

pH-sensitive MRI demarcates graded tissue acidification during acute stroke - pH specificity enhancement with magnetization transfer and relaxation-normalized amide proton transfer (APT) MRI

pH-sensitive amide proton transfer (APT) MRI provides a surrogate metabolic biomarker that complements the widely-used perfusion and diffusion imaging. However, the endogenous APT MRI is often calculated using the asymmetry analysis (MTRasym), which is susceptible to an inhomogeneous shift due to concomitant semisolid magnetization transfer (MT) and nuclear overhauser (NOE) effects. Although the intact brain tissue has little pH variation, white and gray matter appears distinct in the MTRasym image. Herein we showed that the heterogeneous MTRasym shift not related to pH highly correlates with MT ratio (MTR) and longitudinal relaxation rate (R1w), which can be reasonably corrected using the multiple regression analysis. Because there are relatively small MT and R1w changes during acute stroke, we postulate that magnetization transfer and relaxation-normalized APT (MRAPT) analysis increases MRI specificity to acidosis over the routine MTRasym image, hence facilitates ischemic lesion segmentation. We found significant differences in perfusion, pH and diffusion lesion volumes (P<0.001, ANOVA). Furthermore, MRAPT MRI depicted graded ischemic acidosis, with the most severe acidosis in the diffusion lesion (-1.05±0.29%/s), moderate acidification within the pH/diffusion mismatch (i.e., metabolic penumbra, -0.67±0.27%/s) and little pH change in the perfusion/pH mismatch (i.e., benign oligemia, -0.04±0.14%/s), providing refined stratification of ischemic tissue injury.

Stroke onset time estimation from multispectral quantitative magnetic resonance imaging in a rat model of focal permanent cerebral ischemia

Background

Quantitative T2 relaxation magnetic resonance imaging allows estimation of stroke onset time.

Aims

We aimed to examine the accuracy of quantitative T1 and quantitative T2 relaxation times alone and in combination to provide estimates of stroke onset time in a rat model of permanent focal cerebral ischemia and map the spatial distribution of elevated quantitative T1 and quantitative T2 to assess tissue status.

Methods

Permanent middle cerebral artery occlusion was induced in Wistar rats. Animals were scanned at 9.4T for quantitative T1, quantitative T2, and Trace of Diffusion Tensor (Dav) up to 4 h post-middle cerebral artery occlusion. Time courses of differentials of quantitative T1 and quantitative T2 in ischemic and non-ischemic contralateral brain tissue (ΔT1, ΔT2) and volumes of tissue with elevated T1 and T2 relaxation times ( f1, f2) were determined. TTC staining was used to highlight permanent ischemic damage.

Results

ΔT1, ΔT2, f1, f2, and the volume of tissue with both elevated quantitative T1 and quantitative T2 (VOverlap) increased with time post-middle cerebral artery occlusion allowing stroke onset time to be estimated. VOverlap provided the most accurate estimate with an uncertainty of ±25 min. At all times-points regions with elevated relaxation times were smaller than areas with Dav defined ischemia.

Conclusions

Stroke onset time can be determined by quantitative T1 and quantitative T2 relaxation times and tissue volumes. Combining quantitative T1 and quantitative T2 provides the most accurate estimate and potentially identifies irreversibly damaged brain tissue.

A spatiotemporal theory for MRI T2 relaxation time and apparent diffusion coefficient in the brain during acute ischaemia: Application and validation in a rat acute stroke model

The objective of this study is to present a mathematical model which can describe the spatiotemporal progression of cerebral ischaemia and predict magnetic resonance observables including the apparent diffusion coefficient (ADC) of water and transverse relaxation time T2 This is motivated by the sensitivity of the ADC to the location of cerebral ischaemia and T2 to its time-course, and that it has thus far proven challenging to relate observations of changes in these MR parameters to stroke timing, which is of considerable importance in making treatment choices in clinics. Our mathematical model, called the cytotoxic oedema/dissociation (CED) model, is based on the transit of water from the extra- to the intra-cellular environment (cytotoxic oedema) and concomitant degradation of supramacromolecular and macromolecular structures (such as microtubules and the cytoskeleton). It explains experimental observations of ADC and T2, as well as identifying the rate of spread of effects of ischaemia through a tissue as a dominant system parameter. The model brings the direct extraction of the timing of ischaemic stroke from quantitative MRI closer to reality, as well as providing insight on ischaemia pathology by imaging in general. We anticipate that this may improve patient access to thrombolytic treatment as a future application.

T1ρ relaxation time in brain regions increases with ageing: an experimental MRI observation in rats

OBJECTIVE

T1ρ variation is associated with neurodegenerative diseases. This study aims to observe T1ρ relaxation time changes in rat brains associated with normal ageing in Sprague-Dawley (SD) rats, Wistar Kyoto (WKY) rats and spontaneously hypertension rats (SHRs).

METHODS

18 male SD rats, 11 male WKY rats and 11 male SHRs were used. T1ρ measurement was performed at 3-T MR with a spin-lock frequency of 500 Hz. SD rats were scanned at the ages of 5, 8, 10 and 15 months. SHRs and WKY rats were scanned at the ages of 6, 9 and 12 months.

RESULTS

For SD rats, T1ρ at the thalamus, hippocampus and frontal cortices increased significantly from 5 to 15 months (p < 0.05). For the WKY rats and SHRs, the T1ρ values in the thalamus, hippocampus and frontal cortices also increased significantly from 6 to 12 months (p < 0.05). Furthermore, T1ρ in the thalamus, hippocampus and frontal cortices of SHRs were consistently higher than those of WKY rats at the ages of 6, 9 and 12 months (p < 0.05). The percentage regional T1ρ differences between WKY rats and SHRs did not change during ageing.

CONCLUSION

An increase in T1ρ was associated with age-related changes of the rat brain.

ADVANCES IN KNOWLEDGE

An age-related and hypertension-related T1ρ increase in rat brain regions was observed in the thalamus, hippocampus and frontal cortical regions of the rat brain.

Accelerated whole-brain multi-parameter mapping using blind compressed sensing

PURPOSE

To introduce a blind compressed sensing (BCS) framework to accelerate multi-parameter MR mapping, and demonstrate its feasibility in high-resolution, whole-brain T1ρ and T2 mapping.

METHODS

BCS models the evolution of magnetization at every pixel as a sparse linear combination of bases in a dictionary. Unlike compressed sensing, the dictionary and the sparse coefficients are jointly estimated from undersampled data. Large number of non-orthogonal bases in BCS accounts for more complex signals than low rank representations. The low degree of freedom of BCS, attributed to sparse coefficients, translates to fewer artifacts at high acceleration factors (R).

RESULTS

From 2D retrospective undersampling experiments, the mean square errors in T1ρ and T2 maps were observed to be within 0.1% up to R = 10. BCS was observed to be more robust to patient-specific motion as compared to other compressed sensing schemes and resulted in minimal degradation of parameter maps in the presence of motion. Our results suggested that BCS can provide an acceleration factor of 8 in prospective 3D imaging with reasonable reconstructions.

CONCLUSION

BCS considerably reduces scan time for multiparameter mapping of the whole brain with minimal artifacts, and is more robust to motion-induced signal changes compared to current compressed sensing and principal component analysis-based techniques.

The Deakin/Graeff hypothesis: focus on serotonergic inhibition of panic

The Deakin/Graeff hypothesis proposes that different subpopulations of serotonergic neurons through topographically organized projections to forebrain and brainstem structures modulate the response to acute and chronic stressors, and that dysfunction of these neurons increases vulnerability to affective and anxiety disorders, including panic disorder. We outline evidence supporting the existence of a serotonergic system originally discussed by Deakin/Graeff that is implicated in the inhibition of panic-like behavioral and physiological responses. Evidence supporting this panic inhibition system comes from the following observations: (1) serotonergic neurons located in the 'ventrolateral dorsal raphe nucleus' (DRVL) as well as the ventrolateral periaqueductal gray (VLPAG) inhibit dorsal periaqueductal gray-elicited panic-like responses; (2) chronic, but not acute, antidepressant treatment potentiates serotonin's panicolytic effect; (3) contextual fear activates a central nucleus of the amygdala-DRVL/VLPAG circuit implicated in mediating freezing and inhibiting panic-like escape behaviors; (4) DRVL/VLPAG serotonergic neurons are central chemoreceptors and modulate the behavioral and cardiorespiratory response to panicogenic agents such as sodium lactate and CO2. Implications of the panic inhibition system are discussed.

Multiparametric magnetic resonance imaging of acute experimental brain ischaemia

Ischaemia is a condition in which blood flow either drops to zero or proceeds at severely decreased levels that cannot supply sufficient oxidizable substrates to maintain energy metabolism in vivo. Brain, a highly oxidative organ, is particularly susceptible to ischaemia. Ischaemia leads to loss of consciousness in seconds and, if prolonged, permanent tissue damage is inevitable. Ischaemia primarily results in a collapse of cerebral energy state, followed by a series of subtle changes in anaerobic metabolism, ion and water homeostasis that eventually initiate destructive internal and external processes in brain tissue. (31)P and (1)H NMR spectroscopy were initially used to evaluate anaerobic metabolism in brain. However, since the early 1990s (1)H Magnetic Resonance Imaging (MRI), exploiting the nuclear magnetism of tissue water, has become the key method for assessment of ischaemic brain tissue. This article summarises multi-parametric (1)H MRI work that has exploited diffusion, relaxation and magnetisation transfer as 'contrasts' to image ischaemic brain in preclinical models for the first few hours, with a view to assessing evolution of ischaemia and tissue viability in a non-invasive manner.

A fast multiparameter MRI approach for acute stroke assessment on a 3T clinical scanner: preliminary results in a non-human primate model with transient ischemic occlusion

Many MRI parameters have been explored and demonstrated the capability or potential to evaluate acute stroke injury, providing anatomical, microstructural, functional, or neurochemical information for diagnostic purposes and therapeutic development. However, the application of multiparameter MRI approach is hindered in clinic due to the very limited time window after stroke insult. Parallel imaging technique can accelerate MRI data acquisition dramatically and has been incorporated in modern clinical scanners and increasingly applied for various diagnostic purposes. In the present study, a fast multiparameter MRI approach including structural T1-weighted imaging (T1W), T2-weighted imaging (T2W), diffusion tensor imaging (DTI), T2-mapping, proton magnetic resonance spectroscopy, cerebral blood flow (CBF), and magnetization transfer (MT) imaging, was implemented and optimized for assessing acute stroke injury on a 3T clinical scanner. A macaque model of transient ischemic stroke induced by a minimal interventional approach was utilized for evaluating the multiparameter MRI approach. The preliminary results indicate the surgical procedure successfully induced ischemic occlusion in the cortex and/or subcortex in adult macaque monkeys (n=4). Application of parallel imaging technique substantially reduced the scanning duration of most MRI data acquisitions, allowing for fast and repeated evaluation of acute stroke injury. Hence, the use of the multiparameter MRI approach with up to five quantitative measures can provide significant advantages in preclinical or clinical studies of stroke disease.

PANDA-T1ρ: Integrating principal component analysis and dictionary learning for fast T1ρ mapping

PURPOSE

Long scanning time greatly hinders the widespread application of spin-lattice relaxation in rotating frame (T1ρ) in clinics. In this study, a novel method is proposed to reconstruct the T1ρ-weighted images from undersampled k-space data and hence accelerate the acquisition of T1ρ imaging.

METHODS

The proposed approach (PANDA-T1ρ) combined the benefit of PCA and dictionary learning when reconstructing image from undersampled data. Specifically, the PCA transform was first used to sparsify the image series along the parameter direction and then the sparsified images were reconstructed by means of dictionary learning and finally solved the images. A variation of PANDA-T1ρ was also developed for the heavy noise case. Numerical simulation and in vivo experiments were carried out with the accelerating factor from 2 to 4 to verify the performance of PANDA-T1ρ.

RESULTS

The reconstructed T1ρ maps using the PANDA-T1ρ method were found to be comparable to the reference at all verified acceleration factors. Moreover, the variation exhibited better performance than the original version when the k-space data were contaminated by heavy noise.

CONCLUSION

PANDA-T1ρ can significantly reduce the scanning time of T1ρ by integrating PCA and dictionary learning and provides better parameter estimation than the state-of-art methods for a fixed acceleration factor.

Whole brain mapping of water pools and molecular dynamics with rotating frame MR relaxation using gradient modulated low-power adiabatic pulses

Nuclear magnetic resonance (NMR) relaxation in the rotating frame is sensitive to molecular dynamics on the time scale of water molecules interacting with macromolecules or supramolecular complexes, such as proteins, myelin and cell membranes. Hence, longitudinal (T1ρ) and transverse (T2ρ) relaxation in the rotating frame may have a great potential to probe the macromolecular fraction of tissues. This stimulated a large interest in using this MR contrast to image brain under healthy and disease conditions. However, experimental challenges related to the use of intense radiofrequency irradiation have limited the widespread use of T1ρ and T2ρ imaging. Here, we present methodological development to acquire 3D high-resolution or 2D (multi-)slice selective T1ρ and T2ρ maps of the entire human brain within short acquisition times. These improvements are based on a class of gradient modulated adiabatic pulses that reduce the power deposition, provide slice selection, and mitigate artifacts resulting from inhomogeneities of B1 and B0 magnetic fields. Based on an analytical model of the T1ρ and T2ρ relaxation we compute the maps of macromolecular bound water fraction, correlation and exchange time constants as quantitative biomarkers informative of tissue macromolecular content. Results obtained from simulations, phantoms and five healthy subjects are included.

MRI of the human brain at 130 microtesla

We present in vivo images of the human brain acquired with an ultralow field MRI (ULFMRI) system operating at a magnetic field B0 ~ 130 μT. The system features prepolarization of the proton spins at Bp ~ 80 mT and detection of the NMR signals with a superconducting, second-derivative gradiometer inductively coupled to a superconducting quantum interference device (SQUID). We report measurements of the longitudinal relaxation time T1 of brain tissue, blood, and scalp fat at B0 and Bp, and cerebrospinal fluid at B0. We use these T1 values to construct inversion recovery sequences that we combine with Carr-Purcell-Meiboom-Gill echo trains to obtain images in which one species can be nulled and another species emphasized. In particular, we show an image in which only blood is visible. Such techniques greatly enhance the already high intrinsic T1 contrast obtainable at ULF. We further present 2D images of T1 and the transverse relaxation time T2 of the brain and show that, as expected at ULF, they exhibit similar contrast. Applications of brain ULFMRI include integration with systems for magnetoencephalography. More generally, these techniques may be applicable, for example, to the imaging of tumors without the need for a contrast agent and to modalities recently demonstrated with T1ρ contrast imaging (T1 in the rotating frame) at fields of 1.5 T and above.

Relaxation along a fictitious field (RAFF) and Z-spectroscopy using alternating-phase irradiation (ZAPI) in permanent focal cerebral ischemia in rat

Cerebral ischemia alters the molecular dynamics and content of water in brain tissue, which is reflected in NMR relaxation, diffusion and magnetization transfer (MT) parameters. In this study, the behavior of two new MRI contrasts, Relaxation Along a Fictitious Field (RAFF) and Z-spectroscopy using Alternating-Phase Irradiation (ZAPI), were quantified together with conventional relaxation parameters (T1, T2 and T1ρ) and MT ratios in acute cerebral ischemia in rat. The right middle cerebral artery was permanently occluded and quantitative MRI data was acquired sequentially for the above parameters for up to 6 hours. The following conclusions were drawn: 1) Time-dependent changes in RAFF and T1ρ relaxation are not coupled to those in MT. 2) RAFF relaxation evolves more like transverse, rather than longitudinal relaxation. 3) MT measured with ZAPI is less sensitive to ischemia than conventional MT. 4) ZAPI data suggest alterations in the T2 distribution of macromolecules in acute cerebral ischemia. It was shown that both RAFF and ZAPI provide complementary MRI information from acute ischemic brain tissue. The presented multiparametric MRI data may aid in the assessment of brain tissue status early in ischemic stroke.

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.

Sensitivity and source of amine-proton exchange and amide-proton transfer magnetic resonance imaging in cerebral ischemia

PURPOSE

Amide-proton transfer (APT) and amine-water proton exchange (APEX) MRI can be viable to map pH-decreasing ischemic regions. However, their exact contributions are unclear.

METHODS

We measured APEX- and APT-weighted magnetization transfer ratio asymmetry (denoted as APEXw and APTw), apparent diffusion coefficient, T2 , and T1 images and localized proton spectra in rats with permanent middle cerebral artery occlusion at 9.4 T. Phantoms and theoretical studies were also performed.

RESULTS

Within 1-h postocclusion, APEXw and APTw maps showed hyperintensity (3.1% of M0 ) and hypointensity (-1.8%), respectively, in regions with decreased apparent diffusion coefficient. Ischemia increased lactate and gamma aminobutyric acid concentrations, but decreased glutamate and taurine concentrations. Over time, the APEXw contrast decreased with glutamate, taurine, and creatine, whereas the APTw contrast and lactate level were similar. Phantom and theoretical studies suggest that the source of APEXw signal is mainly from proteins at normal pH, whereas at decreased pH, gamma aminobutyric acid and glutamate contributions increase, inducing the positive APEXw contrast in ischemic regions. The APTw contrast is sensitive to lactate concentration and pH, but contaminated from contributions of the faster APEX processes.

CONCLUSION

Positive APEXw contrast is more sensitive to ischemia than negative APTw contrast. They may provide complementary tissue metabolic information.

Quantitative measurements of relative fluid-attenuated inversion recovery (FLAIR) signal intensities in acute stroke for the prediction of time from symptom onset

In acute stroke magnetic resonance imaging, a 'mismatch' between visibility of an ischemic lesion on diffusion-weighted imaging (DWI) and missing corresponding parenchymal hyperintensities on fluid-attenuated inversion recovery (FLAIR) data sets was shown to identify patients with time from symptom onset ≤4.5 hours with high specificity. However, moderate sensitivity and suboptimal interpreter agreement are limitations of a visual rating of FLAIR lesion visibility. We tested refined image analysis methods in patients included in the previously published PREFLAIR study using refined visual analysis and quantitative measurements of relative FLAIR signal intensity (rSI) from a three-dimensional, segmented stroke lesion volume. A total of 399 patients were included. The rSI of FLAIR lesions showed a moderate correlation with time from symptom onset (r=0.382, P<0.001). A FLAIR rSI threshold of <1.0721 predicted symptom onset ≤4.5 hours with slightly increased specificity (0.85 versus 0.78) but also slightly decreased sensitivity (0.47 versus 0.58) as compared with visual analysis. Refined visual analysis differentiating between 'subtle' and 'obvious' FLAIR hyperintensities and classification and regression tree algorithms combining information from visual and quantitative analysis also did not improve diagnostic accuracy. Our results raise doubts whether the prediction of stroke onset time by visual image judgment can be improved by quantitative rSI measurements.

Quantitative chemical exchange sensitive MRI using irradiation with toggling inversion preparation

Chemical exchange (CE) sensitive MRI contrast acquired with an off-resonance irradiation pulse is affected by other relaxation mechanisms, such as longitudinal and transverse relaxations. In particular, for intermediate CEs, the effect of transverse relaxation often dominates CE contrast. Since water relaxation rates can change significantly in many pathological conditions or during physiological challenge, it is crucial to separate these relaxation effects in order to obtain pure CE contrast. Here we proposed a novel acquisition scheme in which a toggling inversion pulse is applied prior to the off-resonance irradiation. By combined acquisition of irradiation images with and without an inversion pulse at both the labile proton frequency and the reference frequency, longitudinal and transverse relaxation contributions are cancelled, and the quantification of CE parameters, such as the exchange rate and the labile proton concentration, can be simplified. Furthermore, the CE-mediated relaxation rate can be readily determined with a relatively short irradiation pulse and without approaching the steady state, therefore, reducing the limitations on hardware and specific absorption rate requirements. The signal characteristics of the proposed method are evaluated by numerical simulations and phantom experiments.

Detecting activity-evoked pH changes in human brain

Localized pH changes have been suggested to occur in the brain during normal function. However, the existence of such pH changes has also been questioned. Lack of methods for noninvasively measuring pH with high spatial and temporal resolution has limited insight into this issue. Here we report that a magnetic resonance imaging (MRI) strategy, T(1) relaxation in the rotating frame (T(1)ρ), is sufficiently sensitive to detect widespread pH changes in the mouse and human brain evoked by systemically manipulating carbon dioxide or bicarbonate. Moreover, T(1)ρ detected a localized acidosis in the human visual cortex induced by a flashing checkerboard. Lactate measurements and pH-sensitive (31)P spectroscopy at the same site also identified a localized acidosis. Consistent with the established role for pH in blood flow recruitment, T(1)ρ correlated with blood oxygenation level-dependent contrast commonly used in functional MRI. However, T(1)ρ was not directly sensitive to blood oxygen content. These observations indicate that localized pH fluctuations occur in the human brain during normal function. Furthermore, they suggest a unique functional imaging strategy based on pH that is independent of traditional functional MRI contrast mechanisms.

Diffusion-weighted imaging-fluid attenuated inversion recovery mismatch in nocturnal stroke patients with unknown time of onset

BACKGROUND

More than a quarter of patients with ischemic stroke (IS) are excluded from thrombolysis because of an unknown time of symptom onset. Recent evidence suggests that a mismatch between diffusion-weighted imaging (DWI) and fluid attenuated inversion recovery (FLAIR) imaging could be used as a surrogate for the time of stroke onset. We compared used the DWI-FLAIR mismatch and the FLAIR/DWI ratio to estimate the time of onset in a group of patients with nocturnal strokes and unknown time of onset.

METHODS

We used a prospectively collected acute IS patient database with MRI as the initial imaging modality. Nineteen selected nocturnal stroke patients with unknown time of onset were compared with 22 patients who had an MRI scan within 6 hours from stroke onset (control A) and 19 patients who had an MRI scan between 6 and 12 hours (control B). DWI and FLAIR signal was rated as normal or abnormal. FLAIR/DWI ratio was calculated from independent DWI and FLAIR ischemic lesion volumes using semiautomatic software.

RESULTS

The DWI-FLAIR mismatch was different among groups (unknown 43.7%; control A 63.6%; control B 10.5%; Fisher-Freeman-Halton test; P = .001). There were significant differences in FLAIR/DWI ratio among the 3 groups (unknown 0.05 ± 0.12; control A 0.17 ± 0.15; control B 0.04 ± 0.06; Kruskal-Wallis test; P < .0001). Post-hoc pairwise comparisons revealed that FLAIR/DWI ratio from the unknown group was significantly different from the control B group (P = .0045) but not different from the control A group. DWI volumes were not different among the 3 groups.

CONCLUSIONS

A large proportion of patients with nocturnal IS and an unknown time of stroke initiation have a DWI-FLAIR mismatch, suggesting a recent onset of stroke.

DWI-FLAIR mismatch for the identification of patients with acute ischaemic stroke within 4·5 h of symptom onset (PRE-FLAIR): a multicentre observational study

BACKGROUND

Many patients with stroke are precluded from thrombolysis treatment because the time from onset of their symptoms is unknown. We aimed to test whether a mismatch in visibility of an acute ischaemic lesion between diffusion-weighted MRI (DWI) and fluid-attenuated inversion recovery (FLAIR) MRI (DWI-FLAIR mismatch) can be used to detect patients within the recommended time window for thrombolysis.

METHODS

In this multicentre observational study, we analysed clinical and MRI data from patients presenting between Jan 1, 2001, and May 31, 2009, with acute stroke for whom DWI and FLAIR were done within 12 h of observed symptom onset. Two neurologists masked to clinical data judged the visibility of acute ischaemic lesions on DWI and FLAIR imaging, and DWI-FLAIR mismatch was diagnosed by consensus. We calculated predictive values of DWI-FLAIR mismatch for the identification of patients with symptom onset within 4·5 h and within 6 h and did multivariate regression analysis to identify potential confounding covariates. This study is registered with ClinicalTrials.gov, number NCT01021319.

FINDINGS

The final analysis included 543 patients. Mean age was 66·0 years (95% CI 64·7-67·3) and median National Institutes of Health Stroke Scale score was 8 (IQR 4-15). Acute ischaemic lesions were identified on DWI in 516 patients (95%) and on FLAIR in 271 patients (50%). Interobserver agreement for acute ischaemic lesion visibility on FLAIR imaging was moderate (κ=0·569, 95% CI 0·504-0·634). DWI-FLAIR mismatch identified patients within 4·5 h of symptom onset with 62% (95% CI 57-67) sensitivity, 78% (72-84) specificity, 83% (79-88) positive predictive value, and 54% (48-60) negative predictive value. Multivariate regression analysis identified a longer time to MRI (p<0·0001), a lower age (p=0·0009), and a larger DWI lesion volume (p=0·0226) as independent predictors of lesion visibility on FLAIR imaging.

INTERPRETATION

Patients with an acute ischaemic lesion detected with DWI but not with FLAIR imaging are likely to be within a time window for which thrombolysis is safe and effective. These findings lend support to the use of DWI-FLAIR mismatch for selection of patients in a future randomised trial of thrombolysis in patients with unknown time of symptom onset.

FUNDING

Else Kröner-Fresenius-Stiftung, National Institutes of Health.