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Purpose: Non‐invasive measurement of cerebral venous oxygenation (Yv) is of critical importance in brain diseases. The present work proposed a fast method to quantify regional Yv map for both large and small veins. Methods: A new sequence was developed, referred to as TRU‐VERA (T2 relaxation under velocity encoding and rapid acquisition, which isolates blood spins from static tissue with velocity‐encoding preparation, modulates the T2 weighting of venous signal with T2‐preparation and utilizes a bSSFP readout to achieve fast acquisition with high resolution. The sequence was first optimized to achieve best sensitivity for both large and small veins, and then validated with TRUST (T2 relaxation under spin tagging), TRUPC (T2 relaxation under phase contrast), and accelerated TRUPC MRI. Regional difference of Yv was evaluated, and test–retest reproducibility was examined. Results: Optimal Venc was determined to be 3 cm/s, while recovery time and balanced SSFP flip angle within reasonable range had minimal effect on SNR efficiency. Venous T2 measured with TRU‐VERA was highly correlated with T2 from TRUST (R2 = 0.90), and a conversion equation was established for further calibration to Yv. TRU‐VERA sequences showed consistent Yv estimation with TRUPC (R2 = 0.64) and accelerated TRUPC (R2 = 0.79). Coefficient of variation was 0.84% for large veins and 2.49% for small veins, suggesting an excellent test–retest reproducibility. Conclusion: The proposed TRU‐VERA sequence is a promising method for vessel‐specific oxygenation assessment. [ABSTRACT FROM AUTHOR]

BACKGROUND: Accurate volumetric segmentation of primary central nervous system lymphoma (PCNSL) is essential for assessing and monitoring the tumor before radiotherapy and the treatment planning. The tedious manual segmentation leads to interindividual and intraindividual differences, while existing automatic segmentation methods cause under-segmentation of PCNSL due to the complex and multifaceted nature of the tumor. OBJECTIVE: To address the challenges of small size, diffused distribution, poor inter-layer continuity on the same axis, and tendency for over-segmentation in brain MRI PCNSL segmentation, we propose an improved attention module based on nnUNet for automated segmentation. METHODS: We collected 114 T1 MRI images of patients in the Huashan Hospital, Shanghai. Then randomly split the total of 114 cases into 5 distinct training and test sets for a 5-fold cross-validation. To efficiently and accurately delineate the PCNSL, we proposed an improved attention module based on nnU-Net with 3D convolutions, batch normalization, and residual attention (res-attention) to learn the tumor region information. Additionally, multi-scale dilated convolution kernels with different dilation rates were integrated to broaden the receptive field. We further used attentional feature fusion with 3D convolutions (AFF3D) to fuse the feature maps generated by multi-scale dilated convolution kernels to reduce under-segmentation. RESULTS: Compared to existing methods, our attention module improves the ability to distinguish diffuse and edge enhanced types of tumors; and the broadened receptive field captures tumor features of various scales and shapes more effectively, achieving a 0.9349 Dice Similarity Coefficient (DSC). CONCLUSIONS: Quantitative results demonstrate the effectiveness of the proposed method in segmenting the PCNSL. To our knowledge, this is the first study to introduce attention modules into deep learning for segmenting PCNSL based on brain magnetic resonance imaging (MRI), promoting the localization of PCNSL before radiotherapy. [ABSTRACT FROM AUTHOR]

Purpose: To develop a model‐based deep neural network for high‐quality image reconstruction of undersampled multi‐coil CEST data. Theory and Methods: Inspired by the variational network (VN), the CEST image reconstruction equation is unrolled into a deep neural network (CEST‐VN) with a k‐space data‐sharing block that takes advantage of the inherent redundancy in adjacent CEST frames and 3D spatial–frequential convolution kernels that exploit correlations in the x‐ω domain. Additionally, a new pipeline based on multiple‐pool Bloch–McConnell simulations is devised to synthesize multi‐coil CEST data from publicly available anatomical MRI data. The proposed network is trained on simulated data with a CEST‐specific loss function that jointly measures the structural and CEST contrast. The performance of CEST‐VN was evaluated on four healthy volunteers and five brain tumor patients using retrospectively or prospectively undersampled data with various acceleration factors, and then compared with other conventional and state‐of‐the‐art reconstruction methods. Results: The proposed CEST‐VN method generated high‐quality CEST source images and amide proton transfer‐weighted maps in healthy and brain tumor subjects, consistently outperforming GRAPPA, blind compressed sensing, and the original VN. With the acceleration factors increasing from 3 to 6, CEST‐VN with the same hyperparameters yielded similar and accurate reconstruction without apparent loss of details or increase of artifacts. The ablation studies confirmed the effectiveness of the CEST‐specific loss function and data‐sharing block used. Conclusions: The proposed CEST‐VN method can offer high‐quality CEST source images and amide proton transfer‐weighted maps from highly undersampled multi‐coil data by integrating the deep learning prior and multi‐coil sensitivity encoding model. [ABSTRACT FROM AUTHOR]

Purpose: Measuring lipid composition provides more information than just total lipid content. Hence, the non‐invasive measurement of unsaturated lipid protons with both high efficiency and precision is of pressing need. This study was to optimize echo time (TE) for the best resolving of J‐difference editing of unsaturated lipid resonances. Methods: The TE dependence of J‐difference‐edited (JDE) MRS was verified in the density‐matrix simulation, soybean oil phantom, in‐vivo experiments of white adipose tissue (WAT), and skeletal muscles using single‐voxel MEGA‐PRESS sequence at 3T. The peak SNRs and Cramér‐Rao lower bounds (CRLBs) acquired at the proposed TE of 45 ms and previously published TE of 70 ms were compared (eight pairs) in WAT, extramyocelluar lipids (EMCLs), and intramyocellular lipids (IMCLs). The lipid composition in skeletal muscles was compared between healthy males (n = 7) and females (n = 7). Results: The optimal TE was suggested as 45 ms. Compared to 70 ms, the mean signal gains at TE of 45 ms were 151% in WAT, 168% in EMCL, 204% in IMCL for allylic resonance, and 52% in EMCL for diallylic resonance. CRLBs were significantly reduced at TE of 45 ms in WAT, EMCL, IMCL for allylic resonance and in EMCL for diallylic resonance. With TE of 45 ms, significant gender differences were found in the lipid composition in EMCL pools, while no difference in IMCL pools. Conclusion: The JDE‐MRS protocol with TE of 45 ms allows improved quantification of unsaturated lipid resonances in vivo and future lipid metabolism investigations. [ABSTRACT FROM AUTHOR]

Background: Water exchange across blood–brain barrier (BBB) (WEXBBB) is an emerging biomarker of BBB dysfunction with potential applications in many brain diseases. Several MRI methods have been proposed to measure WEXBBB, but evidence remains scarce whether different methods can produce comparable WEXBBB. Purpose: To explore whether dynamic contrast‐enhanced (DCE)‐MRI and vascular water exchange imaging (VEXI) could produce comparable WEXBBB in high‐grade glioma (HGG) patients. Study Type: Prospective cross‐sectional. Subjects: 13 HGG patients (58.4 ± 9.4 years, 9 females, 4 WHO III and 9 WHO IV). Field Strength/Sequence: A 3 T, spoiled gradient‐recalled‐echo DCE‐MRI and VEXI containing two pulsed‐gradient spin‐echo blocks separated by a mixing block. Assessments: The enhanced tumor and contralateral normal‐appearing white matter (cNAWM) volume‐of‐interests (VOIs) were drew by two neuroradiologists. And whole‐brain NAWM and normal‐appearing gray matter (NAGM) without tumor‐affected regions were segmented by automated segmentation algorithm in FSL. Statistical Tests: Student's t‐test was used to evaluate parameters difference between cNAWM and tumor, NAGM and NAWM, respectively. The correlation between vascular water efflux rate constant (kbo) from DCE‐MRI and apparent exchange rate across BBB (AXRBBB) from VEXI was evaluated by Pearson correlation. P < 0.05 was considered statistically significant. Results: Compared with cNAWM, both kbo and AXRBBB were significantly reduced in tumor (kbo = 3.50 ± 1.18 sec−1 vs. 1.03 ± 0.75 sec−1; AXRBBB = 3.54 ± 1.11 sec−1 vs. 1.94 ± 1.04 sec−1). Both kbo and AXRBBB showed significantly higher values in NAWM than NAGM (kbo = 3.50 ± 0.59 sec−1 vs. 2.10 ± 0.56 sec−1; AXRBBB = 3.35 ± 0.77 sec−1 vs. 2.07 ± 0.52 sec−1). The VOI‐averaged kbo and AXRBBB were also linearly correlated in tumor, NAWM, and NAGM (r = 0.59). Data Conclusion: DCE‐MRI and VEXI showed comparable and correlated WEXBBB in HGG patients, suggesting that the consistence and reliability of these two MRI methods in measuring WEXBBB. Evidence Level: 2. Technical Efficacy: Stage 1. [ABSTRACT FROM AUTHOR]

Purpose: 3D pulse sequences enable high‐resolution acquisition with a high SNR and ideal slice profiles, which, however, is particularly difficult for diffusion MRI (dMRI) due to the additional phase errors from diffusion encoding. Methods: We proposed a twin navigator‐based 3D diffusion‐weighted gradient spin‐echo (GRASE) sequence to correct the phase errors between shots and between odd and even spin echoes for human whole‐brain acquisition. We then compared the SNR of 3D GRASE and 2D simultaneous multi‐slice EPI within the same acquisition time. We further tested the performance of 2D versus 3D acquisition at equivalent SNR on fiber tracking and microstructural mapping, using the diffusion tensor and high‐order fiber orientation density–based metrics. Results: The proposed twin navigator approach removed multi‐shot phase errors to some extent in the whole brain dMRI, and the 2D navigator performed better than the 1D navigator. Comparisons of SNR between the 2D simultaneous multi‐slice EPI and 3D GRASE sequences demonstrated that the SNR of the GRASE sequence was 1.4–1.5‐fold higher than the EPI sequence at an equivalent scan time. More importantly, we found a significantly higher fiber cross‐section in the cerebrospinal tract, as well as richer subcortical fibers (U‐fibers) using the 3D GRASE sequence compared to 2D EPI. Conclusion: The twin navigator‐based 3D diffusion‐weighted‐GRASE sequence minimized the multishot phase error and effectively improved the SNR for whole‐brain dMRI acquisition. We found differences in fiber tracking and microstructural mapping between 2D and 3D acquisitions, possibly due to the different slice profiles. [ABSTRACT FROM AUTHOR]

Purpose: To propose a novel Numerical fitting method of the Extrapolated semisolid Magnetization transfer Reference (NEMR) signal for quantifying the CEST effect. Theory and Methods: Modified two‐pool Bloch‐McConnell equations were used to numerically fit the magnetization transfer (MT) and direct water saturation (DS) signals at far off‐resonance frequencies, which was subsequently extrapolated into the frequency range of amide proton transfer (APT) and nuclear Overhauser enhancement (NOE) pools. Then the subtraction of the fitted two‐pool z‐spectrum and the experimentally acquired z‐spectrum yielded APT# and NOE# signals mostly free of MT and DS contamination. Several strategies were used to accelerate the NEMR fitting. Furthermore, the proposed NEMR method was compared with the conventional extrapolated semisolid magnetization transfer reference (EMR) and magnetization transfer ratio asymmetry (MTRasym) methods in simulations and stroke patients. Results: The combination of RF downsampling, MT lineshape look‐up table, and conversion of MATLAB code to C code accelerated the NEMR fitting by over 2700‐fold. Monte‐Carlo simulations showed that NEMR had higher accuracy than EMR and eliminated the requirement of the steady‐state condition. In ischemic stroke patients, the NEMR maps at 1 μT removed hypointense artifacts seen on EMR and MTRasym images, and better depicted stroke lesions than EMR. For NEMR, NOE# yielded significantly (p < 0.05) stronger signal contrast between stroke and normal tissues than APT# at 1 μT. Conclusion: The proposed NEMR method is suitable for arbitrary saturation settings and can remove MT and DS contamination from the CEST signal for improved detection of ischemic stroke. [ABSTRACT FROM AUTHOR]

Chemical exchange saturation transfer (CEST) imaging benefits from a longer saturation duration and a higher saturation duty cycle. Dielectric shading effects occur when the radiofrequency (RF) wavelength approaches the object size. Here, we proposed a simultaneous parallel transmission‐based CEST (pTx‐CEST) sequence to prolongate the saturation duration at a 100% duty cycle and improve the RF saturation homogeneity in CEST imaging. The simultaneous pTx‐CEST sequence was implemented by switching the CEST saturation module from the non‐pTx to pTx mode, using the pTx functionality with both transmit channels being driven simultaneously (instead of time‐interleaved). The optimization of amplitude ratio and phase difference settings between RF channels for best B1 homogeneity was performed in phantoms of two different sizes mimicking the human brain and abdomen. The optimal amplitude and phase settings generating the best B1 homogeneity in the phantoms were used in pTx‐CEST scans of the human study. The comparison of the maximum achievable saturation duration between the non‐pTx‐CEST and pTx‐CEST sequences was performed in a protein phantom, healthy volunteers, and a metastatic brain tumor patient. The optimal amplitude ratio and phase difference setting between transmit channels manifested circular and elliptical polarization in the head‐sized and abdomen‐sized phantoms. In the brain, the maximum saturation durations achieved at a 100% duty cycle using the simultaneous pTx‐CEST sequence were prolonged to 2240, 3220, and 4200 ms compared with 980 ms using the non‐pTx‐CEST sequence at repetition times of 3, 4, and 5 s, respectively. The longer saturation duration helped improve the image contrast between the tumor and the normal tissue in the patient. The optimized elliptical polarization mode saturation pulses yielded improved uniformity of CEST signals acquired from the human abdomen. The proposed simultaneous pTx‐CEST sequence enabled essentially arbitrarily long saturation duration at a 100% duty cycle and helped reduce the dielectric shading effects with the optimized RF setting. [ABSTRACT FROM AUTHOR]

Background: Oscillating gradient diffusion MRI (dMRI) enables measurements at a short diffusion‐time (td), but it is challenging for clinical systems. Particularly, the low b‐value and low resolution may give rise to cerebrospinal fluid (CSF) contamination. Purpose: To assess the effect of CSF partial volume on td–dMRI measurements and efficacy of inversion‐recovery (IR) prepared oscillating and pulsed gradient dMRI sequence to improve td–dMRI measurements in the human brain. Study Type: Prospective. Subjects: Ten normal volunteers and six glioma patients. Field Strength/Sequence: A 3 T; three‐dimensional (3D) IR‐prepared oscillating gradient‐prepared gradient spin‐echo (GRASE) and two‐dimensional (2D) IR‐prepared oscillating gradient echo‐planar imaging (EPI) sequences. Assessment: We assessed the td‐dependent patterns of apparent diffusion coefficient (ADC) in several gray and white matter structures, including the hippocampal subfields (head, body, and tail), cortical gray matter, thalamus, and posterior white matter in normal volunteers. Pulsed gradient (0 Hz) and oscillating gradients at frequencies of 20 Hz, 40 Hz, and 60 Hz dMRI were acquired with GRASE and EPI sequences with or without the IR module. We also tested the td‐dependency patterns in glioma patients using the EPI sequence with or without the IR module. Statistical Tests: The differences in ADC across the different tds were compared by one‐way ANOVA followed by post hoc pairwise t‐tests with Bonferroni correction. Results: In the healthy subjects, brain regions that were possibly contaminated by CSF signals, such as the hippocampus (head, body, and tail) and cortical gray matter, td‐dependent ADC changes were only significant with the IR‐prepared 2D and 3D sequences but not with the non‐IR sequences. In brain glioblastomas patients, significantly higher td‐dependence was observed in the tumor region with the IR module than that without IR (slope = 0.0196 μm2/msec2 vs. 0.0034 μm2/msec2). Conclusion: The IR‐prepared sequence effectively suppressed the CSF partial volume effect and significantly improved the td‐dependent measurements in the human brain. Evidence Level: 1 Technical Efficacy: Stage 1 [ABSTRACT FROM AUTHOR]