Shu, Yunhong

PurposeTo investigate the feasibility of substantially reducing acoustic noise while performing diffusion tensor imaging (DTI) on a compact 3T (C3T) MRI scanner equipped with a 42‐cm inner‐diameter asymmetric gradient.MethodsA‐weighted acoustic measurements were made using 10 mT/m‐amplitude sinusoidal waveforms, corresponding to echo‐planar imaging (EPI) echo spacing of 0.25 to 5.0 ms, on a conventional, whole‐body 3T MRI and on the C3T. Acoustic measurements of DTI with trapezoidal EPI waveforms were then made at peak gradient performance on the C3T (80 mT/m amplitude, 700 T/m/s slew rate) and at derated performance (33 mT/m, 10 to 50 T/m/s) for acoustic noise reduction. DTI was acquired in two different phantoms and in seven human subjects, with and without gradient‐derating corresponding to multi‐ and single‐shot acquisitions, respectively.ResultsSinusoidal waveforms on the C3T were quieter by 8.5 to 15.6 A‐weighted decibels (dBA) on average as compared to the whole‐body MRI. The derated multishot DTI acquisition noise level was only 8.7 dBA (at 13 T/m/s slew rate) above ambient, and was quieter than non‐derated, single‐shot DTI by 22.3 dBA; however, the scan time was almost quadrupled. Although derating resulted in negligible diffusivity differences in the phantoms, small biases in diffusivity measurements were observed in human subjects (apparent diffusion coefficient = +9.3 ± 8.8%, fractional anisotropy = +3.2 ± 11.2%, radial diffusivity = +9.4 ± 16.8%, parallel diffusivity = +10.3 ± 8.4%).ConclusionThe feasibility of achieving reduced acoustic noise levels with whole‐brain DTI on the C3T MRI was demonstrated. Magn Reson Med 79:2902–2911, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

PurposeTo assess risks of RF‐heating of a vagus nerve stimulator (VNS) during 1.5 T prostate MRI using body coil transmit and to compare these risks with those associated with MRI head exams using a transmit/receive head coil.MethodsSpatial distributions of radio‐frequency (RF) B1 fields generated by transmit/receive (T/R) body and head coils were empirically assessed along the long axis of a 1.5 T MRI scanner bore. Measurements were obtained along the center axis of the scanner and laterally offset by 15 cm (body coil) and 7 cm (head coil). RF‐field measurements were supplemented with direct measurements of RF‐heating of 15 cm long copper wires affixed to and submerged in the “neck” region of the gelled saline‐filled (sodium chloride and polyacrylic acid) “head‐and‐torso” phantom. Temperature elevations at the lead tips were measured using fiber‐optic thermometers with the phantom positioned at systematically increased distances from the scanner isocenter.ResultsB1 field measurements demonstrated greater than 10 dB reduction in RF power at distances beyond 28 cm and 24 cm from isocenter for body and head coil, respectively. Moreover, RF power from body coil transmit at distances greater than 32 cm from isocenter was found to be lower than from the RF power from head coil transmit measured at locations adjacent to the coil array at its opening. Correspondingly, maximum temperature elevations at the tips of the copper wires decreased with increasing distance from isocenter — from 7.4°C at 0 cm to no appreciable heating at locations beyond 40 cm.ConclusionsFor the particular scanner model evaluated in this study, positioning an implanted VNS farther than 32 cm from isocenter (configuration achievable for prostate exams) can reduce risks of RF‐heating resulting from the body coil transmit to those associated with using a T/R head coil.

PurposeTo develop a fully automated trajectory and gradient waveform design for the non‐Cartesian shells acquisition, and to develop a magnetization‐prepared (MP) shells acquisition to achieve an efficient three‐dimensional acquisition with improved gray‐to‐white brain matter contrast.MethodsAfter reviewing the shells k‐space trajectory, a novel, fully automated trajectory design is developed that allows for gradient waveforms to be automatically generated for specified acquisition parameters. Designs for two types of shells are introduced, including fully sampled and undersampled/accelerated shells. Using those designs, an MP‐Shells acquisition is developed by adjusting the acquisition order of shells interleaves to synchronize the center of k‐space sampling with the peak of desired gray‐to‐white matter contrast. The feasibility of the proposed design and MP‐Shells is demonstrated using simulation, phantom, and volunteer subject experiments, and the performance of MP‐Shells is compared with a clinical Cartesian magnetization‐prepared rapid gradient echo acquisition.ResultsInitial experiments show that MP‐Shells produces excellent image quality with higher data acquisition efficiency and improved gray‐to‐white matter contrast‐to‐noise ratio (by 36%) compared with the conventional Cartesian magnetization‐prepared rapid gradient echo acquisition.ConclusionWe demonstrated the feasibility of a three‐dimensional MP‐Shells acquisition and an automated trajectory design to achieve an efficient acquisition with improved gray‐to‐white matter contrast. Magn Reson Med 79:2024–2035, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

PurposeTo investigate the effect of the asymmetric gradient concomitant fields (CF) with zeroth and first‐order spatial dependence on fast/turbo spin‐echo acquisitions, and to demonstrate the effectiveness of their real‐time compensation.MethodsAfter briefly reviewing the CF produced by asymmetric gradients, the effects of the additional zeroth and first‐order CFs on these systems are investigated using extended‐phase graph simulations. Phantom and in vivo experiments are performed to corroborate the simulation. Experiments are performed before and after the real‐time compensations using frequency tracking and gradient pre‐emphasis to demonstrate their effectiveness in correcting the additional CFs. The interaction between the CFs and prescan‐based correction to compensate for eddy currents is also investigated.ResultsIt is demonstrated that, unlike the second‐order CFs on conventional gradients, the additional zeroth/first‐order CFs on asymmetric gradients cause substantial signal loss and dark banding in fast spin‐echo acquisitions within a typical brain‐scan field of view. They can confound the prescan correction for eddy currents and degrade image quality. Performing real‐time compensation successfully eliminates the artifacts.ConclusionsWe demonstrate that the zeroth/first‐order CFs specific to asymmetric gradients can cause substantial artifacts, including signal loss and dark bands for brain imaging. These effects can be corrected using real‐time compensation. Magn Reson Med 79:1354–1364, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

PurposeImaging gradients result in the generation of concomitant fields, or Maxwell fields, which are of increasing importance at higher gradient amplitudes. These time‐varying fields cause additional phase accumulation, which must be compensated for to avoid image artifacts. In the case of gradient systems employing symmetric design, the concomitant fields are well described with second‐order spatial variation. Gradient systems employing asymmetric design additionally generate concomitant fields with global (zeroth‐order or B0) and linear (first‐order) spatial dependence.MethodsThis work demonstrates a general solution to eliminate the zeroth‐order concomitant field by applying the correct B0 frequency shift in real time to counteract the concomitant fields. Results are demonstrated for phase contrast, spiral, echo‐planar imaging (EPI), and fast spin‐echo imaging.ResultsA global phase offset is reduced in the phase‐contrast exam, and blurring is virtually eliminated in spiral images. The bulk image shift in the phase‐encode direction is compensated for in EPI, whereas signal loss, ghosting, and blurring are corrected in the fast‐spin echo images.ConclusionA user‐transparent method to compensate the zeroth‐order concomitant field term by center frequency shifting is proposed and implemented. This solution allows all the existing pulse sequences—both product and research—to be retained without any modifications. Magn Reson Med 79:1538–1544, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Abstract Due to engineering limitations, the spatial encoding gradient fields in conventional magnetic resonance imaging cannot be perfectly linear and always contain higher-order, nonlinear components. If ignored during image reconstruction, gradient nonlinearity (GNL) manifests as image geometric distortion. Given an estimate of the GNL field, this distortion can be corrected to a degree proportional to the accuracy of the field estimate. The GNL of a gradient system is typically characterized using a spherical harmonic polynomial model with model coefficients obtained from electromagnetic simulation. Conventional whole-body gradient systems are symmetric in design; typically, only odd-order terms up to the 5th-order are required for GNL modeling. Recently, a high-performance, asymmetric gradient system was developed, which exhibits more complex GNL that requires higher-order terms including both odd- and even-orders for accurate modeling. This work characterizes the GNL of this system using an iterative calibration method and a fiducial phantom used in ADNI (Alzheimer’s Disease Neuroimaging Initiative). The phantom was scanned at different locations inside the 26 cm diameter-spherical-volume of this gradient, and the positions of fiducials in the phantom were estimated. An iterative calibration procedure was utilized to identify the model coefficients that minimize the mean-squared-error between the true fiducial positions and the positions estimated from images corrected using these coefficients. To examine the effect of higher-order and even-order terms, this calibration was performed using spherical harmonic polynomial of different orders up to the 10th-order including even- and odd-order terms, or odd-order only. The results showed that the model coefficients of this gradient can be successfully estimated. The residual root-mean-squared-error after correction using up to the 10th-order coefficients was reduced to 0.36 mm, yielding spatial accuracy comparable to conventional whole-body gradients. The even-order terms were necessary for accurate GNL modeling. In addition, the calibrated coefficients improved image geometric accuracy compared with the simulation-based coefficients.

Background: The relationship between cortical lesions (CLs) and white matter lesions (WMLs) in multiple sclerosis (MS) is poorly understood. Pathological studies support a topographical association between CLs and underlying subcortical WMLs and suggest CLs may play a role in both disease initiation and progression. We hypothesized that cortical MS lesions are physically connected to white matter MS lesions via axonal connections. Objective: To assess the presence of CL-WML connectivity utilizing novel magnetic resonance imaging (MRI) methodology. Methods: In all, 28 relapsing-remitting MS patients and 25 controls received 3 T MRI scans, including double inversion recovery (DIR) for CL detection coupled with diffusion tensor imaging (DTI). CL and WML maps were created, and DTI was used to calculate inter-lesional connectivity and volumetric connectivity indices. Results: All patients showed inter-lesional WML connectivity (median 76% of WMLs connected to another WML; interquartile range (IQR), 58%–88%). On average, 52% of detected CLs per patient were connected to at least one WML (IQR, 42%–71%). Volumetric connectivity analysis showed significantly elevated cortical lesion ratios in MS patients (median, 2.3; IQR, 1.6–3.3) compared to null MS and healthy control datasets ( p < 0.001). Conclusion: These findings provide strong evidence of inter-lesional connectivity between CLs and WMLs, supporting our hypothesis of intrinsic CL-WML connectivity.

PurposeTo investigate the effects on echo planar imaging (EPI) distortion of using high gradient slew rates (SR) of up to 700 T/m/s for in vivo human brain imaging, with a dedicated, head‐only gradient coil.Materials and MethodsSimulation studies were first performed to determine the expected echo spacing and distortion reduction in EPI. A head gradient of 42‐cm inner diameter and with asymmetric transverse coils was then installed in a whole‐body, conventional 3T magnetic resonance imaging (MRI) system. Human subject imaging was performed on five subjects to determine the effects of EPI on echo spacing and signal dropout at various gradient slew rates. The feasibility of whole‐brain imaging at 1.5 mm‐isotropic spatial resolution was demonstrated with gradient‐echo and spin‐echo diffusion‐weighted EPI.ResultsAs compared to a whole‐body gradient coil, the EPI echo spacing in the head‐only gradient coil was reduced by 48%. Simulation and in vivo results, respectively, showed up to 25–26% and 19% improvement in signal dropout. Whole‐brain imaging with EPI at 1.5 mm spatial resolution provided good whole‐brain coverage, spatial linearity, and low spatial distortion effects.ConclusionOur results of human brain imaging with EPI using the compact head gradient coil at slew rates higher than in conventional whole‐body MR systems demonstrate substantially improved image distortion, and point to a potential for benefits to non‐EPI pulse sequences. J. Magn. Reson. Imaging 2016;44:653–664.

PurposeRelaxation time constants are useful as markers of tissue properties. Imaging ex vivo tissue is done for research purposes; however, T1 relaxation time constants are altered by tissue fixation in a time‐dependent manner. This study investigates regional changes in T1 relaxation time constants in ex vivo brain tissue over 6 months of fixation.MethodsFive ex vivo human brain hemispheres in 10% formalin were scanned over 6 months. Mean T1 relaxation time constants were measured in regions of interest (ROIs) representing gray matter (GM) and white matter (WM) regions and analyzed as a function of fixation time.ResultsCortical GM ROIs had longer T1 relaxation time constants than WM ROIs; the thalamus had T1 relaxation time constants similar to those of WM ROIs. T1 relaxation time constants showed rapid shortening within the first 6 weeks after fixation followed by a slower rate of decline.ConclusionBoth GM and WM T1 relaxation time constants of fixed brain tissue show rapid decline within the first 6 weeks after autopsy and slow by 6 months. This information is useful for optimizing MR imaging acquisition parameters according to fixation time for ex vivo brain imaging studies. Magn Reson Med 77:774–778, 2017. © 2016 The Authors Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine. This is an open access article under the terms of the Creative Commons Attribution‐NonCommercial‐NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non‐commercial and no modifications or adaptations are made.