Tan, Ek T.

PurposeWe investigate the importance of high gradient‐amplitude and high slew‐rate on oscillating gradient spin echo (OGSE) diffusion imaging for human brain imaging and evaluate human brain imaging with OGSE on the MAGNUS head‐gradient insert (200 mT/m amplitude and 500 T/m/s slew rate).MethodsSimulations with cosine‐modulated and trapezoidal‐cosine OGSE at various gradient amplitudes and slew rates were performed. Six healthy subjects were imaged with the MAGNUS gradient at 3T with OGSE at frequencies up to 100 Hz and b = 450 s/mm2. Comparisons were made against standard pulsed gradient spin echo (PGSE) diffusion in vivo and in an isotropic diffusion phantom.ResultsSimulations show that to achieve high frequency and b‐value simultaneously for OGSE, high gradient amplitude, high slew rates, and high peripheral nerve stimulation limits are required. A strong linear trend for increased diffusivity (mean: 8–19%, radial: 9–27%, parallel: 8–15%) was observed in normal white matter with OGSE (20 Hz to 100 Hz) as compared to PGSE. Linear fitting to frequency provided excellent correlation, and using a short‐range disorder model provided radial long‐term diffusivities of D∞,MD = 911 ± 72 µm2/s, D∞,PD = 1519 ± 164 µm2/s, and D∞,RD = 640 ± 111 µm2/s and correlation lengths of lc,MD = 0.802 ± 0.156 µm, lc,PD = 0.837 ± 0.172 µm, and lc,RD = 0.780 ± 0.174 µm. Diffusivity changes with OGSE frequency were negligible in the phantom, as expected.ConclusionThe high gradient amplitude, high slew rate, and high peripheral nerve stimulation thresholds of the MAGNUS head‐gradient enables OGSE acquisition for in vivo human brain imaging.

BackgroundDistortion‐free, high‐resolution diffusion imaging using DIADEM (Distortion‐free Imaging: A Double Encoding Method), proposed recently, has great potential for clinical applications. However, it can suffer from prolonged scan times and its reliability for quantitative diffusion imaging has not been evaluated.PurposeTo investigate the clinical feasibility of DIADEM‐based high‐resolution diffusion imaging on a novel compact 3T (C3T) by evaluating the reliability of quantitative diffusion measurements and utilizing both the high‐performance gradients (80 mT/m, 700 T/m/s) and the sequence optimization with the navigator acquisition window reduction and simultaneous multislice (multiband) imaging.Study TypeProspective feasibility study.Phantom/SubjectsDiffusion quality control phantom scans to evaluate the reliability of quantitative diffusion measurements; 36 normal control scans for B0‐field mapping; six healthy and two patient subject scans with a brain tumor for comparisons of diffusion and anatomical imaging.Field Strength/Sequence3T; the standard single‐shot echo‐planar‐imaging (EPI), multishot DIADEM diffusion, and anatomical (2D‐FSE [fast‐spin‐echo], 2D‐FLAIR [fluid‐attenuated‐inversion‐recovery], and 3D‐MPRAGE [magnetization prepared rapid acquisition gradient echo]) imaging.AssessmentThe scan time reduction, the reliability of quantitative diffusion measurements, and the clinical efficacy for high‐resolution diffusion imaging in healthy control and brain tumor volunteers.Statistical TestBland–Altman analysis.ResultsThe scan time for high in‐plane (0.86 mm2) resolution, distortion‐free, and whole brain diffusion imaging were reduced from 10 to 5 minutes with the sequence optimizations. All of the mean apparent diffusion coefficient (ADC) values in phantom were within the 95% confidence interval in the Bland–Altman plot. The proposed acquisition with a total off‐resonance coverage of 597.2 Hz wider than the expected bandwidth of 500 Hz in human brain could yield a distortion‐free image without foldover artifacts. Compared with EPI, therefore, this approach allowed direct image matching with the anatomical images and enabled improved delineation of the tumor boundaries.Data ConclusionThe proposed high‐resolution diffusion imaging approach is clinically feasible on C3T due to a combination of hardware and sequence improvements.Level of Evidence3Technical EfficacyStage 1 J. Magn. Reson. Imaging 2020;51:296–310.

BackgroundGradient nonlinearity (GNL) leads to biased apparent diffusion coefficients (ADCs) in diffusion‐weighted imaging. A gradient nonlinearity correction (GNLC) method has been developed for whole body systems, but is yet to be tested for the new compact 3T (C3T) scanner, which exhibits more complex GNL due to its asymmetrical design.PurposeTo assess the improvement of ADC quantification with GNLC for the C3T scanner.Study TypePhantom measurements and retrospective analysis of patient data.Phantom/SubjectsA diffusion quality control phantom with vials containing 0–30% polyvinylpyrrolidone in water was used. For in vivo data, 12 patient exams were analyzed (median age, 33).Field Strength/SequenceImaging was performed on the C3T and two commercial 3T scanners. A clinical DWI (repetition time [TR] = 10,000 msec, echo time [TE] = minimum, b = 1000 s/mm2) sequence was used for phantom imaging and 10 patient cases and a clinical DTI (TR = 6000–10,000 msec, TE = minimum, b = 1000 s/mm2) sequence was used for two patient cases.AssessmentThe 0% vial was measured along three orthogonal axes, and at two different temperatures. The ADC for each concentration was compared between the C3T and two whole‐body scanners. Cerebrospinal fluid and white matter ADCs were quantified for each patient and compared to values in literature.Statistical TestsPaired t‐test and two‐way analysis of variance (ANOVA).ResultsFor all PVP concentrations, the corrected ADC was within 2.5% of the reference ADC. On average, the ADC of cerebrospinal fluid and white matter post‐GNLC were within 1% and 6%, respectively, of values reported in the literature and were significantly different from the uncorrected data (P < 0.05).Data ConclusionThis study demonstrated that GNL effects were more severe for the C3T due to the asymmetric gradient design, but our implementation of a GNLC compensated for these effects, resulting in ADC values that are in good agreement with values from the literature.Level of Evidence: 4Technical Efficacy: Stage 2J. Magn. Reson. Imaging 2018;48:1498–1507

PurposeTo build and evaluate a small‐footprint, lightweight, high‐performance 3T MRI scanner for advanced brain imaging with image quality that is equal to or better than conventional whole‐body clinical 3T MRI scanners, while achieving substantial reductions in installation costs.MethodsA conduction‐cooled magnet was developed that uses less than 12 liters of liquid helium in a gas‐charged sealed system, and standard NbTi wire, and weighs approximately 2000 kg. A 42‐cm inner‐diameter gradient coil with asymmetric transverse axes was developed to provide patient access for head and extremity exams, while minimizing magnet‐gradient interactions that adversely affect image quality. The gradient coil was designed to achieve simultaneous operation of 80‐mT/m peak gradient amplitude at a slew rate of 700 T/m/s on each gradient axis using readily available 1‐MVA gradient drivers.ResultsIn a comparison of anatomical imaging in 16 patients using T2‐weighted 3D fluid‐attenuated inversion recovery (FLAIR) between the compact 3T and whole‐body 3T, image quality was assessed as equivalent to or better across several metrics. The ability to fully use a high slew rate of 700 T/m/s simultaneously with 80‐mT/m maximum gradient amplitude resulted in improvements in image quality across EPI, DWI, and anatomical imaging of the brain.ConclusionsThe compact 3T MRI system has been in continuous operation at the Mayo Clinic since March 2016. To date, over 200 patient studies have been completed, including 96 comparison studies with a clinical 3T whole‐body MRI. The increased gradient performance has reliably resulted in consistently improved image quality.

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.

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.

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.

PurposeTo characterize peripheral nerve stimulation (PNS) of an asymmetric head‐only gradient coil that is compatible with a commercial high–channel‐count receive‐only array.MethodsTwo prototypes of an asymmetric head‐only gradient coil set with a 42‐cm inner diameter were constructed for brain imaging at 3T with maximum performance specifications of up to 85 mT/m and 708 T/m/s. Tests were performed in 24 volunteers to measure PNS thresholds with the transverse (x = left–right; y = anterior–posterior [A/P]) gradient coils of both prototypes. Fourteen of these 24 volunteers were also tested for the z‐gradient PNS in the second prototype and were scanned with high–slew‐rate echo planar imaging (EPI) immediately after the PNS tests.ResultsFor both prototypes, the y‐gradient PNS threshold was markedly higher than the x‐gradient threshold. The z‐gradient threshold was intermediate between those for the x‐ and y‐coils. Of the 24 volunteers, only two experienced y‐gradient PNS at 80 mT/m and 500 T/m/s. All volunteers underwent the EPI scan without PNS when the readout direction was set to A/P.ConclusionMeasured PNS characteristics of asymmetric head‐only gradient coil prototypes indicate that such coils, especially in the A/P direction, can be used for fast EPI readout in high‐performance neuroimaging scans with substantially reduced PNS concerns compared with conventional whole body gradient coils. Magn Reson Med 76:1939–1950, 2016. © 2015 International Society for Magnetic Resonance in Medicine