Bernstein, Matt A.

PurposeTo demonstrate the feasibility of pseudo‐continuous arterial‐spin–labeled (pCASL) imaging with 3D fast‐spin‐echo stack‐of‐spirals on a compact 3T scanner (C3T), to perform trajectory correction for eddy‐current–induced deviations in the spiral readout of pCASL imaging, and to assess the correction effect on perfusion‐related images with high‐performance gradients (80 mT/m, 700T/m/s) of the C3T.MethodsTo track eddy‐current–induced artifacts with Archimedean spiral readout, the spiral readout in pCASL imaging was performed with 5 different peak gradient slew rate (Smax) values ranging from 70 to 500 T/m/s. The trajectory for each Smax was measured using a dynamic field camera and applied in a density‐compensated gridding image reconstruction in addition to the nominal trajectory. The effect of the trajectory correction was assessed with perfusion‐weighted (ΔM) images and proton‐density–weighted images as well as cerebral blood flow (CBF) maps, obtained from 10 healthy volunteers.ResultsBlurring artifact on ΔM images was mitigated by the trajectory correction. CBF values on the left and right calcarine cortices showed no significant difference after correction. Also, the signal‐to‐noise ratio of ΔM images improved, on average, by 7.6% after correction (P < .001). The greatest improvement of 12.1% on ΔM images was achieved with a spiral readout using Smax of 300~400 T/m/s.ConclusionEddy currents can cause spiral trajectory deviation, which leads to deformation of the CBF map even in cases of low value Smax. The trajectory correction for spiral‐readout–based pCASL produces more reliable results for perfusion imaging. These results suggest that pCASL is feasible on C3T with high‐performance gradients.

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.

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.