Purpose
To 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.
Methods
A 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.
Results
In 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.
Conclusions
The 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.
Purpose
Chemical shift‐encoded MRI (CSE‐MRI) is well‐established to quantify proton density fat fraction (PDFF) as a quantitative biomarker of hepatic steatosis. However, temperature is known to bias ...PDFF estimation in phantom studies. In this study, strategies were developed and evaluated to correct for the effects of temperature on PDFF estimation through simulations, temperature‐controlled experiments, and a multi‐center, multi‐vendor phantom study.
Theory and Methods
A technical solution that assumes and automatically estimates a uniform, global temperature throughout the phantom is proposed. Computer simulations modeled the effect of temperature on PDFF estimation using magnitude‐, complex‐, and hybrid‐based CSE‐MRI methods. Phantom experiments were performed to assess the temperature correction on PDFF estimation at controlled phantom temperatures. To assess the temperature correction method on a larger scale, the proposed method was applied to data acquired as part of a nine‐site multi‐vendor phantom study and compared to temperature‐corrected PDFF estimation using an a priori guess for ambient room temperature.
Results
Simulations and temperature‐controlled experiments show that as temperature deviates further from the assumed temperature, PDFF bias increases. Using the proposed correction method and a reasonable a priori guess for ambient temperature, PDFF bias and variability were reduced using magnitude‐based CSE‐MRI, across MRI systems, field strengths, protocols, and varying phantom temperature. Complex and hybrid methods showed little PDFF bias and variability both before and after correction.
Conclusion
Correction for temperature reduces temperature‐related PDFF bias and variability in phantoms across MRI vendors, sites, field strengths, and protocols for magnitude‐based CSE‐MRI, even without a priori information about the temperature.
Background
A low‐cryogen, compact 3T (C3T) MRI scanner with high‐performance gradients capable of simultaneously achieving 80 mT/m gradient amplitude and 700 T/m/second slew rate has been in use to ...study research patients since March 2016 but has not been implemented in the clinical practice.
Purpose
To compare head MRI examinations obtained with the C3T system and a conventional whole‐body 3T (WB3T) scanner in seven parameters across five commonly used brain imaging sequences.
Study Type
Prospective.
Subjects
Thirty patients with a clinically indicated head MRI.
Sequence
3T; T1 FLAIR, T1 MP‐RAGE, 3D T2 FLAIR, T2 FSE, and DWI.
Assessment
All patients tolerated the scans well. Three board‐certified neuroradiologists scored the comparative quality of C3T and WB3T images in blinded fashion using a five‐point Likert scale in terms of: signal‐to‐noise ratio, lesion conspicuity, motion artifact, gray/white matter contrast, cerebellar folia, susceptibility artifact, and overall quality.
Statistical Test
Left‐sided, right‐sided, and two‐sided Wilcoxon signed rank test; Fisher's method. A P value <0.05 was considered statistically significant.
Results
The C3T system performed better than the WB3T in virtually all comparisons, except for motion artifacts for the T1 FLAIR and T1 MP‐RAGE sequences, where the WB3T system was deemed better. When combining all sequences together, the C3T system outperformed the WB3T system in all image quality parameters evaluated, except for motion artifact (P = 0.13).
Data Conclusion
The C3T scanner provided better overall image quality for all sequences, and performed better in all individual categories, except for motion artifact on the T1 FLAIR and T1 MP‐RAGE.
Level of Evidence
2
Technical Efficacy Stage
1
Background
Distortion‐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.
Purpose
To 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 Type
Prospective feasibility study.
Phantom/Subjects
Diffusion 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/Sequence
3T; 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.
Assessment
The 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 Test
Bland–Altman analysis.
Results
The 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 Conclusion
The proposed high‐resolution diffusion imaging approach is clinically feasible on C3T due to a combination of hardware and sequence improvements.
Level of Evidence
3
Technical Efficacy
Stage 1 J. Magn. Reson. Imaging 2020;51:296–310.
Purpose
To 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.
Methods
To 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.
Results
Blurring 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.
Conclusion
Eddy 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.
Purpose
To 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 Methods
Simulation 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.
Results
As 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.
Conclusion
Our 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.
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