Citations

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.

OBJECTIVEAneurysm growth is considered predictive of future rupture of intracranial aneurysms. However, how accurately neuroradiologists can reliably detect incremental aneurysm growth using clinical MRI is still unknown. The purpose of this study was to assess the agreement rate of detecting aneurysm enlargement employing generally used MRI modalities.METHODSThree silicone flow phantom models, each with 8 aneurysms of various sizes at different sites, were used in this study. The aneurysm models were identical except for an incremental increase in the sizes of the 8 aneurysms, which ranged from 0.4 mm to 2 mm. The phantoms were imaged on 1.5-T and 3-T MRI units with both time-of-flight (TOF) and contrast-enhanced MR angiography. Three independent expert neuroradiologists measured the aneurysms in a blinded manner using different measurement approaches. The individual and agreement detection rates of aneurysm enlargement among the 3 experts were calculated.RESULTSThe mean detection rate of any increase in any aneurysmal dimension was 95.7%. The detection rates of the 3 observers (observers A, B, and C) were 98.0%, 96.6%, and 92.7%, respectively (p = 0.22). The detection rates of each MRI modality were 91.3% using 1.5-T TOF, 97.2% using 1.5-T with Gd, 95.8% using 3.0-T TOF, and 97.2% using 3.0-T with Gd (p = 0.31). On the other hand, the mean detection rate for aneurysm enlargement was 54.8%. Specifically, the detection rates of observers A, B, and C were 49.0%, 46.1%, and 66.7%, respectively (p = 0.009). As the incremental enlargement value increased, the detection rate for aneurysm enlargement increased. The use of 1.5-T Gd improved the detection rate for small incremental enlargement (e.g., 0.4–1 mm) of the aneurysm (p = 0.04). The location of the aneurysm also affected the detection rate for aneurysm enlargement (p < 0.0001).CONCLUSIONSThe detection rate and interobserver agreement were very high for aneurysm enlargement of 0.4–2 mm. The detection rate for at least 1 increase in any aneurysm dimension did not depend on the choice of MRI modality or measurement protocol. Use of Gd improved the accuracy of measurement. Aneurysm location may influence the accuracy of detecting enlargement.

PurposeDixon‐based fat suppression has recently gained interest for dynamic contrast‐enhanced MRI, but multi‐echo techniques require longer scan times and reduce temporal resolution compared to single‐echo alternatives without fat suppression. The purpose of this work is to demonstrate accelerated single‐echo Dixon imaging with high spatial and temporal resolution.Theory and MethodsReal‐valued water and fat images can be obtained from a single measurement if the shared initial phase and that due to are assumed known a priori. An expression for simultaneous sensitivity encoding (SENSE) unfolding and fat‐water separation is derived for the general undersampling case, and simplified under the special case of uniform Cartesian undersampling. In vivo experiments were performed in extremities and brain with SENSE acceleration factors of up to R = 8.ResultsSingle‐echo Dixon reconstruction of highly undersampled data was successfully demonstrated. Dynamic contrast‐enhanced water and fat images provided high spatial and temporal resolution dynamic images with image update times shorter than previous single‐echo Dixon work.ConclusionTime‐resolved contrast‐enhanced MRI with single‐echo Dixon fat suppression shows high image quality, improved vessel delineation, and reduced sensitivity to motion when compared to time‐subtraction methods.

BackgroundCarotid plaque imaging with MRI is becoming more commonplace, but practical challenges exist in performing plaque imaging with surface coils.PurposeTo compare the diagnostic performance of a carotid plaque MRI protocol using a standard neurovascular coil (Neurovascular Coil Protocol) to a higher‐resolution carotid plaque MRI using carotid surface coils (Surface Coil Protocol) in characterizing carotid plaque.Study TypeProspective study comparing two MR techniques in plaque characterization.PopulationThirty‐eight consecutive carotid artery disease patients.Field Strength/SequencePatients underwent 3T MRI using 1) a Neurovascular Coil Protocol including the following sequences: 3D‐FSE T1 pre/postcontrast and precontrast 3D IR‐FSPGR, and 2) a Surface Coil Protocol using standard multicontrast MRI sequences.AssessmentPlaque characteristics analyzed by two independent neuroradiologists included intraplaque hemorrhage (IPH), lipid‐rich necrotic‐core (LRNC), and thin/ruptured fibrous cap (TRFC).Statistical TestsDiagnostic performance of the Neurovascular Coil Protocol was compared to the Surface Coil Protocol reference standard using receiver‐operating curves.ResultsFor IPH, sensitivity, specificity, and area under the curve (AUC) of the Neurovascular Coil Protocol were 91.1% (95% confidence interval [CI] = 78.8–97.5%), 87.0% (95% CI = 66.4–97.2%), and 0.92, respectively. For LRNC without IPH sensitivity, specificity, and AUC were 73.3% (95% CI = 44.9–92.2%), 85.7% (95% CI = 67.3–96.0%), and 0.84, respectively. For TRFC, sensitivity, specificity, and AUC were 35.3% (95% CI = 14.2–61.7%), 97.6% (95% CI = 87.4–99.9%), and 0.66 respectively. Interobserver agreement for IPH, LRNC, and TRFC using the Neurovascular Coil Protocol were k = 0.87 (95% CI = 0.75–0.99), k = 0.54 (95% CI = 0.29–0.80), and k = 0.41 (95% CI = 0.08–0.74), respectively.Data ConclusionOur Neurovascular Coil Protocol has high sensitivity, specificity, and accuracy in identifying IPH and LRNC but is limited in assessment of TRFC.Level of Evidence: 1Technical Efficacy: Stage 2J. Magn. Reson. Imaging 2018;47:1264–1272.

PurposeTo investigate the feasibility of using artificial neural networks to estimate stiffness from MR elastography (MRE) data.MethodsArtificial neural networks were fit using model‐based training patterns to estimate stiffness from images of displacement using a patch size of ∼1 cm in each dimension. These neural network inversions (NNIs) were then evaluated in a set of simulation experiments designed to investigate the effects of wave interference and noise on NNI accuracy. NNI was also tested in vivo, comparing NNI results against currently used methods.ResultsIn 4 simulation experiments, NNI performed as well or better than direct inversion (DI) for predicting the known stiffness of the data. Summary NNI results were also shown to be significantly correlated with DI results in the liver (R2 = 0.974) and in the brain (R2 = 0.915), and also correlated with established biological effects including fibrosis stage in the liver and age in the brain. Finally, repeatability error was lower in the brain using NNI compared to DI, and voxel‐wise modeling using NNI stiffness maps detected larger effects than using DI maps with similar levels of smoothing.ConclusionArtificial neural networks represent a new approach to inversion of MRE data. Summary results from NNI and DI are highly correlated and both are capable of detecting biologically relevant signals. Preliminary evidence suggests that NNI stiffness estimates may be more resistant to noise than an algebraic DI approach. Taken together, these results merit future investigation into NNIs to improve the estimation of stiffness in small regions. Magn Reson Med 80:351–360, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

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.

Intracranial aneurysms are heterogeneous in histopathology and imaging appearance. The biological behavior of different types of aneurysms is now known to depend on the structure and physiology of the aneurysm wall itself in addition to intraluminal flow and other luminal features. Aneurysm wall structure and imaging markers of physiology such as aneurysm wall enhancement have been assessed in many prior investigations using conventional-resolution MRI. Recently, high-resolution vessel wall imaging (HR-VWI) techniques with MRI have been introduced. Reports of findings on high-resolution imaging have already emerged for many types of aneurysms demonstrating detailed characterization of wall enhancement, thickness, and components, but many questions remain unexplored. This review discusses the key HR-VWI literature to date. Aneurysm wall findings on conventional-resolution MRI are also discussed as these may help one understand the potential utility and findings on HR-VWI for various aneurysm types. The authors have illustrated these points with several examples demonstrating both features already described in the literature and novel cases demonstrating the potential for future clinical and research applications.