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Background: Quantitative diffusion MRI is a promising technique for evaluating peripheral nerve integrity but low signal-to-noise ratio (SNR) can impede measurement accuracy.Purpose: To evaluate principal component analysis (PCA) and generalized spherical deconvolution (genSD) denoising techniques to improve within-subject reproducibility and peripheral nerve conspicuity.Study Type: Prospective.Subjects: Seven healthy volunteers and three peripheral neuropathy patients.Field Strength/sequence: 3T/multiband single-shot echo planar diffusion sequence using multishell 55-direction scheme.Assessment: Images were processed using four methods: "original" (no denoising), "average" (10 repetitions), "PCA-only," and "PCA + genSD." Tibial and common peroneal nerve segmentations and masks were generated from volunteer diffusion data. Quantitative (SNR and contrast-to-noise ratio [CNR]) values were calculated. Three radiologists qualitatively evaluated nerve conspicuity for each method. The two denoising methods were also performed in three patients with peripheral neuropathies.Statistical Tests: For healthy volunteers, calculations included SNR and CNRFA (computed using FA values). Coefficient of variation (CV%) of CNRFA quantified within-subject reproducibility. Groups were compared with two-sample t-tests (significance P < 0.05; two-tailed, Bonferroni-corrected). Odds ratios (ORs) quantified the relative rates of each of three radiologists confidently identifying a nerve, per slice, for the four methods.Results: "PCA + genSD" yielded the highest SNR (meanoverall = 14.83 ± 1.99) and tibial and common peroneal nerve CNRFA (meantibial = 3.45, meanperoneal = 2.34) compared to "original" (P SNR < 0.001; P CNR = 0.011) and "PCA-only" (P SNR < 0.001, P CNR < 0.001). "PCA + genSD" had higher within-subject reproducibility (low CV%) for tibial (6.04 ± 1.98) and common peroneal nerves (8.27 ± 2.75) compared to "original" and "PCA-only." The mean FA was higher for "original" than "average" (P < 0.001), but did not differ significantly between "average" and "PCA + genSD" (P = 0.14). "PCA + genSD" had higher tibial and common peroneal nerve conspicuity than "PCA-only" (ORtibial = 2.50, P < 0.001; ORperoneal = 1.86, P < 0.001) and "original" (ORtibial = 2.73, P < 0.001; ORperoneal = 2.43, P < 0.001).Data Conclusion: PCA + genSD denoising method improved SNR, CNRFA , and within-subject reproducibility (CV%) without biasing FA and nerve conspicuity. This technique holds promise for facilitating more reliable, unbiased diffusion measurements of peripheral nerves.Level Of Evidence: 2 Technical Efficacy Stage: 1 J. Magn. Reson. Imaging 2020;51:1128-1137. [ABSTRACT FROM AUTHOR]

Purpose: To establish peripheral nerve stimulation (PNS) thresholds for an ultra‐high performance magnetic field gradient subsystem (simultaneous 200‐mT/m gradient amplitude and 500‐T/m/s gradient slew rate; 1 MVA per axis [MAGNUS]) designed for neuroimaging with asymmetric transverse gradients and 42‐cm inner diameter, and to determine PNS threshold dependencies on gender, age, patient positioning within the gradient subsystem, and anatomical landmarks. Methods: The MAGNUS head gradient was installed in a whole‐body 3T scanner with a custom 16‐rung bird‐cage transmit/receive RF coil compatible with phased‐array receiver brain coils. Twenty adult subjects (10 male, mean ± SD age = 40.4 ± 11.1 years) underwent the imaging and PNS study. The tests were repeated by displacing subject positions by 2‐4 cm in the superior–inferior and anterior–posterior directions. Results: The x‐axis (left–right) yielded mostly facial stimulation, with mean ΔGmin = 111 ± 6 mT/m, chronaxie = 766 ± 76 µsec. The z‐axis (superior–inferior) yielded mostly chest/shoulder stimulation (123 ± 7 mT/m, 620 ± 62 µsec). Y‐axis (anterior–posterior) stimulation was negligible. X‐axis and z‐axis thresholds tended to increase with age, and there was negligible dependency with gender. Translation in the inferior and posterior directions tended to increase the x‐axis and z‐axis thresholds, respectively. Electric field simulations showed good agreement with the PNS results. Imaging at MAGNUS gradient performance with increased PNS threshold provided a 35% reduction in noise‐to‐diffusion contrast as compared with whole‐body performance (80 mT/m gradient amplitude, 200 T/m/sec gradient slew rate). Conclusion: The PNS threshold of MAGNUS is significantly higher than that for whole‐body gradients, which allows for diffusion gradients with short rise times (under 1 msec), important for interrogating brain microstructure length scales. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Purpose: To 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. Methods: A‐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. Results: Sinusoidal 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%). Conclusion: The 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 FROM AUTHOR]

Diffusion tensor imaging (DTI) is a noninvasive magnetic resonance imaging (MRI) technique that measures the extent of restricted water diffusion and anisotropy in biological tissue. Although DTI has been widely applied in the brain, more recently researchers have used it to characterize nerve pathology in the setting of entrapment neuropathy, traumatic injury, and tumor. DTI artifacts are exacerbated when imaging off isocenter in the body. Anecdotally, the most significant artifacts in peripheral nerve DTI include magnetic field inhomogeneity, motion, incomplete fat suppression, aliasing, and distortion. High spatial resolution is also required to reliably evaluate smaller peripheral nerves. This article provides an overview of such technical issues, particularly when trying to apply DTI in the clinical setting, and offers potential solutions.Level Of Evidence: 5 Technical Efficacy: Stage 3 J. Magn. Reson. Imaging 2018;47:1171-1189. [ABSTRACT FROM AUTHOR]

Purpose To compare the effectiveness of prospective, retrospective, and combined (prospective + retrospective) EPI distortion correction methods in bilateral breast diffusion-weighted imaging (DWI) scans. Methods Five healthy female subjects underwent an axial bilateral breast DWI exam with and without prospective B0 inhomogeneity correction using slice-by-slice linear shimming. In each case, an additional b=0 DWI scan was performed with the polarity of the phase-encoding gradient reversed, to generate an estimated B0 map; this map or a separately acquired B0 map was used for retrospective correction, either alone or in combination with the prospective correction. The alignment between an undistorted, anatomical reference scan with similar contrast and the corrected b=0 DWI images with different correction schemes was assessed. Results The average cross-correlation coefficient between the DWI images and the anatomical reference scan was increased from 0.82 to 0.92 over the five volunteers when combined prospective and retrospective distortion correction was applied. Furthermore, such correction substantially reduced patient-to-patient variation of the image alignment and the variability of the average apparent diffusion coefficient in normal glandular tissue. Conclusion Combined prospective and retrospective distortion correction can provide an efficient way to reduce susceptibility-induced image distortions and enhance the reliability of breast DWI exams. Magn Reson Med 78:247-253, 2017. © 2016 International Society for Magnetic Resonance in Medicine [ABSTRACT FROM AUTHOR]

This technical report describes use of a novel, conformable receive-only radiofrequency coil for 3T magnetic resonance (MR) neurography in a cohort of patients with occipital neuralgia. Applying a sub-millimeter, isotropic three-dimensional double-echo steady-state sequence, detailed visualization of the occipital nerves and associated pathologies could be achieved.ABBREVIATIONS: ABC＝ definition; XYZ＝ definition. FACE＝ Flexible Array coil for Cervical and Extraspinal; DESS＝ double-echo steady-state; C1, C2, C3＝ First, second, and third cervical vertebrae respectively., (© 2024 by American Journal of Neuroradiology.)

Aims: To report the development of the technique for minimally invasive lumbar decompression using robotic-assisted navigation., Methods: Robotic planning software was used to map out bone removal for a laminar decompression after registration of CT scan images of one cadaveric specimen. A specialized acorn-shaped bone removal robotic drill was used to complete a robotic lumbar laminectomy. Post-procedure advanced imaging was obtained to compare actual bony decompression to the surgical plan. After confirming accuracy of the technique, a minimally invasive robotic-assisted laminectomy was performed on one 72-year-old female patient with lumbar spinal stenosis. Postoperative advanced imaging was obtained to confirm the decompression., Results: A workflow for robotic-assisted lumbar laminectomy was successfully developed in a human cadaveric specimen, as excellent decompression was confirmed by postoperative CT imaging. Subsequently, the workflow was applied clinically in a patient with severe spinal stenosis. Excellent decompression was achieved intraoperatively and preservation of the dorsal midline structures was confirmed on postoperative MRI. The patient experienced improvement in symptoms postoperatively and was discharged within 24 hours., Conclusion: Minimally invasive robotic-assisted lumbar decompression utilizing a specialized robotic bone removal instrument was shown to be accurate and effective both in vitro and in vivo. The robotic bone removal technique has the potential for less invasive removal of laminar bone for spinal decompression, all the while preserving the spinous process and the posterior ligamentous complex. Spinal robotic surgery has previously been limited to the insertion of screws and, more recently, cages; however, recent innovations have expanded robotic capabilities to decompression of neurological structures., Competing Interests: D. R. Lebl discloses being a consultant and on the advisory board for Choice Spine; being a consultant for Depuy Synthes; having an ownership interest in HS2, ISPH, and Vestia Ventures MiRus Investment; being a consultant and having ownership interest in Viseon; being a consultant and having royalties from Stryker; receiving royalties from Nuvasive; and getting research support form Medtronic Sofamor Danek USA, all of which is unrelated to this manuscript. D. B. Sneag declares research support from GE Healthcare, Siemens Healthcare, Medtronic, and AMAG Pharmaceuticals, which are unrelated to this work. J. L. Chazen reports being a consultant for Lexeo Therapeutics, which is also unrelated., (© 2024 Lebl et al.)

Objective. High-resolution MRI of the cervical spine (c-spine) and extraspinal neck region requires close-fitting receiver coils to maximize the signal-to-noise ratio (SNR). Conventional, rigid C-spine receiver coils do not adequately contour to the neck to accommodate varying body shapes, resulting in suboptimal SNR. Recent innovations in flexible surface coil array designs may provide three-dimensional (3D) bendability and conformability to optimize SNR, while improving capabilities for higher acceleration factors. Approach. This work describes the design, implementation, and preliminary in vivo testing of a novel, conformal 23-channel receive-only flexible array for cervical and extraspinal (FACE) MRI at 3-Tesla (T), with use of high-impedance elements to enhance the coil's flexibility. Coil performance was tested by assessing SNR and geometry factors ( g -factors) in a phantom compared to a conventional 21-channel head-neck-unit (HNU). In vivo imaging was performed in healthy human volunteers and patients using high-resolution c-spine and neck MRI protocols at 3T, including MR neurography (MRN). Main results. Mean SNR with the FACE was 141%-161% higher at left, right, and posterior off-isocenter positions and 4% higher at the isocenter of the phantom compared to the HNU. Parallel imaging performance was comparable for an acceleration factor ( R ) = 2 × 2 between the two coils, but improved for R = 3 × 3 with mean g -factors ranging from 1.46-2.15 with the FACE compared to 2.36-3.62 obtained with the HNU. Preliminary human volunteer and patient testing confirmed that equivalent or superior image quality could be obtained for evaluation of osseous and soft tissue structures of the cervical region with the FACE. Significance. A conformal and highly flexible cervical array with high-impedance coil elements can potentially enable higher-resolution imaging for cervical imaging., (© 2023 Institute of Physics and Engineering in Medicine.)

Background The efficacy of ferumoxytol, an ultrasmall superparamagnetic iron oxide particle for three-dimensional (3D) MR neurography, has yet to be evaluated. Purpose To evaluate the effects of low-dose ferumoxytol for vascular suppression and nerve visualization in 3D brachial plexus MR neurography as a pilot study. Materials and Methods Volunteers without anemia were prospectively enrolled in July 2021. Brachial plexus MR neurography was performed 30 minutes following infusion of 25% of the standard (510 mg of iron) therapeutic ferumoxytol dose with use of a 3D short-tau inversion recovery T2-weighted fast spin-echo sequence. The 3D fast spin-echo was acquired with and without the use of additional flow suppression techniques. Two musculoskeletal radiologists qualitatively evaluated examinations for the degree of vascular suppression (0-3, none to complete), nerve visualization (0-2, none to full), and motion artifact (0-4, none to severe). Nerve-to-fat, muscle, or vessel contrast ratios were calculated with use of manually drawn regions of interests. Comparisons of the proportion of scans with adequate image quality (vascular suppression, 3; nerve visualization, 1, 2; motion artifacts, 0, 1) were made with use of the McNemar test. Comparisons of quantitative contrast ratios were performed with use of Wilcoxon signed rank tests. P < .05 was deemed statistically significant. Results There were 12 volunteers (mean age, 25 years ± 3; six women) evaluated. The scans with adequate vascular suppression increased from 0% to 98% with and without ferumoxytol, respectively ( P < .001). All individual nerve assessments of adequate nerve visualization increased from 4%-63% to 36%-100% without and with ferumoxytol, respectively ( P < .001-.010), while motion artifacts were unchanged (from 33% to 52%, P = .212). Quantitatively, nerve-to-vessel contrast ratios increased from 0.6 without to 7.6 with ferumoxytol ( P < .001). The addition of flow suppression did not change nerve-to-vessel contrast ratio quantitatively (from 7.5 to 8.4, P > .99) following ferumoxytol. Conclusion Low-dose ferumoxytol improved vascular suppression and nerve visualization in three-dimensional MR neurography of the brachial plexus compared to imaging without ferumoxytol. © RSNA, 2022.

Multi-atlas based methods have been a trend for robust and automated image segmentation. In general these methods first transfer prior manual segmentations, i.e., label maps, on a set of atlases to a given target image through image registration. These multiple label maps are then fused together to produce segmentations of the target image through voting strategy or statistical fusing, e.g., STAPLE. STAPLE simultaneously estimates the true segmentation and the label map performance level, but has been shown inaccurate for multi-atlas segmentation because it is determined completely on the propagated label maps without considering the target image intensity. We develop a new method, called iSTAPLE, that combines target image intensity into a similar maximum likelihood estimate (MLE) framework as in STAPLE to take advantage of both intensity-based segmentation and statistical label fusion based on atlas consensus and performance level. The MLE framework is then solved using a modified EM algorithm to simultaneously estimate the intensity profiles of structures of interest as well as the true segmentation and atlas performance level. Unlike other methods, iSTAPLE does not require the target image to have same image contrast and intensity range as the atlas images, which greatly extends the use of atlases. Experiments on whole brain segmentation showed that iSTAPLE performed consistently better than STAPLE.