Faculty Bibliography

Objective 
Previous MR guided radiation therapy (MRgRT) radiofrequency coil arrays have been limited to one to two rows of coils in the head-foot direction because of the desire to place radiation-opaque coil circuitry outside the window through which the radiation beam travels. However, such layouts limit parallel imaging undersampling in the head-foot direction. We recently demonstrated a three-row array with a remote coil circuit that improved parallel imaging performance, while preserving the signal-to-noise ratio (SNR) and the radiolucent window. Here we evaluate a four-row prototype design to determine if further parallel imaging advantages could be realized.

Approach
We built remote circuits that allowed radio-opaque components to be placed outside the field of view through which the radiation beam is expected to travel. The circuit consisted of a phase shifter to cancel the phase introduced by the radiolucent coaxial link between the circuit and coil, followed by standard components for tuning, matching, detuning, and preamplifier decoupling. Measurements were performed on an abdominal phantom to compare single-channel coils with remote or local circuits, followed by tests on a 16-channel four-row array.

Main results
The four-row array maintained SNR comparable to two-and three-row designs while supporting 3× head-foot acceleration (minimum reciprocal g-factor = 0.74) and 2×3 multi-directional acceleration (minimum reciprocal g-factor = 0.72), capabilities which were not achievable with previous designs.

Significance
These results demonstrate the technical feasibility of four-row designs, which may benefit MRgRT applications that require high SNR and temporal-resolution.

PURPOSE/OBJECTIVE:This study aimed to quantitatively assess relative arterial contributions to the quadriceps tendon (QT) using 7-Tesla quantitative magnetic resonance imaging (7T-qMRI), histology and high-resolution computed tomography (micro-CT) in a fresh-frozen human cadaveric model. METHODS:Six human cadaveric knee pairs were obtained (mean age: 43 years; range: 23-61 years). Pre- and post-contrast 7T-qMRI scans were performed to quantify tendonous vascularity. Subsequent algorithm-based quantitative histologic analysis was performed using hematoxylin and eosin staining, with validation by CD31 immunohistochemistry. Qualitative analysis was performed on two additional knee specimens using 98 μm micro-CT imaging. RESULTS:The distal QT demonstrated higher median arterial contributions versus central and proximal regions (distal, 47.4% [interquartile range: 30.8%-64.1%]; central, 28.6% [20.4%-41.5%]; proximal, 11.6% [8.7%-18.4%]), with significantly greater contributions distally versus proximally (Cohen's d = 1.58; p = 0.021). These findings aligned with the sagittal sub-analysis (deep-proximal 17.2 ± 19.3% vs. deep-distal 43.4 ± 20.3%; Cohen's d = 1.32; p = 0.050). Histologic analysis (interobserver-reliability: r = 0.95) corroborated the MRI results (distal QT, 43.5 ± 7.9%; central, 30.7 ± 6.4%; proximal, 25.8 ± 4.1%), with significant differences between distal and both proximal (Cohen's d = 2.81; p < 0.001) and central (Cohen's d = 1.78; p = 0.012) regions. The deep layer was found to have significantly higher arterial contributions (61.6 ± 14.2%) versus the superficial layer (38.4 ± 14.2%) (Cohen's d = 1.64; p = 0.018). The medial and lateral QT demonstrated lower arterial contributions versus middle QT (Cohen's d = 0.96-1.26; p > 0.050). CONCLUSION/CONCLUSIONS:7T-qMRI and algorithm-based histological analysis of arterial QT contributions revealed significantly greater arterial contributions in the distal compared to the proximal as well as in the deep compared to the superficial region. While the central region demonstrated higher arterial contributions than the medial and lateral aspects, these differences were not statistically significant. Given that the majority of injuries affect the distal and central portions of the QT, these findings reinforce the rationale for direct tendon-to-bone repair; however, clinical studies are necessary to confirm these findings. LEVEL OF EVIDENCE/METHODS:Level V, cadaveric study.

OBJECTIVES/OBJECTIVE:Quantitative proton density fat fraction (PDFF) and R2* estimation at lower field strengths, such as 0.55 T, is challenging due to lower signal-to-noise ratio, reduced fat water chemical shift, and increased T2* relaxation times. In this study, we propose a 3D hybrid technique for abdominal imaging at 0.55 T that enables the simultaneous acquisition of T2-weighted and T1-weighted images and quantification of fat fraction and R2* parameters. MATERIALS AND METHODS/METHODS:Numerical simulations were performed to optimize a prototype radial hybrid turbo spin echo gradient echo (TSE-GRE) acquisition scheme for improved PDFF and R2* estimation accuracy. Phantom imaging experiments with and without motion were performed to evaluate the sensitivity of the estimation to external motion. Eleven volunteers were imaged on a prototype 0.55 T system. Data were acquired using the proposed technique under free-breathing conditions, and motion-compensated reconstruction was performed using the respiratory signal from a pilot-tone device. Image contrast and estimation performance were compared with conventional acquisition schemes in vitro and in vivo. RESULTS:Numerical simulations indicated R2* estimation accuracy was more sensitive to the choice of echo time compared with PDFF. Performing motion compensation reduced the mean error in R2* from 24 to 5 s-1 while the mean error in PDFF only reduced from 2.7% to 1.6%. The proposed technique generated T2-weighted images with comparable relative liver-spleen contrast as conventional imaging and there were no significant differences (P>0.05) in the PDFF and R2* values estimated from the hybrid technique compared with conventional multi-echo GRE. Further, the free-breathing acquisition allowed improved slice coverage while overcoming breath-hold limitations of conventional acquisition schemes. CONCLUSIONS:The use of a hybrid TSE-GRE acquisition technique can allow simultaneous morphological and quantitative PDFF and R2* estimation at 0.55 T under free-breathing conditions.

PURPOSE/OBJECTIVE:Image quality and resolution in MRI are fundamentally constrained by the performance of radiofrequency (RF) coils used to excite the spins and receive the signal. Electromagnetic (EM) simulations are essential for optimizing coil performance. Existing tools are often slow, memory-intensive, and require expensive licenses. To address these limitations, we introduce a novel, comprehensive, open-source EM simulation tool for RF coil design in MRI. THEORY AND METHODS/METHODS:Our toolbox consists of four complementary software components: (1) a full-wave 3D EM solver based on the wire-surface-volume integral equation (WSVIE), where tensor decompositions reduce memory usage and accelerate simulations for fine body model resolutions; (2) a reduced-order model technique enabling patient-specific coil simulations in minutes; (3) a fully automatic circuit co-simulator for coil tuning, matching, decoupling, preamplifier decoupling, and detuning; and (4) a numerical EM basis generator that can be used to compute ultimate performance metrics. We demonstrated the toolbox with a series of simulations for different applications at 7 T MRI. RESULTS:Our proposed reduced-order model WSVIE solver was 70 times faster compared to a commercial solver for the simulation of a 31-channel 7 T head coil. Our co-simulator tuned, matched, and decoupled the array in 30 min, compared to a few days time of manual interactions needed with the commercial package. The average difference between the signal-to-noise ratio maps was less than 15%. CONCLUSION/CONCLUSIONS:The proposed open-source simulation framework enables fast, memory-friendly, accurate, and anatomy-specific RF coil array design, making it a powerful tool for RF coil engineering.

PURPOSE/UNASSIGNED:To quantitatively assess relative arterial contributions to the patellar tendon (PT) across predefined anatomic regions with 7-Tesla quantitative magnetic resonance imaging (7T-qMRI), algorithm-based histological analysis and high-resolution computed tomography (micro-CT) in a cadaveric model. METHODS/UNASSIGNED:Seven fresh-frozen human cadaveric knee pairs (mean age 41.9 ± 15.5 years) underwent limited vascular dissection and arterial cannulation. Pre- and post-contrast 7T-qMRI, with a volumetric interpolated breath-hold examination (VIBE) three-dimensional T1-weighted gradient echo pulse sequence, quantified tendonous vascularity by measuring contrast enhancement. Subsequent quantitative algorithm-based histologic analysis with hematoxylin and eosin (H&E) staining followed, and two additional specimens underwent high-resolution (98 μm) micro-CT for qualitative vascular assessment. RESULTS/UNASSIGNED: = 0.018). Micro-CT revealed a robust vascular network along the medial PT with smaller branches laterally. Distal to the inferior patellar pole, a peripatellar circular network, extending medially into the posterior PT layers, was qualitatively identified. CONCLUSION/UNASSIGNED:7T-qMRI and histological analyses demonstrated significantly greater arterial supply along the medial border of the PT, while micro-CT revealed a medial and peripatellar circular vascular network extending from the medial margin and the inferior patellar pole into the posterior tendon layers. These findings identify the medial margin as the main vascular source for the PT, with implications for surgical preservation and reducing PT devascularization risk. LEVEL OF EVIDENCE/UNASSIGNED:N/A.

OBJECTIVES/OBJECTIVE:In mild traumatic brain injury, imaging biomarkers are needed to support clinical management. In four antecedent publications, we used two new (sodium and fingerprinting) and two established (spectroscopy and diffusion) MR techniques in a longitudinally followed patient cohort. Here we report final results and combine all data to determine which marker(s) from the four modalities offer the greatest utility for detecting injury and predicting outcomes. We also leverage the independent specificities offered by each modality to explore injury mechanisms. MATERIALS AND METHODS/METHODS:The longitudinal spectroscopy data were analysed to complete a full data set of proton (spectroscopy, fingerprinting, diffusion) and sodium MRI, acquired alongside symptomatic, cognitive, and functional assessments in 27 patients at 1, 3, and 12 months following injury. Twenty-three matched controls were scanned once. Testing for associations between nine MR markers and three outcome measures was standardized across the entire data set, and performed using Spearman correlations and logistic regression. RESULTS:from fingerprinting (marker of the cellular microenvironment). CONCLUSIONS:We identified independent, dynamic, metabolic and ionic changes, with choline and creatine from spectroscopy fulfilling the most criteria for a clinical biomarker.

OBJECTIVE:Real-time imaging is useful for the evaluation of wrist instability. However, currently available real-time magnetic resonance imaging (MRI) methods are limited due to their 2D nature or provide insufficient temporal resolution and image quality for quantitative kinematic analysis. This work introduces a novel approach for volumetric dynamic MRI of the wrist joint during active motion and demonstrates the feasibility of tracking carpal bone motion. MATERIALS AND METHODS/METHODS:A flexible 8-element 3 T wrist receive coil and 3D-printed support platform for guiding motion were designed for dynamic wrist imaging. 2D real-time images were acquired using a fat-saturated FLASH sequence with radial sampling and reconstructed with the GRASP algorithm. Corresponding volumetric dynamic wrist images were obtained by assembling 2D real-time images into 3D snapshots using autodetected MRI-visible markers for slice alignment. The proposed method was demonstrated for radial-ulnar deviation on five healthy volunteers. RESULTS:The flexible wrist coil provided high SNR while allowing a wide range of wrist movements. 2D real-time wrist images were acquired with a temporal resolution of 48 ms/frame with negligible streaking artifacts. Carpal bones and metacarpal bones were properly aligned in the assembled dynamic volumes for all five subjects. The excellent bone-to-tissue contrast enabled accurate segmentation of the individual carpal bones on the assembled dynamic volumes. CONCLUSION/CONCLUSIONS:This work introduces a novel wrist coil and demonstrates that real-time volumetric dynamic examination of the moving wrist is feasible. The achieved image quality and high temporal resolution could enable automatic segmentation of carpal bones and quantitative kinematic assessment for evaluating wrist instability.

Due to limitations in current motion tracking technologies and increasing interest in alternative sensors for motion tracking both inside and outside the MRI system, in this study we share our preliminary experience with three alternative sensors utilizing diverse technologies and interactions with tissue to monitor motion of the body surface, respiratory-related motion of major organs, and non-respiratory motion of deep-seated organs. These consist of (1) a Pilot-Tone RF transmitter combined with deep learning algorithms for tracking liver motion, (2) a single-channel ultrasound transducer with deep learning for monitoring bladder motion, and (3) a 3D Time-of-Flight camera for observing the motion of the anterior torso surface. Additionally, we demonstrate the capability of these sensors to simultaneously capture motion data outside the MRI environment, which is particularly relevant for procedures like radiation therapy, where motion status could be related to previously characterized cyclical anatomical data. Our findings indicate that the ultrasound sensor can track motion in deep-seated organs (bladder) as well as respiratory-related motion. The Time-of-Flight camera offers ease of interpretation and performs well in detecting surface motion (respiration). The Pilot-Tone demonstrates efficacy in tracking bulk respiratory motion and motion of major organs (liver). Simultaneous use of all three sensors could provide complementary motion information outside the MRI bore, providing potential value for motion tracking during position-sensitive treatments such as radiation therapy.

PURPOSE/OBJECTIVE:The purpose of this work was to design and build a coil for quadri-nuclear MRI of the human brain at 7 T. METHODS: RESULTS: CONCLUSION/CONCLUSIONS:While the SNR of the quadruple tuned coil was significantly lower than dual- and single-tuned reference coils, it represents a step toward truly simultaneous quadri-nuclear measurements.