Gregory J. Metzger

Medial meniscus posterior root tear (MMPRT) repairs stabilize root attachment and yield improved clinical outcomes, yet many patients develop persistent meniscal extrusion and osteoarthritis. This study evaluates menisci in MMPRT patients using 3D T2* mapping and assesses T2* correlation with medial meniscal extrusion. Patients underwent 7 T knee MRI before root tear repair and again at 6‐month postoperative follow‐up. Age‐ and gender‐matched controls were scanned once. The protocol included clinical turbo‐spin‐echo sequences and a quantitative 3D GRE sequence, used to calculate T2* mapping of the menisci. Pre‐ and postrepair meniscal extrusion was measured on coronal T2‐weighted images by two independent radiologists. Twenty‐five patients (mean age 53 ± 8 years) and nine controls (51 ± 11 years) were included. Repairs were intact on follow‐up MRI in all patients. T2* values were significantly elevated in patients both pre‐ and postrepair compared with controls across medial and lateral meniscal subregions. Medial meniscal extrusion increased from 3.7 ± 1.1 mm at baseline to 4.9 ± 1.5 mm ( p = 0.002), postoperatively. Prerepair meniscal T2* strongly correlated with postrepair extrusion ( ρ = 0.765; p < 0.001). High‐resolution 3D T2* mapping at 7 T revealed persistently elevated T2* values after MMPRT repair, consistent with ongoing collagen fiber disorganization. Prerepair T2* values were strongly correlated with early postrepair meniscal extrusion despite MRI‐intact repairs, indicating that preoperative tissue quality is associated with postoperative outcomes. These findings support quantitative T2* mapping as a prognostic imaging biomarker that complements conventional morphologic MRI in the evaluation of MMPRTs.

Purpose: To generate accurate EM simulation models of RF coil arrays, improve coil safety validation procedures, and improve RF power management without sacrificing patient safety. Methods: A fast, customized RF co‐simulation formulation was developed for pTx RF coils, enabling the use of experimentally measured maps to optimize the values of lumped elements, phase shifters, and attenuators in the EM simulation model, and demonstrate in a phantom. The proposed method was tested on an 8‐channel pTx knee coil at 7T and a 16‐channel pTx head coil at 10.5T. An RF co‐simulation optimization method that relies solely on matching the simulated and experimental ‐matrices was also implemented as a baseline. 50,000 random RF shims were used to evaluate the error for both co‐simulation optimization methods. Results: The proposed ‐driven co‐simulation optimization method generated accurate maps when compared to experimentally measured maps. The proposed method reduced the average NRMSE of the EM modeling against the measurements from 48.0% to 30.8% for the 7T 8‐channel coil, and from 91.1% to 50.9% for the 10.5T 16‐channel coil when compared to conventional methods using 50,000 random RF shims. Conclusion: The developed ‐driven RF co‐simulation optimization method in this work can be used to generate a more accurate EM simulation model of custom‐built arrays and potentially reduce the need for large safety margins, with improved RF power management.

Purpose: To enable sodium SWIFT imaging, which is a zero‐echo time imaging technique, at ultra‐high magnetic fields through the development of custom electronics hardware, and to showcase this capability with in vivo imaging results. Methods: The custom hardware developed consists of a high‐speed optical trigger with 10 ns resolution, an in‐bore PIN diode driver capable of sourcing high current, and RF switches (both an in‐line switchable attenuator and transmit/receive switch) optimized to achieve sub‐microsecond switching speeds while also producing isolation between the transmitter and receiver. Results: Practical switching speeds of 0.6 μs (receive to transmit) and 1.7 μs (transmit to receive) are achieved on the RF switches, limited by transient‐induced spurious emissions from the low noise amplifier. Transmit to receive isolation of 115 dB is achieved over a 1 MHz bandwidth, with 120 dB of isolation at the Larmor frequency. This was crucial to suppress unblanked RFPA noise. SWIFT images of the human wrist with 1.5 mm isotropic resolution were acquired in less than 5 min to demonstrate the utility of sodium SWIFT imaging at ultra‐high field. SWIFT imaging signal‐to‐noise ratio in a reference phantom with heterogeneous sodium concentrations was comparable to ultra‐short echo time imaging, which provided assurance that hardware had achieved the necessary specifications. Conclusions: The feasibility of sodium SWIFT imaging at ultra‐high field was established, and a musculoskeletal imaging application was demonstrated. The success of this work enables further development of SWIFT at high and ultra‐high fields for imaging of sodium, proton, and other nuclei.

Purpose: To address the limitations of single-distance, 1D performance metrics in RF coil design. This work introduces a multi-objective, volume-of-interest (VOI) based analysis to systematically characterize the trade-offs between power efficiency, pSAR efficiency, and homogeneity as a function of dipole length ( l ) and distance-to-load ( d ) for multiple dipole geometries and target anatomies. Methods: Electromagnetic simulations of straight and end-meandered dipole antennas were performed with varying lengths (100-500 mm) and distance-to-load (1-81 mm) over three anatomical targets (prostate, kidney, heart). Homogeneity, power efficiency, pSAR efficiency, and load sensitivity performance metrics were calculated within each anatomical VOI. Inter-element coupling at variable d was assessed in a 3-element array, and a subset of single-element simulations was experimentally validated using B ₁ ⁺ mapping. Results: A fundamental trade-off was found between power efficiency and pSAR efficiency. Optimal power efficiency was achieved with shorter dipoles (150 mm < l < 300 mm) closer to the sample ( d < 30 mm), while optimal pSAR efficiency and homogeneity were achieved with longer dipoles at further from the sample ( d > 60 mm). Inter-element coupling increased with distance-to-load but could be managed by increasing element spacing. Experimental measurements were in good agreement with simulation trends. Conclusion: Increasing distance-to-load to 40-60 mm, compared with commonly used distances of 20-30 mm, offers a practical strategy for improving pSAR efficiency and homogeneity with a minimal decrease in power efficiency. This work provides a quantitative analysis that enables RF coil designers to make informed, data-driven decisions when developing next-generation body arrays and suggests that unshielded end-meandered dipoles could be an optimal transmit element geometry.

Convolutional neural networks (CNNs) can rapidly predict channel‐wise maps from 7T localizer images, reducing acquisition time to seconds. This paper investigates if a CNN trained on one site's data can generalize to predict maps for brain imaging at unseen sites supporting the feasibility of a universal network for subject‐specific ‐ mapping. We evaluated a U‐Net CNN cross‐site generalization by training on datasets from two different 7T sites and testing its performance across three 7T sites (1 additional testing site) to assess robustness, adaptability, and generalization. The study design included both commercially same systems and the identical physical hardware unit transported between two sites, enabling a more insightful attribution of performance differences to either hardware issues or dataset‐specific variations. To assess prediction quality, we examined magnitude/phase images, error maps, correlation plots, Pearson coefficients, and residual spread plots. Quantitative evaluation included RMSE and SSIM scores. Finally, we calculated 4 kT‐points pulses with both on‐site and cross‐site CNNs to evaluate the effectiveness of the obtained maps for parallel‐transmission (pTx). While on‐site transversal magnitude RMSE scores were as low as 3.0% and 3.1% for the two CNNs, their respective transfer yielded 3.6% and 4.1%. The dynamic pTx‐application showed a CV of 6.5% when using maps predicted by a network trained on its own site. The transfer case, using a map predicted from a network trained on a different site, yielded an increased CV of 13.7%. Although cross‐site applications introduced larger deviations, the predicted maps remained qualitatively plausible and enabled practical use cases, such as calculating dynamic‐pTx. These findings support the potential of cross‐site training, suggesting CNNs trained at one site may generalize sufficiently to unseen sites without additional adjustments. This strengthens the feasibility of a transferable training approach where a single network could be deployed across different institutions without extensive retraining.

Purpose: This study proposes a numerical technique to estimate peak local specific absorption rate (SAR) uncertainty of multi‐channel RF coils during the process of safety validation as an alternative to experimental temperature and electric‐field measurements, and demonstrates its use to enable human studies at 10.5 T. Methods: To ensure patient safety, SAR limits established under international guidelines must not be exceeded. Predicting SAR on state‐of‐the‐art parallel transmit systems relies on electromagnetic simulations, which require extensive experimental validation. Despite a well‐established validation workflow, SAR prediction errors are unavoidable and must be quantified as a safety margin. While MRT tests are commonly used for this purpose, their technical challenges necessitate an alternative. The proposed technique propagates the error between experimentally and numerically acquired distributions to the uncertainty in simulated peak local SAR using Monte‐Carlo simulations without the need for MRT. This method was validated using a 16‐channel transceiver 10.5 T torso coil, as well as an 8‐channel 10.5 T head coil. Results: The proposed numerical technique proved more conservative than existing MRT‐based SAR error quantification methods across all tested scenarios. Its application to validate three state‐of‐the‐art head coils (16Tx/32Rx, 16Tx/80Rx, and 16Tx/128Rx) led to regulatory approval for human head imaging and high‐quality diffusion and functional MRI results at 10.5 T. Conclusion: The proposed technique requires only the experimental acquisition of maps for comparison with simulations, enabling the estimation of SAR prediction uncertainty. This technique was applied to three 16‐channel transmit arrays, each used in conjunction with high‐channel‐count receive arrays for in vivo imaging.

Purpose: Large FOV turbo‐spin‐echo (TSE) imaging at ultra‐high field (UHF) remains challenging due to B ₁ ⁺ inhomogeneity and peak specific absorption rate (pSAR) limitations. This work presents a new time‐interleaved acquisition of modes (TIAMO) framework called SCOPE (Signal and Contrast Optimization with Predicted Excitations), which overcomes these challenges by enabling reduced repetition time (TR) without compromising image quality. Methods: TIAMO is implemented to operate at half the TR of standard static‐shim acquisitions, maintaining scan time parity with traditional acquisitions. To mitigate the expected contrast and signal loss from TR reduction, we developed a novel optimization framework that finds spatially exclusive RF modes, ensuring each voxel is predominantly excited by a single mode, thereby restoring the effective TR. To find these modes, the optimization utilizes an extended phase graph (EPG) model formulated to simulate the TSE signal with interleaved RF excitations. This new signal model is incorporated into the SCOPE framework to compute subject‐specific optimal shims. Previous TIAMO methods are reformulated using this new signal model to elucidate performance differences. Results: SCOPE produced more homogeneous images with improved pSAR efficiency by alternating SAR hotspots between modes. In vivo measurements in the prostate and kidneys strongly correlated to signal prediction and demonstrated superior image quality compared to prior TIAMO methods. The new signal model clarified performance tradeoffs between TIAMO strategies. Conclusion: SCOPE enables rapid, contrast‐preserving TSE TIAMO imaging with reduced TR, addressing the longstanding scan‐time penalty of TIAMO. This work establishes a foundation for real‐time, model‐driven pTx optimization in large‐FOV UHF imaging.

Purpose: Brain MR imaging at 10.5 T ultra‐high field offers significant improvements in signal‐to‐noise ratio (SNR), but faces challenges with B ₁ ⁺ inhomogeneity. Parallel‐transmission (pTx) can be used to achieve a more uniform RF field distribution, but necessitates the use of B ₁ ⁺ calibration in the region of interest. This study explores a universal B ₁ ⁺ shim solution on 10.5 T that could eliminate the need for time‐consuming subject‐specific B ₁ ⁺ calibration. Methods: B ₁ ⁺ data from 7 participants (19 sessions) were used to develop the universal B ₁ ⁺ shim, which was then validated against traditional subject‐specific approaches using T ₁ ‐weighted MP2RAGE structural images in 5 participants (6 sessions). Statistical comparisons of tissue and subcortical segmentations were conducted using popular neuroimaging tools SPM and FreeSurfer, respectively. Results: The universal shim rapidly converged with a small training dataset, likely due to consistent positioning and the simplicity of B ₁ ⁺ shimming used for head imaging. Whole‐brain tissue segmentation showed no statistically significant differences between universal and subject‐specific solutions, with only minor variations near the ventricles and inferior brain regions in the detailed subcortical segmentation. The proposed universal B ₁ ⁺ shim reduces examination time by removing the need for separate data acquisition and optimization. Conclusion: These findings suggest that the universal B ₁ ⁺ shim is a viable substitute for subject‐specific approaches, offering a more efficient solution for neuroimaging applications. Additionally, it confirms that 10.5 T MRI can produce reliable structural brain imaging data, paving the way for broader adoption of ultra‐high field MRI in neuroimaging research.

Purpose: To demonstrate the first in vivo human ²³ Na MRI at 10.5 T using a novel dual‐tuned transceiver body array and to evaluate a self‐gating approach for respiratory motion compensation, focusing on renal imaging. Methods: A custom‐built eight‐channel ²³ Na‐loop ¹ H‐dipole transceiver array was designed, constructed, and characterized. Safe operation was ensured through comparison of electromagnetic simulations against B ₁ ⁺ and SAR phantom measurements. A programmable motion phantom mimicked respiratory motion to assess self‐gating for both the ²³ Na and ¹ H acquisitions. Finally, in vivo human abdominal ²³ Na and ¹ H data were acquired in three healthy volunteers under free breathing, applying the self‐gating approach. Results: The array's electromagnetic model showed good agreement with experimental data, with B ₁ ⁺ NRMSE values of 0.16 for ²³ Na, 0.32 for ¹ H, and SAR NRMSE values of 0.35 for ²³ Na and 0.22 for ¹ H. Total power limits of 125 and 42 W were implemented for ²³ Na and ¹ H, respectively. The motion phantom study confirmed that both ²³ Na and ¹ H self‐gating signals accurately tracked the programmed ground truth motion (correlation coefficients: 0.979 for ²³ Na, 0.995 for ¹ H). Motion binning significantly improved image sharpness as quantitatively evaluated in the phantom. In vivo, self‐gating robustly captured respiratory motion and effectively enabled motion compensation, as shown in binned images with enhanced anatomical detail. Conclusions: We successfully performed the first in vivo human ²³ Na MRI at 10.5 T. The developed dual‐tuned array and validated self‐gating technique enable 10.5 T imaging in humans, paving the way for quantitative studies of sodium homeostasis and advancing diagnostic capabilities.

Purpose: Develop a 32‐channel transceiver array for 7 T body imaging that incorporates an RF shield, improves SNR, lowers g‐factors, and is robust to external loading. Methods: The addition of a local RF shield was first investigated for single resonant blocks consisting of either one loop and a dipole (LD) or three loops and a dipole (3LD). A 32‐channel array consisting of eight shielded 3LD blocks (32LD‐SH) was constructed and validated for in‐vivo use. The SNR, parallel imaging, and transmit performance were compared to a previously published 16‐channel LD array (16LD). The effect of top loading was investigated by placing arms on top of the coils and measuring S‐parameter changes. In vivo imaging of multiple anatomies was performed. Results: In single block experiments, the RF shield impacted SNR and performance by 0.5 efficiency and 30% higher central SNR compared to the 16LD array and supported threefold acceleration in the foot–head direction. Arm placement had no effect on the 32LD‐SH array but reduced the 16LD match to 5.4 dB. Conclusion: A 32‐channel transceiver array was developed for 7 T body imaging that is insensitive to top loading and has higher SNR and lower g‐factors compared to an existing 16‐channel transceiver array. Despite lower transmit performance, parallel transmit optimization permitted the 32LD‐SH to achieve flip angles necessary for high‐quality gradient and spin echo acquisitions of target organs in the chest, abdomen, and pelvis.