Biology subjects

Metzger, G. J.

Publications and source records attributed to Metzger, G. J..

3 recordsLinked to original sources

bioRxiv2026

Distant Dipoles: A Multi-Parameter and Multi-Objective Analysis of RF Coil Performance For 7T Body MRI

PurposeTo 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. MethodsElectromagnetic 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 B1+ mapping. ResultsA 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. ConclusionIncreasing 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.

biophysics↗

bioRxiv2025

A Numerical Alternative to MR Thermometry for Safety Validation of Multi-Channel RF Transmit Coils

PurposeThis study proposes an alternative approach to MR thermometry (MRT) for the safety validation of multi-channel RF transmit coils and demonstrates its use to enable human studies at 10.5T. MethodsTo ensure patient safety, specific absorption rate (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 B+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 coil for imaging the human torso (henceforth referred to as a "body" coil) at 10.5T with two excitation scenarios, as well as an 8-channel 10.5T head coil with four excitation scenarios. ResultsThe 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 functional as well as diffusion MRI results at 10.5T. ConclusionA numerical alternative to MRT requires only the experimental acquisition of B+ maps for comparison with simulations, enabling the quantification of uncertainty in SAR prediction. This technique was applied to three 16-channel transmit arrays, each used in conjunction with high-channel-count receive arrays for in vivo imaging.

neuroscience↗

bioRxiv2024

A 128-channel receive array with enhanced SNR performance for 10.5 tesla brain imaging

PurposeTo develop and characterize the performance of a 128-channel head array for brain imaging at 10.5 tesla and evaluate the potential of brain imaging at this unique, >10 tesla magnetic field. MethodsThe coil is composed of a 16-channel self-decoupled loop transmit/receive array with a 112-loop receive-only (Rx) insert. Interactions between the outer transmitter and the inner 112Rx insert were mitigated using coaxial cable traps placed every 1/16 of a wavelength on each feed cable, locating most preamplifier boards outside the transmitter field and miniaturizing those placed directly on individual coils. ResultsThe 128-channel array described herein achieved 77% of ultimate intrinsic SNR in the center of the brain. Transmit field maps obtained experimentally on a phantom with and without the receive array were similar and matched EM simulations, leading to FDA approval for human imaging. Anatomical and functional data, including with power demanding sequences, were acquired successfully on human volunteers. ConclusionsCounterintuitive to expectations based on magnetic fields [≤]7T, the higher channel counts provided SNR gains centrally, capturing [~]80% uiSNR. Fraction of uiSNR achieved centrally in 64Rx, 80Rx, and 128Rx arrays suggested that a plateau was being reached at 80%. At this plateau, linear to approximately quadratic B0 dependent SNR gains for the periphery and the center, respectively, were observed for 10.5T relative 7T.

neuroscience↗