title: 7 Tesla Magnetic Resonance Imaging of the Heart and Prostate using Parallel Transmission Techniques
title: 7 Tesla Magnetresonanztomographie des Herzens und der Prostata unter Verwendung paralleler Sendetechniken
Creators: name: Egger, Nico
Publisher: name: Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU)
Year: 2025

description: Parallel transmission (pTx) is a method that is based on the use of multiple independent transmit channels, allowing to shape the radio frequency excitation in order to counteract complex interference effects, resulting from the reduced wavelength of the transmit field (B1+) at ultra-high (magnetic) field strengths (UHF). Especially for the large acquisition volumes in body imaging, pTx is essential to facilitate acquisitions with suitable clinical quality and to enable the benefits of an increased signal to noise ratio (SNR) at UHF. However, several additional challenges, such as movement from heartbeat and breathing, as well as limitations caused by restrictions due to the specific absorption rate (SAR), complicate the use of pTx methods in the body and can lead to lengthy calibration times. The overall aim of this work was implementing, evaluating and improving pTx methods for enhanced cardiac and prostate imaging at 7 T. As a first step, B1+ mapping sequences were implemented with a motion robust readout and successfully validated against conventional readouts. With the implementation of these sequences, channel-wise 3D B1+ maps were acquired for 35 heart subjects and 23 prostate subjects. Based on these B1+ maps, tailored and universal phase shims were optimized for both organ regions. Compared to a vendor provided phase shim, the universal shim for the heart decreased the median coefficient of variation (CoV) across all subjects from 29.1% to 18.7% and increased the median excitation efficiency from 53.7% to 63.3%. For the prostate the median CoV was decreased from 14.7% to 7.9% and the median efficiency was increased from 38.5% to 90.9%. Further but smaller improvements were achieved with tailored shims. While phase shims were identified as sufficient for the prostate, due to its smaller size, for the heart, further optimizations focused on tailored and universal 4kT-point pulses. The universal pulse reduced the median CoV to 10.6% and the tailored pulses to 5.7%. Accelerating the B1+ mapping process was evaluated subsequently and demonstrated that accelerations with a factor of at least 12 (acquisition time = 30 s) are feasible. Phase shims for the prostate, based on B1+ maps reconstructed with this acceleration, yielded a median CoV of 6.3% and efficiency of 94.2%, with differences to the universal shim being statistically significant. Tailored-4kT pulses for the heart, optimized on B1+ maps acquired in 30 s, either improved the homogeneity (median CoV = 8.4% vs. 10.6%) or reduced mean SAR by almost 60% at equal homogeneity compared to the universal 4kT-point pulse. Therefore, tailored pTx pulses based on the accelerated maps were demonstrated to offer a good trade-off between reduced calibration time compared to non-accelerated approaches and increased homogeneity and efficiency or reduced SAR compared to universal pulses. In vivo validation measurements were performed for the proposed methods and confirmed the reported results. With the efficiency of the universal and tailored phase shims, acquisitions of turbo spin echo images with the desired contrast in the prostate were enabled, but a need for less restrictive SAR supervision methods was also identified as an important future research goal. Homogeneous excitations of the heart were achieved with the 4kTpoint pulses, however with considerably reduced SED for the tailored pulses. Lastly, an alternative, interleaved acquisition scheme was proposed for the reduction of errors of the relative B1+ mapping method. For this purpose, simulations were performed based on ground-truth phantom, heart and prostate data and validated with phantom measurements. Results demonstrated that compared to the conventional, sequential acquisition scheme, the linear flip angle regime was increased for the interleaved acquisition, reducing the likelihood of errors. Alternatively, at the same level of error, the interleaved acquisition scheme was shown to increase SNR compared to the sequential acquisition.