Search

Purpose: Advancing the development of 7 T MRI for spinal cord imaging is crucial for the enhanced diagnosis and monitoring of various neurodegenerative diseases and traumas. However, a significant challenge at this field strength is the transmit field inhomogeneity. Such inhomogeneity is particularly problematic for imaging the small, deep anatomical structures of the cervical spinal cord, as it can cause uneven signal intensity and elevate the local specific absorption ratio, compromising image quality. This multisite study explores several RF shimming techniques in the cervical spinal cord., Methods: Data were collected from 5 participants between two 7 T sites with a custom 8Tx/20Rx parallel transmission coil. We explored two radiofrequency (RF) shimming approaches from an MRI vendor and four from an open-source toolbox, showcasing their ability to enhance transmit field and signal homogeneity along the cervical spinal cord., Results: The circularly polarized (CP), coefficient of variation (CoV), and specific absorption rate (SAR) efficiency shim modes showed the highest B 1 + efficiency, and the vendor-based "patient" and "volume" modes showed the lowest B 1 + efficiency. The coefficient of variation method produced the highest CSF/spinal cord contrast on T 2 *-weighted scans (ratio of 1.27 ± 0.03), and the lowest variation of that contrast along the superior-inferior axis., Conclusion: The study's findings highlight the potential of RF shimming to advance 7 T MRI's clinical utility for central nervous system imaging by enabling more homogenous and efficient spinal cord imaging. Additionally, the research incorporates a reproducible Jupyter Notebook, enhancing the study's transparency and facilitating peer verification., (© 2024 The Author(s). Magnetic Resonance in Medicine published by Wiley Periodicals LLC on behalf of International Society for Magnetic Resonance in Medicine.)

This contribution discusses the utilization of RF power in an MRI system with RF mode shimming which enables the superposition of circularly polarized modes of a transmit RF coil array driven by a Butler matrix. Since the required power for the individual modes can vary widely, mode-shimming can result in a significant underutilization of the total available RF power. A variable power combiner (VPC) is proposed to improve the power utilization: it can be realized as a reconfiguration of the MRI transmit system by the inclusion of one additional matrix network which receives the power from all transmit amplifiers at its input ports and provides any desired (combined) power distribution at its output ports by controlling the phase and amplitude of the amplifiers' input signals. The power distribution at the output ports of the VPC is then fed into the "mode" ports of the coil array Butler matrix in order to superimpose the spatial modes at the highest achievable power utilization. The VPC configuration is compared to the standard configuration of the transmit chain of our MRI system with 8 transmit channels and 16 coils. In realistic scenarios, improved power utilization was achieved from 17% to 60% and from 14% to 55% for an elliptical phantom and a region of interest in the abdomen, respectively, and an increase of the power utilization of 1 dB for a region of interest in the upper leg. In general, it is found that the VPC allows significant improvement in power utilization when the shimming solution demands only a few modes to be energized, while the technique can yield loss in power utilization in cases with many modes required at high power level.

This article presents the design and realization of 8 × 8 and 16 × 16 Butler matrices for 7 T MRI systems. With the focus on low insertion loss and high amplitude/phase accuracy, the microstrip line integration technology (microwave-integrated circuit) was chosen for the realization. Laminate material of high permittivity (ε(r) = 11) and large thickness (h = 3.2 mm) is shown to allow the best trade-off of circuit board size versus insertion loss, saving circuit area by extensive folding of branch-line coupler topology and meandering phase shifter and connecting strip lines and reducing mutual coupling of neighboring strip lines by shield structures between strip lines. With this approach, 8 × 8 Butler matrices were produced in single boards of 310 mm × 530 mm, whereas the 16 × 16 Butler matrices combined two submatrices of 8 × 8 with two smaller boards. Insertion loss was found at 0.73 and 1.1 dB for an 8 × 8 matrix and 16 × 16 matrix, respectively. Measured amplitude and phase errors are shown to represent highly pure mode excitation with unwanted modes suppressed by 40 and 35 dB, respectively. Both types of matrices were implemented with a 7 T MRI system and 8- and 16-element coil arrays for RF mode shimming experiments and operated successfully with 8 kW of RF power., (Copyright © 2011 Wiley-Liss, Inc.)

Phosphorus (31P) magnetic resonance spectroscopic imaging (MRSI) can serve as a critical tool for more direct quantification of brain energy metabolism, tissue pH, and cell membrane turnover. However, the low concentration of 31P metabolites in biological tissue may result in low signal‐to‐noise ratio (SNR) in 31P MRS images. In this work, we present an innovative design and construction of a 31P radiofrequency coil for whole‐brain MRSI at 7 T. Our coil builds on current literature in ultra‐high field 31P coil design and offers complete coverage of the brain, including the cerebellum and brainstem. The coil consists of an actively detunable volume transmit (Tx) resonator and a custom 24‐channel receive (Rx) array. The volume Tx resonator is a 16‐rung high‐pass birdcage coil. The Rx coil consists of a 24‐element phased array composed of catered loop shapes and sizes built onto a custom, close‐fitting, head‐shaped housing. The Rx array was designed to provide complete coverage of the head, while minimizing mutual coupling. The Rx configuration had a mean S11 reflection coefficient better than −20 decibels (dB) when the coil was loaded with a human head. The mean mutual coupling (S21) among Rx elements, when loaded with a human head, was −16 dB. In phantom imaging, the phased array produced a central SNR that was 4.4‐fold higher than the corresponding central SNR when operating the 31P birdcage as a transceiver. The peripheral SNR was 12‐fold higher when applying the optimized phased array. In vivo 3D 31P MRSI experiments produced high‐quality spectra in the cerebrum gray and white matter, as well as in the cerebellum. Characteristic phosphorus metabolites related to adenosine triphosphate metabolism and cell membrane turnover were distinguishable across all brain regions. In summary, our results demonstrate the potential of our novel coil for accurate, whole‐brain 31P metabolite quantification. [ABSTRACT FROM AUTHOR]