Performance analysis of integrated RF microstrip transmit antenna arrays with high channel count for body imaging at 7 T

Abstract

The aim of the current study was to investigate the performance of integrated RF transmit arrays with high channel count consisting of meander microstrip antennas for body imaging at 7 T and to optimize the position and number of transmit elements. RF simulations using multiring antenna arrays placed behind the bore liner were performed for realistic exposure conditions for body imaging. Simulations were performed for arrays with as few as eight elements and for arrays with high channel counts of up to 48 elements. The B 1 + field was evaluated regarding the degrees of freedom for RF shimming in the abdomen. Worst-case specific absorption rate (SAR wc ), SAR overestimation in the matrix compression, the number of virtual observation points (VOPs) and SAR efficiency were evaluated. Constrained RF shimming was performed in differently oriented regions of interest in the body, and the deviation from a target B 1 + field was evaluated. Results show that integrated multiring arrays are able to generate homogeneous B 1 + field distributions for large FOVs, especially for coronal/sagittal slices, and thus enable body imaging at 7 T with a clinical workflow; however, a low duty cycle or a high SAR is required to achieve homogeneous B 1 + distributions and to exploit the full potential. In conclusion, integrated arrays allow for high element counts that have high degrees of freedom for the pulse optimization but also produce high SAR wc , which reduces the SAR accuracy in the VOP compression for low-SAR protocols, leading to a potential reduction in array performance. Smaller SAR overestimations can increase SAR accuracy, but lead to a high number of VOPs, which increases the computational cost for VOP evaluation and makes online SAR monitoring or pulse optimization challenging. Arrays with interleaved rings showed the best results in the study.

Performance and safety assessment of an integrated transmit array for body imaging at 7 T under consideration of specific absorption rate, tissue temperature, and thermal dose

“… maps and electric fields in the body were extracted and regridded onto a homogeneous grid for the individual channels, and 10 g‐averaged SAR matrices for each voxel were computed from the simulated electric fields. The SAR matrices were compressed using the VOP algorithm 13 with a maximum overestimation of 5 % 8 …”

“…B 1 shimming, 3,4 spokes 5 ) have been developed to optimize the field distribution and diminish these effects. Commonly available eight‐channel RF transmit systems have limited optimization capability, and more channels can increase RF shim capabilities, which is especially useful for body imaging 6–8 …”

“…This configuration provides superior patient comfort, similar to examinations with local receive arrays at 3 T 11 . The additional distance between the surface of the patient and the RF transmit array allows the implementation of higher channel counts for fixed elements size, which can provide higher degrees of freedom (DOF) for pulse optimizations to reduce inhomogeneities caused by wave effects 8 …”

“…The feasibility of integrated coils for performing 1 H imaging at 7 T 11 and the arrangement of the transmit elements have been evaluated previously, 8,12 showing high imaging performance for interleaved array configurations with multiple rings along the z‐direction. Interleaved configurations provide continuous coverage along the field of view (FOV), enable a high element count, and reduce the total array length in the longitudinal direction.…”

“…Interleaved configurations provide continuous coverage along the field of view (FOV), enable a high element count, and reduce the total array length in the longitudinal direction. High‐count arrays, however, also have drawbacks: they can potentially produce a high local specific absorption rate (SAR) in the body, SAR matrices may have a low compression rate 8 if matrix compression algorithms like virtual observation points (VOPs) are used, 13 and RF coupling between the elements can affect the array performance and may require lossy decoupling networks. High worst‐case local SAR (SAR wc ) can lead to high overestimation of low‐SAR shims in the VOP compression and reduced accuracy of the compressed SAR matrices 8 …”