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

• Published in: NMR in biomedicine 2021-07, Vol.34 (7), p.e4515-n/a
• Main Authors: Fiedler, Thomas M., Orzada, Stephan, Flöser, Martina, Rietsch, Stefan H. G., Quick, Harald H., Ladd, Mark E., Bitz, Andreas K.
• Format: Article
• Language: English
• Subjects: Antenna arrays; Arrays; Biological products; body imaging at UHF MRI; Compression; Computer applications; Computer Simulation; Degrees of freedom; Humans; Imaging; integrated transmit coil arrays; Magnetic Resonance Imaging - instrumentation; Male; Microstrip antennas; Models, Biological; Optimization; Radio Waves; VOP compression; Workflow
• ISSN: 0952-3480; 1099-1492
• DOI: 10.1002/nbm.4515

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 B1+ field was evaluated regarding the degrees of freedom for RF shimming in the abdomen. Worst‐case specific absorption rate (SARwc), 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 B1+ field was evaluated. Results show that integrated multiring arrays are able to generate homogeneous B1+ 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 B1+ 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 SARwc, 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. Integrated microstrip antenna arrays, placed behind the bore liner to allow a high number of elements and a clinical workflow, were evaluated regarding body imaging at 7 T. Results show that configurations with three interleaved rings enable homogeneous B1+ field distributions for large, up to 50 cm, FOVs in the body. Furthermore, results show that matrix compressions (e.g. virtual observation points) can lead to substantial overestimation and thus reduced transmit power.