Optimizing a RF Coil for Prostate Imaging at 7 Tesla MRI
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Abstract
Ultra-high ?eld prostate MR imaging (?7T) becomes increasingly difficult due to insufficient radio frequency ?eld excitation penetration. RF coils used at ultra-high ?elds are adapted from lower ?eld strengths but their performance is suboptimal. These coils are designed as near-?eld antennas; however, better signal penetration can be obtained using far-?eld concept, speci?cally in the case of prostate which is located deep in the body. In this project the optimization of an RF coil for prostate imaging at 7T Magnetic Resonance system is investigated. This RF coil was designed following far-?eld-based concept recently proposed by Raaijmakers et al., who has demonstrated that higher transmit B1+ at depth can be obtained; thus increasing the transmit performance necessary to acquire good images. The RF coil consists of a fractionated dipole antenna mounted on an acrylic substrate, and it is used as part of a surface array. The optimization is evaluated in terms of safety (local SAR), transmit performance (B1+), and coupling measurements. A parameters study was performed to improve the performance of the RF coil. Numerical simulations were used to evaluate new designs obtained from the parameters study. From the simulated results, three designs were constructed for validation purposes. Two different measurements were performed to validate the simulations: B1 measurements in the 7 Tesla MR scanner and coupling measurements with a network analyzer; these measurements were performed on a phantom with tissue electrical properties. Finally, an in vivo measurement using two identical sinusoidal-shaped snake antennas (s20) aligned on the pelvis was performed to image a healthy prostate. Both, simulations and measurements in the snake antennas demonstrated that despite the improvement gained in safety with lower levels of SAR, the radiation ?eld penetration pattern is less homogeneous affecting the imaging process. Therefore, these antennas might show more efficiency imaging regions of interest where safety has to be ensured such as the heart, but for prostate imaging the fractionated dipole is preferable.