Conductivity Effects on RF Surface Coils used for MR-Guided HIFU of the Prostate

In an endorectal high-intensity focused ultrasound (HIFU) treatment system the US transducer is surrounded by water to effectively couple the US energy into the adjacent prostate tissue. To provide temperature feedback during HIFU treatment, a local RF coil for the acquisition of temperature-sensitive MR data is required. This coil is in close proximity to the US transducer and is loaded by the dielectric US coupling medium. To optimize the SNR the embedded coils are often tuned within the magnet bore under tissue loading conditions which is impracticable for clinical applications. The purpose of this study is to determine the effect of the conductivity of the surrounding medium on coil performance to be able to predict the changes of the coil quality factor and the resonance frequency for correction. An RF surface coil in a HIFU system can become tedious to tune in match if the influence of the different loading conditions are not account for. In this work we analyzed the detuning of the coil using a simplified circuit model, which is able to account for the salient features of the coil behavior. This modeling concept allows for simple coil creation even in harsh conditions.

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