Correction of proton resonance frequency shift MR-thermometry errors caused by heat-induced magnetic susceptibility changes during high intensity focused ultrasound ablations in tissues containing fat

Abstract

Purpose: In this study, we aim to demonstrate the sensitivity of proton resonance frequency shift (PRFS)-based thermometry to heat-induced magnetic susceptibility changes and to present and evaluate a model-based correction procedure. Theory and Methods: To demonstrate the expected temperature effect, field disturbances during high intensity focused ultrasound sonications were monitored in breast fat samples with a three-dimensional (3D) gradient echo sequence. To evaluate the correction procedure, the interface of tissuemimicking ethylene glycol gel and fat was sonicated. During sonication, the temperature was monitored with a 2D dual flip angle multi-echo gradient echo sequence, allowing for PRFS-based relative and referenced temperature measurements in the gel and T1-based temperature measurements in fat. The PRFS-based measurement in the gel was corrected by minimizing the discrepancy between the observed 2D temperature profile and the profile predicted by a 3D thermal model. Results: The HIFU sonications of breast fat resulted in a magnetic field disturbance which completely disappeared after cooling. For the correction method, the 5th to 95th percentile interval of the PRFS-thermometry error in the gel decreased from 3.8°C before correction to 2.0-2.3°C after correction. Conclusion: This study has shown the effects of magnetic susceptibility changes induced by heating of breast fatty tissue samples. The resultant errors can be reduced by the use of a model-based correction procedure.