A method for MRI guidance of intercostal high intensity focused ultrasound ablation in the liver

High Intensity Focused Ultrasound (HIFU) seems promising for the non invasive treatment of liver tumors. However, the presence of ribs in the HIFU beam path remains problematic since they can create adverse effects (skin burns) by absorption and reflection of the incident beam. This paperpresents a method based on Magnetic Resonance (MR) Imaging for the identification of the ribs in the HIFU beam and selection of the transducer elements to deactivate. The ribs were visualized on anatomical images acquired prior to heating and manually segmented. The resulting regions of interest surrounding the ribs were projected on the transducer surface by ray tracing from the focal point to select the transducer elements to deactivate. The method was validated ex vivo and in vivo in pig liver during breathing under multi slice real-time MR thermometry, using the proton resonant frequency shift method. Ex vivo and in vivo temperature data showed that the temperature increase near the ribs was important when HIFU sonications were performed with all the elements active, whereas the temperature was reduced with deactivation of the transducer elements located in front of the ribs. The temperature at the focal point was similar with and without deactivation of the transducer elements, indicative of the absence of heat efficacy loss with the proposed technique. This method is simple, rapid and reliable and allows to perform intercostal HIFU ablation while sparing the ribs