MR-GUIDED FOCUSED ULTRASOUND ABLATION THROUGH THE RIBCAGE

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This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Introduction: In recent years, MR-guided focused ultrasound (FUS) ablation has shown promise as a non-invasive alternative for the treatment of various diseases. For FUS applications in the upper abdomen and chest, a major limitation is the restricted acoustic window for FUS delivery, due to the higher acoustic impedance of thoracic bone and cartilage compared to soft tissues. Here, we investigate if a human size ribcage provides enough acoustic window to ablate tissue in the heart. Methods and Discussion: The results showed that the maximum ablation temperature varied with the amount of rib obstruction. For comparison, no rib obstruction resulted in a temperature of 45¿C. Areas with less than approximately 40% rib obstruction reached temperature rises of more than 30¿C, sufficient to create tissue necrosis in living tissue (final temperature > 55¿C). However, temperature elevations as low as 5¿C were measured in areas with more than 80% beam path obstruction by the ribs. In addition, an offset from the selected sonication location and the location of the maximum temperature rise was found for most sonications (average of 5 mm in the imaging plane). This suggests that the ribs not only attenuate the heating spot but also influence its position and shape. Surface heating of the ribcage was observed, which highlights the need for adaptive mechanisms such as turning off individual transducer elements with beam paths obstructed by ribs.