Histologic evolution of high-intensity focused ultrasound in rabbit muscle

Rationale and Objectives: The purpose of this study was to examine the histologic evolution over time of rabbit skeletal muscle thermally ablated with high-intensity focused ultrasound. The objectives included determining the extent and focality of damage created by this noninvasive, transcutaneous ablative technology. Methods: Transcutaneous, thermal ablation with an external focused ultrasound transducer was applied to the paraspinous muscles of 19 rabbits. At varying times, up to 100 days after therapy, single sonications were examined histologically. Results: Initially, only subtle staining changes were identified within lesions. In the chronic phase (day 51-100), the muscle was replaced or infiltrated by variable amounts of scar and fat similar to degenerative muscle disorders. Histologic changes were limited to the tissue within the intensity focus of the transducer and were not seen in intervening tissues. Discussion: The current study took a systematic approach to study the long term, in vivo histologic effects of single HIFU lesions in a nonregenerative tissue. This experience in muscle tissue will provide a basis for understanding ultrasound effects for clinical applications such as treatment of uterine fibroids, cardiac tissue, and sarcomas.