Thermal effects of focused ultrasound energy on bone tissue

The effects of focused ultrasound (US) at therapeutic acoustic power levels were studied in vivo on the bone-muscle interface in rabbit thighs. The purpose of this study was to provide direction in establishing safety guidelines for treating tissue masses using focused US on or near bone. A positioning device was used to manipulate a focused US transducer (1.5 MHz) in a magnetic resonance imaging (MRI) scanner. This system was used to sonicate the femurs of 10 rabbits at acoustic power levels of 26, 39, 52 and 65 W for 10 s. The rabbits were euthanized either 4 h or 28 days after the sonications and the bone samples were harvested for histology examinations. In the femurs studied, acoustic power levels from 39 to 65 W resulted in soft tissue damage characterized grossly by coagulated tissue and bone damage depicted by yellow discoloration. Histologic examination of lesions from sonications from 39 to 65 W demonstrated that osteocyte damage and necrosis, characterized by pyknotic cells and empty lacunae, occurred within the ablation area extending through the bone. The follow-up MR images demonstrated an increase in the amount of damage in the femurs at 28 days posttreatment in comparison to images taken immediately after treatment. Focused US directed at the femur caused immediate significant thermal damage to bone in the form of osteocyte necrosis extending through the (approximately) 1 cm bone in this study. The results suggest that, when focused US energy is directed at or near bone-muscle interfaces, precautions should be taken to avoid thermal damage to the bone that can compromise its strength for extended periods.