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 pu rpose of this study was to provide direction in establishing safety guideli nes for treating tissue masses using focused US on or near bone. A position ing device was used to manipulate a focused US transducer (1.5 MHz) in a ma gnetic resonance imaging (MRI) scanner. This system was used to sonicate th e 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 sonicatio ns and the bone samples were harvested for histology examinations. In the f emurs studied, acoustic power levels from 39 to 65 W resulted in soft tissu e damage characterized grossly by coagulated tissue and bone damage deplete d by yellow discoloration. Histologic examination of lesions from sonicatio ns from 39 to 65 W demonstrated that osteocyte damage and necrosis, charact erized by pyknotic cells and empty lacunae, occurred within the ablation ar ea extending through the bone. The follow-up MR images demonstrated an incr ease in the amount of damage in the femurs at 28 days posttreatment in comp arison to images taken immediately after treatment. Focused US directed at the femur caused immediate significant thermal damage to bone in the form o f osteocyte necrosis extending through the (approximately) 1 cm bone in thi s 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. ( C) 2001 World Federation for Ultrasound in Medicine & Biology.