Thermal effects of focused ultrasound energy on bone tissue

The effect of high-intensity focused ultrasound at therapeutic power levels was 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 on or near bone. An magnetic compatible positioning device was used to manipulate a focused ultrasound transducer (1.5 MHz) in a magnetic resonance imaging (MRI) scanner. The system was used to sonicate the femurs of four rabbits (six femurs) at acoustic power levels of 28, 43, 57 and 72 W. From the axial and coronal MR images, there was increasing bow-muscle temperatures and damaged tissue area (lesions) with increasing acoustic powers. Histological examination of the bone cells was performed in two of the treated femurs. In four of the six femurs studied, acoustic powers of 43, 57 and 72 W resulted in soft tissue damage characterized by whitened muscle tissue. At these acoustic powers the bone damage was characterized by yellow discoloration. Acute histological examination of lesions from sonications at 43, 57 and 72 W demonstrated that osteocyte damage and necrosis, characterized by empty lacunae, occurred within the ablation area. Additionally the femurs of two rabbits were allowed to heal over four weeks. MR images from this survival study demonstrated an increase in the amount of vascular damage in the femurs two weeks post treatment in comparison to images taken immediately after treatment. Overall this study demonstrated that high-intensity focused ultrasound directed at the femur caused immediate significant thermal damage to bone in the form of osteocyte necrosis, vascular damage, tissue necrosis and edema in the surrounding soft tissue