Sonographic and histological development of high-intensity focused ultrasound in rabbit muscle

The purpose of this study was to examine the development of the ultrasonic image and histology of rabbit leg muscles irradiated with high-intensity focused ultrasound (HIFU). HIFU was applied to muscles of 40 legs of 20 rabbits. The results of sonication were examined by ultrasound from 10 min to 28 days after HIFU radiation in 19 rabbits. Pathological examination was performed on Day 1 and Day 28. The echo of rabbit leg muscle was revealed to be homogeneous and hypoechoic, as detected by the two-dimensional ultrasound prior to HIFU radiation. The echo of the HIFU lesion was hyperechoic at 10 min after HIFU radiation. The echo at the central point of the HIFU lesion became lower 1 h after HIFU radiation, there was a distinct hyperechoic border zone surrounding the site of sonication. During Days 1–3, the echo of these sites became lower. One week later, there was a hypoechoic border zone surrounding the sites of sonication. Two to four weeks later, the volume of sonication lesions decreased gradually. The echo of sonication lesions was homogeneous and hypoechoic. On Day 1 after HIFU radiation, the HIFU lesion was best observed grossly as pale round areas and was surrounded by hyperemia. The pathology results demonstrated that the cytoplasm was red-stained, the karyon was clear, the muscle cells subjected to HIFU radiation appeared with cytoplasm condensation, and nuclear and cell membrane breakdown was present. Numerous leukocytes and lymphocytes were detected surrounding the coagulation necrosis lesions. Four weeks later, the HIFU lesion was observed as discrete, with sharp differentiation between normal and ablated tissue. The lesion was best observed grossly as pale-yellow round areas and without hyperemia. The leukocyte and lymphocyte infiltration was not apparent in or around the sonications. Fibroblast hyperplasia, fatty infiltration and small scar formation in and around the sonications were observed. Ultrasound examination plays an important role in visualization after HIFU radiation. It provides a basis for understanding ultrasonic image development in clinical applications such as the treatment of uterine fibroids.