[Study on therapeutic dosimetry of HIFU ablation tissue].

It is a difficult problem in high intensity focused ultrasound (HIFU) therapeutic dosimetry that how to use a BFR to ablate a mass in tissue and to determine the energy-efficiency relation, that is, the scale of biological effects of HIFU. A mass lesion was realized in this study according to a treatment principle of damaging tissue from BFRs to fascicle lesions, slice lesions and a mass lesion. A 1.6 MHz transducer, 150 mm in diameter and with a focal length of 120 mm, was used. The focal intensities (I(SATA)) were 0-27 000 W/cm2 and the scanning speeds were 1-4 mm/s. The distance between every fascicle lesion was 5-10 mm and the distance between two slice lesions was 10-20 mm. Different irradiation depths of fascicle slice and mass lesion were observed after HIFU procedures in this study. The dosage of HIFU required for tissue coagulated necrosis was evaluated with energy of HIFU (J) per cubic millimeter (mm3), i.e., J/mm3 which was defined as energy-efficiency factor (EEF). Results showed that EEF needed for producing fascicle lesions increased with the increase of irradiation depth. EEF required for inducing various lesions in biological tissue was different. Generally, it followed the law: EEF(mase)< EEF(slice)<EEF(fascicle). EEF for slice lesion was not simply a summation of EEFs for fascicle lesions at different irradiation depths, although the slice lesion was assembled with fascicle lesions at different irradiation depths in the same treatment slice. In the same way, EEF for a mass lesion was not simple summation of EEFs for slice lesions at different layers. So HIFU therapeutic dosimetry can be carried on investigation by using EEF, Factors of affecting EEF of HIFU include acoustic power, exposure time, irradiation depth, tissue structure, and tissue functional status. Besides, another important factor is the change of the acoustic environment in tissue during the HIFU procedure.