| Objective To explore the formative law of BFR induced by HIFU and B-mode ultrasonography real-time monitoring, to investigate the focal temperature and dosage-effect relation of BFR, as well as to evaluate the curative effect of HIFU through MRI, in vitro. Methods Fresh excised calf liver, muscle, kidney (within 6 hours) were irradiated by HIFU with a frequency of 1.0MHz and 1.6MHz respectively. The focal temperatures were measured using thermocouples adjusted to 0.7mm diameter, the B-mode imaging prior to and posterior to exposure were collected, inspected, and the gray values were visualized. The volumes of coagulation necrosis due to HIFU were estimated with MRI. Finally, the tissues were cut open to examine the maximum injured parts and to take the measure of the focal length.Results ⑴ The region of destroyed tissue produced by HIFU were gray coagulation necrosis, there were clear borderline between destroyed tissue and normal tissue. ⑵ The shape and volume of BFR were different with varied frequency, tissue, duration time, intensity, and exposure depths. ⑶ The ratios of long to short axis of BFR in HIFU were differentfollowing therapeutic dosage. ⑷ The volumes of BFR were decreased with increasing exposure depths of tissue. ⑸ In vitro, at the definite dosage and depth, BFR volume in HIFU of muscle tissue was apparently higher than one of liver and kidney tissue, and the volume in liver tissue was higher than one in kidney. ⑹ Histopathological examination indicated that hepatic lobule of bovine was damaged, and cell volume of it decreased, cell shape became circular, plasmas were deeply stained, nucleus also deeply stained and pycmosised, nucleolus extinct. ⑺ A bright hyper echoic spot was resulted and gray values were increased at the focal region after exposure. SUSI were almost equal to SBFR after one minute duration(t=0.308,p﹥0.05). ⑻ The focal absolute temperature of target tissue were 86.3℃ and the average temperature were 80.5±3.09℃. ⑼The EEF induced by HIFU were increased with the increasing of exposure depth for the "sheaf" formed with the mode of limited overlapping and that of limitless overlapping. While the other parameter was invariable, the EEF of limited overlapping was higher than one of limitless overlapping. ⑽ The EEF on the slice was 2.71±0.85 (J/mm3),while the EEF on the mass 1.73±0.39 (J/mm3). ⑾ The signal of MRI (T1, T2) posterior to exposure by HIFU was decreased. There are correlations between the volume of MRI and the volume induced by HIFU. Conclusions ⑴ The formation of BFR must arrive threshold therapeutic dosage. ⑵ The shape and volume of BFR could be controlled.⑶ It was showed that gross and histopathological finding were consistent.⑷ Real-time ultrasound imaging could be used to visualize the therapeutic dosage and effective. ⑸ Efficacy of ablation with 1.6MHz was higher than that with 1.0MHz in shallow tissue, while conducting ablation indeeper tissue, lower frequency should be used. ⑹ The volume were decreased under exponential law by degrees with the increase of exposure depths, where VBFR=722.74e-0.9369D for 1.0MHz, where VBFR=3489.6e-0.4341D for 1.6MHz. ⑺ When the other parameters fixed, BFR volume in HIFU exposed to different tissue was various. ⑻ The focal average temperature were 80.5±3.09℃,cells of target could be killed completely. ⑼ With the various combination of HIFU-induced BFR and certain treatment planning, bigger tumor or deeper tumor could be completely ablated without destroying the surrounding tissues. ⑽ Under the similar circumstances, the EEF of dot was gradually decreased, following the sequences in which the EEF of dot was higher than that of sheaf, the EEF of sheaf higher than that of slice and that of slice higher than that of mass. ⑾. The lesions in the organism tissue induced by HIFU could be surveilled with MRI.
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