Differences in the lesion formation process between focused ultrasound and microwave ablations

Experimental study on ablation of leiomyoma by combination high-intensity focused ultrasound and iodized oil in vitro

AIM

The aim of the current study was to investigate whether iodized oil (IO) enhances high-intensity focused ultrasound (HIFU) ablation of uterine leiomyoma and to determine the features of hyperechoic changes in the target region.

METHODS

Forty samples of uterine leiomyoma were randomly divided into an experimental group and a control group. In the experimental group, the leiomyoma was ablated by HIFU 30 min after 1 mL of iodized oil had been injected into the center of the myoma. The hyperechoic values and areas in the target region were observed by B-modal ultrasound after HIFU ablation. The samples were cut successively into slices and stained by triphenyltetrazolium chloride (TTC) solution within 1 h after HIFU ablation. The diameters of TTC-non-stained areas were measured and tissues in the borderline of the TTC-stained and -non-stained areas were observed pathologically. All procedures in the control group were the same as those in the experimental group except IO was replaced by physiological saline.

RESULTS

The hyperechoic value in the target region in the experimental group was higher than that in the control group 4 min after HIFU ablation (P < 0.05). Hyperechoic areas in the target region as well as TTC-non-stained volumes in the experimental group were greater than those in the control group (P < 0.05). Routine pathologic observation showed that coagulation necrosis of leiomyoma occurred in the target region in both groups.

CONCLUSION

IO causes coagulation necrosis, enlarges tissue damage, and postpones the attenuation of hyperechoic changes in the target region when HIFU ablation is carried out for leiomyoma in vitro.

Microwave-heating-coupled photoacoustic radar for tissue diagnostic imaging

An investigation of microwave (MW) heating effects on biotissue for enhancing photoacoustic radar (PAR) signals was conducted. Localized tissue heating generated by MWs was used to improve PAR imaging depth and signal-to-noise ratio (SNR). Elevated temperatures were measured with thermocouples in ex vivo bovine muscle. The measured temperature rise on the heated spot surface by MWs was in agreement with theoretical predictions. The study showed localized MW heating can increase the photoacoustic imaging depth by 11%, and the SNR by 5% in ex vivo bovine muscle.

Ablation of leiomyomas using a combination of HIFU and iodized oil in vitro

This study investigates the enhancement effects of iodized oil and the features of hyperechoic focus changes in a target region of high-intensity focused ultrasound (HIFU) ablated uterine fibroids. Leiomyomas in the experimental group of 20 randomly assigned patients were ablated by HIFU under certain parameters 30 min after 1 mL of iodized oil was injected into the center of the myomas. The value of the gray scale and its area were observed by B-mode ultrasound in the target region and were carefully recorded at 0, 2, 4, and 5 min, respectively, after HIFU ablation. The samples were sectioned successively in a thickness of 1∼2 mm and stained by 2,3,5-triphenyltetrazolium chloride solution within 1 h after HIFU ablation. The TTC non-staining volumes were measured afterwards. All procedures in the control group of the other 20 randomly assigned patients were the same except that iodized oil was replaced by physiologic saline. The hyperechoic areas in the target region were observed in all fibroids of both groups. Compared with the control group, the gray scale values in the target region of the experimental group were higher 4 min after HIFU ablation. The enhanced gray scale area in the target region and the 2,3,5-triphenyltetrazolium chloride non-staining volumes in the experimental group were bigger. Based on our experience, HIFU sonication of leiomyomas injected with iodized oil produces hyperechoic foci that are greater than areas injected with saline. More studies are required to understand the clinical implications of these observations.

A novel strategy to increase heating efficiency in a split-focus ultrasound phased array

Focus splitting using sector-based phased arrays increases the necrosed volume in a single sonication and reduces the total treatment time in the treatment of large tumors. However, split-focus sonication results in a lower energy density and worse focal-beam distortion, which limits its usefulness in practical treatments. Here, we propose a new heating strategy involving consecutive strongly focused and split-focus sonications to improve the heating efficiency. Theoretical predictions including linear and thermal-dose-dependent attenuation change were employed to investigate potential factors of this strategy, and ex vivo tissue experiments were conducted to confirm its effectiveness. Results showed that the thermal lesions produced by the proposed strategy could be increased when comparing with the previous reported strategies. The proposed heating strategy also induces a thermal lesion more rapidly, and exhibits higher robustness to various blood perfusion conditions, higher robustness to various power/heating time combinations, and superiority to generate deep-seated lesions through tissues with complex interfaces. Possible mechanisms include the optimization of the thermal conduction created by the strongly focused sonication and the temperature buildup gained from thermally induced tissue attenuation change based on the theoretical analysis. This may represent a useful technique for increasing the applicability of split-focus and multiple-focus sonication techniques, and solve the obstacles encountered when attempting to use these methods to shorten the total clinical treatment time.