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High-intensity focused ultrasound（HIFU）can non-invasively irradiate inside the body. However, when used to treat fetuses, it can cause thermal burns of the mother’s abdominal wall at the skin interface. This study was carried out to determine whether a modified HIFU transducer enabling split-aperture irradiation can prevent thermal burns. Two HIFU transducers were compared: a conventional transducer using full-aperture irradiation and a modified transducer using split-aperture irradiation. The modified transducer was divided into six sectors for split-aperture irradiation and had a larger surface area and a smaller F number（focal length/aperture diameter）than the conventional transducer. HIFU was delivered to eight sites on the left and right leg of a three-month-old baby pig under general anesthesia, and the sites were assessed for thermal burning by two or more dermatologists. The same person performed all irradiations. Full-aperture irradiation with the conventional transducer caused deep dermal burns at all target sites, while split-aperture irradiation with the modified transducer caused only epidermal burns or superficial dermal burns. Split-aperture irradiation using a modified HIFU transducer with six sectors and a smaller F number reduces the severity of skin burns, and thus will improve the safety of HIFU therapy.

In this report, the second-generation high-intensity focused ultrasound （HIFU） systems were clinically evaluated for human fetal therapy in two cases of twin reversed arterial perfusion sequence. The HIFU systems comprised an improved lead zirconate-titanate transducer with an imaging phase array sector probe, a Sonachill cooling system, and three phases of HIFU exposure: a trigger pulse, a continuous heating wave, and an idle period to obtain images with the imaging probe set on the transducer. To evaluate skin surface temperature, a thermal camera was used. To evaluate vessel occlusions, blood flow was measured at fixed timings after exposures. Target vessel occlusion was achieved with HIFU in only one of the cases, but recanalization occurred the following day. Both cases were finally treated with radiofrequency ablation and one infant was successfully delivered without any complications. This case highlighted three advantages with the change to second-generation HIFU systems in human fetal therapy: the simplicity of maneuvers by reduced range of motion disturbance; the ability to observe in real time during the exposure; and a decrease in total ultrasonic output. Treatment interruption due to burns or complaints of heat sensation represented an obstruction to treatment completion. This remains an issue to be addressed in the future.