Hot spots created at skin-air interfaces during ultrasound hyperthermia

It is well known that ultrasound beams will be completely reflected at a soft-tissue-gas interface. The reflected beam is contributing to the power absorption at the tissue interface and may cause a hot spot which could prevent therapeutic temperatures at the treatment volume. In this study the temperature elevation caused by a reflected ultrasound beam at the skin surface has been investigated in dogs' thighs in vivo. The magnitude of the hot spot was quantified and the effect of entrance angle was also investigated. In addition, the possibility of eliminating the hot spot by coupling the beam out of the tissue was studied. The results showed that the temperature elevation can be up to four times larger at the skin-air surface than in resting muscle under similar exposure. The geometry of the reflecting surface had a significant effect on the temperature distribution. When the sound was coupled out of the tissue the magnitude of the temperature elevation at the skin reduced to less than half (depending on the geometry) but was still larger than temperatures measured in muscle. These results suggest the need for computerized treatment planning for scanned focused ultrasound treatments.