title: Prevention of post-focal thermal damage by formation of bubbles at the focus during high intensity focused ultrasound therapy.
abstract: Safety concerns exist for potential thermal damage at tissue-air or tissue-bone interfaces located in the post-focal region during high intensity focused ultrasound (HIFU) treatments. We tested the feasibility of reducing thermal energy deposited at the post-focal tissue-air interfaces by producing bubbles (due to acoustic cavitation and/or boiling) at the HIFU focus. HIFU (in-situ intensities of 460-3500 W/cm2, frequencies of 3.2-5.5 MHz) was applied for 30 s to produce lesions (in turkey breast in-vitro (n = 37), and rabbit liver (n = 4) and thigh muscle in-vivo (n = 11)). Tissue temperature was measured at the tissue-air interface using a thermal (infrared) camera. Ultrasound imaging was used to detect bubbles at the HIFU focus, appearing as a hyperechoic region. In-vitro results showed that when no bubbles were present at the focus (at lower intensities of 460-850 W/cm2), the temperature at the interface increased continuously, up to 7.3 +/- 4.0 degrees C above the baseline by the end of treatment. When bubbles formed immediately after the start of HIFU treatment (at the high intensity of 3360 W/cm2), the temperature increased briefly for 3.5 s to 7.4 +/- 3.6 degrees C above the baseline temperature and then decreased to 4.0 +/- 1.4 degrees C above the baseline by the end of treatment. Similar results were obtained in in-vivo experiments with the temperature increases (above the baseline temperature) at the muscle-air and liver-air interfaces at the end of the high intensity treatment lower by 7.1 degrees C and 6.0 degrees C, respectively, as compared to the low intensity treatment. Thermal effects of HIFU at post-focal tissue-air interfaces, such as in bowels, could result in clinically significant increases in temperature. Bubble formation at the HIFU focus may provide a method for shielding the post-focal region from potential thermal damage.
authors.affiliation: Department of Electrical and Computer Engineering, The George Washington University, Washington, DC 20052, USA.
authors.forename: Vesna
authors.initials: V
authors.lastname: Zderic
authors.forename: Jessica
authors.initials: J
authors.lastname: Foley
authors.forename: Wenbo
authors.initials: W
authors.lastname: Luo
authors.forename: Shahram
authors.initials: S
authors.lastname: Vaezy
mesh_terms: D000818:Animals; D001940:Breast / Q000503:physiopathology / Q000528:radiation effects*; D002056:Burns / Q000209:etiology* / Q000517:prevention & control*; D004307:Dose-Response Relationship, Radiation; D005740:Gases; D006801:Humans; D011829:Radiation Dosage; D011832:Radiation Injuries / Q000209:etiology* / Q000517:prevention & control*; D014422:Turkeys; D014464:Ultrasonic Therapy / Q000009:adverse effects* / Q000379:methods*
chemical_list: D005740:Gases