High-intensity focused ultrasound (HIFU) induced hyperechoic regions for the ultrasound guidance of HIFU therapy
High Intensity Focused Ultrasound (HIFU) treatment of soft tissues has been shown to result in a hyperechoic region in B-mode ultrasound (US) images. The goals of this project were to gain an understanding of the mechanisms involved in the production of a hyperechoic region and to assess the use of the hyperechoic region as a targeting method for ensuring the proper placement of the HIFU focus prior to commencement of therapy. The objectives of this dissertation were to: (1) confirm if cavitation is involved in the appearance of a hyperechoic region; (2) determine if there is a relationship between temperature change and the formation of a hyperechoic region; and (3) determine if a hyperechoic region can be used for targeting of the HIFU focal region by assessing the extent of tissue damage involved.For studying the involvement of cavitation during the appearance of a hyperechoic region, a US-system was developed. The system consisted of (1) a single-element HIFU transducer (3.3 MHz) placed coaxially to (2) a broadband single element unfocused cavitation detector, (0.5--5.5 MHz) and coplanar with (3) an imaging probe (bandwidth of 5--9 MHz). A thermocouple was located near the HIFU focus for thermal measurements. HIFU at 220--1710 W/cm2 was applied for 10 s to porcine muscle in-vivo. Cavitation activity was detected by both active and passive cavitation detection methods with good correlation during the formation of a hyperechoic region in a diagnostic US image. Thermal measurements showed the initial formation of bubbles at the HIFU focus to be due to acoustic cavitation alone or in conjunction with boiling. In addition, the formation of a hyperechoic region resulted in bubble-enhanced heating at the treatment site.The short- and long-term (0 h--14 days) tissue damage associated with the production of a hyperechoic region useful for targeting of the HIFU treatment site was evaluated in vivo in rabbit muscle. Short duration HIFU pulses (31.25 and 62.5 ms at 4390 and 3660 W/cm2, respectively) were found to produce immediate damage to cells and blood vessels within the HIFU focus in 32.5% of 120 samples, followed by normal tissue healing.
- Bioengineering