Brian Rabkin

HIFU‐Induced Hyperecho in Ultrasound Images, Cavitation Activity and Thermal Behavior

High Intensity Focused Ultrasound (HIFU) treatment of soft tissues has been shown to result in a hyperechoic region in B‐mode ultrasound (US) images. This is believed to result from bubble activity at the HIFU focus. Here we report our in vivo results of detecting inertial and stable cavitation in correlation with the appearance of a hyperechoic region, along with in vitro confirmation of these results that included measurement of the temperature at the HIFU focus. The ultrasound system consisted of a HIFU transducer (3.3 MHz), a broadband A‐mode transducer for active and passive cavitation detection (ACD and PCD), and an US‐imaging probe that were all co‐focal and synchronized. HIFU, at in situ intensities of 220– 1,710 W/cm2, was applied for 10 s to pig muscles in vivo or polyacrylamide in vitro at a focal depth of 2 cm. A thermocouple placed at the HIFU focus was added to the above system during the in vitro portion of this study. ACD and PCD results showed a strong correlation between the onset of cav...

Stable cavitation in ultrasound image‐guided high intensity focused ultrasound therapy

Microbubble activity is significantly involved in both diagnostic and therapeutic aspects of ultrasound image‐guided HIFU therapy. Ultrasound interrogation techniques (A‐, B‐, M‐mode, Doppler, harmonic and contrast imaging, and passive and active cavitation detection) were integrated with HIFU. Our results using HIFU devices of 1–5 MHz, and focal, derated intensities of 1,000–10,000 W/cm2, show the formation of microbubbles (about 100 bubbles/mm3, 5–100 microns in size) at the HIFU focus. Boiling, stable, and inertial acoustic cavitation activities were detected during therapy. The presence of bubbles allows the observation of the treatment spot as bright hyperechoic regions in ultrasound images, providing an effective method for guidance and monitoring of therapy. The stable cavitation of microbubbles may provide a mechanism for enhanced HIFU energy delivery, as well as induction of biological responses for stimulation and regulation of specific physiological events such as coagulum and thrombus formatio...

Real‐Time Color‐Doppler Guidance of HIFU for the Selective Avoidance or Occlusion of Blood Vessels

High‐intensity focused ultrasound (HIFU) has been shown to effectively occlude blood vessels deep within tissue. The objective of the current study was to synchronize HIFU and color‐Doppler ultrasound (US) for the real‐time visualization of flow within blood vessels during HIFU treatment. The excitation of the HIFU was synchronized with the color‐Doppler imager by collecting the excitation pulses of one of the elements of either a curved array intracavitary (C 9‐5) or an intraoperative (CL 10‐5) imaging probe. The collected excitation pulse was converted into a TTL‐high pulse, which was delayed and gated to time the excitation duration and location of the HIFU pulse with respect to each imaging frame. The single pulse was used to drive a 3.2 MHz concave HIFU transducer (focal length of 3.5 cm, f‐number 1) while the US imager was not collecting RF signals from the treatment region of the US image. The feasibility of the system was demonstrated in vivo by the selective ablation of tissue adjacent to, or the...

The role of cavitation in therapeutic ultrasound

Ed Carstensen has made many contributions to biomedical ultrasound but among those that are becoming more and more relevant to current clinical practice are those that determine the conditions under which cavitation is induced in vivo. For many years, it was assumed that the medical ultrasound devices were unable to induce cavitation in living tissue because either the acoustic conditions were not sufficient or the nucleation sites that are required were too small. With the advent of lithotripters and high‐intensity focused ultrasound (HIFU) devices, cavitation generation in vivo is commonplace. Our current research at the University of Washington has focused on the role that cavitation plays in stone comminution and tissue damage during lithotripsy, as well as the enhancement or reduction of desirable coagulative necrosis during HIFU application. During HIFU application, we find enhanced heating that results from nonlinear acoustic wave propagation (a key Carstensen contribution) leads to vapor bubble fo...

Hyperechogenicity during high intensity focused ultrasound (HIFU)

Ultrasound guidance of HIFU therapy is attractive because of its portability, low cost, real‐time image processing, simple integration with HIFU instruments, and the extensive availability of diagnostic ultrasound; however, the use of ultrasound visualization for the guidance and monitoring of HIFU therapy often relies on the appearance of a hyperechoic region in the ultrasound image. It is often assumed that the formation of a hyperechoic region at the HIFU treatment site results from bubble activity generated during HIFU exposure. However, it has been determined that this region can be generated with relatively short bursts of HIFU (on the order of 30 ms), bursts so short that negligible temperature elevations are expected to occur. In examining the histology associated with these hyperechoes, there is little evidence of traditional cavitation damage; rather, it appears as if there are many bubbles generated within the individuals cells, suggesting a thermal mechanism. Thermocouple measurements of the t...

Color‐Doppler guided high intensity focused ultrasound for hemorrhage control

To determine efficacy of high intensity focused ultrasound (HIFU) in occlusion of pelvic vessels a 3.2 MHz HIFU transducer was synchronized with color‐Doppler ultrasound imaging for real‐time visualization of flow within blood vessels during HIFU therapy. HIFU was applied to pig and rabbit pelvic vessels in vivo, both transcutaneously and with skin removed. The in situ focal intensity was 4000 W/cm2 on average. Vessel occlusion was confirmed by color or audio Doppler, and gross and histological observations. In rabbits, five out of 10 femoral arteries (diameter of 2 mm) were occluded after 30–60 s of HIFU application. The average blood flow reduction of 40% was observed in the remaining arteries. In pigs, out of 7 treated superficial femoral arteries (2 mm in diameter), 4 were occluded, one had 80% blood flow reduction, and 2 were patent. In addition, 3 out of 4 superficial femoral arteries, punctured with 18 gauge needle, were occluded after 60–90 s of HIFU application. Larger vessels (diameter of 4 mm) ...