Shuki Vitek

500-Element Ultrasound Phased Array System for Noninvasive Focal Surgery of the Brain: A Preliminary Rabbit Study With Ex Vivo Human Skulls

The aim of this study was to test a prototype MRI‐compatible focused ultrasound phased array system for trans‐skull brain tissue ablation. Rabbit thigh muscle and brain were sonicated with a prototype, hemispherical 500‐element ultrasound phased array operating at frequencies of 700–800 kHz. An ex vivo human skull sample was placed between the array and the animal tissue. The temperature elevation during 20–30‐sec sonications was monitored using MRI thermometry. The induced focal lesions were observed in T2 and contrast‐enhanced T1‐weighted fast spin echo images. Whole brain histology evaluation was performed after the sonications. The results showed that sharp temperature elevations can be produced both in the thigh muscle and in the brain. High‐power sonications (600–1080 W) produced peak temperatures up to 55°C and focal lesions that were consistent with thermal tissue damage. The lesion size was found to increase with increasing peak temperature. The device was then modified to operate in the orientation that will be used in the clinic and successfully tested in phantom experiments. As a conclusion, this study demonstrates that it is possible to create ultrasound‐induced lesions in vivo through a human skull under MRI guidance with this large‐scale phased array. Magn Reson Med 52:100–107, 2004.

System and methods for controlling distribution of acoustic energy around a focal point using a focused ultrasound system

A focused ultrasound system includes a plurality of transducer elements disposed about and having an angular position with a central axis. Drive signals drive respective transducer elements that include phase shift values based upon the angular position of each respective transducer element. The phase shift values are based upon an oscillation function that oscillates about the central axis between minimal and maximal phase shift values such that a first on-axis focal zone and a second off-axis focal zone are created. An amplitude and frequency of the oscillation function are controlled to adjust relative acoustic energy levels of the first and second focal zones, and to adjust a radius of the second focal zone, respectively. In addition, the drive signals include an additional predetermined phase shift based upon a radial position of each respective transducer element to adjust a focal distance of the focal zones.

Factors associated with successful magnetic resonance-guided focused ultrasound treatment: efficiency of acoustic energy delivery through the skull

OBJECTIVE Magnetic resonance-guided focused ultrasound surgery (MRgFUS) was recently introduced as treatment for movement disorders such as essential tremor and advanced Parkinsons disease (PD). Although deep brain target lesions are successfully generated in most patients, the target area temperature fails to increase in some cases. The skull is one of the greatest barriers to ultrasonic energy transmission. The authors analyzed the skull-related factors that may have prevented an increase in target area temperatures in patients who underwent MRgFUS. METHODS The authors retrospectively reviewed data from clinical trials that involved MRgFUS for essential tremor, idiopathic PD, and obsessive-compulsive disorder. Data from 25 patients were included. The relationships between the maximal temperature during treatment and other factors, including sex, age, skull area of the sonication field, number of elements used, skull volume of the sonication field, and skull density ratio (SDR), were determined. RESULTS Among the various factors, skull volume and SDR exhibited relationships with the maximum temperature. Skull volume was negatively correlated with maximal temperature (p = 0.023, r(2) = 0.206, y = 64.156 - 0.028x, whereas SDR was positively correlated with maximal temperature (p = 0.009, r(2) = 0.263, y = 49.643 + 11.832x). The other factors correlate with the maximal temperature, although some factors showed a tendency to correlate. CONCLUSIONS Some skull-related factors correlated with the maximal target area temperature. Although the number of patients in the present study was relatively small, the results offer information that could guide the selection of MRgFUS candidates.

Impact of propagating and standing waves on cavitation appearance.

Standing waves play a significant role in the appearance of cavitation phenomena. The goal of this study was to investigate the effect that the relation between standing and propagating waves in a focused field has on acoustic bubble cloud formation. Measurements of the cavitation signals were performed on five different configurations of a hemispheric phased array transducer (230 kHz) representing a wide range of relations between propagating and standing waves. The results show that configurations with a larger propagating component induce bubble clouds at lower pressures than configurations with a larger standing component.