An ultrasound window to perform scanned, focused ultrasound hyperthermia treatments of brain tumors

A series of calculations and tests were performed on polyethylene, polystyrene, acrylic, and a commercially available polymethyl methacrylate cranioplastic to determine which material would best serve as an acoustical window in the skull. The purpose of the window is to transmit focused ultrasound to treat brain tumors with hyperthermia. Each material was evaluated based on its ability to transmit power and to protect the brain. The results revealed that, of the four materials tested, polyethylene transmitted the largest percentage of incident power and is the toughest and best suited material to protect the brain. Further physical tests showed that a polyethylene plate does not significantly distort the sound field. Finally, acute and chronic dog tests (supported by theoretical calculations) using the polyethylene as an acoustical window through the skull showed that it will not reach temperatures greater than the target hyperthermia temperature (42 degrees C) at the required intensity levels unless the tumor is near the skull and the skin surface is not cooled. Since polyethylene effectively transmits power without distorting the sound field of overheating in dog tests, it may now be used in Phase 1 clinical ultrasound hyperthermia tests on human patients. However, the chronic animal studies indicated that the collagen which composes the dura thickens under the cranioplasty and absorbs increasingly greater amounts of power with time. A thick layer of organized fibrous tissue also formed on the external surface of the cranioplasty, filling in the cavity formed by the exterior surface of the cranioplasty and by the excision of the temporalis muscle.(ABSTRACT TRUNCATED AT 250 WORDS)     

A hemisphere array for non-invasive ultrasound brain therapy and surgery

Ultrasound phased arrays may offer a method for non-invasive deep brain surgery through the skull. In this study a hemispherical phased array system is developed to test the feasibility of trans-skull surgery. The hemispherical shape is incorporated to maximize the penetration area on the skull surface, thus minimizing unwanted heating. Simulations of a 15 cm radius hemisphere divided into 11, 64, 228 and 512 elements are presented. It is determined that 64 elements are sufficient for correcting scattering and reflection caused by trans-skull propagation. An optimal operating frequency near 0.7 MHz is chosen for the array from numerical and experimental thermal gain measurements comparing the power between the transducer focus and the skull surface. A 0.665 MHz air-backed PZT array is constructed and evaluated. The array is used to focus ultrasound through an ex vivo human skull and the resulting fields are measured before and after phase correction of the transducer elements. Finally, to demonstrate the feasibility of trans-skull therapy, thermally induced lesions are produced through a human skull in fresh tissue placed at the ultrasound focus inside the skull.     

Recall of awareness during cardiac anaesthesia: influence of feedback information to the anaesthesiologist

We interviewed 303 cardiac surgery patients to evaluate the incidence of intraoperative awareness with recall.
First, we randomly interviewed 99 patients, of whom four patients (4%) reported awareness and recall. We informed the cardiac anaesthesiologists of the results of these interviews, and we also gave general information regarding means to reduce awareness and recall during general anaesthesia. Thereafter, we interviewed 204 consecutive cardiac surgery patients. Now, three of the patients (1.5%) had intraoperative awareness with recall. The reduction in the incidence from 4% to 1.5% was not significant. However, the doses of principal anaesthetic drugs had increased significantly between the two interview phases, while the dose of pancuronium, the main muscle relaxant used, had decreased significantly. Also, there was a significant increase in the number of anaesthesias where anaesthetic agents had been administered continuously instead of bolus or non-continuous dosing techniques. Between the patients with awareness and recall and those without it, there was no difference in the doses of anaesthetic agents given. The patients with awareness were significantly younger than those not aware.
In conclusion, with educational measures and vigilance over the problem, the incidence of intraoperative awareness during cardiac anaesthesia may be reduced. The incidence figure of 1.5% we observed is of the magnitude reported recently by others with modern cardiac anaesthesia techniques.     

Clinical applications of focused ultrasound-the brain.

This paper provides a historic and contemporary overview of the use of focused ultrasound for treating brain disorders.     

MR imaging-guided focused ultrasound surgery of uterine leiomyomas: a feasibility study

The feasibility and safety of magnetic resonance (MR) imaging-guided focused ultrasound surgery for uterine leiomyomas is reported. Sequential sonications were delivered to nine targets. Temperature-sensitive phase-difference MR imaging monitored the location of the focus and measured tissue temperature elevations, ensuring therapeutic dose. MR images and hysterectomy specimens were evaluated. Six leiomyomas received full therapeutic doses, and 98.5% of the sonications were visualized. MR thermometry was successful in all sonications and cases. Focal necrotic lesions were seen in all cases at MR, and five were pathologically confirmed. MR imaging-guided focused ultrasound causes thermocoagulation and necrosis in uterine leiomyomas and is feasible and safe, without serious consequences.     

Local hyperthermia induced by focussed and overlapping ultrasonic fields—An in vivo demonstration

Axial temperature distributions were measured in living and post mortem porcine tissues during sonication with plane, focussed and overlapping ultrasonic fields. With the focussed field it was always possible to induce the temperature maxima at depths up to 50 mm, although the actual temperatures achieved varied from animal to animal. The plane 0.75 MHz transducer produced a maximum temperature close to the skin surface. With 7 overlapping plane fields a relatively uniform temperature distribution was produced in a large tissue volume. The blood perfusion in tissue has a significant effect not only on the magnitude of the temperature increase, but also on the temperature distribution.     

Feasibility and patient tolerance of a novel transrectal ultrasound hyperthermia system for treatment of prostate cancer.

This report describes patient tolerance and toxicity of a transrectal ultrasound hyperthermia system used with external beam radiation therapy in treatment of locally advanced prostate cancer. Nine patients with clinical T2B-T3B (4th edition AJCC criteria) disease received external beam radiation therapy, with two hyperthermia treatments scheduled at least 1 week apart during the first 4 weeks of radiation. Five patients also received hormonal therapy. Interstitial and anterior rectal wall thermometry were performed. Median temperature for each treatment (T50) was 40.8 degrees C and mean CEM T90 = 43 degrees C was 3.4 min. Rectal wall temperature was maintained at < or = 40 degrees C. Treatment duration was limited in three of 17 sessions due to positional discomfort which was alleviated with light IV sedation and use of a 'New Life' mattress (Comfortex, Inc. Winoba, MN, USA). Acute toxicity was limited to NCI common toxicity criteria grade 1 and no excess toxicity was noted with full course radiation therapy +/- hormonal therapy. These findings are consistent with those reported in a previous phase I trial assessing this device. Given the favourable toxicity profile demonstrated to date, modification of treatment parameters for this ongoing phase II study have been instituted that should further the efficacy of transrectal ultrasound hyperthermia for treatment of prostate cancer.     

A focused ultrasound method for simultaneous diagnostic and therapeutic applications--a simulation study.

Similar to other therapeutic methods, ultrasound surgery requires an imaging modality to monitor the extent of tissue damage during treatment. In this paper, we have considered the method of ultrasound-stimulated acoustic emission (USAE) that uses two ultrasonic beams at high frequency (1.7 MHz) (same as that used for ablation) to locally excite the tissue by generating a low-frequency (1-50 kHz) radiation force. Recording of the tissue response at several locations yields an image. The amplitude of the tissue response depends on the mechanical and acoustic tissue properties, namely its stiffness and absorption. These two properties were initially hypothesized to have counteractive effects on the response amplitude, i.e., the amplitude should increase with absorption and decrease with stiffness. To check this hypothesis as well as the degree to which these properties influence the response, finite-element simulations of a uniform lesion formed inside a homogeneous medium were used. The results show that, as expected, the displacement amplitude decreased with increasing lesion stiffness at lower frequencies (except at resonance) while, contrary to our initial hypothesis, it increased with stiffness at relatively higher frequencies (>22 kHz). At resonance, a frequency upshift occurred with increasing stiffness but was found to be highly spatially variant and system dependent, i.e., not yielding a uniform lesion response when imaged. On the other hand, the absorption increase led to a uniform linear increase of the mechanical response amplitude of the lesion. Therefore, at higher frequencies, increase of the two parameters had a synergistic effect on the tissue response to the applied radiation force. This study showed that relatively higher frequencies constitute the optimal range in the use of USAE for coagulation monitoring. A preliminary experimental verification in vitro is also provided.