Objective This work evaluated the clinical feasibility of transcranial MRI-guided focused ultrasound surgery (TcMRgFUS). Methods In initial trials in three glioblastoma patients, multiple focused ultrasound exposures were applied up to maximum acoustic power available. Offline analysis of the MR temperature images evaluated the temperature changes at the focus and brain surface. Instrumentation The method combines a hemispherical phased array transducer and patient-specific treatment planning based on acoustic models with feedback control based on MR temperature imaging to overcome the effects of the skull and allow for controlled and precise thermal ablation in the brain. Technical Development TcMRgFUS offers a potential noninvasive alternative to surgical resection. Results We found that it was possible to focus an ultrasound beam transcranially into the brain and to visualize the heating with MR temperature imaging. While we were limited by the device power available at the time and thus appeared to not achieve thermal coagulation, extrapolation of the temperature measurements at the focus and on the brain surface suggests that thermal ablation will be possible with this device without overheating the brain surface, with some possible limitation on the treatment envelope. Conclusion While significant hurdles remain, these findings are a major step forward in producing a completely noninvasive alternative to surgical resection for brain disorders.

A technique for focusing ultrasound through the human skull is described and verified. The approach is based on a layered wavevector-frequency domain model, which propagates ultrasound from a hemisphere-shaped transducer through the skull using input from CT scans of the head. The algorithm calculates the driving phase of each transducer element in order to maximize the signal at the intended focus. This approach is tested on ten ex vivo human skulls using a 0.74 MHz, 320-element array. A stereotaxic reference frame is affixed to the skulls in order to provide accurate registration between the CT images and the transducer. The focal quality is assessed with a hydrophone placed inside the skull. In each trial the phase correction algorithm successfully restored the focus inside the skull at a location within 1 mm from the intended focal point. The results demonstrate the feasibility of using the method for completely non-invasive ultrasound brain surgery and therapy.

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 T 2 and contrast-enhanced T 1 -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.