Ultrasound focusing using magnetic resonance acoustic radiation force imaging: application to ultrasound transcranial therapy.

PURPOSE: Magnetic resonance guided ultrasonic therapy is a promising minimally invasive technology for constantly growing variety of clinical applications. Delivery of focused ultrasound (FUS) energy to the targeted point with optimal intensity is highly desired; however, due to tissue aberrations, optimal focal intensity is not always achieved. Especially in transcranial applications, the acoustic waves are shifted and distorted mainly by the skull. In order to verify that magnetic resonance acoustic radiation force imaging (MR-ARFI) can be used as a focusing tool in transcranial treatments, such an imaging was applied in vivo on a porcine brain via ex vivo human skull. Then, by the use of MR-ARFI technique, an improved ultrasound focusing algorithm is proposed and demonstrated for both transcranial and none brain applications.
METHODS: MR-ARFI images were acquired on a GE 1.5 T scanner equipped with InSightec FUS systems ExAblate 2000 and ExAblate 4000. Imaging was performed with MR-ARFI sequences of line-scan spin-echo and single-shot gradient-echo echo-planar. The in-plane resolution of both acquisitions was 0.9 x 0.9 mm2. The total acquisition time of MR-ARFI image was 31 s by the line-scan sequence and 1 s by the echo-planar sequence. An in vivo experiment was performed using FUS transducer, which is built out of 1024 ultrasound transmitting piezoelectric elements at 220 kHz frequency. The transducer was focused into the brain of a pig, which was wrapped in a human skull, in degassed water environment to resemble human treatments. The pig underwent a wide bilateral craniectomy to prevent a bone heating from the ultrasound beams. Two focusing experiments were performed in phantoms using 1 MHz and 710 kHz FUS transducers working with 208 and 225 elements, respectively. In the first experiment, aberration was added virtually to the apparatus by adding random phases to the phase map of the transducer. A simple focusing correction scheme was used, in which the corrected phase of a group of elements was chosen such that it maximizes the radiation force at the focal point. In the second experiment, aberrations made by a human skull were corrected using geometrical and phase based adjustments on segments of the transducer.
RESULTS: A maximum displacement of 10 microm was obtained using 1.4 kW acoustic power on a live pig's head that its skull was removed and replaced by ex vivo human skull. Aberration correction using MR-ARFI resulted in near optimal focus, as the radiation force was similar to the nonaberration case. Transcranial, MR-ARFI based aberration correction performed better than CT based aberration correction, a technique that is currently used in brain FUS treatments.
CONCLUSIONS: In the present work, the authors show for the first time a result of MR-ARFI in a live brain through ex vivo human skull. They have demonstrated that aberration correction could be done using MR-ARFI by measuring the radiation force at the focal point. Aberration correction using MR-ARFI is a promising noninvasive technique for transcranial focusing, which may result in near optimal focus and more reliable and safer brain FUS treatments.