FISH: Elisabetta Sassaroli: Focusing Ultrasound through the Skull by an Indirect Wavefront Sensing Approach

Jan 19, 2018 - 12:00 PM to 1:00 PM EST

Speaker: 

Elisabetta Sassaroli (Brigham and Women's Hospital)

Dr. Elisabetta Sassaroli, medical physicist and Research Scientist at Brigham and Women's Hospital, presents "Focusing Ultrasound through the Skull by an Indirect Wavefront Sensing Approach."

"Focusing ultrasound through the skull is a challenging problem. As the ultrasound propagates through the skull, it encounters at first a large impedance mismatch at the skull-water interface, and then owing to the skull heterogeneous structure and composition, its propagation is affected by changes in speed of sound and density, variations in skull thickness, and at the exit it encounters the skull-brain tissue interface, which presents irregularities. This gives rise to ultrasound beam reflection, refraction, scattering, mode conversion, and energy absorption. As result, the incident ultrasound wavefront emerges from the skull highly aberrated (distorted) and the beam is highly attenuated especially at frequencies above about 1 MHz. Recent advancements in ultrasound technology combined with the development of MRI compatible phased arrays and MR thermometry, have made possible focusing ultrasound through the skull. This technology is named transcranial MRI-guided focused ultrasound (tcMRIgFUS). TcMRIgFUS is currently under clinical evaluation for safety and efficacy in functional neurosurgery, brain tumor ablation and targeted drug delivery and has been recently approved by the FDA for the treatment of essential tremors. TcMRgFUS treatments require CT images of the patient’s skull in order to focus ultrasound correctly at the targeted location.

The present status of the technology and how it is implemented clinically will be briefly discussed. Then, some of the techniques which are currently under development for improving it, will be introduced. These techniques are inspired by indirect wavefront sensing techniques developed in adaptive optics and rely entirely on MRI feedback for focusing ultrasound through the skull. More specifically, acoustic radiation force impulse techniques are employed (MR-ARFI). In MR-ARFI, the radiation force induced at the focus by ultrasound pulses of suitable intensity and duration is employed to create local tissue displacement which is primarily along the direction of ultrasound propagation. MRI can be used to image this displacement. Since tissue displacement is related to the focal acoustic intensity, MR-ARFI imaging provides a tool for measuring/imaging the ultrasound beam intensity in the image plane. When attenuation and aberrations are present, this intensity is reduced and the pattern of the focal intensity distribution is distorted, thus providing a way to indirectly image and quantify the effect of attenuation/aberrations."