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Therapeutic uses of transcranial focused ultrasound (tFUS) in the brain are being widely explored in both clinical and research settings from tissue destruction to blood brain barrier opening (BBBO) to neuromodulation. The appeal of tFUS is the ability to target structures throughout the brain with a millimeter scale focus. In order to understand the outcome of the treatment or experiment it is important to ensure the tFUS beam is targeted to and located at the region of interest. Image guidance allows for targeting of anatomical or functional structures within the brain. Here we present a targeting and localization scheme where optical tracking of the transducer is used to target the tFUS focus to a brain region within a previously acquired image and MR acoustic radiation force imaging is used to localize the beam in the brain to ensure the focus is at the target. By combining this targeting and localization scheme with a phased array transducer we are able to then steer the focus to ensure accurate sonications of specific brain regions using tFUS. This talk will discuss the application of this method to two different phased array transducers used for neuromodulation and BBBO in nonhuman primates.

Magnetic particle imaging (MPI) is a tracer-based imaging technique that does not require radiation or a cyclotron. However, it does require bulky MRI-like hardware and does not produce a background tissue image. We propose a new approach to MPI that uses the acoustic radiation force to dynamically move iron nanoparticles through a magnetic field gradient, thereby changing their magnetic moment and producing a detectable signal in a receiver coil. An image could be formed by electronically steering the ultrasound focus to push particles at different spatial locations, like an ultrasonic palpation. This would enable a device that attaches to an ultrasound imaging transducer for simultaneous MPI and inherently registered ultrasound imaging. To test the feasibility of the technique, a cylindrical agar and graphite phantom was placed between a Maxwell coil pair which generated a linear field gradient across it, and particle signals were detected using a receiver solenoid. 2-ms ultrasound pulses were applied using a 6 MHz transducer to generate displacements along the magnetic field gradient. Signal measurements confirmed a lack of signal when iron nanoparticles were absent from the phantom, but the signal was detected when particles were present, with amplitudes that linearly increased with the gradient coil current.