MRI-guided focused-ultrasound is a non-invasive technique that can enhance the delivery of therapeutic agents. The
objective of this work was to develop a focused-ultrasound system for preclinical research in small animals that is
capable of sonicating with high spatial precision within a closed-bore MRI. The system features a computer-controlled,
non-magnetic, three-axis positioning system that uses piezoelectric actuators and linear optical encoders to position a
focused-ultrasound transducer to targeted tissues under MRI guidance. The actuator and encoder signals are transmitted
through low-pass-filtered connectors on a grounded RF-penetration panel to prevent artifacts during image acquisition.
The transducer is attached to the positioning system by a rigid arm and is submerged within a closed water tank. The arm
passes into the tank through flexible bellows to ensure that the system remains sealed. An RF coil acquires high-resolution
images in the vicinity of the target tissue. An aperture on the water tank, centered about the RF coil, provides
an access point for target sonication. Registration between ultrasound and MRI coordinates involves sonicating a
temperature-sensitive phantom and measuring the centroid of the thermal focal zone in 3D with MR thermometry. Linear
distances of 5 cm with a positioning resolution of 0.05 mm can be achieved for each axis. The system was operated
successfully on MRI scanners from different vendors at both 1.5 and 3.0 T, and simultaneous motion and imaging was
possible without any mutual interference or imaging artifacts. This system is used for high-throughput small-animal experiments to study the efficacy of ultrasound-enhanced drug delivery.
A system for performing MRI-guided transurethral prostate thermal therapy has been developed. Ultrasound energy is delivered from a multi-element heating applicator incorporating single or dual-frequency planar transducers. The heating applicators produce a directional heating pattern, and have the capability to generate an arbitrary three-dimensional thermal damage pattern in tissue. The delivery system includes five independent channels, each capable of producing up to 50W of RF power. An MRI-compatible motor has also been developed to control the rotation of the heating applicator inside the bore of a clinical 1.5T MR scanner. The capability to perform quantitative thermometry during heating with these heating applicators has been evaluated in a thermal gel material (TGM) developed in our lab with tissue-mimicking ultrasound and thermal properties.