Effect of the Rat Skull On Focused US, as Measured by MR Thermometry

MR-guided therapeutic ultrasound is becoming increasingly prominent among a range of brain treatments from localized tumors and pain alleviation to neurostimulation. To investigate the influence of the skull on the acoustic field, an appropriate animal model must be established and so the effect of the rat skull needs to be considered and possibly compensated for, when using it as a model to perform therapeutic ultrasound in the brain. Temperature monitoring of both in-vivo and ex vivo experiments was performed in a 3T MRI (GE Signs, Milwaukee WS) using the PRF method. Sonications were performed with a 20 PZT array comprised of 1024 elements at 0.55MHz. Intensities were kept low enough to limit temperature rise in the brain to about 10 degrees C. Temperature maps were acquired 2-3 mm from the inner surface of the skull in phantom material, which was placed behind the skull. Three ex-vivo skulls were used. A grid patters covering the entire skull was laid out for sonications. Only 11/48 sonications created a small single spot showing temperature rise in phantom material. In all other sonications the heat was very diffuse with multiple focal spots. This is compared to the phantom with no skull were 2/41 sonications did not create a small single spots of temperature rise, and in those two spots edge effects can be contributed to the unfocused pattern. All other sonications, performed at the same depth of sonication as in the skull, created an individual thermal spot, which was on average within 0.5mm in any direction from the expected location.