  Transcranial focused ultrasound (FUS) is a non-invasive technique for therapy
and study of brain neural activation. Here we report on the design and
characterization of a new MR-guided FUS transducer for neuromodulation in
non-human primates at 650kHz. The array is randomized with 128 elements, radius
of curvature 7.2cm, and opening diameter 10.3cm (focal ratio 0.7). Simulations
were used to optimize transducer geometry with respect to focus size, grating
lobes, and directivity. Focus size and grating lobes during electronic steering
were quantified using hydrophone measurements in water and a three-axis stage.
A novel combination of optical tracking and acoustic mapping enabled
measurement of the 3D pressure distribution in the cortical region of an ex
vivo skull to within ~3.5 mm of the surface, and allowed accurate modelling of
the experiment via non-homogeneous 3D acoustic simulations. The data
demonstrates acoustic focusing beyond the skull bone, with the focus slightly
broadened and shifted proximal to the skull. The fabricated design is capable
of targeting regions within the S1 sensorimotor cortex of macaques.
