Project Summary/Abstract Transcranial MR-guided focused ultrasound (tcMRgFUS) neurosurgery is a non-invasive treatment for essential tremor, neuropathic pain, and many emerging applications. tcMRgFUS systems have a water bath that couples acoustic energy into the head and is circulated to cool the scalp between sonications. The water bath, however, also acts as a high permittivity dielectric with a much shorter RF wavelength than free space. At the same time, the transducer bowl is electrically conductive, so when the electromagnetic waves generated by the MRI scanner for imaging enter the bath, they travel through the bath in the transducer bowl and reflect off the bowl’s inner surface above the head. The reflected waves then cancel with the incoming waves which causes a dark band with very low MR signal in the images, and generally low signal throughout the brain. The low signal limits the types of scans that can be used to guide and assess tcMRgFUS treatment, especially spin echo scans such as diffusion which could enable earlier lesion and tractography-based assessment. To address this problem, we propose to develop a novel passive reflecting antenna that will sit in the water bath above the head, and will create its own wave reflection that can be controlled so as to add constructively with the incoming wave, thereby restoring the MR signal to levels commensurate with imaging outside the transducer. The wires will require no electrical connections and can be made very thin to avoid disturbing the ultrasound beam. We also propose to develop a complementary ring of passive loop coils that would sit outside the bottom of the transducer and further increase the fields in the brain. We will use electromagnetic simulations of a tcMRgFUS system in an MRI scanner to optimize the antennas and loop coils, design mechanical holders for them that do not disturb the ultrasound beams and do not require modifying the transducer, and then evaluate them in MR imaging of healthy volunteers in a clinical tcMRgFUS system.