Brain shift during neurosurgery results in misregistration between the presurgical MRI or CT scan and the operating field and leads to inaccurate surgical margins. If the degree of brain shift is well known, the presurgical imaging can be adjusted to correspond with the current surgical field. Ultrasound may offer a portable, noninvasive, non-ionizing, rapid, real-time method of tracking brain shift during surgery or in emergency settings. The FUS lab has been testing techniques of 3D motion tracking using an array of low frequency ultrasound transducers positioned around ex vivo human skulls. Current results show that four 500KHz transducers placed exterior to acoustic windows in the skull can accurately track sub-millimeter 3D motion of objects within the skull.