Investigation of standing wave formation in a human calvarium using a large scale, transcranial MR guided focused ultrasound phased-array

A low frequency (<; 0.5 MHz) ultrasound beam is less sensitive to the irregularity of skull density and thickness, and thus experiences less distortion and diffraction by the skull than higher frequency (0.5 to 1 MHz) ultrasound beam. In spite of less distortion and diffraction through the skull, low frequency ultrasound could cause standing wave formation due to reverberations in the closed skull cavity. In this work we measured standing wave in a human calvarium using a large scale, low frequency, MR Guided Focused Ultrasound Surgery (MRgFUS) system. Our measurement demonstrated that standing wave formation in a skull can be significantly minimized with a low f-number phased array.