Bypassing absorbing objects in focused ultrasound using computer generated holographic technique

Purpose: Focused ultrasound (FUS) technology is based on heating a small volume of tissue, while keeping the temperature outside the focus region with minimal heating only. Several FUS applications, such as brain and liver, suffer from the existence of ultrasound absorbers in the acoustic path between the transducer and the focus. These absorbers are a potential risk for the FUS therapy since they might cause to unwanted heating outside the focus region. An acoustic simulation based solution for reducing absorbers' heating is proposed, demonstrated, and compared to the standard geometrical solution. The proposed solution uses 3D continuous acoustic holograms, generated by the Gerchberg-Saxton (GS) algorithm, which are described and demonstrated for the first time using ultrasound planar phased-array transducer. Methods: Holograms were generated using the iterative GS algorithm and fast Fourier transform (FFT) acoustic simulation. The performances of the holograms are demonstrated by temperature elevation images of the absorber, acquired by GE 1.5T MRI scanner equipped with InSightec FUS planar phased-array transducer built out of 986 transmitting elements. Results: The acoustic holographic technology is demonstrated numerically and experimentally using the three letters patterns, "T," "A," and "U," which were manually built into 1 x 1 cm masks to represent the requested target fields. 3D holograms of a focused ultrasound field with a hole in intensity at the absorber region were generated and compared to the standard geometrical solution. The proposed holographic solution results in 76% reduction of heating on absorber, while keeping similar heating at the focus. Conclusions: In the present work we show for the first time the generation of efficient and uniform continuous ultrasound holograms in 3D. We use the holographic technology to generate a FUS beams that bypasses an absorber in the acoustic path to reduce unnecessary heating and potential clinical risk. The developed technique is superior in performance and flexibility compared to the intuitive geometrical technique that is being used in clinical practice. (C) 2011 American Association of Physicists in Medicine. [DOI: 10.1118/1.3651464]