Bypassing absorbing objects in focused ultrasound using computer generated holographic technique Y. Hertzberg School of Physics & Astronomy, Tel-Aviv University, Tel-Aviv 69978, Israel G. Navon School of Chemistry, Tel-Aviv University, Tel-Aviv 69978, Israel (Received 18 July 2011; revised 3 September 2011; accepted for publication 9 September 2011; published 10 November 2011) 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 applica- tions, 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 solu- tion for reducing absorbers’ heating is proposed, demonstrated, and compared to the standard geo- metrical 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  1 cm masks to represent the requested target fields. 3D holograms of a focused ultrasound field with a hole in in- tensity 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 in- tuitive geometrical technique that is being used in clinical practice. V C 2011 American Association of Physicists in Medicine. [DOI: 10.1118/1.3651464] Key words: focused ultrasound therapy, MRI, ultrasound holograms, ultrasound phased-array, phase aberration I. INTRODUCTION The magnetic resonance guided focused ultrasound (MRgFUS) technology provides noninvasive therapeutic technique for tu- mor ablation with real-time monitoring by MRI. The clinical activity of MRgFUS is rapidly growing for the treatment of the uterine fibroids and clinical trials for tumor ablation in the pros- tate, transcostal treatment of the liver and transcranial treatment of the brain. 1–3 The high-accuracy and the noninvasiveness of this novel technology are expected to change significantly sev- eral clinical procedures. FUS therapy of soft tissues is based on the delivery of ultrasound energy to a small focal region of about 1 mm while keeping a relatively low heating in regions outside the focus. The existence of highly absorbing objects near the focus or along the path of the ultrasound beam could result in signifi- cant heat generation outside the focus with potential clinical risk. Such absorbers could be calcifications, bones, air bub- bles, or any other object with different acoustic properties than the surrounding. Calcifications are common inside the brain near the thalamus 4 but exist also in other portions of the body, such as the prostate. 5 Bypassing brain calcifications has been done by the geometrical approach, i.e., by turning off ultrasound phased-array elements which their ray toward the focus passes through the calcification region. Several studies were published recently about reducing heating on the ribcage for the assessment of FUS thermal ablation of the liver tumor. Geometrical approaches for clos- ing phased-array transducer elements were studied numeri- cally 6 and experimentally. 7 Additional techniques, which take into account the wave nature of the ultrasound, were studied for reducing even further the energy density on the ribcage and for improving the focus quality using diffrac- tion 8 and time reversal 9 approaches. In this paper, we introduce new algorithms for ultrasound phased-array generation of continuous ultrasonic holograms 6407 Med. Phys. 38 (12), December 2011 0094-2405/2011/38(12)/6407/9/$30.00 V C 2011 Am. Assoc. Phys. Med. 6407