5435
A fully electronically steered phased array with 4096 elements for ultrasound treatments under MRI guidanceYuexi Huang1, Ben Lucht1, Rohan Ramdoyal1, Samuel Guneseelan1, Tyler Portelli1, Ping Wu1, and Kullervo Hynynen1,2,3
1Physical Sciences Platform, Sunnybrook Research Institute, Toronto, ON, Canada, 2Department of Medical Biophysics, University of Toronto, Toronto, ON, Canada, 3Institute of Biomaterials and Biomedical Engineering, University of Toronto, Toronto, ON, Canada
Synopsis
A flat focused ultrasound array of 13 cm in diameter with 4096 elements was manufactured in house with center-to-center element spacing of half-wavelength at the centre frequency of approximately 500 kHz. The design allows for a much wider steering range. Feasibility for thermal ablation and hyperthermia over large target volumes was demonstrated in animal studies.
Introduction
MR-guided focused ultrasound (FUS) has been demonstrated in various applications1-3 for non-invasive thermal ablations on an out-patient basis without the risk of infection. The current clinical devices for body applications use spherically curved phased arrays, which improve focus quality with a limited number of transducer elements (~200), at the expense of a limited steering range, therefore mainly rely on mechanical positioning for covering a large treatment volume. In this study, we developed a fully populated flat array, which allows for a much wider steering range. Feasibility using this new design4 for thermal ablation and hyperthermia over large target volumes was demonstrated in animal studies.Methods
A flat array of 14 cm in diameter with 4096 elements was manufactured in house with center-to-center element spacing of half-wavelength at the centre frequency of approximately 500 kHz. Custom driving electronics were built using Application-Specific Integrated Circuit (ASIC) technology. The MR-compatible array and driving system were mounted on the standard bed of an MR scanner (MR750, GE Healthcare, Milwaukee, WI, USA). In vivo studies were performed on 13 pigs on thigh muscles. Acoustic power up to 200W over 50s were applied with MR thermometry monitoring (FSPGR, TR 39 ms, TE 20 ms). Focus was either stationary during sonications or steered in circular pattern laterally or along the acoustic beam. Lesions were measured by T2 imaging post treatment (FRFSE, TR 5000ms, TE 100ms). For hyperthermia applications, multi-foci sonication pattern was applied to achieve heating over a large volume (30 cm3) for 15 min at 43 oC.Results
The system focused energy well by using geometric beam-steering phase delays for each of the array elements. New phasing parameters could be loaded while sonicating such that focus could be shifted every 10 ms. The array was able to provide sustained acoustic power with repeated treatments. Good volumes of thermal coagulation were achieved with a wide steering range. Fig.1 shows a steering distance over 12cm between focal lesions. For hyperthermia, heating over a large volume was observed by MR thermometry.Discussion
In this study we demonstrate for the first time that fully electronically steered arrays are feasible. With a flat design and transducer element spacing at half-wavelength, wide steering was achieved without sacrificing focus quality. A wide steering range eliminates the need for a motorized positioning system, which simplifies system design and allows precise MR thermometry without motion induced artifacts. A large number of elements also allow the design of more complicated focal patterns for large volumes, which potentially will increase treatment efficiency and shorten the time of the treatment procedure.Acknowledgements
No acknowledgement found.References
1. Tempany CM et al. Radiology 2003;226(3):897-905.
2. Napoli A et al. Radiology 2013;267(2):514-21.
3. Elias WJ et al. N Engl J Med 2016;375(8):730-9.
4. Ellens NP. Phys Med Biol 2015;60(6):2195-215.
Figures
Fig.1 T2 image shows a
steering range over 12 cm between focal lesions.