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Determination of the Optimal Delay between Sonications during Focused Ultrasound Surgery in Rabbits by Using MR Imaging to Monitor Thermal Buildup in Vivo¹

¹ From the Department of Radiology, Division of Magnetic Resonance Imaging, Brigham and Women's Hospital, Harvard Medical School, LMRC, 007c, 221 Longwood Ave, Boston, MA 02115 (N.J.M., F.A.J., K.H.H.), and the Department of Physics and Astronomy, Tufts University, Medford, Mass (N.J.M.). Received January 20, 1998; revision requested March 17; final revision received August 7; accepted October 6. Supported by National Cancer Institute grants RO1:CA46627 and PO1:CA67165 and a grant from GE Medical Systems. Address reprint requests to N.J.M.

PURPOSE: To use magnetic resonance (MR) imaging to monitor thermal buildup and its effects in treated tissues during sequentially delivered sonications in vivo to optimize the intersonication delay for any set of ultrasound and tissue parameters.

MATERIALS AND METHODS: Sequential sonications were delivered next to each other in both thighs in 10 male New Zealand white rabbits. The time between sonications was 11–60 seconds. Phase-difference MR imaging was used to monitor temperature rise, which was used to estimate the thermal dose delivered to the tissue. T2-weighted and contrast agent–enhanced T1-weighted imaging were used to gauge the extent of tissue coagulation.

RESULTS: With a short intersonication delay (11–40 seconds), the estimated temperature rise and the extent of tissue coagulation increased dramatically in subsequent sonications. However, when the delay was long (50–60 seconds), the size and shape of the destroyed tissue with subsequent sonications was uniform, and the temperature buildup was substantially lower.

CONCLUSION: MR imaging can be used to monitor thermal buildup and its effects due to sequential, neighboring sonications in vivo to produce evenly shaped regions of tissue coagulation. The temperature information obtained from the monitoring can be used to optimize the intersonication delay for any set of ultrasound and tissue parameters.

Index terms: Magnetic resonance (MR), artifact, 44.93 • Magnetic resonance (MR), phase imaging, 44.121411, 44.121412 • Magnetic resonance (MR), thermometry, 44.1214 • Ultrasound (US), focused, 44.1298 • Ultrasound (US), experimental, 44.1298 • Ultrasound (US), therapeutic, 44.1298

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