Optimization of spoiled gradient-echo phase imaging for in vivo localization of a focused ultrasound beam.
The parameters of a spoiled gradient-echo (SPGR) pulse sequence have been optimized for in vivo localization of a focused ultrasound beam. Temperature elevation was measured by using the proton resonance frequency shift technique, and the phase difference signal-to-noise ratio (SNR delta phi) was estimated in skeletal muscle and kidney cortex in 10 rabbits. Optimized parameters included the echo time equivalent to T2* of the tissue, the longest repetition time possible with a 20-s sonication, and the flip angle equivalent to the Ernst angle. Optimal SPGR phase imaging can detect a sonication beam with a peak phase difference of 0.55 radian, which corresponds to a temperature elevation of 7.3 degrees C. The sonication beam can be localized within one voxel (0.6 x 0.6 x 5 mm3) at power levels that are below the threshold for thermal damage of the tissue.
Harvard-Massachusetts Institute of Technology, Division of Health Sciences and Technology, Cambridge, USA.
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Chung
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Hynynen
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Colucci
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Oshio
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Cline
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Jolesz
D000818:Animals; D002138:Calibration; D007672:Kidney Cortex / Q000502:physiology; D008279:Magnetic Resonance Imaging / Q000379:methods*; D008962:Models, Theoretical; D018482:Muscle, Skeletal / Q000502:physiology; D011817:Rabbits; D013696:Temperature; D013821:Thermometers; D014465:Ultrasonics