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 b een optimized for in vivo localization of a focused ultrasound beam. T emperature elevation was measured by using the proton resonance freque ncy shift technique, and the phase difference signal-to-noise ratio (S NR Delta phi) was estimated in skeletal muscle and kidney cortex in 10 rabbits, Optimized parameters included the echo time equivalent to T- 2 of the tissue, the longest repetition time possible with a 20-s son ication, and the flip angle equivalent to the Ernst angle. Optimal SPG R phase imaging can detect a sonication beam with a peak phase differe nce 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 mm(3)) at power levels that are below the threshold for th ermal damage of the tissue.