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.