Optimization of spoiled gradient‐echo phase imaging for in vivo localization of a focused ultrasound beam

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Abstract
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 (SNRδ ϕ) was estimated in skeletal muscle and kidney cortex in 10 rabbits. Optimized parameters included the echo time equivalent to T*2of 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°C. The sonication beam can be localized within one voxel (0.6 × 0.6 × 5 mm3) at power levels that are below the threshold for thermal damage of the tissue.