High sensitivity MR acoustic radiation force imaging using transition band balanced steady‐state free precession

Purpose MR acoustic radiation force imaging (MR‐ARFI) provides a method to visualize the focal spot of a focused ultrasound (FUS) beam without introducing a significant temperature rise. With conventional spoiled MR‐ARFI pulse sequences, the ARFI phase always equals the motion‐encoded phase. In this work, MR‐ARFI using transition band balanced steady‐state free precession (bSSFP) is presented, which improves the sensitivity of MR‐ARFI with high acquisition speed. Theory and Methods Motion‐encoding gradients (MEG) are inserted into bSSFP sequences for MR‐ARFI. By applying an ultrasound pulse during the MEG, motion‐encoded phase is generated, which leads to an amplified change in the image phase when operating in the bSSFP transition band. MR‐ARFI was performed on a homemade gel phantom using both the proposed technique and a spoiled gradient echo ARFI sequence with identical MEG and FUS, and ARFI images were compared. Results The bSSFP‐ARFI sequence generated an ARFI image phase that is more than 5 times larger than the motion‐encoded phase in a few seconds with 2DFT readout. By keeping FUS pulses as short as 1.45 ms, temperature rise was insignificant during the measurement. Conclusion bSSFP‐ARFI has enhanced sensitivity compared with conventional MR‐ARFI pulse sequences and could provide an efficient way to visualize the focal spot. Magn Reson Med 79:1532–1537, 2018. © 2017 International Society for Magnetic Resonance in Medicine.