Object To develop an ultrafast MRI-based temperature monitoring method for application during rapid ultrasound exposures in moving organs.
Materials and methods A slice selective 90 degrees - 180 degrees pair of RF pulses was used to solicit an echo from a column, which was then sampled with a train of gradient echoes. In a gel phantom, phase changes of each echo were compared to standard gradient-echo thermometry, and temperature monitoring was tested during focused ultrasound sonications. Signal-to-noise ratio (SNR) performance was evaluated in vivo in a rabbit brain, and feasibility was tested in a human heart.
Results The correlation between each echo in the acquisition and MRI-based temperature measurements was good (R = 0.98 +/- 0.03). A temperature sampling rate of 19 Hz was achieved at 3T in the gel phantom. It was possible to acquire the water frequency in the beating heart muscle with 5-Hz sampling rate during a breath hold.
Conclusion Ultrafast thermometry via phase or frequency monitoring along single columns was demonstrated. With a temporal resolution around 50 ms, it may be possible to monitor focal heating produced by short ultrasound pulses.