Rapid MR-ARFI method for focal spot localization during focused ultrasound treatments

In MRgFUS, visualization of the focal spot has relied on "test spots" of a small temperature rise. MR acoustic radiation force imaging (MR-ARFI) could be used instead for visualization, without the need to a temperature rise. The goal of this study was to develop a method for rapid focal spot localization method, based on MR-ARFI. In addition, a temperature sensitive alteration of the sequence was proposed to measure any potential temperature rise during MR-ARFI sonications. An EPI pulse sequence was modified to incorporate displacement encoding repeated bipolar gradients. The pulse sequence triggered the FUS system to emit 23 ms ultrasound pulses during the encoding gradients. This imaging method was tested in a gel phantom and porcine tissue on a 3T GE MRI scanner equipped with an InSightec FUS transducer. The results of the study showed that the focal spot can be well depicted in the displacement maps obtained with the new rapid MR-ARFI method. The temperature measurements made during ARFI sonications indicated no temperature increase when optimal sonication duty cycle and power were used.