Magnetic resonance acoustic radiation force (impulse) imaging (MR-ARFI)

Background/introduction MR-ARFI adds to the rich toolbox of MR imaging methods for guiding focused ultrasound. MRI has exquisite soft tissue contrast for targeting focused ultrasound and a variety of contrast mechanisms for assessing the effect of focused ultrasound. In addition, MR can provide temperature mapping in near-realtime. MR-ARFI is an imaging method that compliments these other capabilities: MRARFI can provide a very low temperature rise method to locate the focal spot, calibrate the beam intensity, and potentially evaluate and improve focusing. MR-ARFI typically uses a relatively long ultrasound pulse (1-20 ms) during the application of a magnetic field gradient to encode the displacement of tissue into the phase of the MR image. MR-ARFI is related to elastography, shear-wave imaging, and harmonic motion imaging methods, which evaluate tissue stiffness by evaluation of the ultrasound shear wave that propagates after the ultrasound pulse. However, MR-ARFI is instead visualizing the quasistatic displacement of tissue during the ultrasound pulse, rather than the shear wave after the pulse.