MR-acoustic radiation force imaging (MR-ARFI) and susceptibility weighted imaging (SWI) to visualize calcifications in ex vivo swine brain

Purpose To present the use of MR-acoustic radiation force imaging (MR-ARFI) and susceptibility weighted imaging (SWI) to visualize calcifications in ex vivo brain tissue as a planning indicator for MR-guided focused ultrasound (MRgFUS). Materials and Methods Calcifications were implanted in ex vivo swine brain and imaged using SWI, MR-ARFI, and computed tomography (CT). SWI-filtered phase images used 3D gradient recalled echo (GRE) images with a Fourier-based unwrapping algorithm. The MR-ARFI pulse sequence used a 2DFT spin-echo with repeated bipolar encoding gradients in the direction of the longitudinal ultrasound beam. MR-ARFI interrogations scanned a subregion (14 x 10 x 12 mm) of the brain surrounding the calcification. They were combined into a single displacement weighted map, using the sum of squares method. Calcification size estimates were based on image profiles plotted along the +/- x and +/- z direction, at the full-width half-maximum. Results Both MR-ARFI and SWI were able to visualize the calcifications. The contrast ratio was 150 for CT, 12 for SWI, and 12 for MR-ARFI. Profile measures were 1.35 x 1.28 mm on CT, 1.24 x 1.73 mm on SWI, and 2.45 x 3.02 mm on MR-ARFI. MR-ARFI displacement showed a linear increase with acoustic power (20-80W), and also increased with calcification size. Conclusion The use of SWI-filtered phase and MR-ARFI have the potential to provide a clinical indicator of calcification relevance in the planning of a transcranial MRgFUS treatment. J. Magn. Reson. Imaging 2014;39:1294-1300. (c) 2013 Wiley Periodicals, Inc.