title: MR-acoustic radiation force imaging (MR-ARFI) and susceptibility weighted imaging (SWI) to visualize calcifications in ex vivo swine brain.
abstract: OBJECTIVE
  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).
  
  
  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 × 10 × 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 × 1.28 mm on CT, 1.24 × 1.73 mm on SWI, and 2.45 × 3.02 mm on MR-ARFI. MR-ARFI displacement showed a linear increase with acoustic power (20-80 W), and also increased with calcification size.
  
  
  CONCLUSIONS
  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.
authors.affiliation: Department of Radiology, School of Medicine, Stanford University, Stanford, California, USA.
authors.forename: Rachel R
authors.initials: RR
authors.lastname: Bitton
authors.forename: Kim R Butts
authors.initials: KR
authors.lastname: Pauly
mesh_terms: D000818:Animals; D001927:Brain Diseases / Q000473:pathology*; D002114:Calcinosis / Q000473:pathology*; D038524:Diffusion Magnetic Resonance Imaging / Q000379:methods*; D054459:Elasticity Imaging Techniques / Q000379:methods*; D007090:Image Interpretation, Computer-Assisted / Q000379:methods*; D066298:In Vitro Techniques; D064847:Multimodal Imaging / Q000379:methods*; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D013552:Swine
keywords: MR guided focused ultrasound; MR-ARFI; MR-HIFU; acoustic radiation force; calcification; susceptibility weighted