title: MRI-guided high-intensity focused ultrasound ablation of bone: evaluation of acute findings with MR and CT imaging in a swine model.
abstract: OBJECTIVE
  To evaluate hyperacute (<1 hour) changes on magnetic resonance (MR) and computed tomography (CT) imaging following MR-guided high-intensity focused ultrasound (MRgHIFU) in a swine bone model as a function of sonication number and energy.
  
  
  METHODS
  Experimental procedures received approval from the local Institutional Animal Care and Use Committee. MRgHIFU was used to create distal and proximal ablations in the right femur of eight pigs. Each target was dosed with four or six sonications within similar volumes. The energy dosed to the distal target was higher (419 ± 19 J) than the proximal target (324 ± 17 J). The targeted femur and contralateral control were imaged before and after ablation using MR at 3T. Qualitative changes in signal on T1-weighted, T2-weighted, and T1-weighted postcontrast images were assessed. Ablation dimensions were calculated from postcontrast MRI. The 64-slice CT images were also obtained before and after ablation and qualitative changes were assessed.
  
  
  RESULTS
  MRgHIFU bone ablation size measured on average 8.5 × 21.1 × 16.2 mm (transverse × craniocaudal × anteroposterior). Interestingly, within similar prescribed volumes, increasing the number of sonications from 4 to 6 increased the depth of the intramedullary hypoenhanced zone from 2.9 mm to 6.5 mm (P < 0.001). There was no difference in the appearance of low versus high energy ablations. CT imaging did not show structural abnormalities.
  
  
  CONCLUSIONS
  The number of MRgHIFU focal sonications can be used to increase the depth of treatment within the targeted bone. Unlike CT, T2-weighted and contrast-enhanced MR demonstrated the hyperacute structural changes in the femur and surrounding soft tissue.
authors.affiliation: Department of Radiology and Biomedical Imaging, University of California San Francisco, San Francisco, CA, USA.
authors.forename: Matthew D
authors.initials: MD
authors.lastname: Bucknor
authors.forename: Viola
authors.initials: V
authors.lastname: Rieke
authors.forename: Loi
authors.initials: L
authors.lastname: Do
authors.forename: Sharmila
authors.initials: S
authors.lastname: Majumdar
authors.forename: Thomas M
authors.initials: TM
authors.lastname: Link
authors.forename: Maythem
authors.initials: M
authors.lastname: Saeed
mesh_terms: D000818:Animals; D001842:Bone and Bones / Q000473:pathology / Q000601:surgery*; D005260:Female; D005269:Femur / Q000473:pathology / Q000601:surgery; D057086:High-Intensity Focused Ultrasound Ablation / Q000379:methods*; D008279:Magnetic Resonance Imaging / Q000379:methods*; D053783:Magnetic Resonance Imaging, Interventional / Q000379:methods*; D025321:Surgery, Computer-Assisted / Q000379:methods*; D013552:Swine; D014057:Tomography, X-Ray Computed / Q000379:methods*
keywords: MR-guided focused ultrasound; MRI; bone ablation; high-intensity focused ultrasound