title: A method for MRI guidance of intercostal high intensity focused ultrasound ablation in the liver.
abstract: OBJECTIVE
  High intensity focused ultrasound (HIFU) is a promising method for the noninvasive treatment of liver tumors. However, the presence of ribs in the HIFU beam path remains problematic since it may lead to adverse effects (skin burns) by absorption and reflection of the incident beam at or near the bone surface. This article presents a method based on magnetic resonance (MR) imaging for identification of the ribs in the HIFU beam, and for selection of the transducer elements to deactivate.
  
  
  METHODS
  The ribs are visualized on anatomical images acquired prior to heating and manually segmented. The resulting regions of interest surrounding the ribs are projected onto the transducer surface by ray tracing from the focal point. The transducer elements in the "shadow" of the ribs are then deactivated. The method was validated ex vivo and in vivo in pig liver during breathing under multislice real-time MR thermometry, using the proton resonance frequency shift method.
  
  
  RESULTS
  Ex vivo and in vivo temperature data showed that the temperature increase near the ribs was substantial when HIFU sonications were performed with all elements active, whereas the temperature was reduced with deactivation of the transducer elements located in front of the ribs. The temperature at the focal point was similar with and without deactivation of the transducer elements, indicative of no loss of heat efficiency with the proposed technique.
  
  
  CONCLUSIONS
  This method is simple, rapid, and reliable, and enables intercostal HIFU ablation while sparing ribs and their surrounding tissues.
authors.affiliation: Laboratory for Molecular and Functional Imaging, UMR5231, CNRS, University Victor Segalen Bordeaux 2, 33076 Bordeaux, France. b.quesson@imf.u-bordeaux2.fr
authors.forename: Bruno
authors.initials: B
authors.lastname: Quesson
authors.forename: Mathilde
authors.initials: M
authors.lastname: Merle
authors.forename: Max O
authors.initials: MO
authors.lastname: Köhler
authors.forename: Charles
authors.initials: C
authors.lastname: Mougenot
authors.forename: Sebastien
authors.initials: S
authors.lastname: Roujol
authors.forename: Baudouin Denis
authors.initials: BD
authors.lastname: de Senneville
authors.forename: Chrit T
authors.initials: CT
authors.lastname: Moonen
mesh_terms: D000818:Animals; D006498:Hepatectomy / Q000379:methods*; D057086:High-Intensity Focused Ultrasound Ablation / Q000379:methods*; D007089:Image Enhancement / Q000379:methods; D007090:Image Interpretation, Computer-Assisted / Q000379:methods; D008099:Liver / Q000473:pathology* / Q000601:surgery*; D008279:Magnetic Resonance Imaging / Q000379:methods*; D015203:Reproducibility of Results; D012272:Ribs / Q000473:pathology / Q000601:surgery; D012680:Sensitivity and Specificity; D025321:Surgery, Computer-Assisted / Q000379:methods*; D013552:Swine; D013817:Thermography / Q000379:methods*