title: The targeting accuracy of a preclinical MRI-guided focused ultrasound system.
abstract: OBJECTIVE
  Assess the accuracy, precision, and sources of error using a preclinical MR-guided focused ultrasound system.
  
  
  METHODS
  A preclinical focused ultrasound system, described previously [Chopra et al., Med. Phys. 36(5), 1867-1874 (2009)], was tested on a benchtop and with 3T GE, 3T Philips, and 7 T Bruker MR scanners for spatial targeting accuracy and precision. Randomly distributed water-filled holes drilled into a polystyrene plate were imaged using MRI and targeted using treatment planning software. The ultrasound focus of a 72 mm, f-number 0.8, 1.16 MHz transducer was aimed at the target locations, and 1-2 s continuous-wave sonications were performed on clear polystyrene plates to create localized spots of melted plastic. The distance between target and observed locations was measured and analyzed. Retrospective analysis of targeting accuracy was performed on preclinical data obtained from other experiments at their institution using the same system.
  
  
  RESULTS
  The results suggest that the sources of targeting error under MR guidance can be roughly separated into three components--normally distributed random error; constant shift from inaccuracy in detection of the initial ultrasound focus; and angular misalignment between MR and focused ultrasound (FUS) coordinates. The lower bound on the targeting error was estimated to be 0.25 ± 0.13 mm, while the maximum observed targeting error did not exceed 2 mm. Measures required to reduce errors and improve targeting were developed to reduce the registration and misalignment errors such that maximum error was reduced to 0.36 ± 0.14 mm. Retrospective in vivo analysis indicated that the error was 1.02 ± 0.43 mm, including error extrinsic to the system.
  
  
  CONCLUSIONS
  The FUS system, as described, is capable of precise and accurate sonications. The largest source of error--misregistration of the coordinate systems of the scanner and ultrasound system--was addressed which reduced the error to 0.36 ± 0.14 mm, sufficient for many preclinical applications.
authors.affiliation: Physical Sciences Platform, Sunnybrook Research Institute, Toronto, Ontario M4N 3M5, Canada and Department of Medical Biophysics, University of Toronto, Toronto, Ontario M5G 1L7, Canada.
authors.forename: N P K
authors.initials: NP
authors.lastname: Ellens
authors.affiliation: Physical Sciences Platform, Sunnybrook Research Institute, Toronto, Ontario M4N 3M5, Canada.
authors.forename: I
authors.initials: I
authors.lastname: Kobelevskiy
authors.affiliation: Physical Sciences Platform, Sunnybrook Research Institute, Toronto, Ontario M4N 3M5, Canada.
authors.forename: A
authors.initials: A
authors.lastname: Chau
authors.affiliation: Physical Sciences Platform, Sunnybrook Research Institute, Toronto, Ontario M4N 3M5, Canada and Department of Medical Biophysics, University of Toronto, Toronto, Ontario M5G 1L7, Canada.
authors.forename: A C
authors.initials: AC
authors.lastname: Waspe
authors.affiliation: Physical Sciences Platform, Sunnybrook Research Institute, Toronto, Ontario M4N 3M5, Canada and Department of Medical Biophysics, University of Toronto, Toronto, Ontario M5G 1L7, Canada.
authors.forename: R M
authors.initials: RM
authors.lastname: Staruch
authors.affiliation: Physical Sciences Platform, Sunnybrook Research Institute, Toronto, Ontario M4N 3M5, Canada and Department of Medical Biophysics, University of Toronto, Toronto, Ontario M5G 1L7, Canada.
authors.forename: R
authors.initials: R
authors.lastname: Chopra
authors.affiliation: Physical Sciences Platform, Sunnybrook Research Institute, Toronto, Ontario M4N 3M5, Canada; Department of Medical Biophysics, University of Toronto, Toronto, Ontario M5G 1L7, Canada; and Institute of Biomaterials and Biomedical Engineering, University of Toronto, Toronto, Ontario M5S 3G9, Canada.
authors.forename: K
authors.initials: K
authors.lastname: Hynynen
mesh_terms: D000818:Animals; D002192:Canrenone; D004867:Equipment Design; D008279:Magnetic Resonance Imaging / Q000295:instrumentation / Q000379:methods*; D051379:Mice; D011137:Polystyrenes; D051381:Rats; D014463:Ultrasonography / Q000379:methods*; D014584:User-Computer Interface; D014867:Water
chemical_list: D011137:Polystyrenes; D014867:Water; D002192:Canrenone