title: Treatment envelope evaluation in transcranial magnetic resonance-guided focused ultrasound utilizing 3D MR thermometry.
abstract: BACKGROUND
  Current clinical targets for transcranial magnetic resonance-guided focused ultrasound (tcMRgFUS) are all located close to the geometric center of the skull convexity, which minimizes challenges related to focusing the ultrasound through the skull bone. Non-central targets will have to be reached to treat a wider variety of neurological disorders and solid tumors. Treatment envelope studies utilizing two-dimensional (2D) magnetic resonance (MR) thermometry have previously been performed to determine the regions in which therapeutic levels of FUS can currently be delivered. Since 2D MR thermometry was used, very limited information about unintended heating in near-field tissue/bone interfaces could be deduced.
  
  
  METHODS
  In this paper, we present a proof-of-concept treatment envelope study with three-dimensional (3D) MR thermometry monitoring of FUS heatings performed in a phantom and a lamb model. While the moderate-sized transducer used was not designed for transcranial geometries, the 3D temperature maps enable monitoring of the entire sonication field of view, including both the focal spot and near-field tissue/bone interfaces, for full characterization of all heating that may occur. 3D MR thermometry is achieved by a combination of k-space subsampling and a previously described temporally constrained reconstruction method.
  
  
  RESULTS
  We present two different types of treatment envelopes. The first is based only on the focal spot heating-the type that can be derived from 2D MR thermometry. The second type is based on the relative near-field heating and is calculated as the ratio between the focal spot heating and the near-field heating. This utilizes the full 3D MR thermometry data achieved in this study.
  
  
  CONCLUSIONS
  It is shown that 3D MR thermometry can be used to improve the safety assessment in treatment envelope evaluations. Using a non-optimal transducer, it is shown that some regions where therapeutic levels of FUS can be delivered, as suggested by the first type of envelope, are not necessarily safely treated due to the amount of unintended near-field heating occurring. The results presented in this study highlight the need for 3D MR thermometry in tcMRgFUS.
authors.affiliation: Utah Center for Advanced Imaging Research, Department of Radiology, University of Utah, Salt Lake City, Utah 84108, USA ; Department of Physics and Astronomy, University of Utah, Salt Lake City, Utah 84112, USA.
authors.forename: Henrik
authors.initials: H
authors.lastname: Odéen
authors.affiliation: School of Computing, University of Utah, Salt Lake City, Utah 84112, USA.
authors.forename: Joshua
authors.initials: J
authors.lastname: de Bever
authors.affiliation: School of Computing, University of Utah, Salt Lake City, Utah 84112, USA.
authors.forename: Scott
authors.initials: S
authors.lastname: Almquist
authors.affiliation: Department of Bioengineering, University of Utah, Salt Lake City, Utah 84112, USA.
authors.forename: Alexis
authors.initials: A
authors.lastname: Farrer
authors.affiliation: Utah Center for Advanced Imaging Research, Department of Radiology, University of Utah, Salt Lake City, Utah 84108, USA.
authors.forename: Nick
authors.initials: N
authors.lastname: Todd
authors.affiliation: Utah Center for Advanced Imaging Research, Department of Radiology, University of Utah, Salt Lake City, Utah 84108, USA.
authors.forename: Allison
authors.initials: A
authors.lastname: Payne
authors.affiliation: Focused Ultrasound Foundation, Charlottesville, Virginia 22903, USA ; Department of Neurological Surgery, University of Virginia, Charlottesville, Virginia 22908, USA.
authors.forename: John W
authors.initials: JW
authors.lastname: Snell
authors.affiliation: Department of Bioengineering, University of Utah, Salt Lake City, Utah 84112, USA ; Department of Electrical and Computer Engineering, University of Utah, Salt Lake City, Utah 84112, USA.
authors.forename: Douglas A
authors.initials: DA
authors.lastname: Christensen
authors.affiliation: Utah Center for Advanced Imaging Research, Department of Radiology, University of Utah, Salt Lake City, Utah 84108, USA.
authors.forename: Dennis L
authors.initials: DL
authors.lastname: Parker
keywords: Brain; MR thermometry; PRF; Treatment envelope; tcMRgFUS