title: Improving thermal dose accuracy in magnetic resonance-guided focused ultrasound surgery: Long-term thermometry using a prior baseline as a reference.
abstract: OBJECTIVE
  To investigate thermal dose volume (TDV) and non-perfused volume (NPV) of magnetic resonance-guided focused ultrasound (MRgFUS) treatments in patients with soft tissue tumors, and describe a method for MR thermal dosimetry using a baseline reference.
  
  
  METHODS
  Agreement between TDV and immediate post treatment NPV was evaluated from MRgFUS treatments of five patients with biopsy-proven desmoid tumors. Thermometry data (gradient echo, 3T) were analyzed over the entire course of the treatments to discern temperature errors in the standard approach. The technique searches previously acquired baseline images for a match using 2D normalized cross-correlation and a weighted mean of phase difference images. Thermal dose maps and TDVs were recalculated using the matched baseline and compared to NPV.
  
  
  RESULTS
  TDV and NPV showed between 47%-91% disagreement, using the standard immediate baseline method for calculating TDV. Long-term thermometry showed a nonlinear local temperature accrual, where peak additional temperature varied between 4-13°C (mean = 7.8°C) across patients. The prior baseline method could be implemented by finding a previously acquired matching baseline 61% ± 8% (mean ± SD) of the time. We found 7%-42% of the disagreement between TDV and NPV was due to errors in thermometry caused by heat accrual. For all patients, the prior baseline method increased the estimated treatment volume and reduced the discrepancies between TDV and NPV (P = 0.023).
  
  
  CONCLUSIONS
  This study presents a mismatch between in-treatment and post treatment efficacy measures. The prior baseline approach accounts for local heating and improves the accuracy of thermal dose-predicted volume.
authors.affiliation: School of Medicine, Department of Radiology, Stanford University, Stanford, California, USA.
authors.forename: Rachel R
authors.initials: RR
authors.lastname: Bitton
authors.affiliation: Department of Electrical Engineering, Stanford University, Stanford, California, USA.
authors.forename: Taylor D
authors.initials: TD
authors.lastname: Webb
authors.affiliation: School of Medicine, Department of Radiology, Stanford University, Stanford, California, USA.
authors.forename: Kim Butts
authors.initials: KB
authors.lastname: Pauly
authors.affiliation: School of Medicine, Department of Radiology, Stanford University, Stanford, California, USA.
authors.forename: Pejman
authors.initials: P
authors.lastname: Ghanouni
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D005260:Female; D018222:Desmoid Tumors / Q000175:diagnosis* / Q000503:physiopathology / Q000601:surgery*; D019220:High-Energy Shock Waves / Q000627:therapeutic use; D057086:High-Intensity Focused Ultrasound Ablation / Q000379:methods*; D006358:Hot Temperature; D006801:Humans; D007089:Image Enhancement / Q000379:methods; D008279:Magnetic Resonance Imaging / Q000379:methods*; D008297:Male; D008875:Middle Aged; D011829:Radiation Dosage; D011874:Radiometry / Q000379:methods; D012016:Reference Values; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D025321:Surgery, Computer-Assisted / Q000379:methods*; D013817:Thermography / Q000379:methods*; D016896:Treatment Outcome; D055815:Young Adult
keywords: MR guided focused ultrasound; MR-HIFU; ablation; nonperfused volume; temperature mapping; thermal dose; thermometry