title: Registration accuracy for MR images of the prostate using a subvolume based registration protocol.
abstract: BACKGROUND
  In recent years, there has been a considerable research effort concerning the integration of magnetic resonance imaging (MRI) into the external radiotherapy workflow motivated by the superior soft tissue contrast as compared to computed tomography. Image registration is a necessary step in many applications, e.g. in patient positioning and therapy response assessment with repeated imaging. In this study, we investigate the dependence between the registration accuracy and the size of the registration volume for a subvolume based rigid registration protocol for MR images of the prostate.
  
  
  METHODS
  Ten patients were imaged four times each over the course of radiotherapy treatment using a T2 weighted sequence. The images were registered to each other using a mean square distance metric and a step gradient optimizer for registration volumes of different sizes. The precision of the registrations was evaluated using the center of mass distance between the manually defined prostates in the registered images. The optimal size of the registration volume was determined by minimizing the standard deviation of these distances.
  
  
  RESULTS
  We found that prostate position was most uncertain in the anterior-posterior (AP) direction using traditional full volume registration. The improvement in standard deviation of the mean center of mass distance between the prostate volumes using a registration volume optimized to the prostate was 3.9 mm (p < 0.001) in the AP direction. The optimum registration volume size was 0 mm margin added to the prostate gland as outlined in the first image series.
  
  
  CONCLUSIONS
  Repeated MR imaging of the prostate for therapy set-up or therapy assessment will both require high precision tissue registration. With a subvolume based registration the prostate registration uncertainty can be reduced down to the order of 1 mm (1 SD) compared to several millimeters for registration based on the whole pelvis.
authors.affiliation: Radiation Physics, Department of Radiation Sciences, Umeå University, 90187 Umeå, Sweden. joakim.jonsson@radfys.umu.se
authors.forename: Joakim H
authors.initials: JH
authors.lastname: Jonsson
authors.forename: Patrik
authors.initials: P
authors.lastname: Brynolfsson
authors.forename: Anders
authors.initials: A
authors.lastname: Garpebring
authors.forename: Mikael
authors.initials: M
authors.lastname: Karlsson
authors.forename: Karin
authors.initials: K
authors.lastname: Söderström
authors.forename: Tufve
authors.initials: T
authors.lastname: Nyholm
mesh_terms: D000368:Aged; D001331:Automation; D003952:Diagnostic Imaging / Q000379:methods; D038524:Diffusion Magnetic Resonance Imaging / Q000379:methods; D006801:Humans; D007091:Image Processing, Computer-Assisted / Q000379:methods*; D008279:Magnetic Resonance Imaging / Q000379:methods*; D008297:Male; D008875:Middle Aged; D011467:Prostate / Q000473:pathology*; D011471:Prostatic Neoplasms / Q000175:diagnosis* / Q000473:pathology*; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods; D015203:Reproducibility of Results