title: Validation of an algorithm for the nonrigid registration of longitudinal breast MR images using realistic phantoms.
abstract: OBJECTIVE
  The authors present a method to validate coregistration of breast magnetic resonance images obtained at multiple time points during the course of treatment. In performing sequential registration of breast images, the effects of patient repositioning, as well as possible changes in tumor shape and volume, must be considered. The authors accomplish this by extending the adaptive bases algorithm (ABA) to include a tumor-volume preserving constraint in the cost function. In this study, the authors evaluate this approach using a novel validation method that simulates not only the bulk deformation associated with breast MR images obtained at different time points, but also the reduction in tumor volume typically observed as a response to neoadjuvant chemotherapy.
  
  
  METHODS
  For each of the six patients, high-resolution 3D contrast enhanced T1-weighted images were obtained before treatment, after one cycle of chemotherapy and at the conclusion of chemotherapy. To evaluate the effects of decreasing tumor size during the course of therapy, simulations were run in which the tumor in the original images was contracted by 25%, 50%, 75%, and 95%, respectively. The contracted area was then filled using texture from local healthy appearing tissue. Next, to simulate the post-treatment data, the simulated (i.e., contracted tumor) images were coregistered to the experimentally measured post-treatment images using a surface registration. By comparing the deformations generated by the constrained and unconstrained version of ABA, the authors assessed the accuracy of the registration algorithms. The authors also applied the two algorithms on experimental data to study the tumor volume changes, the value of the constraint, and the smoothness of transformations.
  
  
  RESULTS
  For the six patient data sets, the average voxel shift error (mean +/- standard deviation) for the ABA with constraint was 0.45 +/- 0.37, 0.97 +/- 0.83, 1.43 +/- 0.96, and 1.80 +/- 1.17 mm for the 25%, 50%, 75%, and 95% contraction simulations, respectively. In comparison, the average voxel shift error for the unconstrained ABA was 0.46 +/- 0.29, 1.13 +/- 1.17, 2.40 +/- 2.04, and 3.53 +/- 2.89 mm, respectively. These voxel shift errors translate into compression of the tumor volume: The ABA with constraint returned volumetric errors of 2.70 +/- 4.08%, 7.31 +/- 4.52%, 9.28 +/- 5.55%, and 13.19 +/- 6.73% for the 25%, 50%, 75%, and 95% contraction simulations, respectively, whereas the unconstrained ABA returned volumetric errors of 4.00 +/- 4.46%, 9.93 +/- 4.83%, 19.78 +/- 5.657%, and 29.75 +/- 15.18%. The ABA with constraint yields a smaller mean shift error, as well as a smaller volume error (p = 0.031 25 for the 75% and 95% contractions), than the unconstrained ABA for the simulated sets. Visual and quantitative assessments on experimental data also indicate a good performance of the proposed algorithm.
  
  
  CONCLUSIONS
  The ABA with constraint can successfully register breast MR images acquired at different time points with reasonable error. To the best of the authors' knowledge, this is the first report of an attempt to quantitatively assess in both phantoms and a set of patients the accuracy of a registration algorithm for this purpose.
authors.affiliation: Institute of Imaging Science, Vanderbilt University, Nashville, Tennessee 37232, USA.
authors.forename: Xia
authors.initials: X
authors.lastname: Li
authors.forename: Benoit M
authors.initials: BM
authors.lastname: Dawant
authors.forename: E Brian
authors.initials: EB
authors.lastname: Welch
authors.forename: A Bapsi
authors.initials: AB
authors.lastname: Chakravarthy
authors.forename: Lei
authors.initials: L
authors.lastname: Xu
authors.forename: Ingrid
authors.initials: I
authors.lastname: Mayer
authors.forename: Mark
authors.initials: M
authors.lastname: Kelley
authors.forename: Ingrid
authors.initials: I
authors.lastname: Meszoely
authors.forename: Julie
authors.initials: J
authors.lastname: Means-Powell
authors.forename: John C
authors.initials: JC
authors.lastname: Gore
authors.forename: Thomas E
authors.initials: TE
authors.lastname: Yankeelov
mesh_terms: D000465:Algorithms; D001940:Breast / Q000473:pathology*; D001943:Breast Neoplasms / Q000473:pathology*; D004867:Equipment Design; D019544:Equipment Failure Analysis; D005260:Female; D006801:Humans; D007089:Image Enhancement / Q000379:methods; D007090:Image Interpretation, Computer-Assisted / Q000379:methods*; D008279:Magnetic Resonance Imaging / Q000295:instrumentation* / Q000379:methods; D019047:Phantoms, Imaging; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D013382:Subtraction Technique