title: Voxel-wise uncertainty in CT substitute derived from MRI.
abstract: OBJECTIVE
  In an earlier work, we demonstrated that substitutes for CT images can be derived from MR images using ultrashort echo time (UTE) sequences, conventional T2 weighted sequences, and Gaussian mixture regression (GMR). In this study, we extend this work by analyzing the uncertainties associated with the GMR model and the information contributions from the individual imaging sequences.
  
  
  METHODS
  An analytical expression for the voxel-wise conditional expected absolute deviation (EAD) in substitute CT (s-CT) images was derived. The expression depends only on MR images and can thus be calculated along with each s-CT image. The uncertainty measure was evaluated by comparing the EAD to the true mean absolute prediction deviation (MAPD) between the s-CT and CT images for 14 patients. Further, the influence of the different MR images included in the GMR model on the generated s-CTs was investigated by removing one or more images and evaluating the MAPD for a spectrum of predicted radiological densities.
  
  
  RESULTS
  The largest EAD was predicted at air-soft tissue and bone-soft tissue interfaces. The EAD agreed with the MAPD in both these regions and in regions with lower EADs, such as the brain. Two of the MR images included in the GMR model were found to be mutually redundant for the purpose of s-CT generation.
  
  
  CONCLUSIONS
  The presented uncertainty estimation method accurately predicts the voxel-wise MAPD in s-CT images. Also, the non-UTE sequence previously used in the model was found to be redundant.
authors: Department of Radiation Sciences, Umeå University, Umeå, SE-901 87 Sweden. adam.johansson@radfys.umu.se
authors: Adam
authors: A
authors: Johansson
authors: Mikael
authors: M
authors: Karlsson
authors: Jun
authors: J
authors: Yu
authors: Thomas
authors: T
authors: Asklund
authors: Tufve
authors: T
authors: Nyholm
mesh_terms: D007091:Image Processing, Computer-Assisted / Q000379:methods*; D008279:Magnetic Resonance Imaging / Q000379:methods*; D012680:Sensitivity and Specificity; D014057:Tomography, X-Ray Computed / Q000379:methods*; D035501:Uncertainty