title: Generating patient specific pseudo-CT of the head from MR using atlas-based regression.
abstract: Radiotherapy planning and attenuation correction of PET images require simulation of radiation transport. The necessary physical properties are typically derived from computed tomography (CT) images, but in some cases, including stereotactic neurosurgery and combined PET/MR imaging, only magnetic resonance (MR) images are available. With these applications in mind, we describe how a realistic, patient-specific, pseudo-CT of the head can be derived from anatomical MR images. We refer to the method as atlas-based regression, because of its similarity to atlas-based segmentation. Given a target MR and an atlas database comprising MR and CT pairs, atlas-based regression works by registering each atlas MR to the target MR, applying the resulting displacement fields to the corresponding atlas CTs and, finally, fusing the deformed atlas CTs into a single pseudo-CT. We use a deformable registration algorithm known as the Morphon and augment it with a certainty mask that allows a tailoring of the influence certain regions are allowed to have on the registration. Moreover, we propose a novel method of fusion, wherein the collection of deformed CTs is iteratively registered to their joint mean and find that the resulting mean CT becomes more similar to the target CT. However, the voxelwise median provided even better results; at least as good as earlier work that required special MR imaging techniques. This makes atlas-based regression a good candidate for clinical use.
authors.affiliation: Elekta Instrument AB, Kungstensgatan 18, Box 7593, SE-103 93 Stockholm, Sweden. Center for Medical Image Science and Visualization (CMIV), Linköping University, Sweden. Department of Biomedical Engineering, Linköping University, Linköping, Sweden.
authors.forename: J
authors.initials: J
authors.lastname: Sjölund
authors.forename: D
authors.initials: D
authors.lastname: Forsberg
authors.forename: M
authors.initials: M
authors.lastname: Andersson
authors.forename: H
authors.initials: H
authors.lastname: Knutsson
mesh_terms: D000465:Algorithms; D001271:Atlases as Topic; D001921:Brain / Q000000981:diagnostic imaging*; D003198:Computer Simulation; D006801:Humans; D007091:Image Processing, Computer-Assisted / Q000379:methods*; D021621:Imaging, Three-Dimensional / Q000379:methods; D008279:Magnetic Resonance Imaging / Q000379:methods*; D010363:Pattern Recognition, Automated; D014057:Tomography, X-Ray Computed / Q000379:methods*