title: Geometrical and Monte Carlo projectors in 3D PET reconstruction.
abstract: OBJECTIVE
  In the present work, the authors compare geometrical and Monte Carlo projectors in detail. The geometrical projectors considered were the conventional geometrical Siddon ray-tracer (S-RT) and the orthogonal distance-based ray-tracer (OD-RT), based on computing the orthogonal distance from the center of image voxel to the line-of-response. A comparison of these geometrical projectors was performed using different point spread function (PSF) models. The Monte Carlo-based method under consideration involves an extensive model of the system response matrix based on Monte Carlo simulations and is computed off-line and stored on disk.
  
  
  METHODS
  Comparisons were performed using simulated and experimental data of the commercial small animal PET scanner rPET.
  
  
  RESULTS
  The results demonstrate that the orthogonal distance-based ray-tracer and Siddon ray-tracer using PSF image-space convolutions yield better images in terms of contrast and spatial resolution than those obtained after using the conventional method and the multiray-based S-RT. Furthermore, the Monte Carlo-based method yields slight improvements in terms of contrast and spatial resolution with respect to these geometrical projectors.
  
  
  CONCLUSIONS
  The orthogonal distance-based ray-tracer and Siddon ray-tracer using PSF image-space convolutions represent satisfactory alternatives to factorizing the system matrix or to the conventional on-the-fly ray-tracing methods for list-mode reconstruction, where an extensive modeling based on Monte Carlo simulations is unfeasible.
authors.affiliation: Fundación IDICHUS/IDIS, Complexo Hospitalario Universitario de Santiago de Compostela, Departamento de Física de Partículas, Universidade de Santiago de Compostela, Spain. pablo.aguiar.fernandez@sergas.es
authors.forename: Pablo
authors.initials: P
authors.lastname: Aguiar
authors.forename: Magdalena
authors.initials: M
authors.lastname: Rafecas
authors.forename: Juan Enrique
authors.initials: JE
authors.lastname: Ortuño
authors.forename: George
authors.initials: G
authors.lastname: Kontaxakis
authors.forename: Andrés
authors.initials: A
authors.lastname: Santos
authors.forename: Javier
authors.initials: J
authors.lastname: Pavía
authors.forename: Domènec
authors.initials: D
authors.lastname: Ros
mesh_terms: D000818:Animals; D003198:Computer Simulation; D003201:Computers; D007091:Image Processing, Computer-Assisted / Q000295:instrumentation* / Q000379:methods*; D021621:Imaging, Three-Dimensional / Q000379:methods*; D015233:Models, Statistical; D009010:Monte Carlo Method; D019047:Phantoms, Imaging; D049268:Positron-Emission Tomography / Q000379:methods*; D012984:Software; D013997:Time Factors