title: Analytical, experimental, and Monte Carlo system response matrix for pinhole SPECT reconstruction.
abstract: OBJECTIVE
  To assess the performance of two approaches to the system response matrix (SRM) calculation in pinhole single photon emission computed tomography (SPECT) reconstruction.
  
  
  METHODS
  Evaluation was performed using experimental data from a low magnification pinhole SPECT system that consisted of a rotating flat detector with a monolithic scintillator crystal. The SRM was computed following two approaches, which were based on Monte Carlo simulations (MC-SRM) and analytical techniques in combination with an experimental characterization (AE-SRM). The spatial response of the system, obtained by using the two approaches, was compared with experimental data. The effect of the MC-SRM and AE-SRM approaches on the reconstructed image was assessed in terms of image contrast, signal-to-noise ratio, image quality, and spatial resolution. To this end, acquisitions were carried out using a hot cylinder phantom (consisting of five fillable rods with diameters of 5, 4, 3, 2, and 1 mm and a uniform cylindrical chamber) and a custom-made Derenzo phantom, with center-to-center distances between adjacent rods of 1.5, 2.0, and 3.0 mm.
  
  
  RESULTS
  Good agreement was found for the spatial response of the system between measured data and results derived from MC-SRM and AE-SRM. Only minor differences for point sources at distances smaller than the radius of rotation and large incidence angles were found. Assessment of the effect on the reconstructed image showed a similar contrast for both approaches, with values higher than 0.9 for rod diameters greater than 1 mm and higher than 0.8 for rod diameter of 1 mm. The comparison in terms of image quality showed that all rods in the different sections of a custom-made Derenzo phantom could be distinguished. The spatial resolution (FWHM) was 0.7 mm at iteration 100 using both approaches. The SNR was lower for reconstructed images using MC-SRM than for those reconstructed using AE-SRM, indicating that AE-SRM deals better with the projection noise than MC-SRM.
  
  
  CONCLUSIONS
  The authors' findings show that both approaches provide good solutions to the problem of calculating the SRM in pinhole SPECT reconstruction. The AE-SRM was faster to create and handle the projection noise better than MC-SRM. Nevertheless, the AE-SRM required a tedious experimental characterization of the intrinsic detector response. Creation of the MC-SRM required longer computation time and handled the projection noise worse than the AE-SRM.Nevertheless, the MC-SRM inherently incorporates extensive modeling of the system and therefore experimental characterization was not required.
authors.affiliation: Fundación Ramón Domínguez, Medicina Nuclear, CHUS, Spain and Grupo de Imaxe Molecular, IDIS, Santiago de Compostela 15706, Spain.
authors.forename: Pablo
authors.initials: P
authors.lastname: Aguiar
authors.affiliation: Unitat de Biofísica, Facultat de Medicina, Universitat de Barcelona, Spain and Servei de Física Médica i Protecció Radiológica, Institut Catalá d'Oncologia, Barcelona 08036, Spain.
authors.forename: Francisco
authors.initials: F
authors.lastname: Pino
authors.affiliation: Fundación Ramón Domínguez, Medicina Nuclear, CHUS, Santiago de Compostela 15706, Spain.
authors.forename: Jesús
authors.initials: J
authors.lastname: Silva-Rodríguez
authors.affiliation: Servei de Medicina Nuclear, Hospital Clínic, Barcelona, Spain; Institut d'Investigacions Biomèdiques August Pí i Sunyer (IDIBAPS), Spain; and CIBER en Bioingeniería, Biomateriales y Nanomedicina (CIBER-BBN), Barcelona 08036, Spain.
authors.forename: Javier
authors.initials: J
authors.lastname: Pavía
authors.affiliation: Unitat de Biofísica, Facultat de Medicina, Casanova 143, Spain; Institut d'Investigacions Biomèdiques August Pí i Sunyer (IDIBAPS), Spain; and CIBER en Bioingeniería, Biomateriales y Nanomedicina (CIBER-BBN), Barcelona 08036, Spain.
authors.forename: Doménec
authors.initials: D
authors.lastname: Ros
authors.affiliation: Servicio Medicina Nuclear, CHUS, Spain; Grupo de Imaxe Molecular, Facultade de Medicina (USC), IDIS, Santiago de Compostela 15706, Spain; and Fundación Tejerina, Madrid, Spain.
authors.forename: Alvaro
authors.initials: A
authors.lastname: Ruibal
authors.affiliation: Université de Strasbourg, IPHC, 23 rue du Loess 67037 Strasbourg, France and CNRS, UMR7178, Strasbourg 67037, France.
authors.forename: Ziad
authors.initials: Z
authors.lastname: El Bitar
mesh_terms: D000465:Algorithms; D003198:Computer Simulation; D006801:Humans; D007091:Image Processing, Computer-Assisted / Q000379:methods; D015233:Models, Statistical; D009010:Monte Carlo Method; D019047:Phantoms, Imaging; D015203:Reproducibility of Results; D059629:Signal-To-Noise Ratio; D012984:Software; D015899:Tomography, Emission-Computed, Single-Photon / Q000379:methods*