title: Improved image quality in pinhole SPECT by accurate modeling of the point spread function in low magnification systems.
abstract: OBJECTIVE
  Single photon emission computed tomography (SPECT) has become an important noninvasive imaging technique in small-animal research. Due to the high resolution required in small-animal SPECT systems, the spatially variant system response needs to be included in the reconstruction algorithm. Accurate modeling of the system response should result in a major improvement in the quality of reconstructed images. The aim of this study was to quantitatively assess the impact that an accurate modeling of spatially variant collimator/detector response has on image-quality parameters, using a low magnification SPECT system equipped with a pinhole collimator and a small gamma camera.
  
  
  METHODS
  Three methods were used to model the point spread function (PSF). For the first, only the geometrical pinhole aperture was included in the PSF. For the second, the septal penetration through the pinhole collimator was added. In the third method, the measured intrinsic detector response was incorporated. Tomographic spatial resolution was evaluated and contrast, recovery coefficients, contrast-to-noise ratio, and noise were quantified using a custom-built NEMA NU 4-2008 image-quality phantom.
  
  
  RESULTS
  A high correlation was found between the experimental data corresponding to intrinsic detector response and the fitted values obtained by means of an asymmetric Gaussian distribution. For all PSF models, resolution improved as the distance from the point source to the center of the field of view increased and when the acquisition radius diminished. An improvement of resolution was observed after a minimum of five iterations when the PSF modeling included more corrections. Contrast, recovery coefficients, and contrast-to-noise ratio were better for the same level of noise in the image when more accurate models were included. Ring-type artifacts were observed when the number of iterations exceeded 12.
  
  
  CONCLUSIONS
  Accurate modeling of the PSF improves resolution, contrast, and recovery coefficients in the reconstructed images. To avoid the appearance of ring-type artifacts, the number of iterations should be limited. In low magnification systems, the intrinsic detector PSF plays a major role in improvement of the image-quality parameters.
authors: Unitat de Biofísica, Facultat de Medicina, Universitat de Barcelona, Barcelona 08036, Spain and Servei de Física Mèdica i Protecció Radiològica, Institut Català d'Oncologia, L'Hospitalet de Llobregat 08907, Spain.
authors: Francisco
authors: F
authors: Pino
authors: Unitat de Biofísica, Facultat de Medicina, Universitat de Barcelona, Barcelona 08036, Spain.
authors: Nuria
authors: N
authors: Roé
authors: Fundación Ramón Domínguez, Complexo Hospitalario Universitario de Santiago de Compostela 15706, Spain and Grupo de Imagen Molecular, Instituto de Investigacións Sanitarias de Santiago de Compostela (IDIS), Galicia 15782, Spain.
authors: Pablo
authors: P
authors: Aguiar
authors: Institut d'Investigacions Biomèdiques August Pi i Sunyer (IDIBAPS), Barcelona 08036, Spain and CIBER en Bioingeniería, Biomateriales y Nanomedicina (CIBER-BBN), Barcelona 08036, Spain.
authors: Carles
authors: C
authors: Falcon
authors: Institut d'Investigacions Biomèdiques August Pi i Sunyer (IDIBAPS), Barcelona 08036, Spain and CIBER en Bioingeniería, Biomateriales y Nanomedicina (CIBER-BBN), Barcelona 08036, Spain.
authors: Domènec
authors: D
authors: Ros
authors: Institut d'Investigacions Biomèdiques August Pi i Sunyer (IDIBAPS), Barcelona 080836, Spain; CIBER en Bioingeniería, Biomateriales y Nanomedicina (CIBER-BBN), Barcelona 08036, Spain; and Servei de Medicina Nuclear, Hospital Clínic, Barcelona 08036, Spain.
authors: Javier
authors: J
authors: Pavía
mesh_terms: D000465:Algorithms; D000818:Animals; D005720:Gamma Rays; D007089:Image Enhancement / Q000295:instrumentation / Q000379:methods*; D051379:Mice; D008962:Models, Theoretical; D019047:Phantoms, Imaging; D011786:Quality Control; D059629:Signal-To-Noise Ratio; D015899:Tomography, Emission-Computed, Single-Photon