Purpose: 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 animalPETscannerrPET.
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.
This work was supported in part by Fondo de Investigaciones Sanitarias del Instituto de Salud Carlos III (Project Nos. PI041017, PS09/01206, and CB06/01/1039), by the Ministerio de Ciencia e Innovacion (Grant No. TEC2007-61047), by the Generalitat Valenciana (Grant No. GV06/246), by the Generalitat de Catalunya (Grant No. SGR1049), Ministerio de Ciencia e Innovacion (Project No. SAF2009-08076), and by Ministerio de Ciencia e Innovacion (CDTI-CENIT) AMIT project and Comunidad de Madrid (project ARTEMIS P2009/DPI-1802). P. Aguiar was awarded a “Sara Borrell“ fellowship by Fondo de Investigaciones Sanitarias del Instituto de Salud Carlos III.
II. MATERIALS AND METHODS
II.A. Small animalscanner
II.B. Image reconstruction
II.C. System response matrix calculation
II.C.1. Siddon ray-tracer
II.C.2. Orthogonal-distance ray-tracer
II.C.3. Monte Carlo-based SRM
II.D. PET data
II.D.1. Simulated data
II.D.2. Experimental data
II.E.1. Sensitivity images
II.E.2. Correlation coefficient
II.E.4. Coefficient of variation
II.E.5. Evaluation of the PSF models
II.E.6. Spatial resolution
II.E.7. Time and memory requirements
III.A. Evaluation of the system response matrix calculation
III.A.1. Sensitivity images
III.A.2. OD-RT versus S-RT and MC-SRM
III.B. Evaluation of the PSF models
III.B.1. Sensitivity images
III.B.2. Micro-Derenzo phantom
III.B.3. Spiral phantom: Spatial resolution measurements
III.C. Time and memory requirements
IV. DISCUSSION AND CONCLUSIONS
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