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Estimation of the depth‐dependent component of the point spread function of SPECT
Author(s) -
Beekman Freek J.,
Slijpen Eddy T. P.,
de Jong Hugo W. A. M.,
Viergever Max A.
Publication year - 1999
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.598745
Subject(s) - point spread function , detector , collimator , optics , physics , convolution (computer science) , point source , iterative reconstruction , noise (video) , spect imaging , single photon emission computed tomography , algorithm , computer science , computer vision , artificial intelligence , nuclear medicine , artificial neural network , image (mathematics) , medicine
The point spread function (PSF) of a gamma camera describes the photon count density distribution at the detector surface when a point source is imaged. Knowledge of the PSF is important for computer simulation and accurate image reconstruction of single photon emission computed tomography (SPECT) images. To reduce the number of measurements required for PSF characterization and the amount of computer memory to store PSF tables, and to enable generalization of the PSF to different collimator‐to‐source distances, the PSF may be modeled as the two‐dimensional (2D) convolution of the depth‐dependent component which is free of detector blurring ( PSF ideal ) and the distance‐dependent detector response. Owing to limitations imposed by the radioactive strength of point sources, extended sources have to be used for measurements. Therefore, if PSF idealis estimated from measured responses, corrections have to be made for both the detector blurring and for the extent of the source. In this paper, an approach based on maximum likelihood expectation‐maximization (ML‐EM) is used to estimate PSF ideal . In addition, a practical measurement procedure which avoids problems associated with commonly used line‐source measurements is proposed. To decrease noise and to prevent nonphysical solutions, shape constraints are applied during the estimation of PSF ideal . The estimates are generalized to depths other than those which have been measured and are incorporated in a SPECT simulator. The method is validated for Tc‐99m and Tl‐201 by means of measurements on physical phantoms. The corrected responses have the desired shapes and simulated responses closely resemble measured responses. The proposed methodology may, consequently, serve as a basis for accurate three‐dimensional (3D) SPECT reconstruction.