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ABSTRACT: Collimators determine the imaging properties of the gamma camera in nuclear medicine. Because collimators form images by absorption of radiation, the resolution and sensitivity are related by a geometric trade-off: the number of counts detected is proportional to the square of the FWHM. An optimal collimator design maximizes sensitivity for a fixed FWHM. For parallel holes, the theory of optimal collimator design is well-understood. For example, at a fixed energy, all optimal collimators have the same thickness. A strategy for the optimal design of non-parallel hole collimators is proposed based on the idea that each collimator hole and septa should locally resemble an appropriate optimal parallel-hole design. Based on this strategy, optimal conebeam and fanbeam designs are presented and their performance analyzed
Nuclear Science Symposium, 1999. Conference Record. 1999 IEEE; 02/1999
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ABSTRACT: Collimators, which are just periodic arrays of holes in lead, are
used on gamma cameras in nuclear medicine to create images. Generally,
the collimator hole pattern is not visible in clinical images because
the holes are separated by distances smaller than the intrinsic
resolution of the gamma camera. However, new (CZT) solid-state detectors
are characterized by a regular grid of detector pixels. The interaction
between the collimator-hole lattice and the detector grid almost assures
that moire patterns will appear in the images unless the two grids are
commensurate. Unfortunately, collimators with hole spacings commensurate
with the pixel size are far from optimal in nuclear medicine. A
mathematical analysis of the imaging process in such systems is provided
and used to demonstrate the effects. Two strategies for minimizing the
effects are examined: (1) introducing a gap between the collimator and
the detector and (2) constructing the collimator with numerous small
segments with displaced grid structures. Both strategies have serious
drawbacks
Nuclear Science Symposium, 1998. Conference Record. 1998 IEEE; 02/1998
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ABSTRACT: An experimental measurement of the <sup>99m</sup>Tc point-source response function (PSRF) for a SPECT system is reported in 40 energy windows (80-160 keV). Many researchers have proposed methods for improving SPECT reconstruction by using scattered radiation. Implementation of such reconstruction algorithms requires detailed knowledge of the energy-dependent PSRF, including both the scattering within the patient body and the imaging characteristics of the camera. The authors have measured this PSRF experimentally using a cylindrical phantom filled with water and having a movable point-source immersed inside. The measurements were made using a gamma-camera with a special xyE acquisition interface card that provided both the x-y coordinates and the energy of each event. A spherical capsule filled with <sup>99m </sup>Tc was mounted on a geared armature which moved the source without opening the phantom. Measurements, exceeding 4 million counts at each source position, were made at radial intervals of 1 cm (0-16 cm) and at angular separations of 11.25 degrees. This idealized phantom (cylindrical symmetry and uniform attenuating medium) approximates whole-body imaging in SPECT and provides data for validating Monte Carlo simulations and testing reconstruction algorithms. The authors report fit parameters for an empirical analytic model of the PSRF. The singular value decomposition (SVD) of this SPECT imaging system is computed. From the SVD, energy weighting functions are derived as an alternative to the usual energy windows
Nuclear Science Symposium, 1997. IEEE; 12/1997
