Pixellated CZT detectors provide a new opportunity to improve the image quality of SPECT detector systems. Their performance has to be evaluated in terms of resolution and efficiency, in a similar way as done earlier for NaI detectors.We have developed an analytical model for spatial resolution and geometric efficiency of collimators specifically for pixellated CZT based detectors. We derive an exact description for static and rotating detector concepts, use NEMA performance criteria for detection efficiency, and adapt measures for spatial resolution of pixellated detectors, based on the sampling of the single pixel response function.Tradeoffs among resolution, efficiency, and signal-to-noise ratio (SNR) have been investigated for different applications. Our analysis shows that the concept of rotating collimators suffers from noise accumulation, except for purely hot spot imaging. We propose multi-pinhole, slat-slit or fan beam-slit collimators in a demagnification mode for optimum efficiency and image quality using pixellated solid-state detectors for SPECT cameras.
Quantum-limited-dose (QLD) and noise-equivalent-dose (NED) are performance metrics often used interchangeably. Although the metrics are related, they are not equivalent unless the treatment of electronic noise is carefully considered. These metrics are increasingly important to properly characterize the low-dose performance of flat panel detectors (FPDs). A system can be said to be quantum-limited when the Signal-to-noise-ratio (SNR) is proportional to the square-root of x-ray exposure. Recent experiments utilizing three methods to determine the quantum-limited dose range yielded inconsistent results. To investigate the deviation in results, generalized analytical equations are developed to model the image processing and analysis of each method. We test the generalized expression for both radiographic and fluoroscopic detectors. The resulting analysis shows that total noise content of the images processed by each method are inherently different based on their readout scheme. Finally, it will be shown that the NED is equivalent to the instrumentation-noise-equivalent-exposure (INEE) and furthermore that the NED is derived from the quantum-noise-only method of determining QLD. Future investigations will measure quantum-limited performance of radiographic panels with a modified readout scheme to allow for noise improvements similar to measurements performed with fluoroscopic detectors.Keywords: Flat panel detector (FPD), quantum limited performance range, quantum limited dose (QLD), noise equivalent dose (NED), instrumentation noise equivalent exposure (INEE), radiography, fluoroscopy
We propose a new collimator geometry, the hybrid parallel-slant (HPS) hole geometry, to improve sensitivity for SPECT imaging with large field of view (LFOV) gamma cameras. A HPS collimator has one segment with parallel holes and one or more segments with slant holes. The collimator can be mounted on a conventional SPECT LFOV system that uses parallel-beam collimators, and no additional detector or collimator motion is required for data acquisition. The parallel segment of the collimator allows for the acquisition of a complete data set of the organs-of-interest and the slant segments provide additional data. In this work, simulation studies of an MCAT phantom were performed with a HPS collimator with one slant segment. The slant direction points from patient head to patient feet with a slant angle of 30 . We simulated 64 projection views over 180 with the modeling of nonuniform attenuation effect, and then reconstructed images using an MLEM algorithm that incorporated the hybrid geometry. It was shown that sensitivity to the cardiac region of the phantom was increased by approximately 50% when using the HPS collimator compared with a parallel-hole collimator. No visible artifacts were observed in the myocardium and the signal-to-noise ratio (SNR) of the myocardium walls was improved. Compared with collimators with other geometries, using a HPS collimator has the following advantages: (a) significant sensitivity increase; (b) a complete data set obtained from the parallel segment that allows for artifact-free image reconstruction; and (c) no additional collimator or detector motion. This work demonstrates the potential value of hybrid geometry in collimator design for LFOV SPECT imaging.Index Terms-Collimator geometry, field of view (FOV), hybrid parallel-slant (HPS) hole collimator, SPECT.
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