Improved image quality in pinhole SPECT by accurate modeling of the point spread function in low magnification systems

Abstract

Purpose: 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. C 2015 American Association of Physicists in Medicine. [http://dx

Primary, scatter, and penetration characterizations of parallel-hole and pinhole collimators for I-123 SPECT

“…We used knife-edge apertures, which allowed considerable penetration from the 159 keV photons forming ring-shaped projections and impacting the spatial resolution, as shown in figure 12. These rings can be reduced and hence the spatial resolution can be improved by using keel-edge apertures at the expense of decreased sensitivity (Van Der Have and Beekman 2006) and/or modeling the PSF (Pino et al 2015).…”

Resolution recovery on list mode MLEM reconstruction for dynamic cardiac SPECT system

“…Balancing design aspects necessitates tradeoffs between sensitivity and resolution goals, therefore we have generated 14 design iterations of the DC-SPECT with varying geometry, such as detector pixel size, pinhole size, detector to collimator distances, number of detector heads, etc Previous simulation and modelling results [1][2][3] demonstrated that our system can achieve a calculated 10.0 mm FWHM system spatial resolution and 0.07% sensitivity at the center of FOV. Other than the intrinsic system resolution determined by geometry design, the reconstructed image resolution can also serve as a predictor of the prospective system performance and can be improved by applying iterative algorithms with resolution recovery methods [4][5][6][7].…”

“…Analytical methods can be used to cycle through design iterations but lack accurately reconstructed images without a MC simulated system matrix. Compensating the mismatch between analytic calculation and the acquired data by involving PSF modelling is an option to balance the computation efforts and the accuracy of system matrix modelling [6,7,10,11], and has shown improvement of the image quality [4,12]. The resolution recovery methods based on PSF are usually proposed for modelling the depth of interaction [13], the finite collimator resolution [14], and the detector blurring as well as the misalignment between the pinhole and aperture [8], and is often modelled as a 2D/3D Gaussian [15,16], or Gaussian plus exponential function or B-spline [17].…”