A SPECT imager with synthetic collimation

Abstract

This work outlines the development of a multi-pinhole SPECT system designed to produce a synthetic-collimator image of a small field of view. The focused multi-pinhole collimator was constructed using rapid-prototyping and casting techniques. The collimator projects the field of view through forty-six pinholes when the detector is adjacent to the collimator. The detector is then moved further from the collimator to increase the magnification of the system. The amount of pinhole-projection overlap increases with the system magnification. There is no rotation in the system; a single tomographic angle is used in each system configuration. The maximum-likelihood expectation-maximization (MLEM) algorithm is implemented on graphics processing units to reconstruct the object in the field of view. Iterative reconstruction algorithms, such as MLEM, require an accurate model of the system response. For each system magnification, a sparsely-sampled system response is measured by translating a point source through a grid encompassing the field of view. The pinhole projections are individually identified and associated with their respective apertures. A 2D elliptical Gaussian model is applied to the pinhole projections on the detector. These coefficients are associated with the object-space location of the point source, and a finely-sampled system matrix is interpolated. Simulations with a hot-rod phantom demonstrate the efficacy of combining low-resolution non-multiplexed data with high-resolution multiplexed data to produce high-resolution reconstructions.

Keywords: BazookaSPECT detectors; GPU; SPECT; Single-photon emission computed tomography; high-performance computing; maximum likelihood; molecular imaging; multi-pinhole imaging.

Figure 1

(a) An image of the SyntheticSPECT system. (b) A wireframe image of the multi-pinhole collimator. The 1 mm double-knife-edge pinholes are focused to a common field of view of ~25 mm. There are 110 pinhole apertures but only 46 apertures project the field of view onto the detector with the current 100 mm fiber-optic taper. (c) A typical integrated projection image for one voxel position. The voxel is in the corner of the FOV, and only the pinhole apertures that project this corner are imaged. There are 21 pinhole projections in this image. (d) The integrated projection image for all PSF voxel positions in the sparsely-sampled measurement grid. There is a sharp cutoff at the edge of the image due to the camera aperture. An insensitive area of the image intensifier is clearly seen in the center of one of the pinhole projections. (e) The integrated projection image for all PSF voxel positions in the interpolated grid. The edge of image has been masked due to errors in Gaussian fitting at the boundaries of (d). The insensitive part of the image intensifier at the center of one of the pinhole projections is masked due to inconsistencies in the fitting procedure.

Figure 2

(a) The pinhole projections in Fig. 1c are segmented. (b) A Gaussian distribution is fitted to each pinhole projection in (a), and each distribution is reprojected and centered.

Figure 3

Image of the phantom used in this study. The diameters of the rods were 1.0, 1.2, 1.4, 1.6, 1.8, 2.0 mm, and the separation between the centres of the rods in each section was equal to twice the diameter of those rods. A background activity was also simulated.

Figure 4

Projection data of the hot-rod phantom for system configurations: (a) 1, (b) 2,(c) 3,(d) 4,(e) 5 and (f) 6. The projection data were normalized to 10⁷ counts for each of the test cases. The level of overlap of the pinhole projections increases as the system magnification increases, but the number of projections subtended by the detector is reduced. A small circular insensitive area of the intensifier can clearly be seen in the same location in all images.

Figure 5

The synthetic-collimator images for the four test cases: (a) Test Case 1, (b) Test Case 2, (c) Test Case 3 and (d) Test Case 4. The object (C = 0.15) was reconstructed using MLEM and the activities in the voxels were summed along the axis perpendicular to the collimator face. The edges of each of the images have been masked to highlight the contrast between the rods and the background. (b) suffers from extreme values near the edge of the FOV which would otherwise dominate the image.

Figure 6

The synthetic-collimator images for the four test cases: (a) Test Case 1, (b) Test Case 2, (c) Test Case 3 and (d) Test Case 4. The object (C = 0.50) was reconstructed using MLEM and the activities in the voxels were summed along the axis perpendicular to the collimator face. The edges of each of the images have been masked to highlight the contrast between the rods and the background. (b) suffers from extreme values near the edge of the FOV which would otherwise dominate the image.