Geometric Calibration and Image Reconstruction for a Segmented Slant-Hole Stationary Cardiac SPECT System

Abstract

A dedicated stationary cardiac single-photon emission computed tomography (SPECT) system with a novel segmented slant-hole collimator has been developed. The goal of this paper is to calibrate this new imaging geometry with a point source.

Methods: Unlike the commercially available dedicated cardiac SPECT systems, which are specialized and can be used only to image the heart, our proposed cardiac system is based on a conventional SPECT system but with a segmented slant-hole collimator replacing the collimator. For a dual-head SPECT system, 2 segmented collimators, each with 7 sections, are arranged in an L-shaped configuration such that they can produce a complete cardiac SPECT image with only one gantry position. A calibration method was developed to estimate the geometric parameters of each collimator section as well as the detector rotation radius, under the assumption that the point source location is calculated using the central-section data. With a point source located off the rotation axis, geometric parameters for each collimator section can be estimated independently. The parameters estimated individually are further improved by a joint objective function that uses all collimator sections simultaneously and incorporates the collimator symmetry information.

Results: Estimation results and images reconstructed from estimated parameters are presented for both simulated and real data acquired from a prototype collimator. The calibration accuracy was validated by computer simulations with an error of about 0.1° for the slant angles and about 1 mm for the rotation radius. Reconstructions of a heart-insert phantom did not show any image artifacts of inaccurate geometric parameters.

Conclusion: Compared with the detector's intrinsic resolution, the estimation error is small and can be ignored. Therefore, the accuracy of the calibration is sufficient for cardiac SPECT imaging.

Keywords: cardiac single photon emission computed tomography (SPECT); collimator; geometric calibration; slant-hole.

FIGURE 1

Illustration of proposed 7-segment slant-hole collimator. Projection is elongated at outer sections. Dashed ellipses represent projection on that section of detector when sphere is placed at rotation center.

FIGURE 2

Cross section of common volume (CV) in x,y-plane.

FIGURE 3

Definition of slant angles η and φ, detector coordinates, and object coordinates. z-axis is rotation axis of detector.

FIGURE 4

Setup of point source P.

FIGURE 5

Illustration of geometric response for parallel holes and slant holes.

FIGURE 6

Close-up diagram of footprint of point response function with respect to slant angles α and β.

FIGURE 7

Reconstruction results of point source without (left) and with (right) blurring correction. Five and 50 iterations were used, respectively.

FIGURE 8

Reconstruction results of 2 point sources at 50 iterations. Left one was reconstructed with true parameters. Right one was reconstructed with estimated parameters. Blurring correction was applied in both images.

FIGURE 9

rojection of point source at 90°.

FIGURE 10

Reconstruction results of point source. From left to right: without calibration, with joint estimation calibration but no blurring correction, with joint estimation calibration and blurring correction. Sixty iterations were applied.

FIGURE 11

Projection of heart phantom at 0° and 90°.

FIGURE 12

Reconstruction results of heart phantom without calibration (top), with calibration but no blurring correction (middle), and with calibration and blurring correction (bottom). From left to right: vertical long-axis, short-axis, and horizontal long-axis cuts. All images were reconstructed at 80 iterations.

FIGURE 13

Slices showing defect before (top) and after (bottom) calibration. VLA cut is on left, SA cut on right. Blurring correction was applied. All images were scaled to [0 255] and were displayed using the same scale. Image contrast was enhanced for better visualization of defect. A color version of this figure is available as a supplemental file at http://tech.snmjournals.org.