Reducing Multiplexing Artifacts in Multi-Pinhole SPECT With a Stacked Silicon-Germanium System: A Simulation Study

Abstract

In pinhole SPECT, multi-pinhole collimators can increase sensitivity but may lead to projection overlap, or multiplexing, which can cause image artifacts. In this work we explore whether a stacked-detector configuration with a germanium and a silicon detector, used with 123I (27–32, 159 keV), where little multiplexing occurs in the Si projections, can reduce image artifacts caused by highly-multiplexed Ge projections. Simulations are first used to determine a reconstruction method that combines the Si and Ge projections to maximize image quality. Next, simulations of different pinhole configurations (varying projection multiplexing) in conjunction with digital phantoms are used to examine whether additional Si projections mitigate artifacts from the multiplexing in the Ge projections. Reconstructed images using both Si and Ge data are compared to those using Ge data alone. Normalized mean-square error and normalized standard deviation provide a quantitative evaluation of reconstructed images’ error and noise, respectively, and are used to evaluate the impact of the additional non-multiplexed data on image quality. For a qualitative comparison, the differential point response function is used to examine multiplexing artifacts. Results show that in cases of highly-multiplexed Ge projections, the addition of low-multiplexed Si projections helps to reduce image artifacts both quantitatively and qualitatively.

Task‐based design of a synthetic‐collimator SPECT system used for small animal imaging

“…The Wiener estimator not only has the lowest EMSE for all linear estimators when the data are Gaussian distributed, but it also only requires knowledge of the first- and second-order statistics of the raw image data[14]. Working with direct image data allowed us to bypass the traditional step of image reconstruction, which can introduce additional variables such as iteration number that further complicate the process of determining optimal system designs[19]. The Wiener estimate is given by θ^→W=Kg,θ†Kg−1false(g→−g¯¯→false)+θ¯→,where K g,θ is the cross-covariance of the signal parameters and the data, K g is the data covariance, and g˭ is the mean noise-free image data averaged over Poisson noise and all object distributions.…”

“…12,13 One major advantage of using 123 I is that it emits photons at various energies which allowed us to utilize the multienergy approach described before, with the silicon detector (Si), placed in front, being sensitive to the $ 30 keV photons and the germanium detector (Ge), in the back, being sensitive to the 159 keV photons. 10 The relative abundances of photons at these two energy ranges are relatively similar; however, the exact values were not modeled in our simulations. The germanium detector used in the system has an exquisite energy resolution, while maintaining similar spatial resolutions to conventional detectors.…”

“…14 Working with direct image data allowed us to bypass the traditional step of image reconstruction, which can introduce additional variables such as iteration number that further complicate the process of determining optimal system designs. 19 The Wiener estimate is given bŷ…”

“…Synthetic-collimator SPECT is one design method that improves the sensitivity found in multi-pinhole SPECT systems, while simultaneously maintaining the system’s spatial resolution[8][9][10][11]. In synthetic-collimator SPECT, two planar detectors are used in tandem, with multiplexing possibly occurring on both, but is often found on the more distant detector due to greater magnification (Figure 2).…”

“…as well as for its high specific activity (> 1850 GBq/ μ mol) to avoid mass effects and limit radiation dose [12][13]. One major advantage of using 123 I is that it emits photons at various energies which allowed us to utilize the multi-energy approach described before, with the silicon detector (Si), placed in front, being sensitive to the ~30keV photons and the germanium detector (Ge), in the back, being sensitive to the 159keV photons[10]. The relative abundances of photons at these two energy ranges are relatively similar, however, the exact values were not modeled in our simulations.…”