Quantification of the Multiplexing Effects in Multi-Pinhole Small Animal SPECT: A Simulation Study

Abstract

Our goal is to study the trade-off between image degradation and improved detection efficiency and resolution from allowing multiplexing in multi-pinhole (MPH) SPECT, and to determine the optimal pinhole number for MPH design. We used an analytical 3D MPH projector and two digitized phantoms: the mouse whole body (MOBY) phantom and a hot sphere phantom to generate noise-free and noisy projections, simulating pinhole collimators fitted with pre-studied pinhole patterns. We performed three schemes to achieve different degrees of multiplexing: 1. Fixed magnification and detection efficiency; 2. Fixed detection efficiency and changed magnification; 3. Fixed magnification and changed detection efficiency. We generated various noisy data sets by simulating Poisson noise using differently scaled noise-free projections and obtained 20 noise realizations for each setting. All datasets were reconstructed using 3D MPH ML-EM reconstruction method. We analyzed the quantitative accuracy by the normalized-mean-square-error. We evaluated the image contrast for the hot sphere phantom simulation, and also the image noise by the average normalized-standard-deviation of certain pixels for different degrees of multiplexing. Generally, no apparent artifacts were observed in the reconstructed images, illustrating the effectiveness of reconstructions. Bias increased for increased degree of multiplexing. Contrast was not significantly affected by multiplexing in the specific simulation scheme (1). Scheme (2) showed that excessive multiplexing to improve image resolution would not improve the overall trade-off of bias and noise compared to no multiplexing. However, scheme (3) showed that when comparing to no multiplexing, the trade-off improved initially with increased multiplexing by allowing more number of pinholes to improve detection efficiency. The trade-off reached a maximum and decreased with further multiplexing due to image degradation from increased bias. The optimal pinhole number was 7 for a compact camera with size of 12 cm × 12 cm and 9 for a standard gamma camera with size of 40 cm × 40 cm in this scheme. We conclude that the gains in improved detection efficiency and resolution by increased multiplexing are offset by increased image degradations. All the aforementioned factors must be considered in the optimum MPH collimator design for small animal SPECT imaging.

Evaluation of a Multi-pinhole Collimator for Imaging Small Animals with Different Sizes

“…Although determination of optimal aperture sizes in MPH is usually task-dependent and is beyond the scope of this study, different sizes of pinholes for other applications can be tailored and applied by using other aperture plates. Studies from authors and other researchers have shown that multiplexing leads to image artifacts and should be avoided or kept to minimal [17,25]. However, complete blockage of the multiplexing, while still fully utilizing the whole detector, is difficult to achieve without the use of physical septa for a circular FOV.…”

“…Excessive degree of multiplexing has been recognized to degrade the reconstructed image quality and provides inferior bias and noise trade-off [17,25]. When applying the same collimator length for mice to rats in MPH imaging, the higher degree of multiplexing may cause excessive image artifacts in the reconstructed images.…”

“…Bias (normalized mean square error (NMSE)) between the entire noise-free reconstruction volume and the original phantom was then measured. Noise (normalized standard deviation (NSD)) was measured in a 3D uniform region chosen proximal to the center of the reconstructed images, through an ensemble of 20 noise realizations [17]. All simulations were based on the same acquisition time.…”

“…However, such collimators are difficult to make, and the cost is high especially for rotational systems; therefore, many researchers are still using MPH designs with no septa, such as the one used in this study. For MPH without septa, an optimal number of pinholes, level of magnification, and degree of multiplexing need to be determined to provide the optimal trade-off between image noise and bias [12,13,17].…”

Development of computational small animal models and their applications in preclinical imaging and therapy research

“…These are commonly combined with simulation tools accounting for all physical aspects involved in the image acquisition process and characteristics of the imaging system to generate a simulated dataset that closely mimic clinical and experimental studies. The known features of computational models and simulated datasets provide precise information enabling to evaluate the impact of physical degrading factors inherent to the imaging process, [131][132][133][134][135] assess different design concepts and performance of medical imaging systems, [136][137][138][139][140][141][142][143][144][145][146][147][148][149][150] and advance the development and validation of new image segmentation, [151][152][153][154][155] registration, 156-161 reconstruction, [162][163][164][165][166][167] and processing techniques. [168][169][170][171][172][173][174][175] Likewise, the Digimouse and MOBY models served as optically heterogeneous virtual subjects for light propagation calculations to assess the impact of various parameters involved in optical molecular imaging techniques [176]…”