Younghak Kim a, Wonho Lee b c
a Environmental Radioactivity Assessment Team, Korea Atomic Energy Research Institute, Daejeon 34057, South Korea
b School of Health and Environmental Science, Korea University, Seoul, 02841, South Korea
c Transdisciplinary Major in Learning Health System, Graduate School, Korea University, Seoul, 02841, South Korea
## Abstract
Compton imaging excels at visualizing gamma rays in the range of several hundred kiloelectronvolts to several megaelectronvolts. However, this technique has limitations in the imaging of low-energy gamma rays. In contrast, collimatorless imaging technique determines the location of a source by analyzing the distribution of interactions. Because the collimatorless imaging technique excels at imaging low-energy gamma rays that are easily shielded by detector components, it can compensate for the shortcomings of the Compton imaging technique. In this study, we propose a dual-mode imaging technique that selects the imaging method depending on the target gamma-ray energy and fuses them during reconstruction. The collimatorless imaging method demonstrated high angular resolution at low energy levels, whereas the Compton image surpasses it starting from 200 keV within its reconstructible range. The angular resolution of the dual-mode image was between those of the two methods. The trend of the positional error of gamma ray energy was similar to that of the angular resolution, and the dual-mode method exhibited the lowest average error of 0.7°. The dual imaging method exhibited higher efficiency, figure of merit, and signal-to-noise ratio by utilizing events from both imaging modalities. In addition, we investigated the geometrical effects of various structures.
## 1. Introduction
Gamma ray imaging devices play a crucial role in nuclear safety by accurately identifying and mapping the dispersion of radioactive materials and thereby enabling operators to respond promptly. Their application in industrial fields has been steadily expanding in recent years, including tasks such as contamination measurement, decontamination confirmation, shielding facility assessment, radioactive waste management during operation, and nuclear power plant decommissioning [[1], [2], [3], [4], [5], [6]].
Gamma-ray imaging techniques are generally classified into mechanical and electrical collimations. Mechanical collimations employ a collimator made of a high-atomic-number material, such as lead or tungsten, which is advantageous for attenuating low-energy gamma rays. Coded apertures are commonly used in industrial fields owing to their high sensitivity and fine spatial resolution [7]. Electrical collimation, also known as Compton imaging, is used to determine the distribution of radiation sources by calculating the scattering angle of a scattering-absorption event [8]. This method is effective for imaging gamma rays of intermediate or higher energy, whose main interaction is Compton scattering. Therefore, exceptional performance can be achieved across all energy ranges by selecting a reconstruction method based on the target gamma rays [9,10]. However, fully compensating for the limitations of Compton imaging using a coded aperture is challenging. The coded aperture inherently requires a collimator with specially arranged shielding materials, and its reconstructed image does not perfectly align with the Compton image because of the limited field-of-view (FOV) of the collimator [9].
The collimatorless imaging technique was developed based on the variations in the interaction distribution in the detector depending on the location of a source [11]. The detector response depends on the direction of the gamma-rays, which is influenced by the collimating effect of detector components. It utilizes statistical inference to estimate the source distribution based on pre-obtained detector responses from experiments and computational simulations. The resolution improved as the detector response became more distinct. This method eliminates the need for additional hardware such as collimators. Consequently, it allows for use of the current Compton camera without any modifications; therefore, it does not impose limitations on the FOV. Because it excels at imaging low-energy gamma rays, it can supplement the shortcomings of the Compton imaging technique instead of the coded aperture method.
This study aims to image gamma rays across all energy ranges by selectively utilizing collimatorless and Compton imaging techniques depending on the energy level or by combining both methods based on a modified maximum likelihood expectation maximization (MLEM) equation during reconstruction. To verify the feasibility and merits of each approach, we simulated a cadmium-zinc-telluride (CdZnTe) detector array developed in a previous study and assessed its performance both quantitatively and qualitatively. The analysis included computational simulations to examine the sources with different positions and energies. Furthermore, we simulated additional detectors with different designs and compared their results to analyze the correlation between the geometric structure and performance of the detectors.
CdZnTe Association (CdZnTe.com)
https://www.cdznte.com/thesis/cdznte-semiconductor-based-dual-imager-combining-collimatorless-and-compton-imaging-monte-carlo-simulation.html