Impact of mask thickness on the imaging performance of the coded-aperture gamma camera

Abstract

Coded-aperture gamma imaging techniques have been applied widely in several radiation monitoring fields. However, in terms of the design of the coded-aperture mask, the category and rank of the mask are generally considered, but there is a lack of detailed research on selection of the appropriate mask thickness. Concretely, the imaging performance is affected by the mask thickness in terms of two factors: the full-field image quality of the overall field of view (FOV), which represents the background noise tolerance of the radioactive source; and the image brightness consistency, which represents consistency of the imaging efficiency. We have studied and analyzed these two imaging performance indexes in relation to the mask thickness based on the model of our custom-made gamma camera. Specifically, the full-field image quality initially improves and later deteriorates with the mask thickness increases from 0.1 mm to 30 mm because of the competition and the tradeoff between the transmission background and the oblique irradiation effect; the image brightness consistency initially descends, then improves, and finally decreases again under the same conditions as a result of the tradeoff between the improvement of the transmission background ratio and deterioration of the oblique irradiation coefficient ratio. Furthermore, these two indexes change trends are both related to gamma-ray energy. Finally, based on a comprehensive analysis of the imaging performance, the optimal thicknesses for the customized W-Cu alloy modified uniformly redundant array masks for 59.5 keV, 662 keV, 1250 keV gamma-ray imaging are 0.5 mm, 10 mm, and 16 mm, respectively.

A novel method for assessing imaging quality of coded-aperture gamma-ray camera to radioactive plane source

“…line source -had PSNR values better than 20 dB, whereas those of the rectangular loop line sources#1 and octangular loop line sources were better than 18 dB, implied that with more complex plane source shapes, the imaging quality becomes worse due to more superposed and enhanced background noise. Besides, the PSNR of rectangular loop line source#2 with the largest area was only 17.1 dB, which was worse than that of rectangular loop line source#1, implied that the larger the plane source area, affected by the collimation effect under oblique irradiation [13], image quality becomes worse. Moreover, the SSIM values of the seven experimental images were all above 0.996.…”

“…Based on the advantages of wide energy detection range and high angular resolution of coded-aperture gamma-ray imaging camera [8][9][10], we developed custom-made coded-aperture camera [16] for imaging application on the complex radiation plane source monitoring of nuclear energy facilities, thus the camera was used to conduct the study on assessing imaging quality of coded-aperture gamma-ray camera to radioactive plane source. The custom-made coded-aperture camera (figure 1(a)) consists of three components: a coded-aperture collimator, a position-sensitive detector, and a data acquisition board [13]. Made of W-Cu alloy, the collimator was fabricated from the mask of a mosaic Rank 11 modified uniformly redundant array (MURA) and has a unit size of 4 × 4 × 5 mm 3 (figure 1(b)).…”

“…The position-sensitive detector (figure 1(c)) consists of a 22 × 22 CsI(Na) crystal array with a pixel size of 2 × 2 × 10 mm 3 coupled with an 8 × 8 SiPMs array. The focal length of the camera was set to 82 mm instead of 60.5 mm in [13], therefore the nominal angular resolution and field of view (FOV) of the camera is 3 • and 30 • , respectively.…”

“…in figure 5, the New CNRs of the four point sources located at (0 • , 0 • ), (4.5 • , 4.5 • ), (9 • , 9 • ), (13.5 • , 13.5 • ) in figure 5 were 51.8, 40.3, 38.5, and 30.8, respectively. As a result of the collimation effect under oblique irradiation [13], while the source moved away from the center of FOV, as the mask has a certain thickness, the gamma-ray path through the hole element of the mask was collimated, which deteriorated the projection image and thus the quality of reconstructed image became worse. Thus, our gamma-ray camera provides a reliable imaging capability and position accuracy.…”