Purpose: The three-dimensional polymer gel dosimeter functions as an anthropomorphic phantom that can be applied to dose verification in clinical radiotherapy. The advantage of polymer gel dosimeters is that doses can be evaluated using multiple imaging modalities such as optical computed tomography (CT) and R₂ magnetic resonance imaging (MRI). In this study, we focused mainly on a polymer gel-optical CT system, and aimed to (1) evaluate the noise characteristics of the optical CT system, and (2) improve the image quality and reduce noise by image averaging.

Materials and Methods: Noise power spectra (NPS) were calculated from reconstructed images from an unirradiated gel dosimeter. The NPS of the unirradiated gels ranging from no averaging to averaging 128 images were calculated and compared. The signal to noise ratios (SNR) for the gel dosimeters were also calculated and compared between optical CT, MRI, and X-ray CT (XCT).

Results: The noise components of the dose image from the polymer gel-optical CT system distributed from 0 to 1.5 cycles/mm. The high frequency component of 1.0-1.5 cycles/mm is considered to reflect predominantly the quantum noise component in the optical CT system, and the noise of this frequency component could be reduced by averaging. The results indicated that increasing the number of images averaged from 2 to 128 enhanced image smoothness in visual evaluations. Specifically, the SNR improved by 1.63, 1.55 and 1.51 folds for optical CT, XCT, and MRI, respectively, when averaging 8 images compared to no averaging, with optical CT showing the greatest improvement.

Conclusion: The NPS are useful to quantitatively express the characteristics of each frequency, and we have shown that they can also be analyzed for the polymer gel-optical CT system. The quantum noise component can be improved by image averaging, providing better image quality and reducing granularity of the gel images. Image averaging achieves significantly greater improvement in SNR for optical CT compared with MRI and XCT.