How Does Ambient Light Affect the Image Quality of Phosphor Plate Digital Radiography? A Quantitative Analysis Using Contemporary Digital Radiographic Systems

Abstract

The aim of this study is to quantitatively evaluate the influence of the duration of ambient light exposure on the image quality of digital radiographs obtained with contemporary phosphor plate (PSP)-based systems. Radiographs of an aluminum step-wedge were obtained using VistaScan and Express systems at five X-ray exposure times: 0.10, 0.20, 0.32, 0.40, and 0.50 s; the resulting dose-area products were, respectively, 21.93, 43.87, 70.19, 87.75, and 109.6 mGycm². Before PSP read-out, half of the sensitive surface of the PSP plates was exposed to ambient light for 5, 10, 30, 60, and 90 s. The effect of light exposure on brightness, contrast, contrast-to-noise ratio (CNR), signal-to-noise ratio (SNR), and image saturation was compared using ANOVA with the Tukey test (α = 0.05). Ambient light exposure increased brightness and contrast and reduced CNR and SNR in PSP-based radiographs of contemporary digital systems. At the longest X-ray exposure times, ambient light exposure reduced the dark saturation (mean gray values ≤ 1) observed in Express. In conclusion, the negative effects of ambient light observed on the image quality of PSP-based radiographs are directly proportional to the duration of exposure. Clinicians should be aware of such harmful effects when handling and scanning PSP plates in bright environments.

Keywords: digital dental radiography; image processing; light.

Figure 1

Setup for X-ray and light exposures. (A,B). Upper and lateral view of the aluminum step-wedge longitudinally fixed in the center of a sheathed size-2 PSP plate for X-ray exposure. (C). Upper view of the PSP plate with half of the sensitive surface covered with black paperboard for ambient light exposure. (D). Representative radiograph with the 18 ROIs: 9 in the exposed-to-light half (full-line rectangles) and 9 in the non-exposed-to-light half (dotted-line rectangles).

Figure 2

Representative radiographs of each experimental condition obtained using VistaScan. All images are shown using the same display window and level.

Figure 3

Representative radiographs of each experimental condition obtained using Express. All images are shown using the same display window and level.

Figure 4

Bar graphs showing mean values of brightness and contrast discrepancies (in gray values) and standard deviations (error bars) as a function of exposure times of X-rays and ambient light for both digital radiographic systems.