Abstract
To assess the effect of digital enhancement on the image quality of radiographs obtained with photostimulable phosphor (PSP) plates partially damaged by ambient light. Radiographs of an aluminum step wedge were obtained using the VistaScan and Express systems. Half of the PSP plates was exposed to ambient light for 0, 10, 30, 60, or 90 s before being scanned. The resulting radiographs were exported with and without digital enhancement. Metrics for brightness, contrast, and contrast-to-noise ratio (CNR) were derived, and the ratio of each metric between the exposed-to-light and non-exposed-to-light halves of the radiographs was calculated. The resulting ratios of the radiographs with digital enhancement were subtracted from those without digital enhancement and compared among each other. For the VistaScan system, digital enhancement partially restored brightness, contrast, and CNR. For the Express system, digital enhancement only restored CNR and not the impact of ambient light on brightness and contrast. Specifically, digital enhancement restored 23.48% of brightness for the VistaScan, while percentages below 1% were observed for the Express. Digital enhancement restored 53.25% of image contrast for the VistaScan and 5.79% for the Express; 40.71% of CNR was restored for the VistaScan, and 35% for the Express. Digital enhancement can partially restore the damage caused by ambient light on the brightness and contrast of PSP-based radiographs obtained with the VistaScan, as well as on CNR for the VistaScan and Express systems. The exposure of PSP plates to light can lead to unnecessary retakes and increased patient exposure to X-rays.
Keywords: Dental digital radiography, Light, Image processing, Radiographic image enhancement
Introduction
Photostimulable phosphor (PSP) radiographic systems are widely employed in routine dental practice to indirectly generate intraoral radiographs [1, 2]. These systems utilize PSP plates as image receptor, which have shown partial sensitivity to ambient light [3]; therefore, exposing a PSP plate to light during the period between X-ray exposure and insertion into the scanner can potentially compromise image quality [3–5]. This deterioration can lead to the generation of fading artifacts [3, 6–8], which have shown to impact crucial quantitative image quality metrics, including image brightness, contrast, signal-to-noise ratio, and contrast-to-noise ratio for the VistaScan and Express systems [9], as well as the contrast-to-noise ratio for the DenOptix and ScanX systems [4]. Despite most manufacturers already advising against direct light exposure for PSP plates, the practical reality in dental offices often appears to overlook this concern. Interestingly, a recent study showed that radiographs obtained with PSP plates exposed to light for more than 5 s exhibited reduced perceived overall image quality [10].
The ability to digitally enhance radiographic images is a valuable feature of digital radiography [11–14], and such enhancement can be performed either automatically or manually, with the latter relying on the subjective perception of the observer [15]. Additionally, presets can be used to simplify and standardize the image enhancement process [16]. Digital enhancement has been shown to be specific for each digital radiographic system [12, 13] and has proven effective in improving the objective and subjective image quality in radiographs obtained under suboptimal exposures [14, 17, 18].
The impact of exposing a PSP to ambient light on image quality has been found to vary among digital radiographic systems [10]. Notably, despite the limited information available from the manufacturers regarding the technical specification of PSP plates, a recent study evaluating the microstructure, chemical composition, and image quality of multiple PSP plates found that the VistaScan system exhibits more uniform pixel values, less noise, and smaller, thicker, and more standardized granules when compared to the Express system [19].
Even with strictly controlled quality assurance measures [15], light exposure can occur in clinical situations when handling PSP plates before scanning. Recognizing the value of minimizing the need for retaking radiographic images, the present study hypothesized that image enhancement could potentially restore or partially restore radiographs obtained using PSP plates exposed to light. It is important to evaluate and draw evidence-based conclusions on whether digital enhancement can effectively restore the image quality of PSP-based radiographs that have been damaged by ambient light. Therefore, the aim of this study was to quantitatively assess the effect of digital enhancement on the brightness, contrast, and contrast-to-noise ratio (CNR) of radiographs obtained with PSP plates that were partially exposed to ambient light.
Methods and Materials
Image Acquisition
Ten repeated digital radiographs were acquired of an aluminum step wedge, centrally positioned over the PSP plate of two digital radiographic systems: VistaScan Perio Plus (Durr Dental, Beitigheim-Bissingen, Germany) and Express (Instrumentarium Imaging, Tuusula, Finland). The X-ray source was obtained with the Focus unit (Instrumentarium, Tuusula, Finland), adjusted to operate at 70 kVp, 7 mA, and 0.20 s of X-ray exposure.
Light Exposure, Scanning, and Digital Enhancement
After X-ray exposure, the PSP plates had half of their sensitive surface exposed to light from a fluorescent lamp with an intensity of 80 lx, positioned 2.5 m away, for four different durations: 10, 30, 60, and 90 s, as described in a formerly published methodology [9]. The PSP plate coverage was longitudinal, symmetrically covering the latent image of the aluminum step wedge. The control group comprised 10 PSP plates from each digital radiographic system that were not exposed to ambient light.
Following ambient light exposure, the PSP plates were immediately scanned, and the resulting radiographs, without any pre-processing, were exported as Tagged Image File Format (TIFF) files in both their original form without digital enhancement and with digital enhancement. This process resulted in a total of 160 radiographs (10 repetitions × 2 digital radiographic systems × 4 ambient light exposure duration × 2 enhancement situations). Light exposure and scanning occurred in a lightproof environment to prevent possible interference from external light sources. Representative radiographs of each experimental condition can be observed in Fig. 1.
Fig. 1.
Representative radiographs of each experimental condition
Image Evaluation
Using ImageJ software [20] (National Institutes of Health, Bethesda, MD, USA), eighteen regions of interest (ROI) were selected, such that one ROI was selected in the exposed-to-light half and the other ROI in the non-exposed-to-light half of each step of the aluminum step wedge. From each ROI, mean gray values (MGV) and standard deviation of gray values were obtained to calculate brightness, contrast, and CNR.
Brightness was determined by averaging the MGV, while contrast was calculated by subtracting the MGV from adjacent pairs of aluminum steps. To calculate CNR, MGV from adjacent pairs of aluminum steps were subtracted individually and then divided by the average of the standard deviation values of the same steps.
The ratios of brightness, contrast, and CNR values were calculated with and without digital enhancement for each ambient light exposure duration; this was achieved by dividing the values from the exposed-to-light half by those from the non-exposed-to-light half. Subsequently, the resulting ratios, expressed as a percentage, of the radiographs with digital enhancement, were subtracted from those without digital enhancement. Furthermore, brightness, contrast, and CNR ratios obtained in the radiographs with digital enhancement were compared with those obtained from the radiographs without digital enhancement for each light exposure time, using the absence of exposure as the reference. A flowchart outlining each step of the methodological design is presented in Fig. 2.
Fig. 2.
Flowchart of the methodological design: Ten repeated digital radiographs were acquired of an aluminum step wedge. The PSP plates had half of their sensitive surface exposed to ambient light for four durations (10, 30, 60, and 90 s) and were scanned using two digital systems (VistaScan and Express). The resulting images were exported with and without digital enhancement, and three quantitative metrics were obtained (brightness, contrast, and contrast-to-noise ratio)
Results
Overall, the most expressive restorative effect of digital enhancement on brightness, contrast, and CNR was observed with the longest durations of ambient light exposure. For the VistaScan system, digital enhancement partially restored the effect of ambient light on brightness, contrast, and CNR by bringing the ratios between the exposed-to-light half of the PSP plate and the non-exposed-to-light half closer to 1, which is the condition observed in the absence of light exposure. In contrast, for the Express system, digital enhancement only restored the impact of ambient light on CNR and not on brightness and contrast (Fig. 3).
Fig. 3.
Line graphs illustrating the ratios of brightness, contrast, and contrast-to-noise ratio between exposed-to-light and non-exposed-to-light halves of PSP plates under three conditions: no light exposure, light exposure with digital enhancement, and light exposure without digital enhancement. The ratios are analyzed as a function of the digital system
The most pronounced restorative effect of digital enhancement on brightness for the VistaScan system was observed at 90 s of ambient light exposure, in which image enhancement was able to reduce light damage by restoring 23.48% of brightness (Table 1). In contrast, for the Express, restoration percentages below 1% of brightness were observed, irrespective of the duration of ambient light exposure (Table 2).
Table 1.
Restoring rates (in percentage) on brightness, contrast, and contrast-to-noise ratio (CNR) of digitally enhanced radiographs as a function of the ambient light exposure duration for VistaScan system
| Image quality metric | Ambient light exposure durations (s) | |||
|---|---|---|---|---|
| 10 | 30 | 60 | 90 | |
| Brightness |
0.10 6.34 0.00 |
9.99 30.24 19.88 |
17.50 53.25 40.71 |
23.48 49.36 9.60 |
| Contrast | ||||
| CNR | ||||
Table 2.
Restoring rates (in percentage) on brightness, contrast, and contrast-to-noise ratio (CNR) of digitally enhanced radiographs as a function of the ambient light exposure duration for Express system
| Image quality metric | Ambient light exposure durations (s) | |||
|---|---|---|---|---|
| 10 | 30 | 60 | 90 | |
| Brightness |
0.07 0.07 0.00 |
0.03 0.09 16.00 |
0.52 0.08 35.00 |
0.47 5.79 30.00 |
| Contrast | ||||
| CNR | ||||
Concerning contrast, the highest restorative effect of digital enhancement was observed at 60 s of ambient light exposure for the VistaScan system, resulting in a restoring rate of 53.25%. For the Express system, the maximum restoration rate was 5.79% at 90 s of ambient light exposure. As for CNR, the VistaScan and Express systems achieved their highest restoration rates of 40.71% and 35%, respectively, at 60 s of ambient light exposure.
Discussion
The image quality of radiographs based on PSP plates is affected by ambient light exposure; the impact of such exposure on image quality has been investigated in previous studies, which evaluated artifact formation [3, 6–8], brightness, contrast, contrast-to-noise ratio, image saturation, and signal-to-noise ratio [9]. The outcomes of the present study regarding the impact of PSP plate exposure to ambient light in brightness, contrast, and contrast-to-noise ratio are similar to those obtained in previous studies [4, 9], i.e., longer ambient light exposures had a greater negative impact on image quality. However, our study differs from the previous ones; to the best of our knowledge, no study has yet assessed whether digital enhancement can minimize or eliminate the impairments caused by ambient light exposure and improve image quality. We observed that digital enhancement did restore the values of brightness, contrast, and contrast-to-noise ratio. However, this restorative effect was achieved to a limited extent.
As light exposure leads to information erasure in the PSP plate [4], our initial hypothesis was that digital enhancement could partially restore lost information. For the two assessed digital radiographic systems, the maximum restoration achieved through digital enhancement was 23.48% for brightness, 53.25% for contrast, and 40.71% for CNR. Overall, for the VistaScan system, the positive effect of digital enhancement increased as light exposure duration increased. A similar effect was noticed only for CNR in the Express system. Nevertheless, with longer durations of ambient light exposure, the discrepancy between ratios from radiographs taken with exposed-to-light PSP plates and under digital enhancement, compared to those from the reference standard (radiographs taken without light exposure), became more pronounced.
The ambient light exposure times used in the present study were derived from a previous study [10], which objectively assessed the image quality of PSP plates exposed to ambient light and found a more expressive negative impact on those exposed to light after 5 s. Considering the clinical reality, ambient light exposure times of 30, 60, and 90 s simulated situations commonly encountered in dental educational institutions and workplaces. The light exposure time of 10 s was simulated to replicate situations when the opaque protective envelope is slightly unsealed or when the PSP scanner is configured in a manner that exposes the plate partially to ambient light while being positioned in the scanner’s transport slot aperture. Conversely, the extended durations (30–90 s) replicated the time taken when the professional uses multiple PSP plates as in a full mouth series, and the first exposed PSP plate is in a loose opaque envelope and has to wait to be scanned.
Digital enhancement offers a favorable alternative provided by digital radiographic systems which can be applied after image acquisition, but before the radiograph is displayed on the monitor [14, 21]. It enhances diagnostic accuracy for proximal caries due to insufficient exposures [22]. However, it has also been demonstrated not to interfere with the same diagnostic task [23] and alveolar bone-level measurement [24]. In our study, digital enhancement increased contrast, likely attributable to for nonlinear pixel value modification, resulting in high-contrast radiographs [15, 24, 25]. Regarding CNR, the use of digital enhancement leads to a lower value due to increased noise [21]. However, considering that ambient light exposure also decreases CNR [9], we believe this is why digital enhancement could restore CNR in our study.
Digital enhancement’s variability among digital radiographic systems, with each system exhibiting inherent characteristics influencing the final gray values (e.g. contrast resolution, dynamic range, and integrated imaging software) [14], likely accounts for the different behaviors observed in the VistaScan and Express systems. A previous study revealed that the Express system has a broader dynamic range than the VistaScan and that digital enhancement can transform an unaceptable radiograph into an acceptable one in the VistaScan system [18]. In the present study, digital enhancement in the VistaScan system brought brightness, contrast, and CNR ratios closer to the reference standard established in the absence of light exposure. For the Express system, minimal differences were observed in brightness and contrast ratios, while a more notable effect was observed in CNR.
Due to the in vitro nature of the present study, which utilized an aluminum step wedge, it was possible to specifically evaluate the impact of digital enhancement on image quality in radiographs obtained with PSP plates exposed to light. Considering the objective assessment of this in vitro study, the possible influence of ambient light on the PSP-based image of the dental tissues was not evaluated, thereby limiting the drawing of any conclusions regarding dental diagnostic tasks. Furthermore, our use of two distinct digital radiographic systems underscores the need for caution when extending the present findings to other systems, given the variety in technical aspects across different PSP systems [19]. Further studies are encouraged to qualitatively assess the impact of ambient light exposure on PSP plates with and without digital enhancement in different diagnostic tasks.
Conclusion
Digital enhancement can partially restore the damage caused by ambient light on the brightness and contrast of PSP-based radiographs obtained using VistaScan and on the CNR for those obtained with the VistaScan and Express systems. The exposure of PSP plates to light can result in unnecessary retakes and increase patient exposure to X-rays.
Author Contributions
Sampaio-Oliveira, M. He conceived, did the literature review, performed experimental activities, interpreted the data, drafted the manuscript and did the final revision of the paper. Vieira-Marinho, L.E. He performed experimental activities, evaluated and interpreted the data, drafted the manuscript and approved the paper. Barros-Costa, M. He conceived, did the literature review, drafted the manuscript and approved the paper. Oliveira, M.L. He conceived, did the literature review, performed experimental activities, interpreted the data, drafted the manuscript and did the final revision of the paper.
Funding
This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior – Brazil (CAPES) – Finance code 001.
Declarations
Ethics Approval
Not applicable.
Consent to Participate
Not applicable.
Consent to Publish
Not applicable.
Conflict of Interest
The authors declare no competing interests.
Footnotes
This manuscript has not been published and is not under consideration for publication elsewhere.
Publisher's Note
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