Dentomaxillofacial Radiology (2009) 38, 42-47
© 2009 British Institute of Radiology
doi: 10.1259/dmfr/61622880
How many times can we use a phosphor plate? A preliminary study
S Ergün*,1,
P Güneri1,
D 
lgüy2,
M lgüy2 and
H Boyac
o
lu3
1Faculty of Dentistry, Department of Oral Diagnosis and Radiology, Ege University, Bornova 35100, Izmir, Turkey; 2Faculty of Dentistry, Department of Oral Diagnosis and Radiology, Yeditepe University, Göztepe, Ìstanbul, Turkey; 3Department of Statistics, Faculty of Science, Ege University, Bornova 35100, Izmir, Turkey
*Correspondence to: Dr Selin Ergün, Department of Oral Diagnosis and Radiology, Faculty of Dentistry, Ege University, Bornova 35100, Izmir, Turkey;. E-mail: selin.gogus{at}ege.edu.tr
Received 24 September 2007;
revised 13 January 2008;
accepted 27 January 2008
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Abstract
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Objectives: Digital radiography has become a useful tool in daily dental practice due to the advances in imaging technologies. Charge coupled devices (CCDs) and photostimulable phosphor plates (PSPs) are currently in use for dental imaging; however, the longevity of PSPs in dental practice is not yet established. The aim of this study was to determine the service life of PSPs in a clinical setting.
Methods: Five unused PSPs were exposed with a conventional X-ray device and converted into digital images with Digora Optime (Soredex, Milwaukee, WI). These were recorded as the baseline images. Subsequent digital images of the plates were obtained after 20, 40, 60, 80, 100, 120, 140, 160, 180 and 200 exposures. All radiographic images were subtracted from the first digital image (baseline) and the mean grey values (MGVs) of the subtracted images were established using software. The data were grouped in 3 classes according to the number of exposures (20–80; 100–140; 160–200), and were analysed using variance analysis and 
2 tests.
Results: The MGVs of the subtracted images varied between 126.25 and 127.59, and the difference was not significant among the groups (P = 0.11). However, the differences between the MGVs of the plates on each exposure settings were significantly different than those of the baseline image (P < 0.05).
Conclusions: The findings of this study revealed that even though a slight deterioration occurred after the first exposure, each plate can be used up to 200 times. Further studies are required to reach a more concrete conclusion.
Keywords: digital image, storage phosphor plates, subtraction radiography, durability
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Introduction
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Historically, the first scanned storage phosphor radiographic system was patented in 1975 (Eastman Kodak Co., Rochester, NY) and the first commercial storage phosphor imaging system was introduced by the Fuji Corporation (Tokyo, Japan) in 1981. The first photostimulable phosphor (PSP)-based intraoral system, Digora (Orion Co./Soredex, Helsinki, Finland), became commercially available in 1994.1, 2 Systems using photostimulable phosphor plates (PSPs) – sometimes also referred to as "storage phosphor plates" – utilize a plate covered with phosphor crystals.3 The plates are light sensitive and, therefore, exposure to ambient light must be minimized in the period between removal from their protective cover and placement in the scanner. The length of time that plates are exposed to ambient light during this transfer process determines the level of allowable ambient light at the scanner location. Scanners in which plates are loaded directly into a slot can generally be used in areas of higher ambient light compared with systems in which the plates are loaded on drums prior to placement in the scanner.4
Major advantages of the PSP image receptor are the flexibility of the plates and absence of an electrical cord, which significantly impacts the ease of receptor placement. However, the impact of this flexibility on receptor longevity has not been determined.2, 3, 5 The primary disadvantage of PSP plates is the need for additional time for handling and scanning prior to the final viewing of the radiographic image. Also, plates must be exposed to light to erase the residual image before reusing them. Lastly, the resolution of storage phosphor images is somewhat less than conventional radiographic images.1, 5 It has been reported that phosphor plates can potentially be reused hundreds of times, but are susceptible to scratching. The digital images from phosphor plates which had been repeatedly used demonstrate a lower quality due to the appearance of scratches, which can sometimes render the resulting image completely non-diagnostic and may lead to a remake of the radiograph.1, 5
Mainly due to the presence of structured noise resulting from anatomical details projected over the area of radiographic change, visual comparison of paired radiographs corresponding to different time instances creates confusion.6 The widespread availability and improvement of digital dental imaging has made it easier to use the digital subtraction radiography technique in order to detect the subtle changes between serial radiographs.7 However, it is critical in the subtraction process to account for differences in alignment of the two images to be subtracted. It is also critical to eliminate the differences in contrast due to variations in exposure factors or development. Current methods of image alignment use computer technology to perform image translations, rotations and other more sophisticated image transformations to superimpose the two images in a subtraction pair.8
Researchers usually measure image quality by displaying the images to a group of observers who have to perform a specific diagnostic task, then comparing the observations of one or more other sensor systems with conventional film-based imaging.3 On the other hand, some other authors have utilized the changes within the mean grey values (MGVs) of the pixels of digital images and obtained objective quantitative values for comparison.1, 9, 10 However, we have not encountered any research that investigated the corruption of the plates, which may contribute to the loss of radiographic information and, consequently, to the reduction of the image quality.
The aim of this preliminary in vitro investigation was to evaluate the longevity or the service life of photostimulable phosphor plates in a clinical setting by application of subtraction radiography.
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Materials and methods
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The rationale of using subtraction radiography in order to detect the presence of any corruption of the plates may be explained as follows: in a perfect subtracted image, only the pixels which had been modified between the first and last image acquisition time are revealed, either as black or white. The pixels that showed no difference between the first and last exposures are observed as a homogeneous grey; that is, the MGVs of those pixels are represented as the threshold value, 128. Provided that a plate had remained exactly the same, the subtracted image of the baseline and the subsequent images will have no differences. The subtracted image will therefore be composed of pixels with MGVs of 128.
Employing this rationale, an aluminium stepwedge with four incremental steps (each 2 mm thick) was placed on an unused standard size-2 phosphor plate (measuring 31x41 mm) of the Digora system (Digora Optime; Soredex, Milwaukee, WI). This plate was exposed with a standard X-ray device (Trophy Radiologie; Novalix, Croissy-Beabourg, France) operating at 65 kVp, 8 mA, with a filtration of 2.5 mm Al, 0.02 s exposure time and the focus–plate distance set at 25 cm. This image was considered as the "baseline image" (Figure 1
). Each plate was marked in order to provide subsequent evaluation of the same plate. Then, the plates were used for actual clinical patients. After 20, 40, 60, 80, 100, 120, 140, 160, 180 and 200 exposures, subsequent digital images of the same plate were obtained for the study. Here, the aluminium stepwedge was used for the standardization of the density of the images and to aid to mark the region of interest to measure (Figure 2).
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Figure 2 (a) Subsequent digital image of the plate obtained after the 20th exposure. (b) Subsequent digital image of the plate obtained after the 200th exposure
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The same procedure was repeated for 4 additional unused phosphor plates under the same exposure conditions; thus a total of 5 plates were used throughout the test period in order to obtain 55 digital images (5 baseline and 50 subsequent images).
The resulting images were transferred as 8-bit TIFF files to a personal computer (2.6 GHz Pentium IV CPU, 12 MB of RAM, 40 GB hard disc), converted to RGB format and saved in BMP files with Photoshop 8.0 (Adobe Inc., San Jose, CA). Consequently, all radiographic images were subtracted from the baseline image of its own series by using newly developed software for digital subtraction radiography (Figure 3). This program had the functions of histogram smoothing and image warping for the standardization of the radiographic density, contrast and the geometric alignment of the images using a minimum of four reference points. With this program, histogram smoothing was performed using an improved version of Ruttiman's algorithm that was based on smoothing the empirical distribution of the reference image by using Cardinal splines. Image warping was accomplished by affine, bilinear and biquadratic transforms.11 Correction of contrast and density can be done manually, but some software programs such as ours have the possibility to do a standardized contrast and brightness optimization, which will result in an objective and reproducible image quality.
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Figure 3 All radiographic images were subtracted from the first digital image (baseline) using newly developed software for digital subtraction radiography
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In order to evaluate the "resultant" subtracted images, a standard metric was required to measure the overall difference between the baseline and subsequent images. For this purpose, a well-known criterion that is commonly used to assess the quality of reconstructed images compared with the original ones was preferred: the peak signal-to-noise ratio (PSNR).12, 13 It is known that the more the modified image resembles the reference image, the higher the resulting PSNR value should be; and for a greyscale image, the typical PSNR values range between 20 and 40.14 The PSNR values of the resultant images were equal or over 22 for the subtracted images of plates in our study.
For all measurements, 50x50 pixel areas located above the junction of the second and third steps of the aluminium stepwedge were selected and set as the "area of interest". The MGVs, standard deviations and median values of the selected areas were established using the histogram tool of commercial software (Photoshop 8.0, Adobe Inc.). All measurements were performed by a trained and experienced oral radiologist (SE). (The histogram presents a figure to illustrate how pixels in an image are distributed by graphing the number of pixels at each color intensity level. This reveals whether the image contains enough detail in the shadows, midtones and highlights. The histogram also gives a quick picture of the tonal range of the image, or the image key type). Three measurements were performed from three different areas of interest on each image and the mean histogram value of each subtracted image was noted as "the MGV of the area" (Figure 4).
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Figure 4 The mean grey values, standard deviations and median values of the subtracted images were established with the Photoshop v8.0 histogram tool, and these values were recorded. 50x50 pixel areas were selected and set as the "area of interest". Three measurements were performed on each image
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This procedure was repeated on each set of subtracted images (the digital image of the 20th exposure was subtracted from baseline; the 40th from the baseline; and likewise, subtracting the 60th, 80th, 100th, 120th, 140th, 160th, 180th and 200th images from the baseline image). Then, data were grouped in 3 classes according to the number of exposures (20–80; 100–140; 160–200) and were analysed with variance analysis and 2 tests.
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Results
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The results of this preliminary study were valid only for the present samples and are not attributable to general samples due to the minute size of the samples and the small number of the exposure times.
The MGVs of the subtracted images in each series are presented in Table 1. After the subtraction procedures, the unchanged pixels were noted as homogeneous grey, as observed in Figures 5 and 6; however, the positioning errors of aluminium stepwedge and any artefacts were observed as either black or white areas.
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Table 1 The mean grey values (MGVs) of a perfect subtracted image should be 128. The MGVs of the subtracted images varied between 126.25 and 127.59, and were not different between each group (P = 0.11)
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The mean MGV of the first group which consisted of the subtracted images of the 20th, 40th, 60th and 80th exposures from the baseline image was 126.74. This was 126.89 for the second group, consisting of the subtracted images after the 100th, 120th and 140th exposures. The mean MGV of the third group (subtracted images after 160th, 180th and 200th exposures) was 127.45. The univariate analysis of variance revealed that differences between mean MGVs of the subtracted images among the three groups were not significant (P = 0.11).
On the other hand, the MGVs of the subtracted images on each exposure setting ranged between 126.66 and 127.33. These values were smaller than that of the baseline image, which has 128 as the mean MGV. Analysis of data using the t-test revealed that the differences between MGVs of the subtracted images on each exposure setting were significantly different than that of the baseline image (P < 0.05). This result indicated an initial deterioration of the plates after the first exposure that did not increase with subsequent exposures.
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Discussion
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Accurate oral diagnosis and treatment are very closely linked to the quality of dental radiographs. Digital image quality is a complicated process since the image is produced following a series of procedures and actions, beginning with the digital imaging system and resulting in the clinician's diagnostic decision capabilities. It is known that utilization of radiographic images of reduced quality may hide crucial diagnostic information which may alter the diagnosis and treatment planning. Therefore, a radiographic image of reduced quality usually leads to a "remake/retake" of the radiograph, resulting in unnecessary radiation exposure to the patient.5
The experiences with Denoptix PSPs (Dentsply International, Des Plaines, IL) indicated the durability of this imaging system may be limited despite the manufacturer's claim that the phosphor plates can be used indefinitely.5 Various up-to-date approaches were used to evaluate the presence of radiographic information deterioration on PSPs: Martins et al10 reported that PSPs started to lose information within 5 min of image capture and that almost half of the information was lost within 1 h. Similarly, Akdeniz et al9 had found that the latent image started to fade away when the scanning was delayed for more than 10 min with a subsequent increase in MGVs. They reported that although PSPs do not lose all their information for many days, they recommended scanning Digora PSP-IPs no later than 10 min after exposure.9 Although this agreed with the results of Ang et al1 concerning the increased pixel intensity values with DenOptix storage phosphor-based images scanned with a delay, the researchers found there was no effect on diagnostic image quality.
Employment of PSPs in a radiographic system requires many steps prior to image acquisition and, in the meantime, makes them vulnerable to the environment. Although limited, there are some reports revealing the effects of the environment and/or the image acquisition process on digital image quality.1, 9, 10 To the best of the present authors' knowledge, no previous study has investigated the presence of corruption of the plates that may contribute to the loss of radiographic information and, consequently, to the reduction of the image quality.
To our knowledge, the present study is the first to utilize subtraction radiography in order to examine the deterioration occurring between a "fresh" plate and its descendants. In this technique, subtraction of the pixel values of two images with the same projection geometry shows the differences and suppresses the structures of both images that are similar.3 Subtraction radiography therefore manages to show small differences between radiographs taken a specified time apart.
In this study, the baseline images were considered to be "fresh" since the plates had not been previously used and were kept under standard conditions to avoid any environmental effects. The digital images obtained after subsequent exposures (beginning from 20 up to 200) were saved and then subtracted from the baseline images. The resultant images were used to measure the MGVs of a certain area on the image and the results showed that the measured pixels of all images had undergone some degree of change after the first exposure of the initial plate. On the other hand, the differences between the 20th and subsequent images, up to the 200th exposure, were not significantly different.
The evaluation of the subtracted images also revealed the presence of scratches and noise on some of the final images. These artefacts may lead to reduced image quality, as suggested by Bedard et al.5 Although we agree with their conclusion, we assume that the artefacts observed in the present study may not be discernible to the observer's eye unless subtraction radiography is used.
The findings of the present study were that, even though a slight deterioration occurred after the first exposure, each plate can be used confidently up to 200 times. Unfortunately, our literature search yielded no previous studies addressing this topic to compare with the results of the present study. Knowing that 200 exposures is a very low number to investigate the presence of deformation on the plates during clinical conditions, our results will be commented upon carefully and can be accepted only as preliminary data. In addition, more detailed and large sample size studies are required to reach a more concrete conclusion.
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References
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Abstract
Introduction
