Comparative Study of Two Digital Radiographic Storage Phosphor Systems
 
 

Ana Emília OLIVEIRA
Solange Maria de ALMEIDA
Gisela André PAGANINI
Francisco HAITER NETO
Frab Norberto BÓSCOLO

Departamento de Diagnóstico Oral, Universidade Estadual de Campinas, Piracicaba, SP, Brasil


Braz Dent J (2000) 11(2): 111-116 ISSN 0103-6440

Introduction | Material and Methods | Results | Discussion | Resumo | References


The objective of this study was to evaluate the image quality and dynamic range of two digital radiographic storage phosphor systems: Digora (Soredex, Finland) and DenOptix (Gendex, USA). Four objects were analyzed and eight exposure times employed, totaling 64 images that were analyzed by 5 examiners using a scale from 1 to 4 to classify the images. The scores were submitted to analysis of variance and the results showed statistical differences among the averages attributed to the systems, to the exposition times and to the objects (p£0.01). Digora presented a larger dynamic range and, in general, a better image quality. Although these 2 systems present the same photon detector, they present different results in relation to the evaluated items.


Key Words: dental radiography, radiographic image enhancement.


Introduction

Currently there are 2 different concepts of photon detector for direct digital image acquisition, the CCD (charge-coupled device) and the storage phosphor plate. Most digital systems commercially available are CCD systems, and the image is displayed almost immediately on a computer monitor after exposure of the sensor. However, these systems have several disadvantages such as a relatively bulky detector, the need for a wire connection between the sensor and the computer, a small active area of photon detector and a relatively narrow dynamic range (Grondal et al., 1996; Huda et al., 1997; Wenzel, 1998).

The storage phosphor system was first introduced by Fuji (Tokyo, Japan) in 1981 (Svanaes et al., 1996), but only in the 1990s did it become a reality in intra-oral digital radiographic images, with the release of the Digora system by Soredex (Helsinki, Finland). This image plate consists of phosphor particles embedded in a polymer binder and coated onto a plastic base. Photostimulable luminescence is used to store about 50% of theX-ray energy on the screen producing a latent image (Grondal et al., 1996). The information contained in the plate is released by exposure to a laser scanner. The storage phosphor plate is approximately the same size and bulk as conventional film, there are no connecting wires and the exposure latitude is theoretically extremely wide (Wenzel and Grondal, 1995; Huda et al., 1997; Wenzel, 1998).

Comparative studies of the storage phosphor system and the CCD systems report the good image quality of the phosphor plate (Conover et al., 1996; Moystad et al., 1996; Van der Stelt, 1996) as well as its wide dynamic range (Wenzel and Grondal, 1995; Borg and Grondal, 1996; Versteeg et al., 1997). Recently, Gendex (Gendex Dental X-Ray Division/Dentsply International, Des Plaines, IL, USA) released a new digital system of storage phosphor, DenOptix.

The objective of this research is to evaluate these two storage phosphor systems (Dignora and DenOptix), in terms of the image quality, the dynamic range, and some available resources, acquisition time of the image and some sensor characteristics.


Material and Methods

The storage phosphor systems used in this study were Digora (Soredex) and DenOptix (Gendex). The photon detector used in the Digora system is the phosphor plate similar to standard n° 2 film with the following characteristics: matrix - 416 x 560 pixels; pixel size - 70 x 70 µm; image file - 228 kB in TIFF format. The Digora "calibration mode" was defined in 1 s. The sensor used in the DenOptix system was also the phosphor plate of 31 x 41 mm, with a standard resolution of 300 dpi, matrix of 367 x 485 pixels, pixel size of 85 x 85 µm and image file of 175 kB in TIFF format. An S-VGA monitor, 17-inch plane screen, screen configuration of 1024 x 768 resolution pixels and video plate of 2 mB were coupled to the systems.

Four objects were analyzed: an aluminum stepwedge, a molar area of dry mandible with simulation of soft tissue, three teeth with simulated periapical lesions set in a plaster and sawdust mold for bone simulation, and an aluminum block containing six 0.5 mm wide holes, varying in depth from 0.5 to 3 mm in 0.5 steps.

The X-ray machine used was GE 1000 (General Electric Company, Milwaukee, WI, USA), with the following parameters: 65 kVp, exposure times of 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.8 and 1 s, and a distance of the focal point of 32 cm. Evaluation of the entrance beam dose was made in order to evaluate the relationship between exposition time and dose. The "ion chamber" (Victoreen 06-526; Cleveland, OH, USA), the "X-ray timer" (Victoreen 07-457) and the "kVp meter" (9002; Unfors Instruments, Billdal, Sweden) were used. The applied X-ray machine showed accuracy in relation to exposure time, kilovoltage and dosage with a good linear relationship between radiation dosage and exposure time (Figure 1).

Each object was exposed 8 times in each system, for a total of 64 images. For the dry mandible and set teeth exposures, the phosphor plate was positioned parallel to these objects, with the central beam entrance perpendicular to the object and the plate. The metallic objects were positioned in the central part of the sensor, and exposed at a vertical angle of 90° and horizontal of 0°.

Radiographic analysis was made by five radiologists that used a scale from 1 to 4 to classify the images: 1 - image without the smallest diagnosis possibility; 2 - image with poor diagnosis quality; 3 - image in reasonable conditions for diagnosis; 4 - image in ideal conditions for diagnosis. To avoid differing results among the examiners, software training was given prior to analysis. In this training a group of inherent conditions of the images was standardized, defining a limit for each score, allowing uniformity of image analysis. After training, the examiners were tested and showed satisfactory intra- and inter-examiner results.

Evaluation of images emphasized detection of object details and sharpness. The images were analyzed with software, with reduced room light and monitor brightness. Only the manipulation of brightness and contrast was allowed to avoid the lack of familiarity with the other software resources which could distort the final result. The scores were registered in tables and the data were submitted to statistical analysis (analysis of variance).


Results

Five examiners scored the images of four objects (set up teeth, molar, aluminium stepwedge and block with holes) from 1 to 4 for the two systems, DenOptix and Digora, at 8 exposition times. These scores were submitted to analysis of variance (Table 1) which showed statistical significance for the scores of systems, exposure times and objects. In general, the Digora system produced images with better quality than DenOptix which is clearly shown by more scores of 3 and 4 indicating diagnostic value (Table 2).


Discussion

To evaluate image quality and dynamic range of the 2 storage phosphor systems, it was important that the objects be diversified, because different objects require different exposure times and efficiency levels of the systems to obtain the best image quality, which increases the reliability of the present results. Subjective analysis was chosen for its correlation with dental practice.

In spite of the fact that both systems offer the same principle of image acquisition, the results in relation to the dynamic range of the systems were different, with the Digora system presenting the best performance. This system has a "calibration mode" command that allows the exploration of its wide dynamics range, by means of an adjustment, that makes adaptation of the program possible at the determined exposure time. Digora should be gauged for greater exposure times to explore its latitude to a maximum (Versteeg et al., 1998). In this study, the calibration of 1 s was used. The absence of this automatic calibration resource in DenOptix system reduced its general efficiency with an increase in exposure time. Images with adequate contrast were obtained with Digora, at exposures of 10 to 100% of the calibrated maximum as also reported by others (Hayakawa et al., 1998).

Another important factor which contributed to the results is the way Digora makes the image readout. The Digora scanner first measures the X-ray intensity used in the exposure. This is called the preread phase. Digora uses the preread information to set the final readout level (Digora, 1994).The Digora System captures the images with a dynamic range of 4096:1 and then windows the information into a 256:1 image. This allows the system to correct automatically for overexposure or underexposure (Vandre and Webber, 1995). However, darkness in DenOptix images was observed with an increase in exposure time, where recovery was not possible for diagnosis using "the brightness and contrast" tool. This resource and the equalize function performed well only in cases of images with low density.

DenOptix offers an interesting resource to acquire intra-oral images with different resolutions, in this case, 150, 300 and 600 dpi, leading to a variation in the pixel size, in the spacial resolution of the image (lp/mm) and in its matrix size (DenOptix, 1998). These factors act as intimately related concepts, with direct influence on scanning of the image plate time, the image magnification and the image file, allowing the professional to adjust the resolution in agreement with the interest and demands of the object.

The acquisition time average of digital images for the Denoptix and Digora are higher than CCD systems. For the standard resolution (300 dpi) in DenOptix, scanning takes less than 1.5 min or more than 5 min, according to the number of plates mounted in the intraoral carousel (DenOptix, 1998). Digora accomplishes the readout of its sensors in about 25 seconds; however, this readout is always done individually. Therefore, in terms of image acquisition time, the advantage of one system over another will depend on the number of images to be acquired. However, one important detail to note is that Digora erases the residual energy from the image plate by exposing it to bright light after the readout (Digora, 1994), while DenOptix requires a lightbox to eliminate residual energy from its plates, and these plates must remain two or more minutes, according to the lightbox intensity. This time must be added to the total time of image acquisition.

In relation to the image quality, at all exposure times, Digora provided radiographic images in better diagnostic conditions than DenOptix. The Digora image quality has been rated high, presumably due to the wide dynamic range of the storage phosphor system (Grondal et al., 1996), and this system can provide similar image quality, irrespective of whether the exposure is relatively high or low, in function of its wide recording latitude (Hayakawa et al., 1998). However, the present results showed that DenOptix can offer good image quality, since the selected exposure time respects the limit of its dynamic range, that can be increased or not, in agreement with the object to be exposed.

It is important to cite that the storage plate of the DenOptix system presents some advantages in relation to Digora, such as greater flexibility, smaller external dimensions for not containing borders around its activated face and a smaller thickness, equivalent to standard film.

Therefore we conclude that despite the fact that these systems present the same photon detector, they have peculiarities so that they present different behaviors in relation to the evaluated conditions.


Resumo

Oliveira AE, Almeida SM, Paganini GA, Haiter Neto F, Bóscolo FN: Estudo comparativo de dois sistemas radiográficos digitais de armazenamento de fósforo. Braz Dent J 11(2): 111-116, 2000.

O objetivo deste estudo foi avaliar a escala dinâmica e a qualidade das imagens radiográficas de dois sistemas digitais de armazenamento de fósforo, a citar, Digora e DenOptix. Empregou-se quatro objetos de análise e oito tempos de exposição, totalizando 64 imagens para o estudo, que foram avaliadas por 5 profissionais que se utilizaram de uma escala de classificação de 1 a 4. Os escores aplicados foram submetidos a uma análise de variância e os resultados mostraram diferença estatística em relação as médias atribuídas aos sistemas, aos tempos de exposição e aos objetos (p£0.01). Os resultados gerais, apresentaram o Digora com uma melhor qualidade de imagem e escala dinâmica, demonstrando que apesar destes sistemas apresentarem em comum o mesmo fóton-detector, eles apresentaram diferentes resultados em relação aos itens avaliados.


Unitermos: radiografia dentária, intensificação da imagem radiográfica.


References

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Conover GL, Hildebolt CF, Yokoyama-Crothers N: Comparision of linear measurements made from storage phosphor and dental radiographs. Dentomaxillofac Radiol 25: 268-273, 1996
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Digora Instruction Manual. Soredex, Helsinki, Finland 1994
Gröndal HG, Wenzel A, Borg E, Tammisalo E: An image plate system for digital intra-oral radiography. Dent Update 23: 334-337, 1996
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Correspondence: Frab Norberto Bóscolo, Faculdade de Odontologia de Piracicaba, Avenida Limeira, 901, Areião, 13400-900 Piracicaba, SP, Brasil. Tel: +55-19-430-5203. Fax: +55-19-430-5218. E-mail: boscolof@fop.unicamp.br


Accepted June 1, 2000
Eletronic Publication october, 2000


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