The aim of the present study was to evaluate the image quality of four direct digital radiographic systems. Radiographs were made of the maxillary central incisor and mandibular left molar regions of a dry skull, and an aluminum step-wedge. The X-ray generator operated at 10 mA, 60 and 70 kVp, and images were acquired with 3, 5, 8, 12, 24 and 48 exposure pulses. Six well-trained observers classified the images by means of scores from 1 to 3. Collected data were submitted to nonparametric statistical analysis using Fisher's exact test. Statistical analysis showed significant differences (p<0.01) in image quality with the four systems. Based on the results, it was possible to conclude that: 1) all of the digital systems presented good performance in producing acceptable images for diagnosis, if the exposures of the step-wedge and the maxillary central incisor region were made at 5 pulses, as well as at 8 pulses for the mandibular left molar region, selecting 60 or 70kVp; 2) higher percentages of acceptable images were obtained with the administration of lower radiation doses in CCD-sensors (charge-coupled device); 3) the Storage Phosphor systems produced acceptable images at a large range of exposure settings, that included low, intermediate and high radiation doses.
The conventional intraoral radiographic film remained the only image receptor option for almost a century. However, with the advances in radiology, much research has been done in order to enhance image quality, increase the accuracy of interpretation, reduce radiation dose as well as make it easier to obtain radiographs and decrease the time spent. As a result, direct digital intraoral radiographic systems have become a reality.
In this new imaging modality, the radiographic film is replaced by a sensor for the X-rays. The signal temporarily stored within the sensors is transferred to the computer, which displays an image that may be filed, interpreted, manipulated and quantified. The advantages of the direct digital radiographs are recognized by many authors (1-18). At present, there are two fundamentally different concepts for direct digital image acquisition, the CCD-based (charge-coupled device) and the Storage Phosphor systems.
The CCD-based systems are so-called because they use the CCD as a device for image capturing. In these systems, a chip is used as a sensor for the radiation. A cable connects the sensor to the computer and the image is displayed almost immediately on the monitor after exposure. These sensors are characterized by the reduced dimensions of their active surface in comparison with standard periapical film.
On the other hand, in the Storage Phosphor systems, a photostimulable storage phosphor imaging plate is exposed to radiation and a latent image is stored. The information contained in the plate is released by exposure to a laser scanner. These imaging plates consist of a polyester base coated with a crystalline halide composed of europium-activated barium fluorohalide compounds. When an image plate is irradiated, the absorbed X-ray energy is stored within the phosphor crystals. To read the stored information, a thin collimated helium-neon laser beam scans the surface of the image plate; the stored energy is thereby released as fluorescent blue light. This light is detected by a photomultiplier and converted to electric signals, which are subsequently digitized (19). Photostimulable storage phosphor imaging plates are characterized by having dimensions similar to those of the 1.2 periapical film (approximately 30 x 40 mm).
Since the end of the 1980's, direct digital radiographic systems of the CCD and Storage Phosphor types have been launched worldwide. However, recently, with the launching of a new Storage Phosphor system — DenOptix, the authors proposed to develop this study with the objective of making a comparative evaluation of the images produced by two CCD-based and two Storage Phosphor systems, employing a phantom dry skull, however, having fundamentally clinical implications.
 

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