Digital radiographic method
Digital, filmless, techniques for intraoral radiography have been developed for several important reasons:
1. Conventional film absorbs only a few percent of the x-rays that reach it, utilizing very little of the radiation to which the patient has been exposed.
2. Poor darkroom procedure can lead to both unnecessarily high doses of radiation and loss of diagnostic information.
3. Development of films is time consuming, and the developer and fixing solutions are hazardous to the environment.
For intraoral radiography, digital techniques with direct image acquisition have been available only since the end of the 1980s. Research and development in indirect digital radiography paved the way for direct digital (filmless) techniques. The first to become available, in 1989, was based upon a charged-coupled device (CCD) chip similar to that found in digital videocameras. In most CCD systems, no more than one molar or two premolars and a small amount of periapical bone can be visualized in one image. The small image areas, the relatively bulky sensors, and the need for a wire connection between the sensor and the computer make the clinical use of these systems cumbersome. In addition, these systems have a relatively narrow dynamic range, which means that best image quality can be attained only within a limited
exposure range.
The Digora image plate system is an alternative, with fundamentally different digital image acquisition from that of CCD systems: The radiographic information is captured on a phosphorus storage screen, or image plate (Fig 193). The essential components are the image plate and the readout device¾the scanner, which is connected to a personal computer. The outer dimensions of the scanning unit are 483 x 452 x 135 mm. After exposure, the image plate is placed in the scanner, where the laser beam is deflected across the phosphorus screen. The released light energy is collected in a photomultiplier and converted into an analog signal, which is then digitized. With the Digora system, the anatomic area displayed is almost the same as that shown in modern film-based technology (Fig 194).
Hence, apart from the scanning of the image plate, which corresponds to development of the film, all procedures necessary to obtain an intraoral radiograph are identical to those of conventional radiography. Readout of the image plate takes less than 30 seconds, during which time the image gradually appears on the computer monitor.
The exposure range of the image plate is wide and linear. Because of the wide exposure range, the high sensitivity of the image plate, and the high quality of modern photomultiplier tubes, the image plate system can acquire data over many orders of magnitude in exposure compared to CCD or film systems.
As with other digital images, the Digora images can be altered after exposure to
enable task-specific image characteristics. The system works in a Microsoft Windows
environment, which simplifies all operating procedures. Image brightness and contrast
can be changed by moving and angulating, respectively, a line displayed in a
coordinate system where the gray-level values in the original image and the altered
image are seen on the x-axis and the y-axis, respectively. The image-processing
software also allows edge enhancement and gray-scale inversion.
In addition, different types of measurements, such as measurements of linear
distances (in tenths of millimeters) and angles, can be performed. All values are
displayed on the screen. It is possible to display a histogram of the distribution of gray
levels within a chosen area, the mean gray level value and the deviation around the
mean. The gray-level values may be displayed graphically along a line of selected
position and angulation.
No significant differences have been found among the CCD systems (Durr Vista Ray
Trophy RVG, Sens-A-Ray, and Visualix/Vixa) and the Digora system with respect to
the accuracy with which approximal and occlusal carious lesions can be detected.
However, the recently introduced CMDs/APS sensors have not been compared to the
CCD and Digora systems.
Digital radiography linked to the dental unit offers an attractive design, because the
flat screen is adapted to the bracket table of the dental unit, directly in front of the
patient, facilitating discussion with the patient about findings from the radiographs as
well as from an intraoral camera (Fig 195). Another alternative is Durr`s mobile
system including Vista Ray digital radiography equipment, Vista Cam2 intraoral
camera, flat screen, computer, and printer on a very stable mobile trolley, which can
be easily moved around (Figs 196 (a and b)).
In general, the new digital systems are comparable to conventional radiography,
although contrast enhancement may boost sensitivity at the expense of some loss of
specificity.
