Introduction
Digital
or electronic imaging has been available for more than a decade. The first
direct digital imaging system, RadioVisioGraphy (RVG), was invented by Dr. Frances
Mouyens and manufactured by Trophy Radiologie (Vincennes, France) in 1984 and
described in the U. S. dental literature in 1989.1
Since then, the market has expanded and many digital imaging systems
are available from a variety of dental X-ray machine manufacturers. It
is estimated that 10-20% of dental practitioners use digital imaging technology
in their dental practice.2
It is anticipated these numbers will steadily increase over the next five to
ten years as dentistry continues to move from film-based to digital imaging.
The purpose of this article is to discuss the principles of digital imaging including: direct and indirect digital imaging modalities, intraoral and extraoral imaging applications, diagnostic efficacy, image processing techniques, as well as a variety of factors dentists should consider when purchasing digital imaging systems. A variety of different terms are used to describe aspects of digital imaging and vary somewhat from film-based imaging. Definitions are listed in Table 1.
| Table
1: Terminology |
|
| Film-Based Imaging | Digital Imaging |
| Density – The overall degree of darkening of an exposed film | Brightness - Digital equivalent to density or overall degree of image darkening. |
| Latitude – Measure of the range of exposures that will produce usefully distinguishable densities on a film. | Dynamic Range –The numerical range of each pixel; in visual terms it refers to the number of shades of gray that can be represented. |
| Film Speed – Amount of radiation needed to produce a standard density; refers to the sensitivity of the film to radiation. The faster the film, the less radiation required. | Linearity – Linear or direct relationship between exposure and image density (See Fig.1); contrast is not affected but density can be altered after image acquisition |
| Contrast – The difference in densities between various areas on a radiograph; high contrast images have few shades of gray between black and white while low contrast will demonstrate more shades of gray. | Contrast Resolution – The ability to differentiate small differences in density as displayed on an image. |
| Resolution - Ability to distinguish between small objects that are close together; measured in line pairs per millimeter. | Spatial Frequency – Measure of resolution expressed in line pairs per millimeter. |
| Modulation Transfer Function - Measure of image fidelity as a function of spatial frequency; how close the image is to the actual object. | |
| Radiographic Mottle (Noise) – Appearance of uneven density of an exposed film or graininess | Background Electronic Noise – Small electrical current that conveys no information but serves to obscure the electronic signal |
| Sharpness – Ability of a radiograph to define an edge or display density boundaries. | Signal to Noise Ratio – Ratio between the fraction of the output signal (voltage or current or charge) that is directly related to the diagnostic information (signal) and the fraction of output that does not contain diagnostic information (noise). |
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| Page 2 of 17 | |
| Citation Number: Vol. 3, No. 4, Page 024 |
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