The production of the X-ray beam and interaction of the X-ray photons with the tissues is the same with digital radiography as conventional radiography. The difference between the systems is in the detection of the X-ray photons and the production of the image. Digital systems convert the pattern of photons reaching the detector into an electrical signal. Whilst digital systems have some advantages, the images they produce are often no better than good-quality conventional radiographs.

With conventional film-screen systems, the radiographic image is created by the pattern of photons or light reaching the film. Conventional film-screen-based radiography is an analogue system which means there is a continuous range of shades of grey from white to black on the radiograph. The shade of grey on any part of the film is determined by the number of photons reaching that part. The relationship between the radiographic exposure (number of photons) and optical density of the film is known as the characteristic curve and is sigmoidal (S shaped). If there are too few (underexposure) or too many photons (overexposure) the information given by the pattern of photons is lost. Within the useful (straight) part of the characteristic curve there is a continuous (but logarithmic) linear relationship between the number of photons and the density of the film. When taking conventional radiographs the exposure is set to use the straight part of the characteristic curve. Although there are potentially hundreds of shades of grey on the image, the human eye can only differentiate about 60 shades. Once the film is processed the brightness and contrast are fixed.

The main disadvantages of conventional film based systems are:

 Information stored on the film is fixed and cannot be manipulated following the exposure.

 There is a limited range of useful exposures that make a diagnostic radiograph. If mistakes are made in the choice of exposure or if there are marked differences in thickness of tissue being radiographed then information may be lost.

 Expense and problems with processing.

 Film is bulky and difficult to archive.

 Sharing of images is expensive.

Digital radiographic systems were developed to overcome some of these problems. The term digital comes from the same source as digit (finger) and a useful analogy is that digital information is similar to counting on fingers. In digital radiography the number of photons reaching the detector is converted into a range of discrete values. By comparison an analogue system can measure an almost infinite (continuous) range of values.

With digital radiographic systems there is a direct linear relationship between the exposure and signal. This means that less information is lost (e.g., once film is black extra photons do not result in new information whereas in a digital system the extra photons carry useful information). This means it is possible to view simultaneously areas on the radiograph with marked differences in tissue thickness, or to look at the soft tissues and bones without having to take multiple exposures.

There are two main types of digital radiography. These differ in how the X-ray pattern is converted to an electrical digital signal:

 Computed radiography (CR)

 (Direct) digital radiography (DR)

CR systems use a cassette containing a phosphor screen that can store the latent image for a period of time. When the X-ray photon hits the imaging plate there is transfer of energy to electrons within the phosphor. To read the latent image the imaging plate is loaded into a reader (this is normally automatic, but in some systems this has to be done manually). Electrons within the plate are stimulated to release their energy by a laser scanner. The energy is released as light, with the intensity of light emitted proportional to the number of photons reaching the detector. This light is detected by a photomultiplier tube (detector) and converted into an electrical signal. The electrical signal is then used to create the image. After scanning, the detector is erased by a bright light and the cassette is reloaded.

In DR systems the X-ray photons are converted directly into an electrical signal. This may be performed by using a scintillator to produce light, which is then measured by a light sensitive sensor to produce an electrical signal. The other main method is where the scintillator is incorporated in a plate with a photodiode to detect the light. Other devices are also available. DR systems can give better image quality and faster image production than CR systems, but they are more expensive.

With both CR and DR systems the radiographic image is created from a number of small rectangular picture elements (pixels) arranged in a grid. The resolution of the image is determined mainly by the size of the pixels used to make the image. The smaller the pixel the higher the resolution. For each pixel there is a large (although limited) number of potential shades of grey. The image can be manipulated to use all these potential values, in contrast to conventional film where detection of the full greyscale is limited by the human eye.

In addition to being able to manipulate images after acquisition (e.g., alter window/level, zoom, rotate etc.) there are other potential advantages of digital radiographs:

 Easy to distribute

 No chemicals or film required

 No darkroom needed

To gain the greatest advantage from digital radiographs the images must be viewed on a computer monitor. Special monitors are used for reporting digital radiographs. These have a resolution of at least 2-3 megapixels (MP) and are able to display a wide greyscale range (12 Bit or 4096 shades of grey per pixel) and contrast, e.g., 800:1. These monitors are expensive and contribute to the higher costs of digital radiographic systems.

Digital radiographs are not free from artefacts and careful technique is required (as for analogue radiography). In addition to the artefacts seen with analogue systems, e.g., movement blur, dirt on the animals' coat, dirty screens etc., there are several artefacts specific to digital images. If grids are used with some digital systems there can be artefacts visible on the monitor due to interaction between the grid lines and pixel rows resulting in a corduroy 'moiré' pattern. In some older systems a dark halo artefact may occur around the edges of metal implants.

References

1.  Körner M, Weber CH, et al. Advances in digital radiography: physical principles and system overview. RadioGraphics 2007; 27: 675-686.