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Digital radiography: What is new (Proceedings)

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Apr 01, 2010

This session will discuss the relative advantages and disadvantages of digital radiography. This modality can save time, money and frustration, but comes with an initial up-front financial cost.

Conventional (analog) radiography

Although there are some advantages to conventional radiography, new digital images carry many advantages. The major disadvantages of conventional radiography are significant and rate limiting. Information stored on a film cannot be adjusted following the exposure. There is a limited range of useful exposures that will produce a diagnostic radiograph. This narrow dynamic range means that we need an exposure chart for each body part commonly radiographed and if mistakes are made, either in the chart or the measurement of the part, then the information will be lost. There is a significant expense (time, space and financial) with chemical processing. Film is bulky and difficult to archive. Distribution of film images is time consuming and expensive.

Digital radiographs

Digital images are formed from a discrete number of picture elements (pixels) arranged in a rectangular matrix. The spatial resolution of the image is determined mainly by the size of the pixels used to make the image. The smaller the pixels the greater the spatial resolution. The contrast resolution is determined by the number of shades of grey (typically 4,000) that can be assigned to each pixel. However, the digital image can be manipulated to use all the pixels, so that assigned pixel values are visible at the same time.

Digital systems convert the pattern of photons reaching the detector into an electrical signal that is digitized. Digital systems are only good and the resolution of the detector and the system of converting the digitized signal to a readable image. There is more of a linear relationship between exposure to displayed signal, therefore we avoid the "shoulder" and "toe" regions on conventional radiographs. Once a radiograph is white or black, then all information is gone and no viewing condition recoups that information. The dynamic range refers to the range of exposures which result in a diagnostic image. The dynamic range of digital radiographs is 4-10 times greater than conventional radiographs.

There are two types of digital radiographs, which differ in the way the x-ray photon pattern is converted to an electrical signal.

Computed radiography

Computed radiography (CR) systems use a cassette that's look similar to a conventional radiographic cassette. Within the cassette is an imaging plate made from a photostimulable phosphor. When x-rays photons hit the plate there is a transfer of energy to electrons within the plate. To display an image from this energy transfer the cassette is loaded into a image reader. Within the image reader a laser moves backwards and forward releasing the energy in the form of light. The light is detected by a photomultipier tube that converts the light to electrical energy. The intensity of the light is proportional to the number and energy of the x-ray photons originally striking the plate. The plate is then erased by a bright light and the plate loaded back in the cassette.