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Computed radiography and digital radiography


Computed radiography (CR) and digital radiography (DR) are rapidly replacing screen-film imaging systems in many countries. Both are similar in technology except for the image receptor. CR uses photostimulable phosphor (PSP) plates which must be transported to a digital scanner (or reader) and scanned with a laser beam to convert the stored image to a digital array. DR uses a solid-state detector, or a combination of a light emitting phosphor and a digital converter, which produces digital array and image.

All other aspects of these two technologies are similar including the computer processing of the digital image, transmission and display of the digital image, and digital image storage. These systems rely on picture archiving and communication systems (PACS) for transmission and storage of the digital images.

Important Principles

CR systems use a PSP plate to store the image resulting from the interaction of x-rays with the phosphor. This stored image is then stimulated by a laser beam with a very small focus (0.1 mm or less). This stimulation results in the emission of light which is captured and digitized to produce the digital array. CR is less efficient in terms of image quality for a specific patient dose due to the initial storage, and the stimulation and readout, of the image information.

DR is a more efficient system. One technology uses a light emitting phosphor which is in intimate contact a thin-film transistor (TFT) array. The TFT array converts the light emitted from the phosphor into digital signals. The other technology uses a photoconductor, such as amorphous selenium (aSe) to directly convert the detected x-rays into an electronic signal.

These technologies differ only in terms of the image sensor and the way the image is converted to a digital signal. It is important to understand the difference and advantages of these systems, one of which is reduced cassette handling for the radiographer in the clinical setting.

The display of the image becomes another important aspect of CR and DR. Appropriate display hardware and quality control is essential to obtaining consistent, diagnostic information in digital images.

Patient dose is a concern in digital imaging, both CD and DR. With screen-film imaging an increase in dose usually means the film becomes darker, until the point at which it is unusable. With CR and DR the dose can be increased by two to four times, or more, and it is difficult to observe a difference in the image. With all digital imaging it is possible to use extremely high doses to the patient!

Digital imaging systems, typically, provide an Exposure Index (EI) for each image. The EI is related to the radiation dose received by the imaging plate and should be similar for all images made for a specific projection, e.g., AP lumbar spine. This provides a relatively easy way to monitor the radiation dose used and to determine if there are changes in the dose due to specific radiographers, particular x-ray rooms, or over time. The EI is an excellent quality control tool.

Introduction to References

The References include information on CR and DR systems, dosimetry applicable to these digital imaging systems, and quality control. A document entitled "Assessment of Display Performance for Medical Imaging Systems" is included in the References as an introduction to the display of digital images.