Going digital can help lower radiation dose

Digital imaging first entered diagnostic x-ray departments with the development of CT scanners in the early 1970s. Although digital techniques matured slowly, major advances have been made over the last decade. It will be interesting to assess how patient doses have also changed during this period. Digital technologies are often divided into two groups: indirect and direct readout systems. The first group covers digital fluoroscopy systems with real-time digitization via a television camera output and photostimulable plates that release stored information on laser stimulation. The second group includes systems with an amorphous silicon-based flat-panel readout, where light from phosphors is detected in a photodiode array. It also covers systems with amorphous selenium-based flat-panel detectors that sense charge in a number of ways. CT scanners and charge-coupled devices that allow stored charge to be read out at the end of an exposure fall into this direct category as well. Digital radiography offers a number of capabilities compared with conventional radiography, such as postprocessing, electronic archiving, concurrent access to images, and improved data distribution. Digital radiographic techniques also have a wider dynamic range, allowing systems to form images from a large range of input receptor doses (approximately 0.05 to 300 micrograys per image). This enables operators to vary exposure levels, within certain parameters, without influencing image contrast or visualization of gray values. Patient radiation dose can thus be lowered, assuming the reduction in signal-to-noise ratio is acceptable and the resulting image quality is adequate to answer the clinical question. The potential for using a higher dose than necessary becomes clear in an examination of the radiation operating range of different digital modalities. Overexposure in screen-film imaging is recognized easily on a dark film, but this is not the case with digital radiography. Below a dose of about 10 micrograys per image, a change in dose produces a change in contrast resolution. Above 10 micrograys, however, dose changes produce little or no observable effect on the contrast threshold. Practitioners should ensure that digital imaging equipment does not operate in this range. The next step toward system optimization involves assessing what level of quantum noise can be sustained without compromising diagnostic accuracy. Electronic readout noise may limit contrast resolution at very low doses. Theory predicts that threshold contrast at low to middle values of dose per image will follow N-1/2 (where N is the number of x-ray photons/mm2).(1) Efficient use of digital radiography equipment can be guaranteed only by carefully following rigorous and formalized standards of image quality and dose per image. SYSTEM COMPARISONS Reports suggest that digital fluoroscopy systems can reduce patient radiation dose by up to 50%. The exact reduction depends on whether image quality is being adjusted to suit diagnostic need and varies by the site under examination. One such analysis of 10,000 barium studies, for example, revealed a 50% dose reduction for patients examined with digital rather than conventional fluoroscopy units.(2) Another group reported that it lowered dose by a factor of 10 when using digital fluoroscopy for pelvimetry, as opposed to screen-film radiography or CT techniques.(3) This result was based on adjustment of radiographic exposure to provide adequate information for the measurement required-and no more.