Exposure Factors for Digital Imaging

As it is with film-screen (analog) imaging, digital imaging requires kV and mA and time (mAs) to be selected to acquire radiographic images. Nevertheless, they do not have the same direct effect on digital image quality. Typically, digital imaging tends to be more forgiving on the factors used to acquire an image. This is due to the large latitude or the range of error that a system can fix and still make a diagnostically acceptable image. In digital imaging, like analog, changes in kV does not affect image contrast. Furthermore, when selecting mAs, the digital processing (algorithms) of the image allows for correction of an inaccurate mAs selection. Therefore, an image of optimal quality can thus be obtained with a range of mAs values, in some cases 50% below optimal mAs and 400% above optimal mAs.

Wide latitude in digital imaging systems results in fewer repeats, increased efficiency, and cost reduction. However, wide latitude poses a problem with ALARA when not used responsibly.

Key points to remember about digital:

• Doubling the mAs will NOT double the brightness in digital systems but will change the noise or mottle levels.
• Over Exposed ≠ Dark image
• mAs ≠ density/ brightness
• kVp ≠ Contrast
• Geometric factors still apply in digital imaging
• Although digital systems are not directly affected by kVp and mAs like in film, it is still important to provide accurate exposure techniques to reduce burnout, mottle, scatter, etc.  

previous page | next page