Since the mid-1990s, a steady replacement of analog screen-film detectors with digital radiology detectors has occurred, along with an expectation of lower dose because of minimal retakes and consistent image quality. This is because computed radiology and direct radiology devices have wide exposure latitude/dynamic range, and image post-processing capabilities that provide consistent image appearance even with underexposed and overexposed images. Determining correct exposure parameter settings and patient exposure by image appearance (e.g., density on a film image) is no longer possible. While underexposed images have smaller numbers of X-rays absorbed by the digital detector and can be recognized by a noisy appearance, overexposed images can easily go unnoticed, resulting in unnecessary radiation for the patient.
Today, in the digital X-ray world, modern systems use automatic image processing, so the ratio between detector exposure and image brightness is not always clear. The old visual control function for too-high dosage has been removed, so over/under exposures may go unnoticed on the screen with different brightness as this is corrected automatically.
Maintaining quality and consistency
So how can X-ray examinations maintain quality and secure that the equipment not drift in radiation over time? The Exposure Index, a method by which digital radiology manufacturers provide feedback to the technologist regarding the estimated exposure on the detector, is an indirect way of measuring the image signal-to-noise ratio and an indirect indication of digital image quality. This provides the operator a secured control that the actual exposure has been done with optimal image quality, and that no excess radiation outside of the manufacturer's recommendation has been used.
Manufacturers have begun introducing exposure indicators for digital radiology equipment. The International Electro Technical Commission explains:
"While considerable underexposure results in an increased level of noise, the more alarming aspect (from a radiation protection point of view) is that overexposure cannot be recognized easily in the displayed image.
Therefore, various manufacturers of digital radiology systems have introduced so-called exposure indicators for their equipment. These are numbers, determined from the original image data of each image taken, which allow conclusions about the level of the exposure at the image receptor. However, the exposure indicators are manufacturer of system specific, i.e. they differ for the systems of different manufacturers in their definition and scaling. A unified EXPOSURE INDEX for all digital radiology systems is needed to simplify its usage, e.g. for the establishment of exposure guidelines, particularly when systems of different manufacturers are used within the same department."
