General radiography has a direct relationship between optimal exposure and a diagnostic image.
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Dynamic range
Traditionally, general radiography utilized film technology with a limited dynamic range, in which under or overexposed films either develop ‘too dark’ or ‘too light' 1. Put simply; dynamic range is the series of exposure values that will result in a radiographic image; narrow dynamic range equals a smaller window of optimal exposures 2. In contemporary practice, digital radiography has replaced film technology, and with that, a more forgiving, higher dynamic range 3.
Underexposure
Underexposed images are easy to identify, they contain quantum mottle (noise), appear under-penetrated and often are deemed to be undiagnostic. In the clinical context, an underexposed chest x-ray will appear 'grainy,' and display poor penetration of the mediastinal structures leading to an inaccurate representation of anatomy.
Underexposure errors often occur at the radiographers ends, choosing an inappropriately low exposure (low mAs) for a patient’s examination, or an examination type on the workstation.
Overexposure
Due to the high dynamic range in digital imaging, overexposure is slightly more challenging to identify. Overexposed images will have a distinct lack of quantum mottle while appearing ‘saturated’ or in extreme cases ‘burnt out’ whereby anatomy is completely obliterated from the radiograph. Due to the tendency to correlate a lack of noise with image quality, combined with the high dynamic range of digital imaging, exposure factors in digital imaging will sometimes increase in clinical practice, this is known as 'dose creep' 4.
Exposure indicators
To account for the change in dynamic range, and the growing issue of dose creep, the International Electrotechnical Commission and the American Association of Physicists in Medicine developed standards for the scrutinizing of digital radiography exposures 5,6. Each digital image system provides an Exposure Index (EI), a target EI, and the deviations from that target EI 3,4. The EI is a numerical value related to the signal to noise ratio squared of an image, a marker for exposure quality. Each examination has a target EI, values under that EI are considered underexposed, values above, overexposed 3,4.
Although EI is a useful measure of image quality, it is highly influenced by collimation, gonadal shielding, and medical implants 4. It is therefore essential to assess each image on its merit while considering the EI provided by the vendor.
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