Moving from film to digital x-ray images has many advantages for both patients and x-ray operators. Digital imaging offers a wider latitude of exposure factors when creating the x-ray image, and patients have benefitted due to the fewer number of repeat x-rays needed. When using film, it was easy for the rad tech to tell if they had used too much or too little x-ray dose to make the image, because the film would either be too light or too dark. With digital imaging, it is not always obvious to the radiographer whether he or she is using more x-ray dose than necessary to create the x-ray image.
Each x-ray manufacturer for CR and DR image receptors (plates) provides a way to give the user feedback on whether the appropriate or optimal amount of radiation was used to produce the image. Each manufacturer may have their own proprietary algorithm to calculate a patient’s exposure and their own name for the calculated value. Fuji – one of the largest suppliers of x-ray imaging CR and DR plates in the world – uses what they call the S-number as their exposure indicator.
Fuji’s S-number is defined as 200/Exposure – where Exposure is a median value in mR (milliRoentgens) over the area of interest. This means that a median exposure of 1 mR at the detector in the area of interest yields an S-number of 200. The S-number is an inverse measure of dose. If a dose larger than 1 mR gets through the patient and to the plate then the S-number is less than 200. For example, for a median dose of 4mR will yield an S-value of 200/4 or 25. A low S-number means that you used too much x-ray! Similarly, a high S number (example 200/0.1 mR = 2000) means only a small amount of dose got to the imaging plate – not enough x-ray to create an optimal image. Fuji provides recommended S-values for different body parts. Most of these values range from 200-600.
For images with S-values that are too low or too high, the software may be able to compensate to adjust the image closer to optimal. However, if you are routinely getting S-values that are too low or too high, consult with a physicist to see how you should alter your factors to get better images.