Knowing how number is used is key to controlling exposure
Imaging in a radiology film environment is much like playing Goldilocks and the Three Bears. You take your image, hold it up to the viewbox and say: “This image is too light”; “This image is too dark”; or, “This image is just right!” If you underexpose your image, it will be too light, and if you overexpose the image, it will be too dark (See figure 1). The density and contrast of the image on film is controlled by the kV, mAs and other exposure factors.
However, with digital imaging devices, brightness and contrast are no longer linked to exposure factors. Digital systems produce images with consistent density and contrast regardless of the exposure factors (See figure 2). So how does a radiographer know if a digital image is over- or under-exposed?
The potential for gross overexposure is one issue we encounter when a radiology department or clinic changes to a digital image receptor. The reason for this increased risk is that we’ve lost the visual connection between the exposure and an image’s appearance. That’s why it’s so important for the radiographer to understand how to read and utilize the exposure indicators.
On digital imaging systems, an exposure indicator provides useful feedback to the radiographer about exposures delivered to the image receptor (ASRT, 2010). An over- or under-exposed image will deliver an incorrect exposure indicator; whereas a correct exposure will provide a corresponding exposure indicator. The indicator is a vendor-specific value that provides the radiographer with an indication of the accuracy of their exposure settings for a specific image (ASRT, 2010). The exposure indicator has as many different names as there are vendors in the market. The names include S-number, REG, IgM, ExI and Exposure Index.
Carestream’s computed radiography (CR) and digital radiography (DR) systems both reference their exposure indicator as the exposure index or EI. After an exposure is made, the resulting image appears on the monitor and displays a number in the Exposure Index field. The number is a representation of the average pixel value for the image in a predefined Region of Interest (ROI).
The exposure index allows the radiographer to match the exposure to the desired speed class of operation. The speed class is set in a given department by consulting with an interpreting radiologist. The radiologist’s feedback on sample images helps determine the level of image noise he or she can accept. It’s important to note that, as speed class increases, so does the amount of image noise. Once an acceptable noise level is established, a radiographer can identify the speed class of operation for the imaging system and the corresponding technique charts. It’s the responsibility of the radiographer to select a technique that provides enough exposure to reduce the amount of noise while also adhering to ALARA standards.
The exposure index is indirectly proportional to the speed class of operation. If you’re using the Carestream Exposure Index values, for every 300-exposure-index increase, the speed class is reduced by half. In other words, if the exposure index increases from 1400 to 1700, the speed class is reduced from a 400-speed class to a 200-speed class. The Carestream EI is not necessarily unique to the receptor type. However, CR systems typically operate at a lower speed class than DR systems.
IEC Exposure Index is international standard
Remember that each radiology imaging manufacturer has its own method of providing exposure indicators. This can be confusing to radiographers who have multiple vendors within their facility. Fortunately, there is a standard for exposure index for digital X-ray imaging systems. Developed concurrently by the International Electrotechnical Commission (IEC) and the American Association of Physicists in Medicine (AAPM), in cooperation with digital radiography system manufacturers, the index has been implemented as an international standard. It’s known as the IEC exposure index. Carestream systems are configurable for the user to display the Carestream EI, the IEC EI, or both.
The IEC exposure index is unique to the receptor type being used and to the exam performed. Three default Target Exposure Index (TEI) values are preloaded into the system. The three values represent the default Target EI for bucky, non-bucky and pediatric exams.
Once the operating speed class is determined, the key operator can adjust the Target EIs to correspond to the recommendations made by the facility’s physicist. After an exposure, the IEC EI will display, followed by the deviation index (DI) in parentheses. The deviation index quantifies the difference between the actual EI and the Target EI, and this feedback allows the radiographer to track and adjust his or her exposures. When the actual EI is equal to the Target EI, the DI will equal 0. A positive or negative DI indicates the amount of exposure greater or lesser than the target EI. It does not necessarily mean that an image needs to be
repeated. If the deviation is greater than +3, the exposure index displays in red to indicate a high/low exposure that might need further review.
The DI chart below outlines how to use the deviation index. In the example above, the DI was calculated as 1.06. In the chart you’ll see that a DI of 1 means the resulting exposure was ~26% higher than the Target EI. The initial DI was 1.06, so we can estimate that we are slightly higher, perhaps closer to 30%. Although it might be a good image, it is merely an indicator to the radiographer that he/she might be able to reduce the exposure factors the next time a particular exam is performed- reducing the dose to the patient while still acquiring an acceptable image.
Martin Pesce, RT, is Clinical Development Manager at Carestream