Images comparing analog and digital exposures

Understanding Radiology Exposure Indicators

Reading Time: 4 minutes read

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.

Read the related blog on Diagnostic Reference Levels.

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.

Images comparing analog and digital exposures

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


Martin Pesce, RT, is Clinical Development Manager at Carestream


  • September 17, 2016

    Mary Seib

    good read

  • April 24, 2018


    Dear sir ,
    I am a veterinary surgeon (Southest poin of continental Greece , KALAMATA) , not specialist in radiography. I use a kodak POC 140. How i can find the carestream exposure index (for small animals) and how i can use it , to develop the x-rays <>.

  • April 28, 2018


    I would like to know the value of EI in your X-ray digital radiography for each section on the body

    • April 22, 2019

      Raymalee Frank

      Good afternoon-

      Could you please send me this information for as well? I would like to know the EI for each section of the body.
      Thank you.

  • June 7, 2018


    Can u send me the value of exposure in your X-ray CR/digital radiography for each section on the body?

    • June 8, 2018

      Greetings, Without further detail of the specifics of your location, equipment and local processes, I would like to recommend that you utilize Best Practices of the ASRT (American Society of Radiologic Technologists) or refer to their general site:

      If you are still interested in more information, if you can kindly provide your name, facility name, city, country and phone number – we can have a Carestream representative follow-up to understand your specific requirements. Or you can complete the Contact Us from found here:
      Thank you!

    • January 31, 2019


      Can I also get a copy of this please

        • April 22, 2019

          J Milham

          I need an exposure list for the carestream/vieworks setup. They are in ranges of 300-500 from what I’ve seen. I’m not sure what optimal is for this system.

  • June 21, 2018


    Dear Sir, Madam,

    I am currently trying to implement the EI/DI at our radiology department. I have set a target EI for thorax images using a very life-like, anthropomorphic thorax phantom in collaboration with our radiologists. I now however would like to know how I can set target EIs for the other body parts/views. Can you help me with that?

    Kind regards,

  • August 2, 2018



    I would like to know the value of EI in your X-ray digital radiography for each section on the body for lower and maximum value of EI


  • August 7, 2018


    Hello I’d like a reference EI for the carestream drx ascend model Toshiba E7252X how would I go about acquiring it?

  • August 13, 2018

    Ben Oakes

    Multiple exposure measures confuse radiologists and technologists. Exposure values may not be reported in. PACS 0xc0000185 , Just Cannot access data for analysis. I know, however, would like to know how I can set a target for the other body parts?

  • September 13, 2018


    Hello Sir, I am currently pursuing Radiography in school and would like to know how to utilize the 15% rule of KVp and mAs to diffrent IR types ie. PPS,FPD and Film. Also Thankyou for explaining the need for SI values as well as DI values. Honestly, I now fully understand.

      • October 12, 2018


        Thankyou so much, well explained!

  • September 26, 2018

    Nasrullah khan

    Hello, Sir
    Can u send me the value of exposure in your X-ray CR/digital radiography for each section on the body?

  • October 22, 2018

    Gretchen Koontz

    We have a Carestream 7500 that was installed in mid ’09. I have noticed lately that the exposure indicators for a few exams are not in the 1500 – 1800 range that we were given as acceptable originally for our x-ray unit. For example: Tibia-Fibula – AP 1940, AP 2004, Lat 1981, Lat 2209 (images were a little dark).

  • November 18, 2018


    May i question
    If overexposure, and index exposure high, why the image in komputer is blank/white, see in computer monitor (or light view) ?

  • February 24, 2019

    Walter Errickson

    I have just been given a Kodak Point of Care CR140 system after using DR for years. I am trying to find the EI numbers for various body parts. After reading the above article and the reference article “standard for exposure index ” it appears to me that these numbers must be given to me by my company’s medical physicist. Is this the case or can you provide me with a standard range for this system?

    • February 27, 2019

      Hi Walter. Thank you for reading our article and for your question. Please follow this link to find the appropriate contact in your region who will be able to answer this question for you.

      Thanks and have a great day!

  • March 28, 2019

    Amanda Douglass

    DXR Revolution

    What are the EI recommendation for all body body parts for this machine? Thank you for your time!

  • April 4, 2019

    Chris Hernes

    DRX Revolution
    What are the EI recommendation for all body body parts for this machine?

    DRX Evolution
    What are the EI recommendation for all body body parts for this machine?

    Thank you for your time!

      • April 9, 2019


        Hi CareStream! Is there any way you could send me a copy of this information as well? Thank you so much!


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