The American Association of Physicists in Medicine (AAPM), in cooperation with many manufacturer representatives and the Medical Imaging and Technology Alliance (MITA), and in collaboration with the International Electrotechnical Commission (IEC), has developed a methodology that has evolved into an international standard, IEC 62494-1 [1
], that describes common exposure indices and deviation indices to be implemented across all digital radiography detector types and across all manufacturers and vendors of such equipment. The document explains a method for placing standardized exposure information and content in the DICOM metadata in each image associated with the imaging study. While the details are left to the interested reader [1
], it is the manufacturer’s responsibility to calibrate the imaging detector according to a detector-specific procedure, to provide methods to segment pertinent anatomical information in the relevant image region and to generate an exposure index
(EI) that is linearly proportional to detector exposure.
The user’s responsibility is to define each patient and anatomy-specific examination with a target exposure index (EIT) for the specific detector in use. Determination of EIT values is crucial for the successful implementation of the exposure index standard and requires assessment of image quality. This is an often subjective endeavor, causing a wide variation in response with respect to recommended kVp as well as exposure to the detector from the many stakeholders involved in the process. A significant ongoing effort by the Alliance for Radiation Safety in Pediatric Imaging through the Image Gently campaign has pushed for scientific methods that can objectively establish target values for optimizing the quality of the image while minimizing radiation dose and improving overall care and safety for pediatric (and all) patients.
After image acquisition and any manual adjustments of the automatic processing that updates the EI (e.g., if the anatomical segmentation fails to recognize relevant image regions, such as the presence of a prosthesis, a gonadal or breast shield commonly used in pediatric radiography, or other unlikely attenuator causing the displayed image to appear suboptimal), a feedback signal known as the deviation index
(DI) is calculated according to:
The DI provides feedback to the operator with a value that is equal to 0 when the appropriate exposure to the detector is achieved, a negative number when underexposure has occurred, and a positive number when overexposure has occurred. A DI value of +1 indicates an overexposure equal to 25% more than the target exposure to the detector, while a value of −1 indicates an underexposure equal to 20% less than desired. The range of DI values acceptable for routine clinical work needs further investigation. The range will probably be narrower for examinations using automatic exposure control versus manual parameter setting. DI values of +3 and −3 indicate exposures that are 2 times more and less than the target exposure, respectively.
The DI is intended to be an indication to the technologist on whether the radiographic technique is appropriate for the specific body part and view for an optimal image presentation of the anatomy of interest (proper image brightness and contrast) with acceptable signal-to-noise ratio in the relevant image regions. Therefore, a database of EITvalues must be available in the digital X-ray imaging system for each imaging procedure, and the technologist must specify the correct body part and radiographic view. Otherwise, the calculated DI might be inaccurate because an incorrect EIT value could be used. In imaging situations where deviations from proper positioning and collimation occur, resulting in under- or overexposure in screen-film imaging (for instance, where there is improper positioning of anatomy over an AEC chamber or too small or too large of a collimation area, resulting in lesser or greater amounts of scatter onto the detector and causing the optical density of the processed film to be under- or overexposed), the proper use of the exposure index standard will result in negative or positive values of the DI.
There are many limitations in the use of the standardized EI. The generated EI value depends critically on the relevant image region analyzed; as a result, different EI values are possible when a different image region is segmented and used for estimating the incident exposure. Manufacturers use different analysis methods to determine the relevant image regions and corresponding image histograms. X-ray detectors can have widely different detection efficiencies and respond differently to X-rays of different energies and angles of incidence. Also, the EI is calibrated to only one acquisition condition (kV, filtration, SID, grid) and to the extent that the X-ray energies incident on the detector from a radiographic acquisition are different, inaccuracies in the calculated value can result. Even though the same EI value is reported by different systems, the exposure reaching the detector might be very different. Likewise, significantly different EI values do not necessarily indicate significantly different exposure to the digital X-ray detector. Nevertheless, the ability to overcome segmentation errors to allow for re-computation of the EI by manual adjustment of the image display characteristics and to provide consistent operator feedback via the calculated DI will undoubtedly reduce the confusion and enhance the understanding and safe use of digital radiography detector systems, in particular for pediatric technique factors. The nuances of this new exposure index standard now are at the beginning of clinical implementation and testing, so the next steps are to figure out the appropriate EIT for specific detectors and examinations, to understand the limitations of the EI and to quickly learn about the DI and how to properly compensate for under- and over-exposures with the DI information presented to the technologist as feedback.