|ECR 2019 / C-2411||
|An investigation on the appropriateness of Exposure Index (EI) calibration and Exposure Index target (EIT) values as a method of ensuring diagnostic image quality and optimum patient exposures in Digital Radiography.|
Digital Radiography has a high Detective Quantum Efficiency (DQE) which means that less radiation exposure is needed to achieve the same image quality, relative to conventional film screen radiography (FS).
Digital detectors offer the advantages of a wide dynamic range, adjustable image processing, better image quality, rapid image acquisition and image access at remote locations.  In addition, digital image processing can also compensate 100% for underexposure and more than 500% for overexposure (see figure 1). DR processing amplifies the signal when underexposed and decreases it when overexposed, resulting in an acceptable image. In clinical departments it is well recognised that most staff training is delivered by applications teams and cascaded down through staff in the department . It is imperative that this information is cascaded correctly and radiographers need to be cognisant of the Exposure Index (EI) in DR which is used to give an indication of radiation dose incident on the detector. Problems arise when more than fifteen DR manufacturers have their own proprietary EI metric and range, e.g. ‘the Kodak EI’ will increase if the exposure is excessive whilst Fuji EI ‘S value’ will decrease as exposure increases . This variation causes confusion among radiographic staff and a lack of clarity in the imaging community regarding optimised clinical use and appropriate Quality Control programmes. This is of great significance when staff are working across multiple DR manufacturers in one department with different ranges of acceptability . The International Electrotechnical Commission (IEC) attempted to standardise these variations in EI by publishing an exposure terminology standard in 2008; the IEC62494-1 the aim was to simplify EI, so three terms could be used across all manufacturers.
These three terms important to radiographers were:
(i) Exposure index (EI)- an index of the exposure at the detector in the relevant image region (RIR).
(ii)Target exposure index (EIT)- the target reference exposure obtained when the image receptor is exposed properly. The EIT should provide the exposure that produces a balance between image quality and dose.
(iii)Deviation Index (DI) - measures how far the actual EI value deviates from the EIT.
The optimum DI is 0 indicating a perfect exposure. A negative DI indicates an underexposure whilst a positive DI indicates an overexposure. The higher the value the greater the over or under exposure (see figures 2  and 3).
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