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Image Evaluation and Artifacts
Read the DI: 0 is on target, +3 is about double and -3 about half the target exposure.
Exposure Indicator and DI
The exposure indicator (EI) is a number that estimates the radiation that reached the digital detector in the anatomy of interest; it reflects receptor exposure, not patient dose. The current standard adds a target EI for each exam and a deviation index (DI), which equals 10 times the log of the measured EI divided by the target. A DI of 0 means the exposure hit the target, +1 is about 26 percent over, -1 about 20 percent under, +3 about double and -3 about half. Departments set acceptable ranges, commonly about -1 to +1. Because the EI is calculated from the image histogram, poor collimation, metal or the wrong exam menu can make the number wrong even when the exposure was right.