Is the image diagnostic? (10 point plan)

5. Correct exposure indicator

This is probably the most complex aspect of producing diagnostic images at the optimum radiation dose (ALARA/ALARP).  There is limited visual connection between the exposure given to the patient and the image displayed on the screen used to view the image. It is therefore important for the radiographer / technologist to understand Exposure Indicators (EI) and the Deviation Index (DI). Digital systems generally display images with the optimum density and contrast regardless of the exposure factors. Underexposure can result in image noise, however, overexposed images will be displayed optimally on the display screen often at ten times the exposure. Gross over exposure will result in ‘burned out’ pixels losing anatomical detail usually in the lungs.

Image evaluation for exposure should perhaps be conducted using EIT is the EI for a given radiograph and the target exposure. The 3-point scale for producing diagnostic images is:

1 = Unacceptable, must be repeated

2=Marginal

3=Acceptable

Images should be repeated if relevant anatomical details or ability to see lines or endotracheal tubes in the mobile chest image cannot be distinguished because of poor positioning or exposure making the image undiagnostic. It’s the responsibility of the radiographer / technologist to select a technique that provides enough exposure to reduce the amount of noise while also adhering to ALARA/ALARP standards. 

One major advantage of digital radiography systems is the very wide exposure latitude. Images will be displayed with the optimum density and contrast when the system has been set up with the required algorithm for chest imaging in our examination. As long as the detector is not underexposed or greatly overexposed a diagnostic image will be displayed. Repeat images are rarely required because of exposure errors when anatomically programmed generators are used with the equipment

The milliampere second (mAs), Kilovoltage (kV) and FDD should be recorded for each patient and checked prior to new mobile radiographs. This will allow evaluation of the exposure, which can then be considered for future mobile images.

Patient dosimetry:
The radiation exposure to the patient should be recorded on a log on the mobile equipment for future imaging and in PACS/RIS for record keeping. This can be recorded as the exposure factors and the mobile used. DR mobiles may provide a Dose Area Product reading (DAP) measured in centi-Gray centimetre squared (cGy × cm2).

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 exposure index has been implemented as an international standard. It’s known as the IEC exposure index. The IEC exposure index is unique to the receptor type being used and to the exam performed.

This will allow audit of and between systems to further optimise exposures in diagnostic imaging