Saturday, February 20, 2010

Towards the Standardization of Exposure Indices

Exposure index is a tool used by Radiological Technologists in digital radiography to assess radiographic exposure. Exposure index is, broadly speaking, a measure of the amount of exposure detected by the image receptor. Accordingly, it depends on the mAs and kV used, the detector area irradiated, and the beam attenuation (Bontrager & Lampignano, 2005). 

Under the reign of film-screen radiography, exposure is directly related to the final density of the image. A measure of exposure is not needed because it is plainly evident on the image. In digital radiography, overexposed images may not be dark and underexposed images may not be light. Post processing ultimately controls the brightness of the image on the monitor. Exposure thus becomes evident only through the noise in the image, more noise coinciding with underexposure. Because of the low resolution displayed by the monitor used by the Radiological Technologist in assessing the image, noise level is sometimes not evident. The exposure index is the indication of an appropriate exposure and resultant image quality (AAPM, 2009)

Depending on what system you use, the concept of an exposure index is represented by one of many terms.  Sensitivity number, log median exposure (lgM), f-number, reached exposure (REX), and detector exposure index (DEX) are just a few used terms, demonstrating the miscellany in nomenclature. It comes as no surprise that all of these ways of describing exposure are actually different, either in the information conveyed or the way it is described (Carlton & Adler, 2005).

Fujifilm's S-number is the oldest exposure indicator, closely mimicking the "speed class" system used in film-screen radiography. Exposure index increases with a decrease in exposure. Kodak's exposure index (EI) is representative of the average pixel value for the clinical area of interest. A change of 300 in the EI indicates a change of a factor of 2 in the exposure. Agfa CR utilizes the "lgM" or log median exposure. A change in the lgM by 0.301, or a logarithm of 2, indicates a change of a factor of 2 in the exposure. Philips EI is inversely proportional to air kerma, and the scale used is represented in bigger discrete steps (200, 250, 320…) which requires a 25% change in exposure per step (AAPM, 2009). This illustrates the large differences in the source and calculation of the exposure indicator.

The American Association of Physicists in Medicine (2009) suggest the standardization of an indicator of exposure to the detector. Not only would it be useful to monitor differences in exposure between systems in a given institution, but it would provide unambiguous information for Technologists working with more than one system. A standardized exposure indicator should, according to the AAPM (2009), reflect the radiation exposure to the detector and image noise. 

The deviation index (DI) is the proposed index to be displayed to the Radiological Technologist immediately after every exposure. The DI is a measure of the relative deviation from the targeted value for a particular body part and view (AAPM, 2009).

American Association of Physicists in Medicine. (2009). An Exposure Indicator for Digital Radiography. Retrieved from http://www.aapm.org/pubs/reports/rpt_116.pdf

Bontrager, K. L., & Lampignano, J. P. (2005). Textbook of radiographic positioning and related anatomy (6th ed.). Elsevier Science. 

Carlton, R. R. & Adler, A. M. (2005). Principles of radiographic imaging: An art and a science. Delmar Learning. 

1 comment:

  1. That AAPM report, although long and difficult to read, is a excellent resource for this topic. Thanks for pointing that out.I hope these can be standardized soon but it looks like it could be very difficult.

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