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Many modern imaging systems come equipped with
automatic collimation, in which the system senses the size of
the receptor being used and collimates to its outer edges. This
may be acceptable when imaging a hand on an 18 x 24 cm
cassette, which will fill the entire field. However, when using a
larger cassette, this leaves areas of unattenuated beam, which
promotes scatter production and decreased contrast in the
image. The recommendation is always to collimate close to the
skin line of the patient, whenever possible. When you’re using
digital imaging equipment, it will also help the imageprocessing software better identify the correct region of
interest for optimal image processing.
Keep in mind that CR and DR receptors are more sensitive to
scatter radiation than film/screen cassettes. About 50-90
percent of a radiograph’s density may result from scattered
radiation, so that restriction of the primary beam requires an
increase in exposure to compensate for the loss of density.2
Radiographers need to be aware of the effects that scatter
radiation will have on image quality, and collimate
appropriately to reduce these effects.
These basic principles are the cornerstones of high-quality,
consistent imaging. Radiographers who understand and
routinely apply these principles are vital to the success of any
imaging facility. Modern imaging equipment has
revolutionized the industry by performing many of these
functions automatically, but radiographers must understand
these principles, and adjust or override these features to
ensure correct operation and reduce unnecessary exposure to
patients. Getting back to the basics can save you time, money
and aggravation by reducing repeat images, materials used
and patient exposure. In other words, sometimes to move
forward, it’s best to look backwards.
Martin S. Pesce, B.S.R.T (R), is a Worldwide Applications
Engineer, DMS Product Commercialization Team, X-ray
Solutions, Carestream Health, located in Rochester, NY.
1Richard R. Carlton, M.S., R.T. (R) (CV), FAERS, Arlene M. Adler, M.Ed., R.T. (R),
FAERS, Principles of Radiographic Imaging an Art and a Science, Delmar
Publishers, Albany, pg. 505, 1996.
2Joseph Selman, M.D., FACR, FACP, The Fundamentals of X-Ray and Radium
Physics, Charles S Thomas Publisher, Springfield, IL, pg. 387, 1994.
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