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Department of Radiology, Brigham and Women's Hospital
Harvard Medical School

V. Artifacts (Attenuation)

The class of photons which is most useful for nuclear medicine imaging consists of photons that originate from a nuclear decay and follow a straight-line trajectory through a collimator hole.

Attenuation is the loss of these useful photons, either by photoelectric absorption in the patient, or by scatter through an angle sufficiently large that they can no longer be detected.

In terms of image quality, the problem with attenuation is that it is not constant everywhere in the image. Fewer photons can pass through a longer pathlength of intervening material (eg, soft tissue) than can pass through a shorter pathlength.

This effect leads to image artifacts (eg, "breast shadows" on SPECT myocardial perfusion studies, "bowl-shaped" SPECT reconstructions of uniform sources, etc). Attenuation also degrades our ability to obtain accurate quantitative information from nuclear medicine images.

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Lesson Author: Stephen C Moore, PhD, scmoore@bwh.harvard.edu
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Updated June 1, 1998