Answer to Question #4177 Submitted to "Ask the Experts"

I have several metal surgical clips on the anterior surface of my right kidney. Three of my abdominal CT scans were done with these clips in place. What happens when the CT beam hits something metal, such as this? Is the CT beam blocked, does it go bounding around, or something else? I have not seen this topic covered in any of the literature I've reviewed.

When photon radiations, like x rays, interact with matter, it can be absorbed or scattered. With surgical clips, the radiation beam is blocked from the CT detector directly behind the metal clip. However, since the CT image is created from several detectors as the beam moves around the patient, the detectors not directly shielded by the metal clip will detect the radiation that is obliquely scattered off of the clip. (See "What is Computed Tomography?" on the Food and Drug Administration Web site.) This generates a streaking artifact. Since the CT image is reconstructed by a computer, it is possible to correct for these artifacts.

John P. Jacobus, MS
Certified Health Physicist

Editor's Notes: For an extreme example of streaking see the Washington University in St. Louis Web site.

For a properly corrected image, click on "Image Count: 3" at the MedPix Web site and note the bright clip in the upper right edge of the abdominal wall.