Monday Case of the Day Physics Author: James Kofler, Ph.D., Jonathan Morris, MD Mayo Clinic, Rochester MN History A head CT study of a patient shows diffuse hypodensity throughout the white matter of the right hemisphere and left paramedian hemisphere (Figure 1), while a study the previous day was normal. A second patient the same day shows a similar abnormality. What action should be taken? Retake the studies with different acquisition parameters Reconstruct the images with different parameters Call in the service engineer Check the daily QA images Figure 1: Diffuse right hemisphere abnormalities

Diagnosis Check the QA images. On this occasion, the daily QA images showed structures similar to those seen in the clinical images (see Figure 2, below). Contrast material was found on the x-ray window of the gantry; after it was cleaned, the phantom images were reacquired and the artifact was no longer present. Figure 2: Daily QA water phantom image

Discussion There are steps that technologists can take to increase the probability of catching this kind of problem in the QA images. All of the images should be viewed, not just the first image. Artifacts can show up in some of the image planes but not others. Also, the images should be viewed with a narrow window setting. Figure 3 is a water QA image taken with standard abdominal settings. Figure 4 is the same image, but with the settings recommended by the ACR. Figure 3: WW= 400, WL= 0 Figure 4: WW=100, WL=0