Figure 28a. Partial volume artifact in a 52-year-old woman with dyspnea. (a) On a 3.75-mm-thick CT scan, partial volume averaging of vessel and lung creates an artifact that mimics pulmonary embolism within the anterior segment of the left upper lobe pulmonary artery (arrow). The apparent pulmonary embolism is ill defined. (b) Contiguous CT scan obtained inferior to a demonstrates normal lung adjacent to the left upper lobe pulmonary artery. (c) Contiguous CT scan obtained immediately superior to a demonstrates a contrast material-filled pulmonary artery, a finding that confirms that the low attenuation seen in a was due to partial volume artifact.