Abstract:
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Purpose: CT pulmonary angiogram images can demonstrate flow and other artifacts which mimic pulmonary embolism (PE). The purpose of this study is to define the normal range of Hounsfield Unit (HU) values of acute PE. In addition, to compare the mean HU values of acute and chronic PE.
Methods and Materials: This retrospective study was from July 2001 to January 2003. CTs for evaluation of PE were acquired with 1.25 mm collimation. All patients with a diagnosis of chronic PE were identified. Chronic PE criteria included: Complete occlusion of a vessel which is smaller than peers; eccentric or concentric thickening of the vessel wall with obtuse angles to the wall; contrast flowing through a thickened smaller artery; a web or flap within contrast filled artery; thrombus present for more than 3 months. Acute PE criteria included: Arterial occlusion with failure to opacify the entire lumen due to a central filling defect (the artery enlarges in comparison to peers); partial filling defect surrounded by contrast; the filling defect makes acute angles to the vessel wall when in contact. Two readers independently made HU measurements using the above and the following criteria: The largest diameter thrombus was identified for an individual patient; the region of interest (ROI) was measured from the middle image of three which demonstrated thrombus; ROI diameter was selected to be half the diameter of the thrombus being measured; ROI was positioned in order to avoid streak artifacts. 28 patients with chronic PE were identified; 13 were excluded because the lesions were too small to measure, 2 cases were excluded due to streak artifacts. 27 consecutive patients with acute PE were identified; 2 were excluded because of streak artifacts. Statistical analysis included calculation of means and standard deviations, the t-test and the Bland-Altman plot.
Results: Reader 1; chronic PE HU mean =100 +/- 36 (range 51-159), acute PE HU mean = 33 +/- 15 (range 6-63), p < 0.0001. Reader 2; chronic PE HU mean = 82 +/- 32 (range 32-135), acute PE HU mean = 33 +/- 14 (range 13-65), p < 0.0001. The Bland-Altman plot demonstrated very good agreement between readers.
Conclusion: The range of acute PE to 3 standard deviations is from -12 to 78 HU, above or below this range of values the chance of acute PE is 0.13%, meaning that flow or other CT artifact becomes more likely. Within this range the CT criteria of PE are necessary to differentiate between acute PE, chronic PE and artifact. Chronic PE has a statistically significant higher mean HU value than acute PE, likely due to enhancement of organizing thrombus.
Questions about this event email: cwittram@partners.org
Wittram MBChB, C,
The Hounsfield Unit Values of Acute and Chronic Pulmonary Emboli. Radiological Society of North America 2003 Scientific Assembly and Annual Meeting, November 30 - December 5, 2003 ,Chicago IL.
http://archive.rsna.org/2003/3102565.html
