How good was that CT Pulmonary Angiogram You Ordered?
CT is currently the gold standard imaging modality for pulmonary embolism. Since we order these quite a bit in the ED, we should know some of the important nuances regarding interpretation of the scan. All of us at some point have looked at a pulmonary CTA and thought that it looked a bit "fuzzy" or perhaps it didn't "look right" This happens more often in obese patients. There is good literature to show that a suboptimal CTA misses clinically significant PE. So, it is important for emergency physicians to know a little about the CT scan ordered for our patients.
How can you know if the CT scan YOU ordered to rule out PE is really "good enough" to rule out PE?
Well, you can rely on the radiologist. But remember they may not comment of the quality of the scan. Or, they may simply recommend another test.
Look at the Hounsfield Units (HU). For those who have PACS or some other computer radiology display,all you need to do is move the cursor to the main pulmonary artery and see what value (usually on the bottom of the screen) is displayed.
A HU >280 indicates that the CT is "good" (i.e. good enough contrast bolus to detect clot). By the way, >350 looks white.
So, a 34 yo obese patient who gets a CT scan to rule out PE, who has 170 HU in the main pulmonary artery, has not had an optimal CT. Thus, you really haven't ruled out PE even if the read is "negative." Often this is due to poor bolus timing.
References
American College of Radiology, Appropriateness Criteria, 2006
