There's something satisfying about getting a perfectly-timed arterial liver CT. I think its a combination of the cotton wool appearance of the spleen, partially-filled portal vein, and any hypervascular lesions lighting up like a Christmas tree. So if you're ever wondering if you have timed your multiphase CT liver correctly, those are a few things to look for on the arterial phase.
I always use bolus tracking for my multiphase liver protocol. I inject 150 ml @ 5 ml/sec and track on the abdominal aorta. As soon as I see contrast in the aorta, I initiate my arterial phase BUT I set a scan delay of 15 seconds from this point. In other words, the arterial phase is scanned at 15 seconds post aortic transit time (ATT). This usually equates to 35 seconds post injection.
The portal venous phase is then automatically set to scan 30 seconds after the arterial phase has finished, which is usually 65 seconds post injection. You will notice the spleen parenchyma on the portal venous phase is now homogeneous, and the hepatic vein has contrast which was not present on the arterial phase.
Lastly, here's a quick question for you:
Can you find a structure that has the SAME appearance on both arterial and PV phases?
If you guessed the splenic vein, then you are correct! Contrast quickly moves from the splenic artery to the vein and then onto the portal vein. The late arterial phase of the liver should occur during this transient stage of contrast moving from the splenic to the portal vein. So if you don't see contrast in the splenic vein, you've scanned too early. And if contrast has moved right through and filled up the portal vein, you've scanned too late.
If you use less than 150 ml of contrast, the splenic vein may look more washed out on the portal venous phase because the contrast bolus has moved right through. This is technically not a big issue in terms of diagnostic value, but it is something to keep in mind when critiquing your image.
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