I had my breasts done on different days (a pain, given that it's an hour away, and one of my scans was at 7 am, the other at 9:30 pm). They told me it's that they need to inject contrast fluid mid-scan, then watch its progress through the part they're scanning, and if they do it on the same day, the stuff's still in the body, it's not a "fresh" contrast injection. They can only do one foot at a time, at any rate, so they inject the stuff, do one foot . . . then they can't see the contrast fluid go through the other foot. (I keep wanting to type "boob" for "foot.)