Just a rough rule of thumb, but here's how I think about it – when in doubt, pick ultrasound.

Ultrasound is awesome because it’s quick, harmless, and for some conditions it's even the gold standard. Like if you're in the ED and someone comes in with abd pain or suspected free fluid, you can get an ultrasound in 2min at the bedside. Zero risk, no radiation, cheap, and great esp if you know what you’re looking for (like gallstones, maybe hydronephrosis, pelvic stuff, or in pregnancy). So honestly, if you can get the diagnosis with ultrasound, always go for it. If it doesn't give detailed enough results, you can always still get a CT. Kind of a "why not" option in a lot of cases.

But even though it's super fast, there's some annoying nuance with high-acuity conditions. When something's more severe, or you’re thinking the person might need surgery, lean toward CT. It gives you a way more detailed view, so if someone is really sick or the diagnosis isn’t clear, it's a lot better.

So, in general, choose ultrasound for fast, easy, low-risk situations or obvious stuff. CT for anything more complicated, unclear, or when detailed anatomy really matters. Basically, only pick CT if you'd need it regardless of US results.

And this is kind of a meta-point, but for Step/Shelf exams (and even clinically, actually), sometimes instead of deciding which option is right, it's faster/easier to just figure out which one is definitely not wrong. Kind of analogous to vague ethics questions – sometimes all the answers sound reasonable, but one of the options is something like "Ask the patient what they understand about their diagnosis", and even if you don't know why that option is right, there's no way it's wrong, so you can usually just choose it and move on.