Imaging such as MRI's, CT's, Octreotide Scans, and Gadolinium Scans (newer) will and may be used in the search for the source of your Cushing's Syndrome, though don't worry, it's not usually all of these, nor all at once!
Because a pituitary source is the most likely location of an ACTH-producing tumor by far, the pituitary is usually the first place they look unless laboratory testing strongly suggests an adrenal or ectopic source. (And, I'll probably give the most info here since this is where the bulk of my experience has been)
The imaging of choice for pituitary tumors is a 3T dynamic pituitary MRI.
3T denotes the strength of the magnet used (3 Tesla's). There are still a lot of 1.5 T machines in use, but they do NOT give as clear an image as a 3T machine for pituitary tumors, which is why 3T is now the standard when looking for pituitary microadenomas (more common than macro's, which are larger and could be more easily seen on a lower-strength machine). Microadenomas are under 1 cm in size, and are often as small as 3mm (or even less!). Look at a pencil eraser, then cut it in half...now picture this chunk inside a bean and sitting at the base of your skull, almost in the center of your head. Think it'd be hard to see? Yep. Many, many radiologists and doctor's miss them, which is why it is important to get copies of your imaging and not just the report, AND to be sure a Cushing's-experienced radiologist, endocrinologist and/or neurosurgeon looks at the actual images. (Ask me how I learned this one the hard way? Yep. Wasted a year of my life sick with no known course of action because I didn't know this...okay, I also didn't know when to test and testing at the wrong time. So it was a double whammy, but either of those having been different would have suggested I needed to keep testing.)
The word "dynamic" refers to how the dye is injected. They first run a number of scans without contrast dye. Then they either have two tech's work together in unison, one to inject the dye as the other starts the machine with an audible count-down to be sure they inject and start at the same time OR the tech will hook you up to an IV-infusion type machine that he can set to inject the medication automatically as the tech starts the MRI. It is VERY time-specific. The reason it must be done this way is that these tiny tumors generally look like normal tissue on a pre-contrasted MRI. And, they uptake dye like normal tissue and thus will often not show up on a post-contrast MRI. So why do the MRI? Because they absorb the dye at a slightly slower rate. A healthy pituitary should absorb the dye in a uniform way. These ACTH-producing microadenoma's will tend to look like a dark spot when that happens, but as you keep looking through the timed scans, eventually it will lighten up like hte rest of the pituitary. Dynamic is important. You want this done right. Most surgeons won't go looking for a tumor no one can see. It isn't exactly low-risk. But staying sick isn't low-risk either. You want that proof so you can get well.
Sometimes the paperwork will say with/without contrast, but even so, it has to be done a VERY specific way as explained above. You will know they aren't doing a dynamic if it is just one person administering the MRI, and they walk in with an injection needle and then expect to walk out and start the scan afterwards on their own. I had somehow been scheduled wrong once, and the tech came in in just this manner. I KNEW it wasn't right, so refused to let him inject me (once it is injected, there is no way of doing a dynamic for some time, and I was on a trip with no way of coming back later). I knew better and it wasn't going to happen! I told him no, it was supposed to be a 3T dynamic pit MRI. He argued with me, I didn't relent, and finally he listened when I told him to call the neurosurgeon's office to ask them (it was a post-surgery MRI ordered by the neurosurgeon this time, and I was at one of their hospital's imaging facilities). Sure enough, I was right. They had me on the wrong machine, even! A 1.5 T. Something had gotten lost in the shuffle of scheduling. The tech's attitude then switched from irritation and frustration with me, to being impressed I knew it wasn't the right protocol. He asked if I was a tech myself. lol It was a mess of a day as they tried to get me fit into the right type of machine (which were fully scheduled for the day already), but they were kind and it worked out. I got my 3T MRI. When you're making decisions about whether to have repeat brain surgery, the right testing (and imaging) is obviously important; it was worth a day of work.
"Pituitary" should be obvious, right? I mean, that's what we're trying to see! But no, sometimes doctors will order a brain MRI. Yes, both take pictures of your brain AND your pituitary, but the pituitary is tiny and the brain is big, and a brain MRI's "slices" (pictures) are spread farther apart. We need the focus on the pituitary, and we need those super-fine, close-together slices. Brain MRI's aren't likely to catch microadenoma's, and as many of us know from experience, it's hard enough to find them with the right protocol.
The next most common cause of endogenous Cushing's Syndrome (meaning outside the pituitary), is an adrenal source...meaning a tumor on an adrenal gland. Laboratory testing can suggest this is likely, because often ACTH is low or near zero with these patients. Adrenal glands can be hard to visualize adequately, as they're fairly small and sit like little pyramid hats atop each kidney. Sometimes an MRI is used to image them, but I believe more often, an abdominal CT is the imaging of choice.
Suspected Ectopic Cushing's Syndrome (meaning a source not in the pituitary or adrenals) usually produces very HIGH amounts of ACTH, so lab testing may suggest this is the type of source to look for. It also often requires an MRI or CT, but these may not be sufficient to identify a tumor. In this instance, as well as sometimes after a failed Bilateral adrenalectomy, more specialized, whole-body testing is required. In many ways it is the same sort of scan, but they use a contrast dye that can help pinpoint a tumor specifically (ie, the dye will be attracted to that type of tissue, I believe tissues with somatostatin receptors) and will "light up" on the scan. The most common type is an Octreotide scan, but there is a new type of scan, that may be more sensitive, that is starting to be used for testing also called a Gadolinium scan. Neither of these types of tests are widely available, again it is specialized testing and generally patients have to travel to find a facility with the ability to preform them.