Found this concise definition: A SPECT scan is a type of nuclear imaging test, which means it uses a radioactive substance and a special camera to create 3-D pictures. While imaging tests like X-rays can show what the structures inside your body look like, a SPECT scan produces images that show how your organs work.

Sounds to me like the scan, as amandalouise posted, is generally used to check for any structural abnormalities (a mass, or tumor, for example).

However, from the tiny bit I've looked at online certain brain activities can be determined by the scan (seizure activity, perhaps) and that type of neurological activity can cause some psychiatric symptoms. I mean...no...mental illness can't exactly be diagnosed with a scan but certain symptoms of what appears to be mental illness can show up as abnormal brain structure or activity.

This type of thing is something that only a medical professional can really interpret. It sounds to me like your psychiatrist is covering all the bases, which I think is a good thing. My current p-doc sent me for a brain scan right after our first appointment and I was impressed. I thought...yeah...it's about time one of these doctors took a look at my brain and made sure there's no abnormality within my brain structure itself.