My son had one but its a lot of radiation. So unless your looking for something specific I don't think it is worth it. For my son he fell out thewindow and they opted for an MRI instead of xrays.

Stroke. Of course that was long before any concerns about Autism. I just recommend having it done somewhere that is experienced in pediatric sedation, a plus if experience with pediatrics on spectrum.

We did a MRI and learned nothing while putting my son through a lot of stress. I think the only thing an MRI does is check the physical structure of the brain. I haven't heard from anyone that it was a useful procedure. Personally I think you are better off finding someone that does Neurotherapy and have them do a QEEG. This brain imaging tool will map the level of electrical activity in your child's brain. Then this information can be used to develop a treatment plan to balance that activity. Huge benefits for most kids that have done Neurotherapy.

How about doing it when your son is sleeping. We participated in a clinical trial (no gain to us, just contribution to science in hope that if this continues, future kids and parents get better help). They did not want sedation, so we reached there at 10.00pm, DS took really long to sleep but once he slept, he didn't move (had supports as well) for almost 50 minutes. This was fMRI and DS was 3.

The question is why are they wanting an MRI? My son had one this year because he has multiple issues and they wanted to know for sure if he had any physical brain abnormalities such as enlarged ventricles, papilledema (swelling), etc. He also has issues with his optic nerves and the MRI gave them a better look at those.
If you aren't sure why they want an MRI done, ask. They should be looking for a physical problem.
It is not harmful but can be stressful. My son has a needle phobia and had to be held down by 4 nurses + me just to inject the contrast dye.
But at 3, I would assume sedation would be necessary so that might be easily avoided...