I recently received a question about radiologists and whether or not you can see a radiologist without a physician referral.
According to http://www.radiologyinfo.org/, "a radiologist is a physician who has specialized training in obtaining and interpreting medical images, which makes him or her an imaging expert. These images are obtained by using x-rays (radiographs, CT, fluoroscopy), or radioactive substances (nuclear medicine), others by means of sound waves (ultrasound) or the body's natural magnetism (MRI)."*
As far as I know from my experience in the medical field, a radiologist is usually referred by another physician trying to diagnose a disease process, or injury. Radiologists are also responsible for treating cancer and assisting with non-invasive surgeries using the body's magnetism (usually used for brain surgeries).
So, to answer your question Zombie, a radiologist is typically always used with the referral from a doctor because of the need for an x-ray or radiology to cure, or improve the condition of a disease.
Inshallah, this helps Zombie, and any others who may have been wondering.
*Info gathered from radiologyinfo.org.