As mentioned, there are various types of contrast that may be administered prior to a diagnostic procedure. Iodine is used in most cases, but depending upon the type of test being conducted or the patient’s allergies to a particular substance, other suitable solutions can be used. Iodine and barium sulfate based contrast materials are typically best for an X-ray or CT scan while gadolinium is ideal for MRIs, and saline works well for diagnostic examinations of the heart.