Just getting an MRI scan for back pain that is “not serious” without other significant symptoms doesn’t make sense. There must be more going on to require an MRI. Is the pain severe? Are you impaired with your activities or work? Do you have weakness or bowel and bladder symptoms? Has the pain persisted for more than three months without relief from physical therapy, chiropractic or medications?
You mention contrast injection with the MRI. Contrast (Gadolinium) enhances blood vessels and is useful if you have had a prior surgery or are suspected of having an inflammatory condition but is normally never used on the first MRI if either of those are not present.
The term “slipped disc” is a poor one for a diagnosis. The disc cannot “slip” as it is firmly attached to the bone of the vertebra. The top vertebra can slip on the bottom one but this is easily seen on an X-ray and can be made more apparent on flexion-extension X-rays. The doctor may think you have a disc herniation.
Ask your doctor why you need an MRI. He will probably be able to give you the indications for this imaging study. Ask why you need contrast also. Make sure the MRI machine is at least 1.5 Tesla strength. There are “open” MRIs that have much lower strength (0.3 Tesla) and many times, the imaging is poor and will need to be repeated.