Which Test When?
Before making a diagnosis, your health care provider may order Computer Tomography (CT or CAT scan) or Magnetic Resonance Imaging (MRI) if you’re having neurological issues involving the brain, spine, head or neck. Here are some basic differences:
- Timing. A CT typically lasts just a few seconds, while a typical neuro MRI takes about 20 minutes.
- Physics. A CT scan takes hundreds of X-rays in just a few seconds that are then sent to a computer, which takes these individual images and reconstructs them into 3-D cross-sectional images. MRI uses a permanent magnet and radio waves that react with different tissues in your body. These reactions are measured by a receiver on the machine and generate similar 3-D cross-sectional images.
- Technique. Both CT and MRI require you to lay on your back on a table that slides into an enclosed tube (although “open magnet” MRI machines are not fully enclosed to accommodate claustrophobic patients). CT scans are very quiet, while the MRI magnets can make loud “knocking” noises that can be unpleasant. Both studies are otherwise painless.
- Radiation. Because CT scan utilizes X-rays, it imparts radiation into your body that is accumulated over your lifetime. The radiation with each CT scan is very low. The overall risk to your health is negligible to minimal. MRI has no radiation dose.
- Exclusions. If not MRI compatible, certain metals implanted in your body (pacemakers or cochlear devices) can react with the magnet and potentially move or heat up. Please let the technologist know about any previous surgeries or such devices, as each case is handled individually. Implanted metal does not preclude CT scan, but can in certain cases produce artifacts (misrepresentations of body structures). Certain CT and MRI studies will be ordered with contrast (such as dye)—please let the technologist know about any preexisting allergies or previous contrast reactions.
Both CT and MRI play critical roles, with each having advantages over the other in specific situations.
In general, bones are better evaluated by CT, and soft tissue (muscles, ligaments, tendons, spinal cord, etc.) with MRI. Here are some common indications for CT or MRI studies:
- Chronic back pain. MRI is a better option for disc herniations, spinal cord and nerve roots.
- Oncology. MRI is most often used to diagnose and follow brain and spine cancers. At North Mississippi Medical Center, CT is used to follow up most neck cancers. This could differ between institutions and even between physicians at the same location.
- Acute/new medical problems or trauma/injury. CT’s speed and ability to diagnose or exclude the most common pathology is ideal for these scans. MRI may also be used to evaluate soft tissues in an acute traumatic injury or compliment CT in certain “problem-solving” situations.
- Chronic neurological problems. When the initial CT doesn’t uncover the cause of headaches, imbalance or dizziness, MRI is often utilized to evaluate the brain, inner ear structures and cranial nerves.
- Vessel imaging. Both CT and MRI are exceptional at visualizing arteries and veins within the head and neck, though CT is slightly superior. However, MRI may be used, especially in younger patients, to avoid radiation exposure or in patients who are unable to receive iodinated contrast for CT because of either allergy or renal dysfunction.
We use both CT and MRI daily—both are very safe. CT and MRI are often used together to diagnose and sort out the most common and obscure neurologic and neurosurgical problems. If you have any questions or concerns, please ask your health care provider, technologist or even the radiologist.