Do I need an MRI or CT? is one of the most frequently asked questions we hear on a daily basis and often followed by the question Is it safe?
First, Magnetic Resonance Imaging, MRI, is an imaging technique that uses a combination of magnetic fields, radio waves and computers to generate detailed images of the human body without exposure to radiation.
To acquire the images takes from thirty to sixty minutes. The MRI unit comes in two "sizes": the ‘open MRI' designed for claustrophobic or larger patients and ‘standard' that most of us find quite comfortable. Today's newer ‘open' systems create images of equal quality to ‘standard' MRI units. To maximize resolution and accuracy, studies are tailored to specific body parts with specialized antennas used gain signal from areas such as the knee, brain or spine. There are no known harmful effects to an MRI scan, but there are certain patients who should avoid being scanned. For example, patients with pacemakers, implanted stimulators, or certain brain aneurysm clips should not be scanned. Pregnancy is not a contraindication since there are no known harmful effects to the fetus, but if possible one should avoid a scan in the first trimester. Some MRI studies require a contrast injection. Generally speaking, contrast is very safe though some patients have been known to develop a reaction and should be avoided for any patient with Nephrogenic Systemic fibrosis. Therefore, contrast is only given when it will provide additional information. The final recommendation to use contrast should rest with the radiologist.
Computed Tomography, CT or CAT scan, is an imaging modality using X-rays, specialized detectors, and computers. Over the past few years we have seen dramatic improvement in the resolution and image acquisition time of CT scanners. A typical CT study takes approximately 15 minutes. Current generation CT images of the chest can take seconds and it is now possible to acquire images of the coronary arteries in a single breath hold or heartbeat. Most CT studies are performed using an iodine based intravenous contrast which is often well tolerated, although side effects are more common with CT contrast than MR contrast. CT contrast should be avoided for patients with preexisting kidney disease or who are on certain medications, necessitating pre-screening. Oral contrast can also be administered, which helps when evaluating the bowel. There have been several recent scientific papers and media reports detailing the potential risk for developing cancer following exposure to radiation. Obviously, these reports are disconcerting because a CT can result in exposure to radiation approximately 20 times more than a typical chest X-ray. Radiation doses will vary by the body part scanned and more detailed studies are underway. On balance, it is critical to weigh the medical necessity of the CT study while keeping radiation exposure to a minimum. For example, avoid repeat exams of the same body area by keeping a record of when and were you had your scan, ask the facility to forward your medical results to your physician or specialist rather than undergoing another study and make sure the facility where you receive your scan has a radiation exposure reduction program in place. You should feel empowered to ask questions and be actively engaged in managing your medical care.
When recommending an MRI or CT study to a patient, the physician assesses the patient's condition and medical appropriateness of each modality and often seeks input from the radiologist. There are also several resources available to physicians to aid in the decision-making process including "Appropriateness Criteria" developed by the American College of Radiology. In this instance, the patient's symptoms, indications and suspected disease are paired against various imaging procedures, assigning each a relative value. That is, if a patient presents with short term acute low back pain without leg pain or other symptoms, the criteria would suggest no imaging at all. However, if the patient presents with low back pain, leg weakness and a fever or history of cancer, the criteria would indicate that MRI is the most appropriate test. Similarly, if a patient presented with simple headache, no imaging is necessary, as opposed to headache with a history of cancer or significant trauma which may recommend MRI or CT.
In summary, it is important for patients and physicians, working together, to carefully consider when an imaging study is appropriate in diagnosing the patient's issue. Communicating with your physician will also help reduce, if not eliminate, redundant and inappropriate imaging, avoid potential risks, and help you manage your care in a more medically appropriate manner toward better health.