CT Scan vs. MRI: Differences between Machines, Costs, Uses

Picture: A medical technician prepares a patient for an MRI to check for a possible brain tumor

• CT scans utilize X-rays to produce images of the inside of the body while MRI (magnetic resonance imaging) uses powerful magnetic fields and radiofrequency pulses to produce detailed pictures of organs and other internal body structures.
• CT scans use radiation (X-rays), and MRIs do not.
• MRIs provide more detailed information about the inner organs (soft tissues) such as the brain, skeletal system, reproductive system, and other organ systems than is provided by a CT scan.
• CT scans are quick, painless, and noninvasive.
• MRI scans are not invasive, but they are noisy, take more time, and may cause claustrophobia (anxiety due to being in the enclosed space of the machine).
• MRI scans are costlier than CT scans.
• MRI scanners may cause a safety issue due to their strong magnets.

• A CT (computerized tomography) scan is a combination of a series of X-ray images taken at different angles; the CT uses a computer to create images from these X-rays.
• An MRI (magnetic resonance imaging) is a scan that uses magnetic fields and radio waves to produce a detailed image of the body’s soft tissues and bones.

A CT scan works by taking multiple X-rays at various angles and then utilizing those X-rays to form a three-dimensional image of whatever organ system is being examined. A computer examines all of the various X-rays taken at different angles and synthesizes the images to form a three-dimensional computer model of internal organs.

MRIs use and send superconducting magnet and radiofrequency waves into the body. The magnetic field lines up atoms either in a north or south position with a few atoms that are unmatched (keep spinning in a normal fashion). When radiofrequency is added, the unmatched atoms spin in an opposite direction, and when the radiofrequency is turned off those unmatched atoms return to the normal position emitting energy. The energy emitted sends a signal to the computer and the computer uses mathematical formulas to convert the signal into an image.

In general, both CT and MRI scans are relatively safe. However, there can be problems. MRI scans should not be done on patients that have aneurysm clips (clips of the vessels within the brain) unless these clips are known to be MRI safe, as these clips can be pulled off and the patient could die from bleeding into the brain.

Another problem with the MRI is the presence of some cardiac pacemakers or defibrillators because the magnets can cause malfunctions in these battery-operated devices. Any metal devices that can interact with a magnetic field, for example, the presence of metal shavings in an organ, the eye, or extremity may be pulled out by the magnetic field. Moreover, other canisters that are metal (like some oxygen tanks) need to be kept away from MRI machines because they can be attracted to the magnet and injure or kill the patient.

CT scans do not have these problems; however, they do expose the patient to radiation, though it’s a relatively low dose. Certain types of CT scans may not be appropriate during pregnancy.

CT scans are quick, painless, and provide good details about your condition to your doctor. MRI scans also are painless and provide greater detailed images of soft tissue than CT scans. MRI scans have no ill effects on the body unless the person has some contraindications for the scan as mentioned previously. However, MRI scans are not quick, and some patients find the narrow opening claustrophobic, generating anxiety, especially when the noisy magnets reset. This can hurt the comfort level of the patient. Moreover, the patient must lay completely still while the images are being taken.

Less confining or “open” MRIs are available, but they do not produce images in detail as seen in the regular MRI scans.

Examples of diseases or conditions that CT scans are used to help diagnose include:

• Take images of the brain to help differentiate between an ischemic or hemorrhagic stroke.
• Evaluate head trauma patients, especially facial trauma.
• Help determine the diagnosis of abdominal pain and/or pain in the pelvis, small bowel, colon, and other internal organs.
• Determine the cause of unexplained pain.

CT scans are also useful in some traumatic injuries to show subtle fractures in bones.

MRI scans are principally used for imaging detailed pictures of organs, soft tissue, ligaments, and other features that are more difficult to see, with CT, for example, MRI provides more images that were more detailed when the lumbar spine is viewed to determine if a herniated disc is present.

Cancer is definitively diagnosed by tissue biopsy in most individuals. CT and MRI scans can show “masses” that are likely tumors (aggregations of cancer cells) but is not a definitive diagnostic tool for cancers. Both CT and MRI scans may be used to determine the best site to do a biopsy to definitively diagnose cancer. Moreover, once cancer has been identified in the patient, CT and MRI can give your oncologist (a doctor that specializes in cancer) a better idea about if or where cancer has spread (metastasized) in the body.

Both CT and MRI scans can see bones and bony structures of the body; however, MRI provides detail, especially of the soft tissues surrounding the bones. Both have a wide scope of application in medicine.

Both CT and MRI scans can see soft tissues of the body; however, MRI provides greater detailed images of soft tissues.

Most current MRI machines cannot rapidly scan the brain to help determine the cause of a stroke (ischemic versus hemorrhagic). In general, CT scans are rapid (quick) and give your doctors, especially in the emergency department, a very useful diagnostic tool.

MRI is usually reserved for non-emergency situations when time can be taken to get a detailed look at the brain or soft tissues of a patient.

An MRI costs about twice as much (about $1,200-$4,000) as a CT scan, especially if a contrast dye also is used with the MRI.

A CT scan is limited in detail when used for diagnosing soft tissue problems. A good example is what happens to a professional athlete that is injured. He or she may get an X-ray or CT of their ankle or knee to see if there is any fracture. If there is no fracture, the doctor will most likely order an MRI to get a much better-detailed picture to assess the damage level, if any, to the ligaments and other soft tissues of the injured area. MRI information also can help determine the best treatment program for that athlete.

Some types of metal implants or orthopedic hardware are magnetic and are not compatible with an MRI, especially older types. If you are unsure about having any metal fragments in your body (war shrapnel or metalworking injuries), you may need an X-ray before your MRI.

The following list is examples of things that are not compatible with MRI scans, particularly metal implants, or fragments containing iron. (NOTE: not all of these things are definite contraindications for getting an MRI. Talk to your doctor about your circumstance before the test.)

• Artificial (prosthetic) metal heart valves
• Artificial joints
• Brain aneurysm clips
• Cochlear implants
• Dentures/teeth with magnetic keepers
• Drug pump implants
• Eye implants
• Implantable cardioverter-defibrillators
• Implanted metal wires, rods, screws, or plates
• IUDs intrauterine device
• Medication patches (transdermal patches) that contain metal foil
• Metallic fragments in or near the eyes or blood vessels
• Nerve stimulators
• Obesity. Some MRI machines have weight limits and may vary from 300 to 500 pounds, and such patients need to fit into the scanner opening.
• Pacemakers
• Penile implants
• Permanent cosmetics or tattoos
• Pregnancy - MRI scans are not recommended during pregnancy, particularly in the first three months.
• Tubal ligation clips
• Surgical clips or staples

In general, if you are pregnant you should not get a CT scan unless necessary; however, you and your doctor have to decide whether the risks are worth the benefits of a CT scan. Like the MRI scanners, various CTs have patient weight limits (which may vary from about 300 – 500 pounds) and/or limits on the size of the patient to fit through the scanner.