Last week I had a young patient ask me what the difference is between an MRI and a CAT scan. Not long after that, I noticed an error in a newspaper article that mixed up the two technologies. 

Radiologic imaging of the human body has revolutionized our diagnostic accuracy. However, it also has the negative effect of reducing our reliance on a good medical history and physical examination. 

There is also a real concern about patients receiving too much radiation over their lifetimes as a result of having too many CT scans (more below). This is particularly concerning in children who may receive numerous scans over their lifetimes that may increase their risk of cancer.

We are the only country in the world where a CT and/or MRI scanner is in the neighborhood of virtually every citizen. While this is convenient, it leads to over-utilization of these very expensive and sometimes unnecessary technologies.

CAT (CT) scanners have been around longer than MRI scanners. CAT stands for Computed Axial Tomography, but we typically use the shortened abbreviation CT for Computed Tomography.  This test uses ionizing radiation (X-rays) to produce the medical image. 

When performing a CT scan, the patient lies on a table that is surrounded by a big ring called a gantry that contains one or more X-ray tubes that rotates around the patient. As the tube is spinning around the patient, the table moves the patient through the ring.

The X-rays pass through the body and activate detectors on the opposite side of the patient. The more detectors there are, the more detailed the image is, and the faster the scan can be done. X-rays pass differently through various body tissues. Through the marvels of high speed computing, the scanner produces digital pictures of the body.  The pictures typically look like 2-dimensional slices through the body. The machine can also reconstruct 3-D images that can be manipulated to provide striking images – almost like seeing the object in question with the naked eye.

The first MRI scan of a human was produced in 1977. MRI stands for Magnetic Resonance Imaging. Unlike CT scanning, this test does not use ionizing radiation to produce images.  The patient lies on a table that slides into the MRI machine, a tube that can vary in length from a few feet to the full length of the body. Some scanners have open sides and are usually used for obese or claustrophobic patients.

MRI scanners work by placing the patient inside a powerful helium-cooled magnet that causes atoms (usually hydrogen) in the tissues in the body to align with the magnetic field. These atoms are present in varying amounts depending on the tissues being studied. The patient is then systematically exposed to radio waves that cause the atoms to produce tiny magnetic fields. The scanner detects the differences in the fields and a very powerful computer is used to produce the image.

You may be wondering why a doctor would choose to use CT instead of MRI or vice versa. Each procedure has strengths and weaknesses as far as imaging particular tissues. MRI scanners do a better job at imaging tissues containing water that contains lots of hydrogen atoms. It is therefore an excellent tool for looking at the brain, spinal cord, nerves, muscle, tendons, ligaments, cartilage as well as various tumors.

CT scanners are particularly useful for imaging bone fractures and internal bleeding. CT scans are superior to MRI for imaging the chest, particularly the lungs to look for pneumonia, tumors, blood clots, etc. CT scans also take much less time to perform, therefore are more useful in trauma situations when a doctor needs to rapidly identify internal injuries.

CT and MRI technologies continue to improve. CT scanners used to employ a single X-ray tube and detector. The machines now utilize multiple tubes and detectors that can collect massive amounts of image data as the patient is moved through the machine. An added bonus to high speed computing is that it allows much more rapid scanning, requiring 30 to 70 percent less radiation exposure. MRI scanners are also employing more powerful magnets to improve the resolution (sharpness) of the images.

- Dr. John Roberts is a licensed medical physician. He writes a weekly column exclusively for Sagamore News Media publications.