What is the difference between MRI, CAT scan, myelogram and radiographs? All the modalities are used in some way to image the nervous system, however they each have their advantages and disadvantages.
MRI: This stands for Magnetic Resonance Imaging. It is the “gold standard” for imaging the nervous system. The MRI uses a very strong magnetic field to excite hydrogen ions which in turn causes a release of energy that can be measured and turned into a picture of the tissue that we are focusing on. Since hydrogen is present in all tissues in various amounts, it gives us great images of all tissues, with the exception of the lungs (not a lot of hydrogen in air filled spaces) and the heart (too much movement). The best images produced by the MRI are those of tissues that are in a very straight line like the brain and the spine. The MRI allows us to see the brain and spinal cord along with the spinal nerves, muscles, bones and all other structures that are nearby. It also allows us to see fluid in tissue that may be from blood, swelling (edema), infection or spinal fluid (CSF). This produces the best image possible of the nervous system, arguably in all circumstances of disease. Images can be made lengthwise (like a side view) and in cross section. Think of slicing a loaf of bread. We can “slice” through the brain or spine and look at the individual slice. MRI is safe, painless and non – invasive. It requires anesthesia to make sure the patient does not move during the scan and takes about 30 – 45 minutes to produce a complete study a body part, comprised of numerous individual scan. It is typically the most expensive imaging modality.
CAT Scan: This stands for Computed Axial Tomography. A CAT scan or “CT” scan uses radiation (x-rays) to look at body parts. One way to think of x-rays and how they are used is to think of a shadow. When you hold your hand up against a light and see the shadow against the wall, your hand is blocking the light rays from hitting the wall. This creates an image of your hand. X-rays work in the same way. X-rays are a form of energy just like light rays only you cannot see them with your eyes. They are a very powerful form of energy that can penetrate through the skin of the body. The patient’s bones block the x-rays and the machine then creates a picture of the shadow of the image that is blocking the x-rays. With a CAT scan we can look at body parts in cross section and lengthwise just like an MRI. However, because we are “blocking” energy (the x-rays) some tissues do a better job at this than others. Bones for instance are very dense and the powerful x-rays are blocked better than a soft tissue such as the brain. Therefore, you get a great image (or shadow) of bones with a CAT scan, but a poor image of soft tissue like the brain. Because CAT scans use radiation to create the image, it is not as safe as MRI. However, it would take a lot of CAT scans to be dangerous to the patient. To look at the spinal cord with a CAT scan a spinal puncture is performed and a chemical is injected into the spinal canal, through a needle to help the CAT scan “see” the spine (to create a myelogram – see below). Therefore, it can be more invasive than MRI. A spinal puncture where a contrast agent (“dye”) is injected around the spine carries risk of seizure, damage to the spine by injecting the material into the spinal cord instead of around it and risk of infection. CAT scans require the patient to be under anesthesia for the test. The test typically takes a few minutes depending on the individual machine. It is not as expensive as MRI.
Here is one way to compare the MRI to a CAT scan of the brain or spine. If you were driving down the highway on a snowy night and saw brake lights ahead of you, you would recognize that a vehicle of some sort was in front of you. Think of that as a CAT scan. Now, if you were driving on the highway on a clear sunny day and a vehicle put on its brakes in front of you, you could tell what kind of vehicle it was, how many people were in it, what its license plate was, what color it was, make and model of the vehicle etc. That would be an MRI (sunny day) compared to a CAT scan (snowy night). Granted, if you were interested in bones and certain specialized scans, a CAT scan would be better than MRI. However, for imaging the brain and spinal cord, there is no comparison (see below).
Radiograph: Commonly called “x-rays”. This is an image of a patient produced by x-rays. Much like a CAT scan, it is a “shadow” of the tissue being imaged. It works best for bones and is commonly used as a screening tool for looking inside the body. Usually only a side view and downward view of the area of interest is produced. It is commonly used to look for broken bones. It uses radiation to look at the area of interest; so repeated exposure can be dangerous. It is a quick test and does not always require anesthesia. Radiographs can be helpful when looking at the bones of the spine prior to an MRI. We can usually see broken bones, infected bones and cancer that affect the bones (although not always). It is considered a non – invasive test…usually (see myelogram below).
Myelogram: With this test a radiograph is taken after a contrast agent (sometimes called a “dye”) is injected around the spinal cord. The dye surrounds the spinal cord and blocks x-rays so the spine can be seen. A spinal puncture carries risk of seizures, damage to the spine by injecting the material into the spinal cord instead of around it and risk of infection. Myelograms were commonly used to image the spine, however, they have been replaced by MRI. Myelograms do not give as much information regarding the location of a problem such as a herniated disk. With a myelogram there is a risk the surgeon may approach the spinal cord from the wrong side, making disk surgery significantly more risky to the patient.
The best imaging modality of the nervous system for your pet should be determined by your veterinarian and veterinary neurologist / neurosurgeon. Not all tests are equal. Sometimes, multiple tests are requested depending on circumstances.
Side view of a dog brain with a brainstem mass. The front of the patient is on the left side of the image. The mass is “more white” than surrounding tissue and is labeled with an arrow.
MRI Scan Side view of a dog brain with a mass in the forebrain. The front of the patient is on the left side of the image. The mass is “more white” than surrounding tissue and is labeled with an arrow.
MRI Scan of the spine
Side view of a dog spine with a ruptured disk causing spinal cord compression. The front of the patient is on the left side of the image and the tail is on the right. The disk is “black” and is protruding up into the spinal cord. You can also see a little “white” area in front of and behind the disk in the spinal cord. This represents fluid (blood or edema – swelling).
Myelogram of the spine
Side view of a dog spine after a contrast agent (“dye”) has been injected into the spinal canal. The front of the patient is on the left side of the image and the tail is on the right. The contrast is the white lines that extend in front of and behind the needles that are inside the spinal canal.