Magnetic Resonance Imaging (MRI)
Dr. Mauricio Solano
Most people have heard of an MRI but are not quite sure what it is. The MRI (magnetic resonance imaging) is similar to X-rays, but it is a method that gives the most precise anatomical information on patients today. An X-ray can show the size and shape of various organs but it does not show the interior of an organ. Diseases that were once impossible to detect without an autopsy can now be diagnosed and treated in the living animal.
Tufts University School of Veterinary Medicine in Grafton Massachusetts, currently uses MRI for diseases that affect the brain and spine. These diseases, known as neurological diseases, show a wide array of symptoms and in many cases can be life threatening. Some symptoms that may prompt your veterinarian to recommend an MRI include seizures, circling, depression and behavioral changes such as aggression. Staggering, paralysis of one or several limbs and spinal pain may also be present.
Any animal showing signs of a neurologic disease should have a thorough neurological exam. Often, physical examination and various diagnostic tests will diagnose the underlying problem. In those few animals that are not diagnosed through routine tests, an MRI may be recommended. At this time, an MRI is typically only performed on animals to detect abnormalities in the brain and spinal cord. Unlike human medicine, MRI of the joints, muscle and abdominal cavity are not performed as routinely. But, as the availability for MRI studies in animals increases, so does the likelihood that the MRI will be used for a variety of ailments.
Though an MRI seems like the best test you could do to find out what is causing your pet's illness, it is not a test that can be performed during your first visit to the veterinarian. As with any diagnostic technique, deciding whether or not your pet needs an MRI comes after a complete examination of your pet. If it is decided that your pet would benefit from an MRI, your family veterinarian is likely to refer your pet to a local referral hospital or veterinary university where a team of specialists can examine your pet and perform the MRI.
In order to perform an MRI, the animal must remain still while the machine takes the necessary images. No one can be in the room to hold the pet so animals are anesthetized for this procedure. Prior to anesthesia, a wide array of laboratory tests are performed. Some of these tests may include radiographs, blood work, urinalysis and electroencephalograms (EEG) among others.
Where an MRI Performed?
Though the availability of MRI for pets is increasing, there are still only a few institutions or hospitals in the United States that offer MRI services to animals. Large institutions that offer the service may have long waiting periods, up to a couple of weeks.
The primary reason that MRI is limited for pets is expense. With equipment prices reaching $1,000,000 and more, it is unlikely your local practitioner owns one. In addition, the cost of running the unit is prohibitive to most professionals with the exceptions of well-funded private institutions or subsidized universities. In fact, many veterinary hospitals offering the service have arranged access to the units at nearby human hospitals instead of having the unit on their premises. It's possible that your pet may have an MRI at the same hospital that treats you.
What Is Involved
Depending on the seriousness of the disease, your pet may or may not be hospitalized several days before the scan. The day of the scan your pet will not be allowed to eat; although water will be available. A catheter will be placed to give medications and anesthesia. Once the pet is under anesthesia, he will be transported to the MRI unit. A scan of the brain takes approximately two hours to generate. Though the scanning times are getting shorter with newer units, MRI is a modality in which imaging of the area of interest takes a long time. MRI uses magnetic fields and radio waves to form an image. The technology is therefore considered less invasive than radiographs, which uses potentially harmful X-rays to make images of the body. Since there is no ionizing radiation involved with MRI, no harmful effects to the body have been noted with its use to this date. The risk of doing the exam actually comes from the need for general anesthesia, which is considered very low, and not from the use of the magnetic field or the radio waves generated by the unit.
Prior to the scan, it is very important that you inform the specialists if your pet has any metal implants. This includes pins, wires and screws that may have been used in the past to repair a fracture. Microchips, old bullets and metallic clamps used during surgeries such as spays are also important to know. Preliminary X-rays will often reveal these metallic items even if you are unaware of their presence. Metal will degrade the quality of the MRI images. It can also distort and obscure an important area.
Immediately after the examination is completed, your pet will be transferred to the recovery room. It will take him about two hours to be fully awake. Often, the imaging specialist makes the diagnosis as soon as the images appear on the screen. Common brain diseases that are diagnosed with MRI include brain tumors, brain infarcts (lack of blood to an area of the brain), brain abscesses and inflammation of the external layers of the brain (meninges). In the spine, disease such as herniated discs and tumors are commonly seen.
What Does the MRI Unit Look Like?
The MRI unit looks like a large cube with a small hole in the center. This cube houses a powerful magnet. The patient is placed completely inside the MRI unit, actually in the middle of the magnet. MRI takes advantage of the many protons found at the atomic level in the body. To understand this technology, imagine these protons as spinning tops. These protons are all spinning in a random manner within the body. When your pet is placed in the magnetic field, all the protons in his body start spinning in a synchronized fashion. This spinning creates a low energy field. When the unit's radio waves reach the animal, the protons move to a high energy level. Simply stated, the patient is absorbing energy in the form of radio waves. When the radio waves are turned off, the protons will release the energy they have absorbed, in the form of more radio waves. The MRI unit then recovers the radio waves emitted from the body and uses them to generate a computer image.
What Can an MRI Show?
The technology of the MRI allows a veterinary radiologist to take a closer look at the inside of the patient's head. An X-ray of the head shows only the bones of the skull. The contents of the head (that is the brain) remain hidden inside the skull. In other words, the bones of the skull cover the brain. A popular analogy to explain this concept is the "sliced bread analogy." Without slicing the bread, you see only the oven-baked golden external surface. If we want to look at the white part of the bread we have to slice the bread and remove one of the pieces. A slice of bread is the equivalent of one MRI image. Taking this analogy further, thinner or thicker slices can be made and the bread can even be sliced in any orientation. This is a major advantage over CT.
At Tufts University School of Veterinary Medicine, MRI is now considered the standard technique to assess the brain and the spinal cord. During the last two years there has been an accelerated interest in making MRI more accessible to pet owners and the public is taking advantage of it. However, the best initial treatment for your pet is the treatment given by your family veterinarian. Despite wonderful imaging modalities such as MRI, a complete physical examinations and routine veterinary checkups is what will ensure the good health and long life of your pet.