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MRI and CT Imaging- Questions and Answers
Q. What is MRI?
A. MRI uses a combination of magnetic fields, radio waves and computer technology to produce high detailed pictures of many parts of the body. It is especially good at imaging the soft tissues of the body. Because MRI can show problems that cannot be seen with other diagnostic imaging techniques, it has rapidly become the most powerful diagnostic imaging modality in human medicine.
Q. Are there any risks with MRI studies?
A. MRI in itself is a very safe procedure. The magnetic field and the radio waves have no known harmful effects (MRI studies can even be performed during pregnancy). Because patients need to be absolutely still during and MRI study, however, general anesthesia is required for animals. To minimize risks associated with anesthesia, the renal, hepatic and cardiac status should be evaluated prior to MRI studies. Animals with cardiac pacemakers should not be scanned.
Q. What work-up is required before the MRI?
A. Obtain any appropriate lab tests as indicated for evaluating the suspected underlying disease, and to ensure that anesthesia can be safely performed. Pets occasionally have internal metallic objects (bone plates, pins and wires used to repair fractures) or BB shots at various locations. These are rarely dangerous for the patient but they can cause large magnetic artifacts that make MRI studies useless. Therefore recent x-rays are required prior to MRI examinations to rule out the presence of metallic objects that could interfere with the study. These x-rays should be obtained by your veterinarian.
Q. How long does an MRI scan take to perform?
A. Because MRI scans allow us to obtain images in multiple planes they take from 45 minutes to 90 minutes to complete, and consist of 4 to 6 different individual scans, often providing 50 to 100 different pictures. Scans are performed with the animal under general
anesthesia.
Q. How does MRI work?
A. MRI stands for Magnetic Resonance Imaging. Water makes up about 95% of living organisms, therefore hydrogen is the most common atom in our bodies. Like our planet, each hydrogen proton spins around a magnetic North pole and South pole. The North and South poles are normally oriented at random. When a patient is placed inside a strong magnetic field, however, the poles line up. This is the first stage of the process. Then the MRI system emits pulses of radio waves that knock the hydrogen protons out of alignment. Because the protons are still within the strong magnetic field, they rapidly return to alignment. In doing so, they release the energy they had absorbed, also in the form of radio waves. The returning radio waves are detected by sensitive antennas called coils and a computer produces images showing the distribution and intensity of the radio waves emitted by the hydrogen protons. All protons do not release the same amount of energy at the same rate. Therefore different molecules will produce different MRI signals that can be used to differentiate tissues and abnormalities within tissues. This is why MRI can see certain types of tissues and lesions better than any other imaging technique: it displays information at the molecular level.
Q. Why does MRI and CT scanning require anesthesia?
A. Anesthesia is required in order to insure that the animal is perfectly still during the examination so that high quality images can be obtained for proper diagnosis.
Q. Are there side effects to the anesthesia?
A. Most animals have little or no side effects from the anesthetic. Some animals may be drowsy or a little clumsy in walking and may have some vomiting after the anesthesia. General anesthesia is a serious matter, especially in a critically sick or unstable animal, and there can occasionally be severe reactions to the procedure that can be life threatening. There can also be allergic or other adverse reaction to the anesthetic in some animals.
Q. MRI scans seem expensive. Why is that?
A. MRI scanners are expensive to purchase, install and maintain and they often require expensive upgrades. MRIs also require lots of electricity for generating the magnetic field and for cooling the magnet, and some require expensive liquefied gasses for cooling. Our MRI scan prices not only include performing the scan itself, but also includes the services of our anesthesia and all drugs and anesthetic agents. In addition our prices include the services of a radiologist consultant for interpretation. When one considers all of these factors plus the enormous amount of information gained about the animals condition, the cost is generally well justified. An MRI requiring general anesthesia in a human would cost 2 or 3 times as much.
Q. What conditions are MRI scans useful in diagnosing?
A. MRI is very useful in diagnosing disorders of the Brain, Spine, Joints, Soft tissues, Abdomen and Pelvis. Common conditions diagnosed are the causes of seizures, paralysis, personality change or other neurologic conditions (brain tumors, hemorrhage, abscess, encephalitis and stroke), spine conditions (arthritis, disk herniation, spinal canal narrowing, spinal tumors), joint problems (ligament tears, meniscus tears, arthritis, tumors) abdominal and pelvic disorders (tumors, cysts, abscesses, aneurysms), sinus disorders (infections, cysts, and tumors), and eye disorders (tumors, infections).
Q. What if contrast enhancement is required?
A. In order to visualize some abnormal structures such as tumors, a magnetic enhancing material that contains gadolinium is injected intravenously during the test. These substances are very safe and are usually tolerated very well. Rarely, severe allergic reactions occur.
Q. How is a CT Scan different from an MRI
A. CT (or CAT) stands for Computed Axial Tomography. This technique of imaging uses an X-ray tube that rotates in a large circle around the subject and the X-rays that pass through the body slice are detected by a large number of detectors located around the same circle. The resulting information is analyzed by a powerful computer and a picture is provided of slices representing the relative X-ray densities of the slice being imaged.
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