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MRI

Magnetic Resonance Imaging (MRI) is a painless way to look inside the body without x-rays. It uses a powerful magnet, radio waves and a computer to produce remarkably detailed images of internal tissues and organs. 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.

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 because they are influenced by surrounding atoms. 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.

Until recently, imaging of the brain and spinal cord have been the most common MRI applications, but the list of indications is growing rapidly. Any animals with unexplained seizures, altered mentation, behavior changes or any other evidence of cerebral or cerebellar disease are candidates for MRI studies. Disc herniations and other spinal cord diseases can be diagnosed accurately without a painful myelogram (injection of contrast material around the spinal cord). MRI provide the most accurate and detailed images of the nasal cavities, eye, peri-orbital area, as well as middle and inner ear. Neck lesions, cranial mediastinal lesions, and tumor staging are also best done with MRI. Because of their exquisite soft tissue detail, MRI studies are very helpful to delineate masses prior to surgical removal. MRI is also the least invasive way to diagnose porto-systemic hunts, other vascular anomalies, and many cardiac problems. MRI is the best imaging technique for joint problems in people and protocols for animals are being developed. Table I lists the most common MRI indications. This list grows constantly as we learn more about MRI in animals. Your veterinarian and a radiologist will help you determine whether your pet would benefit from an MRI.

Table I. Some indications for MRI