MRI in veterinary medicine
What is MRI?
MRI stands for magnetic resonance imaging. It is a type of imagine that relies on magnets, rather than ionising radiation, as is produced by x-ray machines and CT scanners.
How does MRI work?
MRI relies on powerful magnets that manipulate the "spin" of hydrogen ions in the body. Coils detect changes in the spin of the hydrogen ions, allowing computers to map the position of these ions in the body. Since most hydrogen ions are found in water within the body, at its simplest, an MRI scan creates a map of body water. Many pathological processes, such as cancer and inflammation cause an increase in water in a region, and this can be detected by the scanner.
Why use MRI?
We have lots of imaging options in veterinary medicine, including ultrasound, x-ray and computed tomography (CT). These all have their place, and their pros and cons. MRI produces highly detailed images of soft tissue in thin slices through the area of interest. It also has the ability to "see through" bone, which means that it can image regions surrounded by bone, such as the brain, the spinal cord and the nasal cavity. CT also has some ability to see through bone, but MRI is generally superior to CT for examination of soft tissues.
What are the disadvantages of MRI?
There are a few disadvantages. Firstly, the equipment is expensive, to purchase and for some machines to run, which means MRI scans are often expensive. This is made worse by the fact that compared to CT, images are slow to acquire. The slowness of acquiring images also means that it is poor at imaging moving parts such as the chest, the heart and parts of the abdomen, and CT is preferable for this. MRI is not as good as CT at looking at bone, although it is useful for looking at bone marrow and bony tumours.
What is MRI used for in veterinary medicine?
MRI has revolutionised veterinary neurology, allowing us to image the brain and spinal cord, and thereby allowing us to diagnose conditions such as brain tumours, hydrocephalus and syringomyelia which previously it was only possible to diagnose on post mortem examination. MRI gives good images of the spinal cord, including helping to diagnose intervertebral disc disease. MRI is also useful for nasal disease, allowing the fine bones within the nose to be imaged, to help diagnose causes of nasal discharge and nose bleeds (such as tumours and fungal infections). It can be useful for working out how far tumours extend, allowing surgeons to plan how much tissue to remove to give the best chance of a cure without taking out too much healthy tissue. It can also give clues as to the presence of abscesses, cysts and foreign bodies with soft tissues.
MRI is a safe and powerful tool for detecting a number of disease conditions in dogs and cats. Other imaging techniques and diagnostic tests may be more appropriate for certain diseases, and your vet will be able to advise which are the best tests to run. MRI can be expensive, due to the running costs and costs of equipment, but for some conditions, it is the only way of making a diagnosis and deciding on the best treatment.