Magnetic Resonance Imaging (MRI)
Magnetic resonance imaging or MRI is a very new type of imaging technique discovered America in the 1970s and which only became available as a clinical tool in the 80s.
MRI uses a magnetic field rather than x-rays or ultrasound waves to produce an image. MRI has some similarities to computed tomography or CT in that the patient lies flat on a table which is then moved into a circular tunnel and the images produced are slices or cross sections through the patient.
The magnetic field used can be of different strengths broadly categorised as high or low field. Irrespective of the strength of the magnetic field, the radiation used is not of the ionising type as seen in x-rays. To the best of our knowledge, the magnetic fields produced do not cause any significant damage to living cells and are considered extremely safe.
In addition to its safety, MRI’s major advantage is its ability to show marked differences in soft tissue and fluid contrast for example differences between the cerebrospinal fluid surrounding the brain and the brain itself, between grey and white matter within the brain or between tumours, oedema or swelling within the brain and surrounding normal tissue. Similarly, the intervertebral discs in the spine, the spinal cord itself and the nerves exiting the spine may be clearly seen.
MRI is now being increasingly used in veterinary medicine in particular to image the brain and spine. It is frequently used in the investigation of seizures, brain tumours, encephalitis and meningoencephalitis (inflammation of the brain and its surrounding membranes), ‘stroke’ or haemorrhage. The internal architecture of the brain could be clearly seen including abnormalities such as hydrocephalus, the location and extent of tumours and secondary changes such as swelling or haemorrhage. Abnormalities in the shape of the skull such as caudal occipital malformation and secondary changes such as syringohydromyelia can be assessed noninvasively.
Bulging or rupture of the intervertebral discs is a common problem in dogs and is best appreciated on MRI. Another major advantage of MRI is it’s non-invasive nature. Prior to the advent of MRI, a dye would have to be injected into the spinal canal surrounding the spinal cord in order for many these changes can be seen on an x-ray (myelogram) or CT study. Secondary involvement of the spinal cord such as swelling or haemorrhage is also clearly seen on MRI. MRI is also used in the investigation of infection of the intervertebral discs or discospondylitis.
MRI may also be the best method in the investigation of diseases of the nasal cavity, sinuses, orbit and the middle and inner ear. Nasal tumours, inflammation and infection are common in dogs. MRI is tremendously useful in identifying the presence and extent of disease, the potential involvement of the surrounding nasal bones and skull bones and in differentiating between the different types of disease.
Many people are familiar with the use of MRI in the investigation of shoulder and knee pain in people. Similarly MRI is very useful in dogs and cats also. The joint fluid, ligaments, tendons and the cartilage and underlying bone be clearly visualised in a non-invasive fashion. MRI is particularly useful in the investigation of some chronic unexplained lameness conditions in dogs. Stress fractures, bone bruises, tendon or ligament tears, joint effusions, peripheral nerve sheath tumours such as brachial plexus or lumbosacral plexus tumours for example may be best evaluated with MRI. Myositis or inflammation of muscle tissue is also best seen on an MRI scan.
Because of its inherent ability in differentiating soft tissue contrast, MRI is frequently employed in the investigation of soft tissue lumps or masses or in investigating chronic discharging wounds or sinuses. Examples include investigation of swellings of the neck or in the search for foreign bodies such as pieces of wood, grass awns or small pieces of glass.
MRI is also used less frequently in the evaluation of abdominal, chest and heart disease.
