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Magnetic resonance imaging (MRI) is an advanced imaging modality which does not use ionising radiation and produces detailed soft tissue images. The other main advanced imaging modality available to veterinary patients is computed tomography (CT). This utilises x-rays and computer technology to form an image. CT images are useful for assessing bony changes, and for imaging of the thorax. CT scans are also generally cheaper than MRI scans. However, MRI is thought to be safer than CT since it does not use ionising radiation. Compared to CT, MRI also has advantages of the ability to image in any plane, better spatial resolution, better tissue contrast and increased sensitivity to disease processes which alter the water content of tissue. Essentially, MRI is looking at the density of hydrogen ions or protons within the body. These ions are most common in water, so MRI is sensitive at picking up changes in the water content of tissues, as may be seen with inflammation or neoplasia.
MRI is now the gold standard imaging modality for a number of disease conditions, both in human and veterinary medicine. It can also be complementary to other imaging modalities such as computed tomography (CT), ultrasonography and radiography.
The major application of MRI in veterinary medicine is in the field of neurology. This is because of the ability of MRI to “see through” bone, and image the soft tissue within. Conventional radiography and ultrasonography are both extremely limited in visualising the central nervous system, and MRI and to some extent CT are the imaging modalities of choice for this area. However, neurology is not the only field to benefit from MRI technology. Ophthalmologists find it important, for example to assess the retrobulbar space. Delineation of soft tissue masses elsewhere in the body can be important for oncologic surgical planning, and there are a number of internal medicine and orthopaedic uses.
The field of neurology has been revolutionised by MRI, with the ability to visualise nervous tissue ante-mortem and without invasive surgery. The main neurological regions that MRI is used to investigate are the brain and the spinal cord.
Because MRI is a costly investigation, and requires a general anaesthetic which has inherent, if small risks, its use should be restricted to cases in which a full neurological examination has been performed, and there is evidence of intra-cranial neurolocalisation. A CSF tap is a vital part of most investigations of intracranial disease, since inflammatory brain conditions will not always show up on MRI. Conditions in which there is raised intra-cranial pressure, such as large space occupying lesions, increase the risk of herniation through the foramen magnum during CSF collection, and so MRI prior to this procedure is recommended.
The Bath MRI Unit
Brain tumours in 2 dogs and a cat
Before the advent of MRI, the diagnosis of most spinal diseases relied on contrast radiography of the spine (myelography). Myelography has inherent risks, including exposure to ionising radiation and the risk of a reaction to the contrast, which can occasionally be fatal. Both techniques require general anaesthesia. Myelography is generally limited to diagnosing the location of spinal cord compression, by demonstrating an interruption to the flow of contrast. MRI gives much more information on the pathology involved, for example disc disease or neoplasia.
The nasal cavity suffers with regards to conventional imaging techniques from the same drawbacks as the brain, namely it is enclosed within a bony shell which makes visualisation of soft tissue within by radiography or ultrasonography difficult. Unlike the brain the nasal cavity can be accessed by endoscopy, and this modality can be complementary to MRI. Endoscopy may enable direct visualisation of a lesion, and enable biopsy material to be taken directly. However, certain parts of the nasal cavity may be hard to access with an endoscope, and factors such as nasal discharge, epistaxis, operator experience and equipment can limit the diagnostic utility. MRI allows evaluation of soft tissue changes, destruction of the turbinates and presence of masses.
MRI is of great utility in the diagnosis of orbital disease, showing a high degree of sensitivity for detection of changes.
Some parts of the abdomen, such as the retroperitoneal space, remain motion free under anaesthesia. Good quality scans can therefore be obtained of kidneys, adrenal glands, and prostate.
MRI can be of great utility in assisting the planning of surgical removal of neoplasia. Radiography and ultrasonography can be limited when examining the extent of tumours, and visual examination at the time of surgery can miss extensions of neoplasia into surrounding tissue.
The RF ROOM housing the MRI unit