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MRI (magnetic resonance imaging) scanning is a diagnostic test that is used to look at various parts of the body, including the brain, spine, eye sockets, joints and soft tissues. MRI scans are commonly used in people, and they are also frequently performed at Willows. Unlike X-rays, MRI scanning uses a magnetic field rather than radiation.
During an MRI scan the patient is placed in a magnetic field, and this temporarily alters the atoms of hydrogen in the body. As the magnetic field is altered, parts of the hydrogen atoms change their alignment, giving off faint radio waves which are then picked up by sensitive, specially designed coils placed around the part of the patient that is being imaged. A powerful computer then analyses the signals that are received by the coils, and this builds up a picture of the parts of the body which are being investigated. Sophisticated changes can be made to the magnetic field and radio frequencies to highlight different tissues and different diseases within the tissues.
An MRI scan is made up of lots of images which are obtained as slices or cross-sections of the patient. These slices allow individual structures to be imaged without having to look at them through other structures that might get in the way (this superimposition of structures is a drawback of traditional X-rays or radiographs). The direction of the slices can be changed to give the best information about any particular structure.
Is there anything special about the MRI scanner, facilities and imaging personnel
The short answer is, yes! Not only does Willows have some of the most advanced MRI scanning equipment available in veterinary facilities in the UK, but the imaging department is staffed by a highly experienced team of accredited, recognised Specialists. The facility and the staff are available 24 hours a day, 365 days of the year in order to give the best levels of patient and client care we can provide. This combination of facilities and staffing levels is unsurpassed in the UK.
Different scanners have different strengths of magnetic field, and the scanner at Willows has a strong magnetic field (1.5 tesla- a so-called 'high field' scanner) – this helps to give good quality images (assisting our Specialists in making a diagnosis), and it also allows the scans to be obtained relatively quickly. Willows’ scanner is constructed so that the whole body of a human, and therefore of a dog or cat, can pass within the magnetic field – as a result any part of a patient’s body can be scanned, as required.
Although the high field scanner at Willows allows scans to be completed more quickly than low field scanners, each of the multiple sequences (components of the scan) that must be performed take about 5 minutes, and the patient must remain completely still throughout the examination. As a result, MRI scans in our patients are performed under general anaesthesia (unlike the situation in humans). Because the main part of an MRI scanner is a large magnet, special ‘MRI-compatible’ non-magnetic anaesthetic monitoring equipment (that shows the patient’s blood pressure, breathing and blood oxygen levels etc) must be used, and like all the equipment in the MRI scanning room, this comes with a hefty price tag – the patient monitor alone in our scanning room cost about £40,000!
MRI is a diagnostic imaging technique which provides superb detail and contrast in soft tissues (as opposed to bones). As the magnetic field is not interfered with by surrounding air or bony tissue, exquisite detail can be seen in areas not easily imaged in any other way, specifically the brain and spinal cord (the nerves that run from the brain inside the spine). Brain and spinal cord imaging make up more than 90% of the MRI caseload at Willows, and we see over 700 cases a year that require brain and spinal cord imaging.
As the brain is surrounded by a hard bony case (the skull), other types of imaging such as X-rays struggle to produce useful images of the brain tissue. Computed tomography (CT scanning) can be used to image the brain, but the images are not as clear as with MRI. In very young patients, or animals with skull defects such as open fontanelles (the ‘soft spot’ on the skull in newborn infants), an ultrasound scan can be used to image the brain, but the view obtained is limited and can be difficult to interpret.
An MRI scan of the brain (arrow), showing the detail which can be seen – in this scan the cross section of the brain seen is viewed from the side
Another MRI scan of the brain (arrow) This view is a cross-section seen from the back
An ultrasound scan of the brain (arrow) through the soft spot on a skull – the detail seen is very limited compared to the MRI scan. This view is a cross-section seen from the back
A CT scan of a brain (arrow), again showing much less detail than the MRI scan. This view is a cross-section seen from the back (the skull around the brain is shown as bright white)
Traditionally conventional radiography (X-ray) has been used to look at the spinal cord.
A normal X-ray of the spine – the bones are clearly visible, but the area of the spinal cord (arrow) shows no soft tissue detail
Normal X-rays are very limited in their ability to show the spinal cord, so a technique called myelography can be used to improve the information that is obtained. Myelography involves injecting a dye (a contrast agent) which shows up on X-rays (similar to the way barium shows up on a barium meal X-ray study) around the spinal cord, to try to highlight any compression (squashing) or swelling of the cord. Myelography is not without risk, as the contrast agent has to be injected very precisely around the spinal cord to avoid damage and, in addition, some animals can suffer adverse reactions to having the dye injected e.g. some patients may have seizures after myelography.
An myelogram of the spine showing dye, seen as white on the X-ray (arrow) injected around the spinal cord
CT scanning shows up the bones of the spine very well, but provides only limited information about the nerves of the spinal cord itself. The same dye that is used in an X-ray myelogram can also be injected around the spinal cord when a CT scan is performed. This provides much more information compared with conventional X-ray myelography, but the risks associated with injecting contrast agent around the spinal cord remain.
A CT scan of the spine showing a cross section through a vertebra (seen as white) with the spinal cord (arrowed) in the middle of the bone
A CT myelogram with dye (seen as white) injected around the spinal cord (arrows)
In comparison, MRI provides superb detail of the spinal cord and surrounding soft tissues in a safe, non-invasive fashion.
MRI image of the spine showing abnormal material (a ‘slipped disc’ – blue arrow) next to the spinal cord (red arrow), with no need for dye injection
MRI scan of the spinal cord (this time in a slice viewed from above), showing slipped disc material (blue arrows) bulging out and pressing against the spinal cord (red arrows), with no need for dye injection