Introduction to Magnetic Resonance Imaging (MRI)
Magnetic resonance imaging or MRI is a medical imaging technique which can be utilised to visualise internal structures with relatively great detail. Magnetic resonance scanners generate strong magnetic fields which pulse and cause individual nuclei of atoms within the body to spin. Gradients in the strength of the magnetic field causes different nuclei to spin at different speeds. Powerful 3D imaging software is able to translate these variances in spin in to a visual interpretation of the body. The result is a 3D image (viewed in cross-sections) that provides great contrast between the soft tissues of the body. This makes MRI an important tool in veterinary imaging.
Pathologies will alter the properties of tissue, resulting in the production of a different image to healthy tissue. This is why MRI is becoming increasing popular as a diagnostic tool; especially for heart, brain and muscle diseases, as these are tissues which would typically be difficult to image with traditional methods such as X-rays or computed tomography (CT). Another benefit of MRI is that, unlike X-rays and CT scans, MRI does not use ionizing radiation to create an image.
Clinical veterinary MRI started around 25 years ago, with veterinary institutions installing their own scanners during the 1990s. Initial MR scanners had low strength magnetic fields coupled with basic software, they were also extremely expensive.
Clinical uses for the early MR scanners mainly involved the scanning of the canine head and brain, however as MR scanners have become more common place, their applications in veterinary practices have become more widespread. Scanners are now increasingly used for spinal and orthopedic conditions as well as central nervous system disorders and the diagnosis of joint diseases such as osteoarthritis.
Types of MRI Scanners Used in Practice
MRI scanners are typically either high or low strength. Low strength scanners are open field and do not fully enclose the patient whilst high strength scanners are closed field and thus require full enclosure of the patient.
Open field scanners have become increasingly popular due to their flexibility in design, smaller size and the lower price tag. Closed field scanners however provide greater quality images with stronger contrast of soft tissues – making them more adept at distinguishing smaller details.
The increased strength of closed field scanners comes at a price as they require a stronger magnetic field to operate with greater detail. Closed field scanners operate at 1.5 Tesla, that’s 7.5x as powerful as open field scanners (0.2 Tesla) and a massive 2.5 million times as strong as the magnetic field of the Earth (0.000006 Tesla).
Open field scanners may not provide the detail which closed field scanners can, but they can be extremely useful in situations where it would be near impossible to get the patient into an enclosed MR scanner. For example, it can prove a great technical difficulty to get a horse in to an enclosed MR scanner, as such a ‘standing MRI’ system has been developed. This system eradicates the need for general anaesthesia and increases the ease at which MRI can be preformed.
Applications of MRI
MRI is a powerful diagnostic tool and as such one of the most common applications of MR scanning in practice is the diagnosis of disease or injury such as spinal injuries, investigation of central nervous system abnormalities and the diagnosis of neoplastic diseases (tumours). In the equine world MRI is becoming increasingly useful in diagnosing the possible causes of lameness.
Other applications include:
- Diagnosis of soft tissue injuries such as damaged ligaments
- Joint injuries such as osteoarthritis
- Injury or disease of internal organs such as the brain, heart or digestive organs
Note: MRI cannot provide detailed images of bones and as such X-ray remains the most popular tool for investigating damage to bones
Symptoms which may prompt recommendation for an MRI scan include:
- Behavioural changes
- Paralysis of limbs
- Spinal pain
- Visible distress caused by the limbs or joints
MRI in Practice
MR scanning typically requires that the patient undergoes general anaesthesia, this ensures that the patient remains still during the often noisy and lengthy procedure. Considering some procedures can last upwards of an hour the importance of anaesthesia becomes clear. It is imperative that the patient remains still to produce clear and detailed final images. Failure to produce clear images will hinder the diagnostic process and may create the need for another scan.
Because MR scanners can produce unwanted heat, this is compensated for by increasing the circulation of cool air. As such, patient body temperature can decrease. It is therefore often necessary to provide wraps or blankets to keep body temperature up and reduce the risk of hypothermia. Conversely, the powerful radio transmission required to spin atoms may, in some conditions, heat the body to a point of hyperthermia.
During a scan, the rapid switching of the magnetic field between the on and off state can cause nerve stimulation. Stimulation can cause twitching of limbs, but poses no danger.
The condition of the patient prior to the scan is another factor to take in to account, for example, patients with spinal injuries will require extra attention during movement to and from the scanner. As MRI is often used for investigating the brain, the use of general anaesthesia must also be carefully considered. For example brain pathologies may alter how the respiratory system reacts to anaesthesia.
MR scanners can produce a large amount of noise reaching levels of up to 120dB, fortunately, animals are typically anaethetised and therefore not subject to these harsh noise levels. Staff on the other hand may wish to use ear protection.
As MR scanning is a non-invasive procedure, no special aftercare is required other than the typical care provided following the administration of general anaesthesia.
MR scanners produce strong magnetic fields and as such can pose a number of dangers. The large magnets used in closed field scanners are superconductors that require cooling by liquid hydrogen. These superconductors cannot be turned off except for under exceptional circumstances, this is due to the fact that the shutting down process (quenching) involves the venting of helium which poses an asphyxiation risk.
Whilst running, the strong magnetic field produced by MR scanners can attract metal items with lethal force – even larger metal items such as tables and trollies. As such staff must take extreme care to ensure that patients and the staff themselves are free from magnetic metals.
Staff should be fully briefed on MRI safety and appropriate warnings should be in place in practices with MR scanning units, however more information on MRI safety can be found here.