Magnetic Resonance Imaging (MRI)

MRI

Uses a powerful magnetic field and radio waves to proved an image - does not use radiation making it a lot safer

The two types of MRI scanners used in veterinary practice

1. Superconducting magnets

2. Permanent magnet

Superconducting magnets

• Usually tube shaped, and the animal being scanned is moved into the centre of the scanner on a moveable table top 

• Usually used by companies providing mobile MRI services, some referral practices and universities 

• Unlikely to see this type of scanner in general practice due to the level of power source that’s needed 

Permanent Magnet 

• Tend to be C-shaped and open at the sides 

• Whilst they do not provide such a string magnetic field and scan may take longer to acquire, they have the advantage of being less expensive to purchase and maintain, and are often the scanner of choice for centres where the electricity supply is inadequate for running a superconducting magnet 

• Not as powerful, take longer, lower power source 

Magnetic Field Strength

• The type of MRI scanners available also vary depending on their magnetic field strength (measured in Tesla), low-field and high-field

• Low field (usually 0.2-0.5T) 

• High field (1.5T and above) 

• The majority of MRI scanners in veterinary use are between 0.4T and 1.5T (Perspective - 1T magnet on a crane is strong enough to pick up a car) 

• The image provided is cross-sectional 

• It gives good soft-tissue detail as the tissues contain differing amounts of water and therefore hydrogen nuclei

• Bone is less easily imaged - because bone has got an incredibly high specific gravity

MRI safety

• Due to the strength of the magnetic field great care must be taken with equipment, staff and personnel when working with MRI scanners

• Equipment should be made of NON-FERROUS materials. This includes GA equipment and trolleys, and can be very expensive

Pre MRI checks - Health and Safety!

• Does the patient have a microchip? Some microchips can distort the MRI image 

• Is there any chance the patient could have eaten any foreign body containing metal? - an x-ray could be advised to check before having an MRI 

• Does the patient have any implants? If so where are they and what are they made of? 

• Even if the patient has implants which aren’t made of ferrous metal, they can heat up in an MRI scanner which can cause pain, discomfort and can damage the implants 

Check personnel!

Do you or anyone who will be working with the MRI scanner have:

• Any metal objects on their person?

• Any implants? 

• A pacemaker? 

Fringe Field

• If the MRI scanner can affect implants within the body, it follows that other objects taken into the room could also be affected

• The magnetic field is strongest within the scanner itself, but the field reaches out into the area surrounding the scanner. This is called the fringe field from weak to string very rapidly as you approach the scanner

• Fringe field goes from weak to string very rapidly as you approach the scanner

Patient preparation

• All patient have to be anaesthetised as the image takes several minutes or up to an hour to acquit, therefore any movement can blur the image and render it non-diagnostic (can take from half and hour up to an hour) 

• All collars and metal tags should be removed, and patient with metal implants are not suitable for MRI 

• The magnets can even heat up non-ferrous metal causing potential problems (e.g. pain, discomfort, damage to implants, burns)

Indications for use

• Head trauma 

• Epilepsy (diagnosing brain tumours) 

• Spinal disease

• Middle/Inner ear disease 

• Lameness/orthopaedic conditions (ruptured cruciate) 

MRI is generally considered ‘gold standards’ for imaging of the brain and spinal cord 

Advantages

• Does not use ionising radiation, so is considered safer than x-rays

• Helps to show the full extent of a disease and plan surgery on the animal 

• Can be used on pregnant patients (no effect on foetus reported) 

• Good detail shown on scans, i.e. investigation of the nervous system. Also shows muscle and joint composition ad thoraces abnormalities (i.e. tumour) and vascular abnormalities 

Disadvantages

• Not ideal for investigating bone structure containing calcium 

• It is a noisy and long process 

• People or patient with metal implants should not enter the room or undergo MRI scanning 

• It is expensive 

Recording Information 

• Images are produced in digital format; this can be emailed, burnt to CD or copied to an x-ray film and stored accordingly 

• Note that when compared to x-rays and CT, calcified material is black on an MRI scan 

Use of contrast media 

• Contrast media can be used to highlight parts of the body to assist in achieving a diagnosis 

• With MRI scanning, the most commonly used contrast media is Gadolinium