​Clinical Procedures – Magnetic Resonance Imaging (MRI)

​Clinical Procedures – Magnetic Resonance Imaging (MRI) 
Indications
• Indications are manifold and too numerous to list. 
Contraindications
• As there is no ionizing radiation, radiation precautions do not apply
• D All ferromagnetic materials will be strongly attracted to the scanner creating missiles which may prove extremely dangerous. MRI-safe trolleys, resuscitation equipment, and wheelchairs must be employed
• D Implanted ferromagnetic devices, aneurysm clips, and retained foreign bodies (e.g. shrapnel or metallic fragments in the eyes) will also move towards the scanner potentially causing major injury
• D Although electronic pacemakers are not made of ferromagnetic material, they may be reset' or stop altogether
• A strict questionnaire is employed before anyone (staff or patient) is allowed near the magnet. If in doubt, access is denied
• Magnetic tape and credit cards may be 'wiped by the magnet
• D Many brands of mascara contain ferromagnetic filaments which may heat and cause burns to the eyelids
• D Caution should also be taken with tattoos some contain iron
• The patient must be able to lie flat and still for the duration of the scan.
Most scanners are relatively tight, larger patients may not fit.
Technology
• The MRI scanner houses a very large electromagnet which is always on
• Radiowaves are produced by the machine, which interact with hydrogen atoms in the patient. Radiowaves are, in turn, produced by the interaction with the hydrogen atoms and are detected by the machine which converts the data into images. The scanner has no moving parts
• The suite houses a Faraday cage preventing radiowaves getting in or out.
Procedure
• This depends on indications and the part of the body examined
• The patient lies on the scanner table. 'Coils' may be placed over the body-part of interest
• Most examinations do not involve IV contrast being given. If this is given, contrast containing gadolinium (Gd) is usually used:
• This is usually hand-injected immediately before the scan
• Depending on the part of the body examined, the patient may be asked to hold their breath via speakers in the machine
• The scan itself can last up to 40-50 minutes for some body parts.

Risks
• Nephrogenic systemic fibrosis (NSF): linked to gadolinium exposure in 2006. Symptoms may begin up to 3 months from exposure and may include pain, swelling, erythema, fibrosis of internal organs, and death. Patients with renal impairment are at greatest risk (no cases
recorded in those with GFR >60) and at least 9 hours of haemodialysis is required to remove it from the bloodstream. 
• Metallic artefacts: twisting or movement of artefacts within the body
• Biological effects: the magnetic fields employed may induce voltages within the body. The most common effect is 'magnetophosphenes' or visual flashes seen by the patient as the optic nerve is stimulated.
Stimulation of other nerves and muscles may occur
• Tissue burns may occur if conducting loops (e.g. ECG leads) are in contact with skin
• Temperature: the oscillating voltages create tissue heating. Overall body temperature may rise by 0.3°C
• Noise: may reach up to 95dB. Headphones or earplugs are usually
worn
• Claustrophobia: experienced in up to 10% of patients.