Magnetic Resonance Imaging (MRI)

Authors: Mr Richard McIntyre*
                            Prof Stacy Goergen *

What are the generally accepted indications for magnetic resonance imaging (MRI)?

Magnetic resonance imaging is increasingly being used for diagnosis and follow up. The wide range of indications can be grouped under:

• Brain – indications include stroke, temporal lobe epilepsy, infection, inflammation, tumour, multiple sclerosis (MS), dementia, post-trauma, metabolic disorders, congenital malformations, internal auditory canal pathology, vascular pathology, pituitary fossa pathology, nerve palsies and metabolic disorders.
• Spinal cord – indications include radiculopathy, myelopathy, MS, inflammation, infection, tumour, congenital malformation, postoperative investigation and post-trauma.
• Musculoskeletal (MSK) – indications include all MSK system: joints for derangement, infection, inflammation, post-trauma, tumour and vascular pathologies. Plain films are still very useful.
• Abdomen and pelvis – investigates pathology of the various organs including tumours, vascular pathologies, infection, inflammation, congenital abnormalities and metabolic disorders. Used for detection of local invasion of rectal, prostatic and cervical carcinomas, and assessing the anatomy in peri-anal fistulae.
• Cardiac – indications include ischaemia, tumour, infiltrative diseases, congenital malformation and cardiomyopathy
• Vascular studies – increasingly being carried out without contrast medium (so with no risk of contrast allergy or nephrogenic systemic fibrosis (NSF) (see Gadolinium contrast medium (MRI contrast agents)), as new techniques are developed.
• Pregnancy – indications for the placental position and invasion, as well as reviewing foetal anomalies, particularly cerebral.

Gadolinium contrast media is used for similar cases to iodinated contrast in computed tomography (CT), but only totals approximately 30% of studies. This is mainly due to MSK, spine and cerebral MRI scans, which are most frequently carried out, not usually requiring a contrast injection.

What are the prerequisites for having MRI done?

Because of the potentially serious complications from safety issues with MRI (interaction of the magnetic field with implanted devices and ferromagnetic objects), each patient is required to answer an MRI safety screening form. This will be checked during a verbal interview with the patient before the scan to ensure they do not have any implants that are contraindicated.

The make and model of certain implants will need to be checked against relevant databases and MRI safety websites by the staff in the MRI department to ascertain their safety. This might take some time, particularly if medical notes or operation reports need to be obtained from another hospital to verify the exact nature of the implant or device. Therefore, if a patient is known to have an implant, it is best to obtain this information before the appointment so that the scan does not need to be delayed or cancelled.

The decision of whether or not to undertake an MRI scan should be made by the referring doctor depending on the clinical and medical indications of the patient. Discussion with the radiologist might be useful to determine if MRI is the most suitable imaging modality.

What are the absolute contraindications for MRI?

Metal-containing implants comprise a set of possible contraindications to MRI scanning, as they might heat or move during the procedure. The functioning of mechanical or electronic implants might be interrupted or permanently altered, so many of these devices or patients who have these devices in situ will not be allowed to enter the scan room or be scanned. Devices or metal foreign bodies that are most likely absolute contraindications include:

• Pacemaker, defibrillator or wires other than sternal wires – the exact mechanism of malfunction is not certain, but death has been recorded. More recent studies report minimal complications or malfunction of modern pacemakers, but caution is required. There is now at least one model of pacemaker that is marketed as MRI safe; however, scanning is to be completed under strict guidelines. These include switching the device off before the commencement of the procedure, and the presence of specialised medical personnel and company representatives. Scanning is carried out only on specific body areas under strict field regulations, and so should be considered only in extreme cases.
• Metallic foreign body in the eye – these might move or heat during scanning resulting in serious eye injury. They require exclusion using orbital X-rays on the day of scanning (or before the day of scanning, providing no further eye injuries with metal fragments occur in the interim between the X-ray and the MRI).
• Deep brain stimulator – possible thermal injury along the wires, malfunction of device.
• Swan-Ganz catheter – wire causes melting of adjacent catheter and malfunction.
• Bullets or gunshot pellets – near great vessels or vital organs, such as the lungs, heart or brain, which might move because of insufficient adjacent scar/tissue and cause damage.
• Cerebral aneurysm clips – if magnetic, can move. Also not scanned if type unknown.
• Cochlear implant – malfunction.
• Magnetic dental implants – loss of magnetic hold to keep the implant in place.
• Drug infusion devices – might malfunction.

Please contact the MRI department if you are unsure about any contraindications or requirements necessary.

What are the relative contraindications for MRI?

There are a multitude of implants within patients that need to be assessed as safe. Many are safe and will be reviewed on the safety screening form before the scan.

These implants include, but are not restricted to:

• AAA stent
• stapes implant
• implanted drug infusion device
• neuro or bone growth stimulator
• surgical clips, wire sutures, screws or mesh
• ocular prosthesis
• penile prosthesis
• joint replacement or prosthesis
• other implants, in particular mechanical devices

There are many other implants that might be contraindicated and should be discussed with the MRI department individually. Some implants are compatible, but might require assessment befpre and after the scan; for example, programmable shunt devices.

Other conditions can act as relative contraindications for MRIs:

• Large patients might find it difficult to fit into the bore of the MRI. Some MRI facilities have wider bore magnets that can more easily accommodate larger patients.
• Claustrophobic patients might require sedation. This can be prescribed to be taken before the procedure or intravenous sedation can be given in the MRI department. It is useful to note this on the request form.
• Significant pain might limit a patient’s ability to lie still (such as during an MRI scan for sciatica). Movement degrades the images, limiting interpretation and reducing the accuracy of the report.
• Surgery in the previous 6 weeks.

In addition:

• Children can be scanned without sedation from about the age of 6 or 7 years.
• Pregnancy is not a contraindication to MRI, although discussion with the MRI facility is suggested. No teratogenic effect or acoustic damage has been documented after foetal MRI scanning.

What are the adverse effects of MRI?

There are no known side-effects of an MRI, providing the patient does not have a contraindicated implant in situ.

Scanning while the patient is in the first trimester of pregnancy is generally avoided, although the risks are theoretical and probably small, but not clinically established. Scanning at this time should be discussed with the radiologist. Gadolinium-based contrast media are usually contraindicated in pregnancy.

In patients with poor renal function, there is a risk of nephrogenic systemic fibrosis (NSF) associated with gadolinium chelate injections (see Gadolinium contrast medium (MRI contrast agents)). Patients with known, or at risk of, renal impairment need to have their renal function assessed before MRI in order to determine whether administration of gadolinium contrast is best avoided.

Gadolinium contrast is generally very safe, but does have a low rate of allergic reactions (about 1 in 1,000 minor reactions and 1 in 10,000 significant reactions). MRI departments using contrast should have appropriate staff and medication to deal with reactions to contrast.

Is there any specific post procedural care required following MRI?

There is no specific post-procedural care required.

Are there alternative imaging tests, interventions or surgical procedures to MRI?

The advantage of MRI should be determined on a case-by-case basis. If unsure whether MRI is the modality of choice, clinical discussion of the case with the radiologist is warranted.

Often it is best to start imaging investigations with the simpler modalities – especially plain films in the musculoskeletal system. Normal ultrasound and CT might obviate the need for more complex imaging.

Ultrasound, like MRI has no ionising radiation, and is often the first line of investigation. It has a well-established role, and is also an evolving imaging modality.

CT has the advantage of being a much faster imaging technique, and is thus especially useful for trauma/road trauma, where time is critical. CT has no immediate safety issues, but does involve ionising radiation. It also has the contraindication of contrast medium injection with poor renal function (but not leading to NSF).

CT is better for acute haemorrhage, calcification, fine bony detail and most fractures. Imaging of the lung parenchyma remains in the CT field, and it is also currently used for pulmonary emboli and the coronary arteries (over MRI at the present time). Claustrophobia is unusual in a CT scanner.

If imaging is not helpful in determining the nature of a lesion, surgical intervention or biopsy might be required.

Further information about MRI:

There are many types of MRI scans (referred to as sequences) that can be carried out during an MRI procedure. These sequences are determined by the clinical indications given on the request and so specific clinical indications must be given to ensure the correct sequences are carried out.

Useful websites for MRI:

1. Safety of implants:
   www.mrisafety.com
2. General MRI information:
   www.users.on.net/~spinupdownunder/

*The author has no conflict of interest with this topic.

Page last modified on 18/8/2017.