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Effective uses of MRI
MRI is only effective when it supplements history and physical examination and when it is likely to improve the health outcome for the patient.
Although able to offer detailed images, MRI scanning is not always the most appropriate imaging.4 In many cases, a physical examination by an experienced practitioner is able to provide an equal or better diagnosis than an MRI scan and at a substantially lower cost.
MRI is often the first imaging choice for many brain and spinal cord conditions, due to its superior detail. However, in emergency situations or when looking for bony abnormalities (e.g. occult spine fractures), computed tomography (CT) is usually recommended due its speed and superior imaging of bone.
Benefits and risks of MRI
MRI does not use ionising radiation. In situations where it will provide better or similar information to CT or X-ray, MRI may be a safer choice. This is particularly relevant for children and young adults, who are at a greater risk from ionising radiation than older adults.
To obtain a quality MRI scan, a patient must remain immobile for the duration of the scan and MRI scanning times are typically much longer than other modalities. This may mean that some patients will require sedation or anaesthesia for MRI. The risks of sedation or anaesthesia need to be balanced with the potential for a better health outcome with information obtained by MRI.
MRI is able to produce detailed (two- and three-dimensional) images of soft tissue, including of the brain, nerves, organs, cartilage, tendons, muscles and ligaments. This makes it a superior modality for imaging in some clinical situations.
However, the ability to see the components of the body in high detail does not necessarily mean improved outcomes for patients. GPs need to keep in mind whether any diagnostic imaging will alter treatment and the potential for revealing incidental findings.
The more sensitive the imaging modality, the more likely a finding of an ‘abnormality’. Many abnormalities are not clinically relevant and will not progress. Only a very small number of people will benefit from early detection of an ‘incidentaloma’, while others will suffer anxiety and the effects of investigations and treatment for something that would never have caused harm.5
Contraindications and safety
The following information has been adapted from the RANZCR Guidance for GP referrals for MRI studies.6
- prostheses and implants (e.g. pacemakers, internal hearing devices, neurostimulators, orthopaedic and dental implants, programmable shunts, vascular clips)
- newer implants and prostheses, such as titanium, many types of steel and almost all joint prostheses, are usually MRI compatible
- electrically activated devices, such as pacemakers, may be damaged or disrupted by MRI. Some pacemakers can be safely scanned with cardiological supervision – note that not all MRI sites can provide this service
- implanted infusion pumps can often be put in ‘safe’ mode for scanning
- note that if imaging near an implant or prosthesis, there may be a reduction in image quality
- metallic foreign bodies (e.g. small metal fragments in the eye)
- X-rays (or CT) may be required to confirm or exclude the presence of a foreign body
- conductors (e.g. wires, metallic surgical staples, some dermal medication patches and some tattoos)
- MRI can induce electric currents in these conductors and generate heat. Serious burns have been recorded. If the conductor cannot be removed, external cooling can be used.
Safety considerations include:
- hearing loss
- the loud mechanical vibrations in the scanner can aggravate pre-existing hearing loss and tinnitus. This is usually temporary. While hearing protection is routinely offered to patients, it does not always prevent symptoms
- between 2% and 5% of patients cannot tolerate the enclosed space of an MRI scanner. Most MRI sites can provide sedation (usually intravenous) to patients, where it is medically appropriate
- there are no known adverse effects of MRI in pregnancy. It is considered reasonable to perform MRI during pregnancy if the result is required for management during pregnancy and is not available from other tests. Otherwise, it is prudent to defer the MRI scan until after pregnancy (or at least after the first trimester)
- MRI contrast agents are relatively contraindicated in pregnancy
- breastfeeding is not a contraindication to MRI or MRI contrast agents
- it is not necessary for women to stop breastfeeding before or after an MRI, nor does breast milk need to be manually expressed and discarded after MRI.
Intravenous contrast is not routinely required for MR imaging. It is usually confined to looking for tumours or inflammatory lesions.
Contrast agents used for MRI are different to those used for CT and X-ray contrast studies. MRI contrast agents have lower rates of anaphylactoid reaction and are given at much lower doses than those used for CT. There is minimal risk of causing or aggravating renal impairment. However, patients with severe renal disease are at risk of nephrogenic systemic sclerosis if given MRI contrast agents (i.e. gadolinium). This is a rare but serious condition and deaths have occurred.
Prior to referral, GPs need to inform the MRI site if the patient has known significant renal impairment (i.e. eGFR <30mL/min/1.73m2). If risk factors for potential renal impairment are present, an eGFR result (taken up to 3 months before intended MRI) will be required before administration of contrast for MRI.
For patients with significant medical illness in the 3 months preceding MRI, and for hospital inpatients, a more recent eGFR (timing will be related to the nature, severity and timing of the illness) is a wise precaution.
- Does the patient have any metallic implants? Surgical? Traumatic?
- Are there any implanted devices? Pacemakers? Infusion pumps?
- Have any wires been left in the patient? Pacing leads? Wire markers in catheters?
- Does the patient have significantly impaired renal function or risk factors for this (if MRI contrast agents need to be given)?
- Is the patient claustrophobic?
- Identify the implant if possible (the patient may have received an information brochure about the implant at the time of surgery or you may need to obtain operative records from the surgeon or hospital where the device was implanted).
- Consult with the MRI service if you have questions about the safety of a device in the MRI environment.
- Establish safety requirements for the implant and patient.
- Consider the feasibility and risk vs. potential benefit from the proposed scan.
- Warn the MRI service if renal impairment or risk factors are present and MRI contrast agent use is likely or possible; send a recent eGFR result with request, if possible.