MRI scanners, although free from potentially cancer-inducing ionizing radiation found in plain radiography and CT, have a host of safety issues which must be taken very seriously. MRI safety can be divided into:
- main magnetic field
- varying magnetic (gradient) fields
- radiofrequency
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Main magnetic field
The main magnetic field (B0) of a 1.5 T magnet is about 30,000 times the strength of the earth's magnetic field. It is strong enough to pull heavy-duty floor buffers and mop buckets into the bore of the magnet, move trolleys across the room and turn oxygen bottles into hazardous projectiles (missile effect). Deaths have occurred from trauma as a result of these effects. Smaller objects such as coins, hairpins and pens have been yanked off the person carrying them.
The strong field can affect cardiac pacemakers. Older pacemakers had a magnetic reed switch which could be disturbed by a strong magnetic field resulting in possible deleterious effects to the patient with one implanted. In addition their components often had a significant ferromagnetic content. Therefore in the past, having a pacemaker was an absolute contraindication to undergoing an MRI.
Modern pacemakers often have solid-state switches which are much less likely to be affected and much reduced ferromagnetic material. Due to this technical innovation, radiological societies in both North America and Europe have issued revised guidelines stating that having an implanted cardiac device is now only a relative contraindication 1.
Many older intracranial aneurysm clips are ferromagnetic and as a result experience a torque (twisting) in a magnetic field. Not everyone with an aneurysm clip experiences a fatal hemorrhage when placed in a magnet, but several cases have been reported.
Programmable ventriculoperitoneal shunts are adjustable shunts to treat hydrocephalus by diverting cerebrospinal fluid to the peritoneum. The programmable valve settings are changed by a device with a strong magnetic field supplied by the manufacturer which changes the opening pressure of the ventricular system in the brain. Some shunts are designed so that they are not effected by the magnetic field of MRI scanners however it is recommended that all programmable shunt settings be checked with a plain radiograph of the head following an MRI study to ensure that the settings were not altered4.
Some types of heart valves (e.g. Starr-Edwards) are torqued in a magnetic field: however, this torque is less than the stresses that occur normally as a result of blood flow. Therefore, heart valves are usually only a relative contraindication for MRI.
More of an annoyance than a safety problem is the ability of the magnetic field of a MRI machine to erase the information contained on the magnetic strip on ATM and credit cards. This may occur a short distance inside of the scanner room of a MRI machine. In addition mechanical watches will "freeze up" in a strong field, sometimes permanently.
Some metallic objects that are usually safe near an MRI machine are gold jewelry, shirt cuff-links and eyeglass frames.
Varying magnetic (gradient) fields
Varying magnetic fields are necessary in order to obtain images from MRI scanners. Changing magnetic field induce electrical currents in conductors (this is how an electrical generator works). In patients with metal in their body, the potential exists for electrical currents being induced in the metal with subsequent heating. This may occur with metal foreign bodies or some surgical implants. It does not universally occur and some patients with hip prostheses, for example, may be scanned without harm.
Very rapidly changing magnetic fields as may be achieved with echo planar imaging can cause nerve stimulation. This stimulation can affect motor nerves with resulting muscle contraction as well as the retina with resulting flickering lights called magnetophosphenes. This is particularly an issue with ultra high field magnets (i.e. ≥7 T).
Radiofrequency
The radiofrequency (RF) power that is capable of being produced matches that of many small radio stations (15-20 kW). As a result there is the presence of heating effects from the RF, known as the specific absorption rate. In most pulse sequences, the heating is insignificant and does not exceed FDA guidelines. However echo planar imaging (EPI) and some spectroscopy localization techniques are capable of exceeding FDA guidelines. Monitoring of the power deposition in patients is a requirement for FDA approval of clinical MRI scanners.
Potential for electrical shock exists with RF coils so proper grounding and insulation of coils is necessary. Any damage to coils or their cables needs prompt attention. Also looping of the cable to a coil can result in burns to patients that come into contact with them. It is best to avoid all contact with the RF coil cables.
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