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Safety of Magnetic Resonance Imaging
Magnetic resonance imaging (MRI) injuries have occurred from projectiles created by the magnetic field. The powerful magnetic field of the magnetic resonance (MR) system will attract iron-containing (also known as ferromagnetic) objects and may cause them to move suddenly and with great force. This can pose a possible risk to the patient or anyone in the object's "flight path." Great care is taken to be certain that objects such as ferromagnetic screwdrivers and oxygen tanks are not brought into the MR system area. As a patient, it's vital that you remove all metallic belongings in advance of an MRI exam, including hearing aids, watches, jewelry, and items of clothing that have metallic threads or fasteners.

The stray magnetic field outside the bore of the magnet is known as the fringe field and this is a 3 dimensional field measured in Gauss. MRI systems are shielded to confine the fringe field within the scan room. Magnetic fields less than 5 Gauss are inconsequential to MRI safety. In most systems the 5 Gauss field is confined within the scan room, so the fringe field doesn't affect any area external to the magnet room.

The 30 Gauss field demarcates the point where projectile hazards become significant and only MRI compatible equipment can safely enter this region. Each MRI system has its own unique fringe field due to varying magnetic design, shielding characteristics, and field inhomogeneity. Each site must be supplied with a schematic that clearly defines the fringe field of the magnet. The schematic must demarcate the 30 Gauss and 5 Gauss lines.

This section summarizes the different zones of a MR suite and points out specific safety issues of greatest concern. MRI sites are divided into 4 safety zones based on the American College of Radiology guidelines:
• Zone 1: General public area outside the MR environment. This area is the reception and waiting areas.
• Zone 2: Area between Zone 1 (Public Access) and the strictly controlled Zone 2 (Control Room) and Zone 3 (Magnet). This is the area just outside of the restricted area Zone 3. This is the area of travel that patients are brought into their procedure.
• Zone 3: Control Room. All access to Zone 3 is to be strictly restricted with access to regions within it controlled by and entirely under the supervision of MR personnel. This zone is restricted from general public access by a reliable restricting method that can differentiate between MR and non-MR personnel.
• Zone 4: Magnet Room. No individual is allowed in the scan room without being supervised by trained MRI personnel. The scan room door is always locked when unattended. Only MR compatible equipment approved by the MR Safety Committee may be brought into Zone 4. The MR technologists must be able to directly observe and control via line of sight the entrances or access to Zone 4 from their normal positions when stationed at their desks in the scan control Room.

Acoustic Noise

Position information can then be recovered from the resulting signal by the use of a Fourier transform. These fields are created by passing electric currents through specially-wound solenoids, known as gradient coils. Switching of field gradients causes a change in the Lorentz force experienced by the gradient coils, producing minute expansions and contractions of the coil itself.

Since these coils are within the bore of the scanner, there are large forces between them and the main field coils, producing most of the noise (clicking or beeping) that is heard during operation. This is most marked with high-field machines and rapid-imaging techniques in which sound intensity can reach 120 decibels (dB) (equivalent to a jet engine at take-off).

As a reference, 120 dB is the threshold of loudness causing sensation in the human ear canal — tickling, and 140 dB is the threshold of ear pain. Since decibel is a logarithmic measurement, a 10 dB increase equates to a 10-fold increase in intensity — which, in acoustics, is roughly equal to a doubling of loudness.

The use of ear protection is essential for anyone inside the MRI scanner room during the examination.

Contrast Agents

The most commonly used intravenous contrast agents are based on a chemical compound of gadolinium (gd). In general, these agents have proved safer than the iodinated contrast agents used in X-ray radiography or CT. Anaphylactoid reactions are rare, occurring in approximately 0.03 to 0.1%. Of particular interest is the lower incidence of nephrotoxicity, compared with iodinated agents, when given at usual doses — this has made contrast-enhanced MRI scanning an option for patients with renal impairment, who would otherwise not be able to undergo contrast-enhancement.

Although gadolinium agents have proved useful for patients with renal impairment, in patients with severe renal failure requiring dialysis there is a risk of a rare but serious illness, nephrogenic systemic fibrosis, that may be linked to the use of certain gadolinium-containing agents. The most frequently linked is gadodiamide, but other agents have been linked too. Although a causal link has not been definitively established, current guidelines in the United States are that dialysis patients should only receive gadolinium agents where essential, and that dialysis should be performed as soon as possible after the scan to remove the agent from the body promptly.

Contrast agents may be injected intravenously to enhance the appearance of blood vessels, tumors, or inflammation in the case of MS. Contrast agents are typically used for MS patients to help determine if there is any active disease progression. Unlike computed tomography (CT), MRI uses no ionizing radiation and is generally a very safe procedure. Nonetheless the strong magnetic fields and radio pulses can affect metal implants, including cochlear implants and cardiac pacemakers. In the case of cardiac pacemakers, the results can sometimes be lethal, so patients with such implants are generally not eligible for MRI.

MRI is used to image every part of the body, and is particularly useful for tissues with many hydrogen nuclei and little density contrast, such as the brain, spinal cord, muscle, connective tissue and most tumors.

Implants and Foreign Bodies

Pacemakers are generally considered an absolute contraindication towards MRI scanning, though highly specialized protocols have been developed to permit scanning of select pacing devices. Several cases of arrhythmia or death have been reported in patients with pacemakers who have undergone MRI scanning without appropriate precautions. Other electronic implants have varying contraindications, depending upon scanner technology, and implant properties, scanning protocols and anatomy being imaged.

In the case of pacemakers, the risk is thought to be primarily RF induction in the pacing electrodes/wires causing inappropriate pacing of the heart, rather than the magnetic field affecting the pacemaker itself.

Ferromagnetic foreign bodies such as metal fragments, or metallic implants such as surgical prostheses and aneurysm clips are also potential risks, and safety aspects need to be considered on an individual basis. Interaction of the magnetic and radio frequency fields with such objects can lead to trauma due to movement of the object in the magnetic field, thermal injury from Rf induction heating of the object, or failure of an implanted device. These issues are especially problematic when dealing with the eye. Most MRI centers require an orbital x-ray to be performed on anyone suspected of having metal fragments in their eyes, something not uncommon in metalworking.

Because of its non-ferromagnetic nature and poor electrical conductivity, titanium and its alloys are useful for long term implants and surgical instruments intended for use in image-guided surgery. In particular, not only is titanium safe from movement from the magnetic field, but artifacts around the implant are less frequent and less severe than with more ferromagnetic materials such as stainless steel. Artifacts from metal frequently appear as regions of empty space around the implant that are frequently called a "black-hole artifact". For example, a 3 mm titanium alloy coronary stent may appear as a 5 mm diameter region of empty space on MRI, whereas around a stainless steel stent, the artifact may extend for 10 to 20 mm or more.

Pregnancy

No known effects of MRI on the fetus have been demonstrated. In particular, MRI avoids the use of ionizing radiation, to which the fetus is particularly sensitive. However, as a precaution, current guidelines recommend that pregnant women undergo MRI only when essential. This is particularly the case during the first trimester of pregnancy, as organogenesis takes place during this period. The concerns in pregnancy are the same as for MRI in general, but the fetus may be more sensitive to the effects. However, one additional concern is the use of contrast agents; gadolinium compounds are known to cross the placenta and enter the fetal bloodstream, and it's typically recommended that their use be avoided.

Despite these concerns, MRI is rapidly growing in importance as a way of diagnosing and monitoring congenital defects of the fetus because it can provide more diagnostic information than ultrasound and it lacks the ionizing radiation of CT.

Claustrophobia and Discomfort

Due to the construction of some MRI scanners, they can be potentially unpleasant to lie in. Older models of closed bore MRI systems feature a fairly long tube or tunnel. The part of the body being imaged must lie at the center of the magnet, which is at the absolute center of the tunnel. Because scan times on these older scanners may be long (occasionally up an hour for the entire procedure), people with even mild claustrophobia are sometimes unable to tolerate an MRI scan without management. Modern scanners may have larger bores and scan times are shorter. This means that claustrophobia is less of an issue, and many patients now find MRI an innocuous and easily tolerated procedure.

Alternative scanner designs, such as open or upright systems, can also be helpful where these are available. Though open scanners have increased in popularity, they produce inferior scan quality because they operate at lower magnetic fields than closed scanners. However, commercial 1.5T open systems have recently become available, providing much better image quality than previous lower field strength open models.