Ensuring safety and well-being of patients undergoing MRI

Understanding the potential hazards of Magnetic Resonance Imaging requires some knowledge of the basic principles. Living tissue contains a large amount of water and fat molecules, which contain hydrogen atoms. Magnetic Resonance Imaging uses the nuclei of these hydrogen atoms, also known as protons, to produce images. MRI can be used to show the position and concentration of the protons in the body corresponding to the position of the various tissues, as well as the tissue characteristics.

Magnetic resonance imaging requires three main components: the magnet, the gradient coils and the radio frequency transmit and receive coils. The magnet creates a very strong magnetic field that aligns the protons in the body. The gradient coils change the magnetic field in three planes so that the targeted protons have a different vibration frequency from their surroundings. This allows a localization of the protons, which is a necessary step for the generation of the images that an MRI system produces.

The magnetic field attracts objects made of steel or other ferromagnetic materials with enormous force. This force is 30,000 to 60,000 times greater than the Earth's magnetic field. Even heavy objects can fly into the magnet. The magnetic field can also create torque or twisting forces on ferromagnetic objects. Objects such as some implants may experience torque due to the magnetic force, potentially causing tissue damage. The magnetic field of an MRI system with a superconducting magnet is always on and must always be treated with caution.

The main magnetic field is not completely contained within the bore of the magnet. All magnets are surrounded by a magnetic field, represented by the magnetic field lines. The magnetic attraction force varies around the scanner. The force is highest at the opening of the bore. More information regarding the field lines and attraction forces for different field strengths can be found in the technical description. During the site planning, a controlled access area around the MRI system is identified. Outside this area the static magnetic field does not pose a hazard to the general public. Inside the controlled access area, there is a potential hazard. For most sites the controlled access corresponds with the examination room. The examination room must be marked with the safety marking plate that includes the warning sign "strong magnetic field".

Do not bring objects made of magnetic materials into the controlled access area. The magnetic attraction force pulls ferromagnetic objects towards the magnet. In this way, large metal objects such as an oxygen tank or cleaning machine, can become powerful projectiles. Objects are pulled into the magnet at very high speeds, posing a significant risk to anyone in the trajectory of the object. Even small objects can become dangerous projectiles. An item such as a small paperclip or hairpin can easily reach speeds of 65 kilometers per hour (or 40 miles per hour) and higher, when pulled into an MRI magnet and can cause serious damage or injury.

Larger and heavier objects experience a stronger force, as attraction is roughly proportional to the mass of the object. The attraction force increases rapidly within a strong field gradient in the fringe field when you approach the magnet. An object that does not appear to demonstrate ferromagnetic properties may suddenly be pulled into the magnet as you take one step closer. The attraction force experienced increases exponentially when approaching the magnet. The magnetic field also twists ferromagnetic objects to align with the direction of the magnetic field. The twisting or torque forces can be dangerous to patients with ferromagnetic implants because this force can displace the implant, tearing tissue or rupturing blood vessels. Brain aneurysm clips are particularly at risk from this effect.

To locate the position of structures within the human body, each MR system applies switched gradient magnetic fields. These switching gradient fields are only active during scanning and do not extend outside the magnet. Every MRI system includes a set of three gradient coils. These gradient coils create the gradient magnetic fields used for the spatial encoding of the MR signal. The gradient fields are applied in three orthogonal directions to create different imaging plains. The gradient fields are switched on and off during scanning. 

The hazards associated with the fast switching and high gradients are hearing damage due to acoustic noise, patient discomfort due to peripheral nerve stimulation and the potential malfunction of active implants.

Acoustic noise results from the rapidly switched electrical currents running through the gradient coils. This electromagnetic force is known as the Lorentz Force. Since the gradient coils are not free to move the sudden application of force results in the typical knocking sound during scanning. This knocking sound is louder for scans with high dB/dt values. The sound that is generated can approach acoustic noise levels high enough to cause discomfort or result in tinnitus or hearing damage.

The predicted sound pressure level for each scan sequence is displayed in the user interface. The sound pressure level shows the number of decibels, dB relative to the recommended maximum sound level. International MRI safety standards for patients allow a maximum sound level of 99 dB(A) for up to one hour. Hearing protection must be worn by the patient during scanning as the system's acoustic noise can be experienced as uncomfortable and may exceed 99dB(A). Hearing protection must include appropriately fitted earplugs that provide sufficient sound damping of greater than 30 decibels. Additional use of the Philips headset is always recommended. The acoustic noise generated by the system is not only a hazard to the patient, but also affects anyone present in the examination room during scanning. Always provide hearing protection to anyone present in the examination room during scanning.

Another effect of the high dB/dt values is peripheral nerve stimulation, PNS. PNS is caused by the rapid changes in gradient fields. This rapid change induces electric fields in the human body and can cause a tingling sensation or superficial twitching. PNS is unlikely to occur outside the imaging volume and is therefore generally only experienced by the patient. The location and nature of the PNS differs for each individual. Not all patients will experience PNS. PNS is temporary and there are no known long term health effects related to nerve stimulation. PNS is more likely to occur during scans that require rapid gradient switching, such as those used in diffusion imaging and fMRI.

The expected PNS level can be viewed in the user interface, it is expressed as a percentage of the mean threshold level. For example, a PNS of 55% means there is a 55% chance that the patient will experience PNS. Scans with a high PNS can be identified by the warning icon. If the expected PNS level exceeds 80%, a warning message is displayed. The warning message displays the first time a scan with high PNS level is started.

During an MRI examination RF energy is transferred to the body. This can potentially result in warming of the patient. The patient temperature rise is proportional to the total energy delivered to the patient. This is known as the “specific energy dose” (SED). SED head is determined by SAR and scan time. SAR, specific absorption rate, is the RF power absorbed by the patient per unit mass expressed in watts per kg. International standard set limits to the level of SAR. SAR levels do not take into account the scan duration. SED is equal to SAR multiplied by scan time and is expressed in kilojoules per kilogram and provides an indication for warming of the patient.

Both the SAR and SED values are displayed for each scan. For SED the scheduled, delivered and total SED levels are displayed throughout the examination. On the MR console the SED is displayed as both numerical values and as an overall SED status indicator. The SED status indicator is displayed in the exam dashboard.

Scanning with high SAR levels, greater than 2 watts per kg or scanning for a prolonged period of time can warm up the patient, resulting in increased perspiration. Patient perspiration may result in the unintended RF circuits between body parts leading to burn injuries. Scan in normal operating mode, with whole body lower than SAR 2 watts per kg. When possible, the use of scans with high SAR levels should be minimized. But where this is needed, it is advisable that you distribute the SAR scans across the entire examination by interleaving the high SAR scans with lower SAR scans. Avoid running high SAR scans at the end of a high SED examination. To support the cool down mechanism of the patient, set the patient ventilation at the maximum level when scanning high SAR sequences that are greater than 2 watts per kg.

It should be noted that the thermal load and the associated patient temperature rise of an MR examination is a separate phenomenon from local RF energy related thermal injuries or burns. The transmitted RF field results in the induction of the electrical currents in the body. These currents flow through the electrically conductive tissue of the human body, resulting in heat dissipation. The transmitted RF energy and the associated heat dissipation is a potential hazard for all patients. Heating sensations and tissue heating can occur if the patient's body cannot effectively dissipate the generated heat.