Alec Titan
Magnetic resonance imaging (MRI) is a safe and effective way to diagnose a wide variety of diseases and conditions. However, the magnetic fields and radiofrequency energy produced by an MRI scanner may cause side effects such as peripheral nerve stimulation, muscle twitching, and heating of the body and surrounding tissue. This is due to the rapidly changing magnetic gradients and electric fields induced in the body, which can lead to nerve and muscle stimulation and, in rare cases, more severe and painful muscle contractions. While MRI is generally considered safe, careful screening of people and objects entering the MR environment is critical to ensure safety and reduce potential side effects.
| Characteristics | Values |
|---|---|
| Cause of muscle spasms | Peripheral nerve stimulation (PNS) |
| PNS cause | Variation of the local magnetic field over time |
| PNS effect | Currents in nerve and muscle tissues |
| PNS symptoms | Tingling, tapping, twitching, muscle contractions |
| PNS intensity | Varies between individuals |
| PNS safety | IEC 60601-2-23:2015 sets acceptable levels to protect against PNS |
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What You'll Learn
Peripheral nerve stimulation
PNS typically occurs in pulse sequences that use rapid gradient switching, such as echo planar, turbo-SE, or SSFP techniques. The largest gradient fields may occur outside the field of view, and the strongest induced electric fields are usually located in the more superficial portions of the patient, where many peripheral nerves are present. These electric fields are also concentrated in areas where tissues of different conductivity, such as bone, fat, and muscle, are in close proximity or near metallic implants.
When mild, PNS may be perceived as a tingling or tapping sensation, often surprising the patient but causing no real discomfort or danger. However, as the intensity of stimulation increases, motor nerve depolarization can lead to severe and painful muscle contractions.
The intensity of the electric field causing PNS is directly proportional to the rate of change of the magnetic gradient field. Therefore, larger axons are more susceptible to depolarization, as are those with anatomic turns and kinks where current densities may be higher.
PNS is not considered a significant danger during MRI scans. The stimulation is usually mild, and the thresholds for respiratory and cardiac stimulation are much higher than the PNS thresholds accessible by typical MRI parameters.
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Radiofrequency energy and magnetic fields
The RF coils are an integral part of the MRI machine, which is typically enclosed within a copper-lined room known as a Faraday shield. This shield helps to block potential electromagnetic noise, ensuring a controlled environment for accurate imaging. The RF coils work in conjunction with strong magnetic fields to produce high-quality images.
The interaction between radiofrequency energy, magnetic fields, and human tissue can lead to various effects, including tissue heating. This heating phenomenon is particularly significant during long MRI examinations. It is important to note that the RF energy and magnetic fields can also induce electrical currents in the human body, specifically in nerve and muscle tissues, resulting in a phenomenon known as Peripheral Nerve Stimulation (PNS). PNS can cause sensations such as tingling, tapping, or twitching, and in more intense cases, painful muscle contractions.
The use of radiofrequency energy and magnetic fields in MRI carries specific safety considerations. For instance, the magnetic fields can attract magnetic objects, potentially causing damage to the scanner or harm to individuals in the vicinity. Additionally, the RF energy and magnetic fields can cause heating in implanted medical devices and surrounding tissues, posing a risk of burns. Furthermore, these fields may cause electrically active medical devices to malfunction, impacting the delivery of intended therapy.
To ensure patient safety, rigorous screening procedures are implemented before MRI examinations. This includes careful screening of individuals and objects entering the MRI environment to prevent any magnetic objects from entering the magnet area, as they could become projectiles. Additionally, hearing protection is provided to mitigate the loud knocking noises generated by the magnetic fields, which can harm hearing if adequate protection is not used.
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Cochlear implants
If you have a cochlear implant, it is important to be aware of the potential risks posed by MRI scanners and to take the necessary precautions. MRI scanners use strong magnetic fields and radio waves (radiofrequency energy) to create images of the internal structures of the body. The strong magnetic field can affect implants that contain metal or magnets, causing them to move or twist inside the patient's body, resulting in discomfort, pain, or injury.
To ensure a safe MRI scan, patients with cochlear implants should carry their implant card, which contains important information about the implant and MRI safety. This card should be presented to the healthcare provider and MR technologist, who should also be informed about the presence of the implant. The healthcare provider will then be able to follow the specific recommendations and instructions for the type of cochlear implant.
It is important to note that not all cochlear implants are compatible with MRI scanners. Some implants may be rated as "MR Unsafe," indicating that a patient with that type of implant should not receive an MRI exam while they have the implant. Other implants may be rated as "MR Conditional," meaning that the patient can undergo an MRI exam under certain specific conditions and with proper precautions. Overall, while it is possible to undergo an MRI with a cochlear implant, careful consideration and adherence to safety guidelines are essential to ensure a safe procedure.
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Time-varying gradient fields
PNS can cause a range of sensations, from mild tingling or tapping to more severe and painful muscle contractions as the stimulation intensity increases. The intensity of the electric field (E) causing nerve or muscle depolarization is directly proportional to the rate of change of the magnetic gradient field (dB/dt). This means that higher dB/dt values result in stronger electric fields and more intense PNS. However, it's important to note that current safety regulations restrict dB/dt values to levels below those that could induce pain or cardiac stimulation.
The use of specific pulse sequences, such as echo planar imaging (EPI), turbo-SE, or SSFP techniques, can result in rapid gradient switching and higher dB/dt values, increasing the likelihood of PNS. Additionally, the largest gradient fields often occur outside the field-of-view, and the induced electric fields are typically strongest in the more superficial portions of the patient's body where peripheral nerves are prevalent. Therefore, it is crucial for patients to be informed about these potential sensations and instructed to alert staff immediately if they experience any discomfort during the scan.
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Magnetic force and metal implants
Magnetic resonance imaging (MRI) involves the use of a very strong static magnetic field, time-varying (gradient) fields, and radiofrequency energy. The strong magnetic field of an MRI scanner will attract magnetic objects, potentially causing them to move or become projectiles. This is a significant concern in the presence of metal implants, which can experience a force in the scanner and may be displaced.
Metal implants can be affected by the magnetic field in several ways. Firstly, they can be physically displaced or moved due to the magnetic force, which could be dangerous for the patient. Secondly, long wires in implants, such as pacemakers, can result in induced currents and heating from the RF magnetic field, potentially damaging surrounding tissue. Thirdly, metals can cause the static (B0) magnetic field to become inhomogeneous, leading to severe image degradation and potentially inaccurate diagnoses.
However, it is important to note that not all metal implants are ferromagnetic and susceptible to magnetic forces. Titanium, for example, is paramagnetic and is not affected by the magnetic field of MRI. Studies have shown that most non-ferromagnetic implants are safe for patients undergoing MRI, and even some ferromagnetic implants have been found to exhibit only minimal deflection relative to their in vivo applications. Nevertheless, careful screening is critical to ensure the safety of patients with implants, and the decision to undergo MRI should consider the potential for image distortion due to implants.
In recent years, advances in technology have been made to address the challenges of imaging near metal implants. These developments aim to reduce noise and distortion in magnetic field maps, allowing for better visualization of the anatomy near the implant. Additionally, novel techniques enable non-invasive temperature measurement using MRI in the vicinity of metallic devices. These advancements improve the safety and accuracy of MRI procedures for patients with metal implants.
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Frequently asked questions
The magnetic fields and radiofrequency energy produced by an MRI scanner may cause peripheral nerve stimulation, which can lead to muscle spasms or twitching.
PNS is the excitation of nerves in the extremities from electrical voltage potentials induced by rapidly changing magnetic gradients.
When mild, PNS may be perceived as a tingling or tapping sensation. As stimulation intensity increases, it can lead to painful muscle contractions.
Some people feel warm or experience a tingling sensation during the scan, but this usually goes away once the test is over. There may also be side effects associated with the use of contrast dye, such as allergic reactions, vomiting, headache, dizziness, or nausea.
It is important to carefully screen people and objects entering the MRI room to ensure that no metallic items are present, as these can be attracted to the strong magnetic field of the scanner and may cause injury. Additionally, all patients should be provided with sound-dampening hearing protection to mitigate the loud noises created by the scanner, which may otherwise cause harm to hearing.
