Magnetic resonance imaging (MRI) is a medical imaging technique that uses strong magnetic fields and radio waves to generate images of the inside of the human body. MRI is an extremely useful diagnostic tool, allowing doctors to see detailed images of organs and tissues without using ionizing radiation. However, MRI does carry some risks that patients should be aware of.
Safety Concerns with MRI
The three main safety concerns with MRI are:
- Projectile accidents from ferromagnetic objects
- Burns from implants and devices
- Health effects from exposure to strong magnetic fields
Let’s explore each of these risks in more detail.
Projectile Accidents
The strong magnets used in MRI can attract ferromagnetic objects like iron and some alloys towards the machine. This can turn ordinary objects into dangerous projectiles. There have been accidents where heavy metal objects were pulled into the MRI at high speeds, seriously injuring or even killing patients and operators.
To avoid projectile accidents, MRI facilities have strict policies about not allowing any loose metal objects in the MRI room. Patients may be asked to remove watches, jewelry, glasses, hair clips, and any clothing with metal parts like zippers. The magnetic field can wipe credit cards, disable pacemakers, and pull on metal implants or fragments inside the body.
Burns
The radiofrequency energy used during MRI scanning can produce significant heating of metal implants and devices. This can lead to burns or tissue damage. Implants that are known to pose heating risks include:
- Cardiac pacemakers and implantable defibrillators
- Aneurysm clips
- Metal screws, plates, or rods
- Older types of breast tissue expanders
- Insulin pumps and other metal medical devices
Patients with any internal or external metal implants will need to discuss the safety issues with their doctor and MRI technician. External devices may need to be removed, and internal devices may require special preparation or monitoring.
Magnetic Field Effects
There are potential health risks from being exposed to the strong magnetic fields used in MRI. Research on the long-term effects of MRI fields is limited, but possible concerns include:
- Temporary dizziness or vertigo during MRI due to the vestibular system being stimulated
- Distorted senses of smell and taste
- Headache
- Fatigue
- Tingling or numbness in the extremities
- Nausea
- Muscle twitching
No harmful effects have been consistently demonstrated when MRIs are used as directed. However, the risks may be greater for very long scan times or research protocols with unapproved equipment or procedures.
Risks for Specific Groups
While MRI is quite safe for most people when proper precautions are taken, the risks may be higher for certain vulnerable groups. Let’s look at some of the main ones.
Pregnant Women
There are no known harmful effects from MRI on the developing fetus. However, as a precaution, MRI exams are generally avoided during the first trimester unless there is a strong medical need.
Contrast agents are avoided during pregnancy since their effects are unknown. Gadolinium contrast crosses the placenta and enters the fetal bloodstream. After birth, babies rapidly excrete the drug.
Infants and Children
MRI is considered very safe in children as long as sedation is not required. General anesthesia has risks for children. Also, some facilities require children to be a certain age (like infants older than 4-6 weeks).
Hearing protection is especially important for safe pediatric MRI. Children’s developing ears are more sensitive to noise. Younger children may also need immobilization devices to hold still.
Claustrophobia and Anxiety
Up to 5% of adults have claustrophobia (fear of enclosed spaces) severe enough to interfere with MRI. Confined space, noise, and scan time can also provoke anxiety. Sedation may help, but also carries risks.
Open MRI machines can accommodate claustrophobia, but have lower image quality. Other accommodations include music, communication with technicians, or virtual reality goggles to distract patients.
MRI Safety Procedures and Restrictions
MRI facilities take extensive precautions to screen patients and prevent accidents. Safety begins before the patient enters the MRI room.
Below are typical restrictions and safety procedures for MRI:
| Safety Procedure | Purpose |
|---|---|
| Pre-screening questionnaire | Identify metal implants, devices, health conditions, or factors like claustrophobia |
| Change into hospital gown | Remove personal clothing and objects |
| Identification bracelet and paperwork double check | Confirm patient identity and scan parameters |
| Visual inspection | Look for any external metal prior to entering |
| Earplugs or headphones | Protect hearing from machine noise |
| Magnetic wand wave | Final check for loose metal objects |
| Control room observation | Technician monitors patient at all times |
| Emergency stop button | Allows patient to immediately stop if needed |
| Metal detector arch | Sounds alarm if metal objects pass through |
By following safety protocols, the chances of an accident are extremely low. Patients also play a vital role by providing full information to their doctor ahead of time.
MRI Contrast Agents
Contrast agents are drugs used to improve the visibility of internal body structures in MRI images. About 30-40% of MRI exams utilize intravenous contrast.
The most common MRI contrast agents contain the metal gadolinium. They carry additional risks beyond the MRI itself.
Allergic Reactions
MRI contrast agents may cause allergic reactions or anaphylaxis in susceptible patients. Reactions are more common with gadolinium compared to iodinated CT contrast. Patients are monitored during and after injections.
Nephrogenic Systemic Fibrosis
Nephrogenic systemic fibrosis (NSF) is a rare but serious condition linked to gadolinium MRI contrast. It results in thickening and tightening of the skin and connective tissues. NSF is seen almost exclusively in patients with severe kidney disease.
Special precautions are now taken when administering gadolinium agents to patients with impaired kidney function. The dose may be lowered or a more stable agent used to reduce NSF risks.
Gadolinium Retention
Studies show gadolinium contrast agents may deposit in the brain and other body tissues, where the metal can remain for years. The clinical significance of gadolinium retention is still being investigated.
To reduce retention, doctors avoid unnecessary repeat MRIs with gadolinium. Kidney function tests help identify patients who may poorly excrete the metal.
Conclusion
MRI is a versatile and valuable medical tool for imaging the body without x-rays. Its main risks come from the strong magnets involved. With proper screening and safety protocols, MRI exams are very safe for most people. Certain groups like pregnant women, infants, and patients with metal implants may need extra precautions.
Being well-informed and prepared helps patients have a smooth MRI experience. Patients should ask their doctor and MRI staff any questions and express any concerns before the scan. Following staff instructions also ensures maximum safety inside the MRI suite.