MRI does not use radiation in the way most people mean when they ask this question. It produces zero ionizing radiation, the type linked to DNA damage and cancer risk. Unlike X-rays and CT scans, an MRI scanner uses a powerful magnetic field and radiofrequency pulses (a form of non-ionizing energy, similar in principle to radio waves) to create detailed images of your body. Your radiation dose from an MRI is effectively zero millisieverts, compared to 7 mSv for a chest CT or 0.1 mSv for a single chest X-ray.
How MRI Creates Images Without X-Rays
Your body is full of water, and water contains hydrogen atoms with single protons at their cores. An MRI scanner surrounds you with a magnetic field strong enough to force those protons into alignment. The scanner then fires short bursts of radiofrequency energy, which knock the protons out of alignment. When the bursts stop, the protons snap back into place and release small amounts of energy as they do. Sensors pick up that released energy and use it to build cross-sectional images of your tissues.
The entire process relies on magnetism and radio waves. Neither one strips electrons from atoms or molecules in your body, which is the defining feature of ionizing radiation. X-rays and CT scans work by sending ionizing radiation through tissue, which is what makes them useful but also what gives them a small, cumulative cancer risk. MRI sidesteps that mechanism entirely. Neither MRI nor ultrasound appears to damage DNA or increase cancer risk.
What “Non-Ionizing” Actually Means
Radiation is a broad term that covers a wide spectrum of energy. At the high-energy end you find X-rays and gamma rays, which carry enough energy to ionize atoms, meaning they can knock electrons loose and potentially damage DNA. At the lower-energy end you find radio waves, microwaves, and visible light. These are all technically forms of radiation, but they lack the energy to ionize tissue. The radiofrequency pulses used in MRI fall squarely in this non-ionizing category. So while an MRI scanner does emit a form of electromagnetic energy, it is not the kind that carries a cancer risk.
Side Effects That Can Happen During a Scan
Just because MRI avoids ionizing radiation doesn’t mean the experience is completely without physical effects. The most noticeable one is noise. The rapidly changing magnetic fields inside the scanner produce loud knocking and banging sounds that can reach levels high enough to damage hearing if you aren’t given ear protection. Most facilities provide earplugs or headphones before the scan begins.
The radiofrequency energy can also warm your body slightly. The potential for heating increases during longer scans. The FDA sets limits on how much energy the scanner can deposit into tissue, measured as the specific absorption rate: no more than 4 watts per kilogram averaged over the whole body during a 15-minute window, and no more than 3.2 watts per kilogram for the head over 10 minutes. Some people also report a mild twitching sensation from nerve stimulation, and a small number experience temporary dizziness when moving in or out of the magnetic field.
Contrast Dye Is the Bigger Safety Question
Some MRI exams require an injection of a contrast agent to make certain structures easier to see. The most common type is gadolinium-based. For most people, side effects are minor: brief discomfort at the injection site, nausea, a mild headache, or itching. Serious reactions are uncommon.
The more significant concern involves people with severe kidney problems, who face a higher risk of a rare condition called nephrogenic systemic fibrosis, which causes thickening and tightening of the skin and connective tissue. Additionally, research published in recent years has shown that trace amounts of gadolinium can remain in the body, including the brain, for months to years after an injection. Most of the contrast agent leaves through urine shortly after the scan, but the small residual amount has prompted the FDA to require class-wide safety warnings on all gadolinium-based contrast products. This is particularly relevant if you need repeated contrast-enhanced MRIs over time.
Metal Implants and MRI Safety
The magnetic field inside an MRI scanner is powerful enough to pull ferromagnetic metal objects toward it at high speed and to interfere with electronic medical devices. This makes certain implants genuinely dangerous during a scan. Absolute contraindications include most older-style pacemakers and implantable defibrillators, cochlear implants, certain neurostimulation devices, and drug infusion pumps. Metallic fragments in the body, such as shrapnel or bullet fragments, can shift position inside tissue. Even small metallic foreign bodies in the eye can migrate and cause injury.
Other items that may be hazardous include some types of cerebral aneurysm clips, magnetic dental implants, catheters with metallic components, certain prosthetic limbs, hearing aids, and body piercings. Before any MRI, you’ll be asked detailed screening questions about your medical and surgical history. If there’s any doubt about whether an implant is safe, the MRI team will check the device’s specific compatibility rating before proceeding.
MRI During Pregnancy
Because MRI uses no ionizing radiation, it is considered one of the preferred imaging options during pregnancy alongside ultrasound. No published human studies have documented harm to a developing fetus from the magnetic field or radiofrequency energy alone. The American College of Radiology does not recommend treating the first trimester any differently from later trimesters when it comes to MRI safety. Animal studies have largely shown no increased risk, and research in humans has not found significant increases in congenital anomalies, cancer, or hearing or vision loss in children exposed to MRI before birth.
Gadolinium contrast during pregnancy is a different matter. The contrast agent can cross the placenta and enter the amniotic fluid, where it may linger and potentially dissociate into a less stable form. A large retrospective study found a small but statistically significant increase in skin and inflammatory conditions in children whose mothers received gadolinium during pregnancy. For this reason, guidelines recommend using gadolinium in pregnant patients only when it would significantly change the diagnosis or improve outcomes for the mother or baby.