How Does an MRI Work? A Simple Guide to Magnetic Resonance Imaging
Updated: Oct 29
An MRI works by using a strong magnet, radio waves, and a computer to make detailed pictures of the inside of your body. The magnet lines up tiny particles in the water of your tissues, radio waves briefly knock them out of line, and the signals they give off as they settle back are turned into images. MRI does not use X-rays or other ionizing radiation. This guide walks through the science, the parts of the machine, what MRI is used for, and what to expect during your scan.
The basics of how an MRI works
MRI stands for magnetic resonance imaging. It is based on a science called nuclear magnetic resonance.
Your body is made up mostly of water, and water contains hydrogen. The center of each hydrogen atom is a single proton. These protons act like tiny magnets, which makes them respond well to a strong magnetic field. That is why MRI focuses on hydrogen.
Here is the process, step by step:
Magnetic field: The scanner's strong magnet lines up the hydrogen protons in your body.
Radio wave pulses: Short bursts of radio waves push the protons out of line.
Relaxation: When the pulse stops, the protons settle back into line. As they do, they give off a small signal.
Signal detection: Special coils in or around the scanner pick up these signals.
Image formation: A computer turns the signals into detailed cross-section pictures of your body.
Parts of an MRI machine
The magnet
The magnet is the heart of the machine. There are three main types:
Superconducting magnets: the most common type; they make a strong, steady field using wire coils cooled to very low temperatures with liquid helium
Resistive magnets: they make a field with electric current and use a lot of electricity
Permanent magnets: they are used in some smaller systems and make a weaker field
The magnet is always on, even when no one is being scanned. That is why the MRI team is so careful about metal in the scanner room.
Gradient coils
Gradient coils slightly change the strength of the magnetic field across your body. This lets the scanner pick out one thin "slice" at a time and figure out exactly where each signal came from. Gradient coils switching on and off is also what causes the loud knocking and buzzing sounds. Learn more in why MRIs are so loud.
Radiofrequency (RF) coils
RF coils send the radio wave pulses and pick up the signals coming back from your body. Different coils are made for different body parts, such as head coils, knee coils, and body coils. The technologist may place a lightweight coil over or around the area being scanned.
The computer system
The computer collects the signals and turns them into images. Software helps sharpen the pictures and lets the team create different types of images from the same scan.
How MRI creates different kinds of images
Magnetic field strength
Magnet strength is measured in Tesla (T). Many clinical MRI machines run at 1.5T or 3T, and some systems use lower field strengths. A stronger field can give more signal, but image quality also depends on the coils, the software, and the scan settings. Read more about magnetic strength on MRI machines.
Pulse sequences
An MRI exam is made up of several sets of pictures called sequences. Each sequence uses a different pattern of radio pulses and gradient changes. Common ones include:
T1-weighted: good for showing anatomy, such as the structure of the brain
T2-weighted: makes fluid look bright, which helps show swelling and inflammation
Proton density: a middle ground that is often used for joints
Image contrast
Different tissues settle back into line at different speeds. These speeds are called relaxation times:
T1 relaxation: how fast protons line back up with the magnetic field
T2 relaxation: how fast the protons fall out of step with each other after the pulse
Because fat, muscle, fluid, and other tissues all have different relaxation times, they show up as different shades of gray. That is what lets MRI tell tissues apart so well.
What MRI is used for
MRI can image almost any part of the body. Common uses include:
Brain and spinal cord: tumors, multiple sclerosis, stroke, aneurysms, and spinal cord injuries
Muscles and joints: torn ligaments and cartilage, muscle injuries, bone infections, and arthritis
Heart and blood vessels: heart defects, heart muscle damage, blood vessel blockages, and aneurysms
Belly and pelvis: the liver, kidneys, pancreas, and reproductive organs
Breast: sometimes used along with mammograms, often for people at higher risk of breast cancer
For more examples, see what an MRI scan can diagnose.
Benefits of MRI
No ionizing radiation: unlike X-rays and CT scans, MRI does not use radiation, which helps when repeat scans are needed
Excellent soft tissue detail: MRI shows the brain, nerves, muscles, ligaments, and organs very clearly
Non-invasive and painless: no cuts or needles are needed, unless contrast dye is used
Versatile: it can be used on almost any part of the body
Risks and things to consider
Metal, implants, and devices
The strong magnet can affect metal and some medical devices, such as pacemakers, cochlear implants, and certain other implants. Many implants are safe for MRI, but each one has to be checked. Always tell your doctor and the MRI team about any implant, device, or metal in your body before the scan.
Claustrophobia
Some people feel anxious or closed in inside the scanner. If this worries you, tell your doctor ahead of time so you can talk about options. Our tips on overcoming claustrophobia during an MRI may help.
Contrast dye
Some MRI exams use a contrast dye given through an IV. Allergic reactions are rare. Tell your doctor about any allergies and any kidney problems before your scan.
Preparing for an MRI
Clothing: you may be asked to change into a gown and remove metal items like jewelry, watches, and hair clips
Food and drink: for most MRIs you can eat normally, but some exams have special instructions, so follow what you are told
Medical history: share any health conditions, allergies, implants, or past surgeries
What to expect during an MRI
Positioning: you lie on a padded table that slides into the scanner.
Ear protection: you get earplugs or headphones, because the machine is loud.
Staying still: you hold as still as you can during each sequence so the images stay sharp.
Communication: the technologist can see and hear you, and you can talk through an intercom.
Breathing: for some scans, you may be asked to hold your breath for a few seconds.
How long the scan takes depends on the body part and the exam. See how long an MRI scan takes for more detail.
After the MRI
Most people can go back to normal activities right away. If contrast was used, a few people notice brief side effects, such as mild dizziness or a metallic taste. Follow any instructions the care team gives you.
A radiologist, a doctor trained to read medical images, reviews your scan and sends a report to the doctor who ordered it. Your doctor will go over the results with you and talk about any next steps, such as more tests, treatment, or a follow-up scan.
New and advanced MRI techniques
MRI keeps improving. Some advanced techniques include:
Functional MRI (fMRI): measures brain activity by tracking changes in blood flow
Diffusion tensor imaging (DTI): maps how water moves through the brain's white matter pathways
Magnetic resonance spectroscopy (MRS): gives chemical information about tissues
Faster scanning: newer methods aim to shorten scans while keeping images clear
Learn more in our guides to diffusion tensor imaging and AI in MRI.
Frequently asked questions
How is an MRI different from a CT scan?
MRI uses a magnet and radio waves. CT uses X-rays. MRI is usually better for soft tissues, while CT is often used for bones and quick emergency imaging. Read more in MRI vs. CT scan.
Is an MRI safe for everyone?
MRI is safe for most people. Some implants and devices need to be checked first, and a few may not be safe in the scanner. Always tell the team about any metal or devices in your body.
Does an MRI use radiation?
No. MRI uses a magnet and radio waves, not ionizing radiation.
Can I move during an MRI?
You should stay as still as possible. Movement can blur the images, and part of the scan may need to be repeated.
Does an MRI hurt?
No. The scan itself is painless. Some people find it hard to lie still or are bothered by the noise.
How long does it take to get MRI results?
At Millenium MRI, results are usually sent to your referring doctor within 24 hours. Your doctor will then share the results with you.
Related reading
Schedule an MRI at Millenium MRI
Millenium MRI has centers in Ocala, Largo, and Tamarac, Florida, and in North Little Rock, Springdale, Jonesboro, and Marion, Arkansas. Board-certified radiologists read every scan, and ARRT-certified technologists perform them. Same-day appointments are often available, and most patients are in and out in about 35 minutes. We accept most insurance plans, workers' compensation, personal injury cases, and self-pay. Call your nearest center or request an appointment online.
This article is for general information and is not medical advice. Talk with your doctor about your specific situation.
Comments