An MRI machine uses powerful magnets and radio waves to create detailed images of the inside of your body, without any radiation. Unlike X-rays or CT scans, which use ionizing radiation, MRI produces images by manipulating the behavior of hydrogen atoms in your tissues. The result is exceptionally clear pictures of soft tissues like the brain, spinal cord, muscles, ligaments, and organs.
How an MRI Machine Works
Your body is mostly water, and every water molecule contains hydrogen atoms. Each hydrogen atom has a single proton at its center that behaves like a tiny bar magnet, spinning on its axis with a north and south pole. Normally, these protons spin with their axes pointed in random directions. But when you lie inside the MRI scanner’s powerful magnetic field, the protons snap into alignment, all pointing the same way. This creates a uniform magnetic signal throughout the tissue being scanned.
The machine then sends a burst of radio waves into your body at a precise frequency. This extra energy knocks the aligned protons out of position. When the radio wave pulse switches off, the protons gradually return to their aligned state, and as they do, they release energy in the form of a faint radio signal. Sensors inside the machine detect that signal. Different types of tissue (fat, muscle, fluid, bone marrow) release energy at different rates, and the machine translates those differences into a highly detailed image.
Main Components Inside the Machine
Three core systems work together inside every MRI scanner. The main magnet produces the primary magnetic field that aligns your hydrogen protons. In most hospital scanners, this magnet is always on, running at either 1.5 or 3 Tesla (units of magnetic field strength). For reference, 1.5 Tesla is roughly 30,000 times stronger than Earth’s magnetic field.
Gradient coils sit inside the main magnet and create smaller, variable magnetic fields. These coils let the machine pinpoint exactly where in the body each signal is coming from, encoding location in three dimensions. They’re also responsible for the loud knocking and banging you hear during a scan. The rapid switching of electrical current through the gradient coils causes them to vibrate against the main magnetic field, producing mechanical noise that regularly exceeds 95 decibels and can reach above 105 dB on 3T scanners. That’s comparable to standing near a jackhammer, which is why you’re given earplugs or headphones.
The radiofrequency (RF) coil is the component placed closest to your body. It sends the radio wave pulses that knock protons out of alignment and then acts as an antenna to pick up the signals they emit. Different RF coils are shaped for different body parts: a cage-like coil fits around the head for brain imaging, while flat, flexible coils wrap around a knee or shoulder.
What MRI Is Best At Imaging
MRI’s greatest strength is soft tissue contrast. It distinguishes between different types of soft tissue far better than CT or X-ray. Research comparing MRI and CT for evaluating soft tissue tumors found MRI was significantly better at displaying contrast between tumor and muscle and between tumor and blood vessels. This makes MRI the go-to imaging tool for brain conditions, spinal cord injuries, torn ligaments and cartilage, joint problems, and many cancers.
It’s also the preferred choice for imaging the brain and nervous system, detecting multiple sclerosis lesions, evaluating heart disease, assessing liver tumors, and diagnosing conditions in the wrist, jaw, and other complex joints. CT scans still outperform MRI for bone fractures, lung imaging, and emergency situations where speed matters, but for anything involving soft tissue detail, MRI is typically superior.
1.5T vs. 3T Scanners
Most clinical MRI machines run at either 1.5 Tesla or 3 Tesla. The signal strength used to compose an image is directly proportional to the magnetic field strength, so a 3T scanner collects roughly twice the signal of a 1.5T machine. In practice, though, the clinical difference is more nuanced than “stronger is better.”
A systematic review by the Canadian Agency for Drugs and Technologies in Health found that 3T and 1.5T scanners performed similarly for most clinical outcomes. Where 3T did show clear advantages was in detecting multiple sclerosis lesions, identifying coronary artery disease, visualizing nerves and small joint structures in the wrist, and spotting liver metastases. Interestingly, 1.5T was actually better in some cases, such as prostate tumor imaging, where the stronger field introduced artifacts that made interpretation harder. Most hospitals use 1.5T scanners as their workhorses, reserving 3T for situations where the extra detail is clinically useful.
Open vs. Closed Machines
A closed MRI is the traditional tunnel-shaped design, where you slide into a cylindrical bore roughly 60 centimeters wide. It feels confining, and patients with claustrophobia often struggle with it. Open MRI machines address this by removing the surrounding walls. Instead of a tube, two flat magnets sit above and below you with open sides.
The tradeoff is image quality. Open scanners typically operate at around 0.23 Tesla, a fraction of the 1.5 to 3T strength found in closed machines. The resulting images have noticeably less detail and lower resolution. For a straightforward scan of a large structure, an open MRI may suffice, but for anything requiring fine detail, most radiologists prefer the closed design.
Portable and Low-Field MRI
A newer class of compact, portable MRI machines is changing where scans can happen. The Hyperfine Swoop, the first FDA-cleared portable MRI system, operates at just 0.064 Tesla and is designed for brain imaging at the bedside. It can be wheeled into an ICU or emergency department, bringing imaging directly to patients who are too unstable to transport to a traditional scanner. Other portable systems cover neonatal brain imaging, spine and joint imaging, and even MRI-guided prostate biopsies.
Researchers have even built a wearable MRI “cap” weighing just 8.3 kilograms with a construction cost under $450. These ultra-low-field devices sacrifice the crisp resolution of hospital scanners, but they’re opening access to MRI in settings where it previously wasn’t possible, from rural clinics to disaster zones.
Contrast Agents
Some MRI scans require a contrast agent, most commonly a compound based on the element gadolinium, injected into a vein before or during the scan. Gadolinium is paramagnetic, meaning it interacts with the MRI’s magnetic field and changes how nearby water molecules behave. This shortens the time it takes for protons to return to their resting state, which brightens the signal on the resulting image.
The practical effect is that areas where the contrast agent accumulates (tumors, inflamed tissue, blood vessels) light up more clearly against surrounding tissue. Contrast-enhanced MRI is commonly used to evaluate tumors, detect inflammation, image blood vessels, and assess liver lesions. The agent circulates through your bloodstream and is filtered out by the kidneys over the following hours.
What the Scan Feels Like
MRI scans typically last between 30 minutes and two hours depending on what’s being imaged and how many sequences are needed. You’ll lie still on a padded table that slides into the scanner bore. The machine is loud, producing repetitive knocking, buzzing, and thumping sounds as the gradient coils cycle through different sequences. Each sequence sounds different, and there are brief quiet pauses between them.
You won’t feel the magnetic field or the radio waves. The bore is lit and usually has a small fan blowing air to reduce any feeling of confinement. You’ll be given earplugs or noise-canceling headphones, and you can communicate with the technologist through an intercom at any time. Some facilities offer music or even video goggles. If contrast is used, you may feel a brief cool sensation at the injection site, but the scan itself is painless.
Safety and Metal Restrictions
Because the MRI magnet is extraordinarily powerful and always active, metal objects pose serious risks. The magnetic field can pull ferromagnetic objects toward the scanner at high speed or cause implanted metal to heat up, shift, or malfunction. Before any scan, you’ll be screened for metal in or on your body.
Certain implants are absolute contraindications. Cardiac devices like pacemakers and defibrillators can malfunction, deliver inappropriate shocks, or overheat during a scan, though newer MRI-conditional versions of these devices are increasingly available and can be scanned with specific precautions. Metallic foreign bodies in the eyes are particularly dangerous because the magnetic field can cause them to move and damage delicate tissue. If you have a history of metal fragments near your eyes from welding or trauma, you’ll need an orbital X-ray before the scan is approved.
Other items that cannot go into the scanner include cochlear implants, certain drug infusion pumps, cerebral aneurysm clips, bullets or shrapnel fragments, and hearing aids. Devices like coronary stents, joint replacements, IUDs, and surgical clips require case-by-case evaluation. If you have any implant, the MRI team will look up its specific make and model in a safety database to determine whether scanning is safe. Everyday items like jewelry, watches, credit cards, and phones must be removed before entering the room.