From the outside, an MRI machine looks like a large white doughnut. From the inside, you see a smooth, curved plastic tunnel with built-in lighting, small air vents, and sometimes a tiny mirror or screen angled toward the open end. The bore (the tunnel you lie in) is typically about 60 centimeters wide, roughly two feet, though wide-bore machines stretch to 80 centimeters. What you can’t see are the layers of sophisticated hardware wrapped concentrically around you, hidden behind that smooth interior shell.
What You See as a Patient
When you slide into the bore on the motorized table, the first thing you notice is how close the ceiling is. In a standard 60-centimeter bore, the walls curve just inches from your face and shoulders. The interior surface is a smooth, light-colored plastic shell designed to feel as uncluttered as possible. Small vents push a steady stream of air across your face to keep you cool and reduce the feeling of confinement.
Most modern machines have a strip of soft lighting along the bore and a two-way intercom so you can talk to the technologist at any time. Some newer systems go further: Philips, for example, offers an in-bore display with dynamic lighting, video, and sound designed to distract patients, particularly children, during the scan. You’ll usually be given headphones or earplugs, a squeeze-ball panic button, and sometimes a small angled mirror mounted on the head coil that lets you see out the end of the tunnel.
If your scan requires a specific body part to be imaged, the technologist may place a rigid plastic device over that area before you go in. These are surface coils or array coils, custom-shaped receivers that look like cages, helmets, or padded frames. A head scan uses a helmet-like coil that fits snugly around your skull. A knee scan uses a cylindrical cradle. These aren’t decorative; they’re radio antennas tuned to pick up the faint signals your body emits during the scan.
The Hidden Layers Around the Bore
Behind the smooth interior shell, the machine is built in concentric layers, each performing a distinct job. Think of it like nested cylinders, each one slightly larger than the last. Starting from the tunnel wall and moving outward, here’s what’s packed inside.
Radiofrequency Coils
The innermost hidden layer is a large built-in RF coil called the body coil. It’s a cylindrical arrangement of wire loops running the length of the bore, and its job is twofold: it sends pulses of radio energy into your body to excite hydrogen atoms, then listens for the faint energy those atoms release. The body coil works like a speaker and microphone in one. The surface coils placed directly on your body serve as higher-sensitivity microphones for specific areas, picking up signals with greater detail because they sit closer to the tissue being imaged.
Shim Coils
Positioned near the RF coils are shim coils, small independent electromagnets strategically placed around the imaging area. Their sole purpose is to fine-tune the magnetic field so it’s perfectly uniform across the space your body occupies. Even tiny variations in field strength can blur the final image, so these coils generate correction fields that cancel out distortions. Some systems place shim coils inside the RF coil layer, others outside it, depending on the design tradeoff between precision and efficiency.
Gradient Coils
The next layer out contains three sets of gradient coils, one for each spatial direction (left-right, head-to-toe, front-to-back). These coils are large copper wire patterns mounted on a cylindrical support structure. They create controlled variations in the magnetic field that let the computer pinpoint exactly where in your body each signal is coming from, essentially giving the image its spatial map.
Gradient coils are also responsible for the machine’s signature loud knocking and buzzing. When the scanner rapidly switches electrical current through these coils, the coils experience strong forces from the surrounding magnetic field. They vibrate like a drumhead, and those vibrations radiate outward as sound. Different scan sequences switch the gradients at different speeds and patterns, which is why the noises change throughout your exam. Sound levels can reach 100 decibels or more, comparable to a jackhammer, which is why hearing protection is mandatory.
The Main Magnet
The outermost and largest component is the superconducting magnet itself. This is what gives the MRI its power and accounts for most of the machine’s weight, often several tons. The magnet is made of wire, traditionally a niobium-titanium alloy, wound into massive coils. When cooled to an extreme temperature, this wire loses all electrical resistance and can carry a powerful, stable current indefinitely without any external power supply.
To reach that superconducting state, the wire must be chilled to around negative 269 degrees Celsius, just a few degrees above absolute zero. This is accomplished by bathing the coils in liquid helium. A conventional clinical scanner can require over 1,000 liters of liquid helium, all contained within a vacuum-insulated vessel called a cryostat. The cryostat works like a high-tech thermos, with layers of vacuum and radiation shielding to keep the helium from boiling off. If you could peel back the outer casing of the machine, this gleaming, insulated tank is the dominant structure you’d see.
How Strong the Magnet Actually Is
Most clinical MRI scanners operate at either 1.5 Tesla or 3.0 Tesla. For reference, Earth’s natural magnetic field is roughly 0.00005 Tesla, so a 1.5T scanner generates a field about 30,000 times stronger than the planet itself. A 3.0T machine produces an even stronger signal, which translates to sharper, more detailed images because the energy emitted by hydrogen atoms in your body scales directly with field strength. Some research facilities use 7.0T machines, though these remain uncommon in everyday clinical settings.
This extreme magnetic strength is why you’re asked to remove all metal objects before entering the scan room. The field is always on, not just during your scan, and it’s powerful enough to pull metal objects across the room at dangerous speed.
Open MRI Machines Look Different
Not all MRI machines use the traditional enclosed tunnel. Open MRI machines have magnets that don’t completely surround your body. Instead of a narrow cylinder, the most common open design positions two large flat magnets above and below the table, leaving the sides open. This wider design is significantly more comfortable for people who are claustrophobic, anxious, or too large for a standard bore.
The tradeoff is image quality. Open MRI machines typically use lower-strength magnets, which produce less signal and therefore less detailed images. For many routine scans this is perfectly adequate, but certain diagnostic questions still require the stronger field of a closed-bore machine. Wide-bore scanners, with their 80-centimeter openings, offer a middle ground: more room than a standard 60-centimeter tunnel but still using a full cylindrical magnet for higher image quality.
What Holds It All Together
From the outside, the MRI looks like a sleek, featureless unit. The outer casing is a fiberglass or plastic shell that hides all the engineering underneath. Beneath that shell, every layer is precisely positioned. The gradient coils sit inside the bore of the main magnet. The shim coils are threaded between layers. The RF body coil lines the inner wall. Cooling lines, electrical cables, and vibration-dampening materials fill the gaps. The entire assembly is mounted on a reinforced floor, sometimes with its own concrete pad, to handle the weight and minimize vibration transfer to the rest of the building.
The scan room itself is also part of the system. The walls, floor, and ceiling are lined with a continuous copper or aluminum shield called a Faraday cage, which blocks outside radio signals from contaminating the incredibly faint signals the machine is trying to detect. Even a nearby cell phone could interfere with image quality without this shielding. So while the inside of the bore looks deceptively simple, you’re lying at the center of one of the most precisely engineered environments in modern medicine.