The inside of an MRI machine is a narrow, well-lit tunnel lined with smooth plastic paneling. From a patient’s perspective, it looks surprisingly simple: a rounded tube with a flat surface to lie on, soft lighting, and sometimes a small mirror or screen near your face. But behind that clean interior shell sits an extraordinary stack of engineering, with layers of magnets, coils, and cooling systems packed into the walls around you.
What You See as a Patient
When you slide into an MRI scanner on the motorized table, you enter a cylindrical tube called the bore. A traditional bore measures about 60 centimeters (roughly 23.6 inches) across, which means the walls are close on either side. Wide-bore machines open that up to around 70 centimeters, giving noticeably more room. Either way, the interior walls are a smooth, curved plastic housing, usually white or off-white, with no visible hardware or moving parts.
The bore is not dark. Built-in lighting runs along the length of the tunnel, and many newer machines take this further with ambient lighting that can change color to create a calmer environment. Philips, for example, offers systems with dynamic lighting, sound, and even video projected inside the bore to keep patients (especially children) relaxed during scans. A steady flow of air moves through the tunnel to keep you cool and reduce any sense of stuffiness. You’ll also typically have a call button in your hand and may hear the technologist’s voice through a speaker or headphones.
Depending on which body part is being scanned, a plastic frame called a coil may be placed over or around that area before you go in. A head scan, for instance, involves a cage-like structure that fits over your face with openings so you can see. A small angled mirror is sometimes attached to it, letting you look out toward the room or at a screen. Beyond those accessories, the inside looks plain and featureless by design.
What’s Hidden Behind the Walls
The smooth interior hides several layers of specialized hardware, each nested inside the next like concentric rings. Starting from the outermost layer and working inward, here’s what surrounds you during a scan.
The Superconducting Magnet
The outermost and largest component is the main magnet, which generates the powerful magnetic field that makes MRI possible. Most clinical scanners run at 1.5 or 3 tesla, while research scanners can reach 7 tesla or higher, with some experimental machines going up to 11 tesla. For reference, even a 1.5-tesla magnet is roughly 30,000 times stronger than Earth’s magnetic field.
These magnets are built from coils of a specialized alloy that becomes superconducting (meaning it loses all electrical resistance) when cooled below about 9 kelvin, or roughly minus 264 degrees Celsius. Once current starts flowing through these coils, it circulates indefinitely without any additional power, as long as the temperature stays low enough. That’s why MRI magnets are “always on,” even when no scan is happening. The coils sit in a sealed chamber filled with liquid helium to maintain that extreme cold. Some newer systems are moving toward helium-free cooling designs that use gaseous refrigerants instead, but liquid helium remains the standard in most installed machines.
Shim Coils and Ferromagnetic Blocks
Inside the main magnet sits a set of components designed to make the magnetic field as uniform as possible. No magnet produces a perfectly even field on its own, so during installation, small ferromagnetic blocks are placed inside the bore to correct for irregularities. On top of that, a set of resistive coils called shim coils generate small corrective fields that vary by position. Together, these create the highly uniform field needed to produce clear images. You’d never see or feel them, but without them, your scan would be unusably distorted.
Gradient Coils
The next layer inward is a set of gradient coils, and these are responsible for the loud banging and knocking sounds you hear during a scan. Their job is to create small, precisely controlled variations in the magnetic field so the scanner can pinpoint exactly where in your body each signal is coming from.
Three separate sets of gradient coils handle the three spatial dimensions. The coil that controls depth along the length of the tunnel uses a paired-ring design called a Maxwell coil, with two loops separated by a specific distance relative to their radius. The other two directions use saddle-shaped coils (called Golay coils) with wires running along the length of the bore. These coils switch on and off rapidly during a scan, and the force of that switching against the main magnetic field is what vibrates the machine and produces all that noise.
The RF Coil
The innermost layer of hardware is the radiofrequency (RF) coil system. This is what actually sends radio waves into your body and listens for the signals that come back. The built-in RF coil is typically a “birdcage” design: a ring of evenly spaced wires running along the bore, arranged so the radio energy they produce is distributed evenly across a large area. This is the volume coil, designed to cover broad regions like your whole torso or head.
For more detailed scans of specific body parts, technologists place smaller surface coils directly on or around the area of interest. These are the plastic-encased accessories you see being positioned before your scan. In their simplest form, a surface coil is a loop of wire paired with a capacitor, tuned to resonate at exactly the right frequency to pick up signals from hydrogen atoms in your tissue. Because they sit close to the target, they capture much sharper detail than the built-in volume coil alone.
How These Layers Work Together
During a scan, the main magnet holds every hydrogen atom in your body in a stable alignment. The gradient coils then briefly alter the field in precise patterns, selecting one thin “slice” of your body at a time. The RF coil fires a pulse of radio waves tuned to the natural frequency of hydrogen atoms, which tips them out of alignment. As those atoms snap back into place, they emit faint radio signals of their own. The RF coil (or the surface coil placed on you) picks up those return signals, and the scanner’s computer translates the timing and strength of each signal into a detailed cross-sectional image.
This entire cycle repeats hundreds of times per scan, with the gradient coils switching rapidly to build up data from different slices and angles. Each round of switching produces a burst of noise, which is why scans involve long stretches of rhythmic knocking, buzzing, or hammering sounds that can reach 100 decibels or more. You feel none of the magnetic or radio activity, but you definitely hear the gradient coils doing their work.
Why It All Looks So Simple
The clean, minimal appearance of the bore interior is deliberate. Every component that could interfere with the magnetic field or the radio signals is either made from nonmagnetic materials or sealed behind shielding. The plastic housing keeps you from contacting any hardware and provides a smooth, easy-to-clean surface. There are no sharp edges, no visible wiring, and no metal fasteners. The result is a space that looks like little more than a lit tunnel, even though just centimeters behind that wall sits one of the most powerful and precisely engineered magnets most people will ever encounter.