MRI uses powerful magnets, radio waves, and hydrogen atoms already in your body to create detailed images of organs, tissues, and joints. Unlike X-rays or CT scans, it involves no ionizing radiation. Instead, it relies on the magnetic behavior of water molecules in your tissues, which is why it excels at imaging soft structures like the brain, spinal cord, muscles, and ligaments.
Strong Magnetic Fields
The core of every MRI machine is a large magnet that creates a stable, uniform magnetic field. Most clinical scanners operate at 1.5 or 3 Tesla, which is roughly 30,000 to 60,000 times stronger than Earth’s natural magnetic field. A small number of advanced facilities now use 7-Tesla scanners, which produce more than twice the field strength of a standard 3T machine and can reveal finer anatomical detail.
These magnets are almost always superconducting electromagnets, made from special wire that loses virtually all electrical resistance when cooled to near absolute zero (around -273°C). Liquid helium keeps the wire at that extreme temperature, allowing electrical current to flow continuously without any energy loss. Once the magnet is “ramped up,” it stays on permanently, even when no one is being scanned. This is why MRI machines consume about 8 to 14 kilowatts of electricity around the clock just to maintain the cooling system and keep the magnet stable.
How Your Body Produces the Signal
Your body is mostly water, and every water molecule contains hydrogen atoms. Each hydrogen atom has a single proton at its center that acts like a tiny spinning magnet. Normally these protons point in random directions and cancel each other out. But when you lie inside the scanner’s magnetic field, they snap into rough alignment, like compass needles lining up with a bar magnet. Most settle into a low-energy state (aligned with the field) while a smaller number point in the opposite, high-energy direction. The slight excess of low-energy protons creates a faint net magnetization in your tissues.
The scanner then sends a burst of radio waves tuned to a very specific frequency. At 1.5 Tesla, that frequency is about 64 megahertz, similar to an FM radio station. This precise tuning matters because hydrogen protons will only absorb energy at their natural “resonance” frequency (called the Larmor frequency). When they absorb that energy, they flip from the low-energy state to the high-energy state and tilt away from the main magnetic field. The moment the radio pulse switches off, the protons relax back to their original alignment and release the absorbed energy as a faint radio signal. That released signal is what the machine detects and uses to build an image.
Different tissues relax at different speeds. Fat protons return to alignment quickly; water-rich tissues like cerebrospinal fluid take longer. These timing differences are what give MRI its remarkable ability to distinguish between tissue types, producing the contrast you see between gray matter and white matter in a brain scan, or between a healthy ligament and a torn one.
Gradient Coils for Spatial Mapping
A uniform magnetic field alone would make every hydrogen proton in your body resonate at the same frequency, producing one undifferentiated signal with no way to pinpoint where it came from. To solve this, the scanner contains three sets of gradient coils, one for each spatial direction (left-right, top-bottom, head-to-toe). These coils slightly vary the magnetic field strength across your body so that protons at different locations resonate at slightly different frequencies.
By switching these gradients on and off in carefully timed sequences, the scanner can encode spatial information into the signal. A computer then uses that frequency and timing data to reconstruct a precise, slice-by-slice image. The rapid switching of gradient coils is also what produces the loud banging and knocking sounds you hear during a scan.
Radiofrequency Coils
RF coils are the components that both transmit the radio wave pulses and pick up the returning signals from your body. Some coils do both jobs, while others are specialized as transmit-only or receive-only. The coils you’re most likely to notice are the ones placed close to the body part being imaged: a helmet-like cage around your head for brain scans, a flat paddle across your knee, or a wraparound coil for shoulder imaging. Placing the coil close to the target area improves signal quality and produces sharper images.
Contrast Agents
Some MRI exams require an injected contrast agent to make certain structures or abnormalities stand out more clearly. The most common type is gadolinium-based. Gadolinium is a metal that brightens areas of the image as it moves through your bloodstream, making it easier to spot things like tumors, inflammation, or blood vessel abnormalities. Because gadolinium can be harmful on its own, it’s bound to a carrier molecule (called a chelating agent) that keeps it stable and allows your kidneys to flush it out, typically within hours.
Not every MRI requires contrast. Your radiologist will order it only when the additional detail is needed for diagnosis.
What Can’t Go Into an MRI
Because the scanner produces such a strong magnetic field, certain metal objects and implants are dangerous inside the MRI room. Items containing iron or other ferromagnetic materials can be pulled violently toward the magnet or heat up during scanning. Some specific contraindications include metallic foreign bodies in the eye (which could shift and damage surrounding tissue), certain gastric reflux devices that contain magnetic beads, insulin pumps (both external and implanted types), and temporary cardiac pacing leads, which can conduct the scanner’s radio wave energy and cause burns.
Not all metal is a problem. Many modern joint replacements and surgical screws are made from titanium or other non-ferromagnetic metals and are safe in most scanners. Before any MRI, you’ll fill out a screening questionnaire about implants, metal exposure, and medical devices so the technologist can determine whether it’s safe to proceed.
Power and Infrastructure
MRI machines are significant energy consumers. During active scanning, a typical system draws between 22 and 47 kilowatts, depending on the model and the type of scan being run. Even in idle mode, ready to accept the next patient, the machine pulls around 13 to 15 kilowatts to keep the magnet cold and the electronics ready. In low-power overnight mode, consumption drops to about 8 to 9 kilowatts. Over a year, a single MRI scanner can use as much electricity as several homes combined, which is one reason imaging centers carefully schedule scans and manage machine downtime.