How Does an MRI Scanner Work? Listening to Hydrogen in a Magnetic Field
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A person lies inside a magnet thousands of times stronger than the earth's field, a radio pulse is transmitted, and the body transmits a faint radio signal back. From that signal a machine reconstructs a picture of soft tissue that no X-ray can produce, without using ionising radiation at all. The whole thing rests on the behaviour of hydrogen nuclei, which the body is full of because it is full of water.
The physics in order
The process has four stages and each does something specific:
- •Alignment: a hydrogen nucleus is a single proton with a property called spin, which behaves like a tiny magnet, and inside a strong field a very slight excess of them line up with it, producing a small net magnetisation along the field
- •Excitation: a radio pulse at exactly the right frequency, set by the field strength, tips that magnetisation sideways and makes the protons precess together in step
- •Relaxation: once the pulse stops, the protons return to alignment and lose their synchrony, and while doing so they induce a faint radio signal in a receiving coil
- •Encoding: additional gradient coils make the magnetic field vary slightly across the body, so the frequency and phase of the signal differ by position, which is what allows the machine to work out where each contribution came from
- •Reconstruction: the collected signals form a set of spatial frequency data, and a mathematical transform converts it into an image
- •The loud knocking noise is the gradient coils being switched rapidly, which makes them flex against the main magnetic field
Why soft tissue shows up
The contrast comes from the fact that protons in different tissues return to equilibrium at different rates, and the machine can emphasise different aspects of that. Two independent processes are involved: the recovery of magnetisation along the main field, described by a time constant called T1, and the loss of synchrony among the protons, described by T2. Fat, water, muscle, grey matter and white matter have different values of both, and by adjusting when the signal is measured relative to the pulses, the operator chooses which difference dominates the picture. That is why the same anatomy looks completely different on a T1-weighted and a T2-weighted scan, with fluid appearing dark on one and bright on the other. Further sequences target other properties: diffusion weighting detects how freely water molecules move, which changes within minutes of a stroke and makes it the standard urgent brain scan; contrast agents containing gadolinium shorten T1 locally and highlight tissue with a disrupted blood supply or an abnormal blood vessel network.
What it does that other imaging cannot
The comparison with other methods is mostly about what each is sensitive to. X-rays and computed tomography measure how much radiation is absorbed, which depends on density and atomic number, so bone and metal show brilliantly and soft tissues differ only slightly, and both deliver ionising radiation. Ultrasound measures reflected sound, is cheap, portable and real-time, and cannot see through bone or air. Nuclear medicine and positron emission tomography image function by tracking an injected radioactive tracer, at the cost of a radiation dose and poor anatomical detail. Magnetic resonance produces exceptional soft tissue contrast with no ionising radiation, which is why it dominates imaging of the brain, spinal cord, joints, muscles and many tumours. Functional magnetic resonance imaging exploits the fact that oxygenated and deoxygenated haemoglobin behave differently in a magnetic field, so local changes in blood oxygenation can be tracked, which is the basis of most of modern cognitive neuroscience and measures blood flow as a proxy for neural activity rather than activity itself.
The costs and the hazards
The trade-offs are real. Scans take minutes rather than seconds, so motion blurs them and children and distressed patients may need sedation. The bore is narrow and enclosed, and a significant minority of people cannot tolerate it. The machines are extremely expensive to buy, install and run, requiring liquid helium to keep the superconducting magnet cold, and a global helium supply that is genuinely finite. The safety hazard that dominates practice is the magnet itself, which is always on, even when the machine is not scanning, and which will accelerate any ferromagnetic object into the bore with lethal force, a mechanism that has killed people who brought oxygen cylinders, chairs and tools into the room. Implanted metal is a serious screening question, with many devices now designed to be conditionally safe under specified conditions. Radiofrequency energy deposits heat, which is limited by regulation, and gadolinium contrast is avoided in severe kidney disease because of a rare and serious complication.
The takeaway
Magnetic resonance aligns hydrogen nuclei in a strong field, tips them with a radio pulse, and listens to the faint signal they emit while returning to alignment, using gradient fields to encode position. Contrast comes from tissues relaxing at different rates, which is why the same anatomy looks different on differently weighted scans. It gives soft tissue detail no X-ray can match with no ionising radiation, at the cost of time, expense and a magnet that is permanently on and will pull steel objects into the bore.