MRI Physics for Radiographers: From Protons to Pictures

Every image you plan on the console rests on the same chain of physics — a proton tipped, a signal induced, a frequency decoded into a pixel. This is that chain, in the order it happens.

Why we image hydrogen

MRI could image several elements; in practice it images hydrogen almost exclusively. Three facts explain why. Hydrogen is abundant — the body is two-thirds water and rich in fat. Its nucleus is a single proton with a large magnetic moment, giving a strong, detectable signal. And because water content shifts with oedema, inflammation and tumour, hydrogen is a natural reporter of disease.

Spin, precession and the Larmor frequency

Every hydrogen proton behaves like a tiny bar magnet. Placed in the scanner's strong static field (B₀), the protons do not simply snap into line — they precess, wobbling around the field like a spinning top. The rate of that wobble is the most important number in MRI, the Larmor frequency (ω₀ = γB₀). For hydrogen γ is about 42.58 MHz per tesla, so at 1.5 T protons precess at roughly 64 MHz — in the radio band.

Resonance: tipping the magnetisation over

The net magnetisation sits along B₀, but a receiver coil can only detect magnetisation rotating across it. So a radiofrequency pulse (B₁), applied at exactly the Larmor frequency, tips it into the transverse plane. Because B₁ oscillates in step with the precessing protons — resonance — its small torque adds up cycle after cycle. How far the magnetisation rotates is the flip angle: a 90° pulse tips it fully over for maximum signal; a 180° pulse inverts it.

Relaxation: where contrast comes from

The moment the pulse switches off, the system returns to equilibrium by two independent routes — and these are the origin of soft-tissue contrast. T1 relaxation is longitudinal recovery: fat recovers quickly (short T1) and is bright on T1-weighted images. T2 relaxation is transverse decay: fluid dephases slowly (long T2) and is bright on T2 — which is why T2 lights up oedema and most pathology.

The operator's controls: TR and TE

T1 and T2 are fixed properties of tissue. What the radiographer controls is how strongly each shows up, through two timing parameters — TR sets T1 weighting, TE sets T2 weighting.

Spatial encoding and k-space

Resonance and relaxation give a signal, not a picture. Gradients locate it in space: slice selection, phase encoding and frequency encoding. The encoded signal fills k-space, a raw matrix whose centre carries image contrast and whose edges carry fine detail. A Fourier transform — which separates a mixed signal into its component frequencies, like an ear picking single notes out of a chord — converts filled k-space into the grayscale image on the console.

Common pulse sequences

Spin echo uses a 180° refocusing pulse for a true, high-quality T2 image but takes longer. Gradient echo skips it for speed at the cost of T2*-weighting and metal sensitivity. Diffusion-weighted imaging images the random motion of water — the emergency brain sequence, where an acute infarct restricts diffusion and stays bright while normal tissue fades.

Image quality and artifacts

Every MRI image is a compromise between signal-to-noise, contrast and spatial resolution, all against scan time. When an image isn't real anatomy, it's an artifact — and most are fixable at the console once you can name them.

The chain, end to end

Read it back and the machine stops being a black box. A proton precesses at the Larmor frequency; a resonant pulse tips the magnetisation over; the rotating magnetisation induces a signal; T1 and T2, shaped by TR and TE, give it contrast; gradients encode position and fill k-space; a Fourier transform turns k-space into the image. Every parameter on the console is a lever somewhere on that chain — and knowing which lever moves which outcome is the difference between running a protocol and understanding it.