Magnetic resonance imaging (MRI) is a measurement technique that produces spatial maps of nuclear magnetic resonance signals, most often from the hydrogen nuclei (protons) abundant in water and fat. An MRI scanner places a sample in a strong static magnetic field, excites the nuclei with radiofrequency pulses, and records the faint signal the nuclei emit as they return to equilibrium. This article describes the physics of how that signal is produced, localised and reconstructed. It is a methods explainer about the instrument and the measurement, not an account of how any individual image should be interpreted.
The starting point: nuclear spin in a magnetic field
Certain atomic nuclei, including the single proton of hydrogen-1, possess an intrinsic quantum property called spin, which gives them a small magnetic moment. In the absence of an external field these moments point in random directions and there is no net magnetisation. When the sample is placed inside the scanner’s strong static field, conventionally labelled B0 and measured in tesla, the moments adopt a slight preferential alignment with the field. The result is a small bulk magnetisation along the field direction.
Within that field the nuclei precess, wobbling like a spinning top, at a characteristic frequency. This is the Larmor frequency, and it is proportional to the field strength: stronger fields produce higher precession frequencies and, all else being equal, a larger signal. The proportionality constant is the gyromagnetic ratio, a fixed property of each nucleus. For hydrogen at clinical and research field strengths the Larmor frequency falls in the radiofrequency band, which is why radio waves are the natural tool for manipulating the spins.
Excitation: the radiofrequency pulse
To generate a measurable signal, the scanner transmits a radiofrequency (RF) pulse tuned precisely to the Larmor frequency. Because the pulse matches the precession frequency, energy is transferred efficiently to the spin system, a condition known as resonance. The pulse tips the bulk magnetisation away from the static-field axis and into the transverse plane, where its rotation induces a small oscillating voltage in a nearby receiver coil. That induced voltage is the raw MRI signal.
Once the pulse is switched off, the magnetisation begins to return to its equilibrium alignment. The way it returns carries information about the local molecular environment, and this is the basis of MRI contrast.
Relaxation: T1 and T2
Two largely independent relaxation processes govern the recovery. T1, the longitudinal or spin-lattice relaxation time, describes how quickly the magnetisation re-grows along the static-field axis as the spins release energy to their surroundings. T2, the transverse or spin-spin relaxation time, describes how quickly the rotating magnetisation loses coherence as individual spins drift out of phase with one another. A related quantity, T2*, includes additional dephasing caused by small imperfections in the field.
| Parameter | What it describes | Physical cause |
|---|---|---|
| T1 | Recovery of magnetisation along B0 | Energy exchange with the molecular lattice |
| T2 | Loss of phase coherence in the transverse plane | Spin-spin interactions |
| T2* | Faster transverse decay including field inhomogeneity | Local field variations plus spin-spin effects |
Different materials have different T1 and T2 values, so by choosing when to excite and when to measure, an experimenter weights the signal towards one relaxation property or another. This is what produces the visible distinction between tissues in a reconstructed image, and it is a property of the measurement parameters rather than an interpretation of the sample.
Spatial encoding: turning signal into an image
A plain RF excitation tells you the total resonance signal but nothing about where in the sample it came from. MRI solves this with gradient coils that superimpose small, controlled, spatially varying magnetic fields on top of B0. Because the Larmor frequency depends on field strength, a gradient makes the precession frequency vary with position. The scanner uses three orthogonal gradients in a carefully timed sequence: a slice-selection gradient applied during the RF pulse so that only one plane is excited, a frequency-encoding gradient that makes frequency map onto one in-plane axis, and a phase-encoding gradient that imprints position onto the signal phase along the other axis.
The signals collected under these gradients fill a raw data matrix known as k-space, which represents the spatial frequencies of the object rather than the picture itself. Applying an inverse Fourier transform to k-space converts those spatial frequencies into the familiar image. The reconstruction is a mathematical operation on measured data, and understanding it is essential to reporting a study reproducibly; our guide on reporting analytical methods reproducibly covers how acquisition parameters should be documented.
Why MRI sits alongside other measurement techniques
MRI is one of several physical techniques researchers use to probe matter without contact. It shares the pulse-and-detect logic of ultrasound, and like time-domain spectroscopy it relies on a Fourier transform to move between a measured signal and an interpretable representation. Standard vocabulary for describing such methods in the research record is maintained in the CASRAI dictionary, and the broader context of where measurement sits in a project appears across our research lifecycle coverage.
Frequently asked questions
Why does MRI use hydrogen nuclei?
Hydrogen is overwhelmingly the most abundant magnetically active nucleus in water- and fat-containing samples, and it has a relatively large gyromagnetic ratio, which together give the strongest signal. Other nuclei such as phosphorus-31 or carbon-13 can be imaged in specialised research spectroscopy, but they produce much weaker signals.
What does field strength change in the measurement?
A higher static field raises the Larmor frequency and increases the equilibrium magnetisation, which generally improves signal-to-noise ratio and allows finer spatial detail or faster acquisition. It also changes relaxation behaviour and engineering demands, so field strength is a key parameter to record when reporting a method.
What is k-space?
K-space is the raw data domain in which MRI signals are collected. Each point encodes a spatial frequency of the object, and the full image is obtained by an inverse Fourier transform of the completed k-space matrix. It is a representation of the measurement, not the picture itself.
Is MRI quantitative?
The underlying parameters, including T1, T2 and proton density, are physical quantities that can in principle be measured numerically. Quantitative MRI sequences aim to recover these values rather than weighted contrasts, which is why precise reporting of acquisition settings is essential for reproducibility, as discussed in our reproducibility coverage and the guidance for authors.
