Grades 9–12 · Machines
How can an MRI scanner make pictures using magnets?
An MRI scanner can form an image because magnetic responses from tissue produce measurable signals. Hydrogen nuclei, abundant in water and fat, have magnetic moments. A strong static field creates a small net magnetization across their population; the large arrow represents this combined effect, rather than every nucleus pointing in the same direction.
A magnetic moment in the field can precess: its direction turns around the field axis. The precession frequency depends on field strength. A radiofrequency pulse near the appropriate resonance can transfer energy efficiently and tip the net magnetization away from its equilibrium direction. Resonance means that the driving frequency matches the relevant magnetic response.
After excitation, transverse magnetization precesses and changes the magnetic flux through a receiver coil, inducing an electrical signal. The signal evolves as magnetization returns toward equilibrium and different contributions lose phase alignment. Tissues differ in these behaviors, and the pulse sequence determines which differences influence the recorded measurements.
One combined signal would not reveal where its contributions originated. Gradient coils add controlled spatial changes to the magnetic field. These make precession frequency or accumulated phase depend on position, providing labels within the mixed signal. Repeating measurements with selected gradients gives a computer enough information to reconstruct spatial structure rather than simply plotting signal strength as a picture.
The large arrow inside Tissue represents net magnetization in the Static field. The precessing-signal route leads to a receiver coil and reconstruction; the field-gradient label identifies the additional position encoding needed to form an image.
In the diagramMRI measures magnetic resonance signalsLarge arrow: net magnetization of an ensembleTissue inside magnetic fieldStatic fieldPrecessing signal → receiver coil → reconstructionField gradients encode position • schematic only
MRI combines magnetic fields and radiofrequency energy rather than ionizing x-rays. Its reconstructed images depend on the measurement sequence and are not direct photographs of individual nuclei. The drawing isolates net magnetization, excitation, encoded reception and reconstruction without calculating tissue brightness or interpreting a clinical scan: detecting a response and locating its source are connected but distinct steps.
About this explanation
Starting knowledge: Recognize magnetic fields, resonance and radiofrequency signals.
Sources
- National Institute of Biomedical Imaging and Bioengineering — Magnetic Resonance Imaging (MRI)
- RadiologyInfo · ACR and RSNA — MRI Safety: What is MRI and how does it work?
- National Research Council, NCBI Bookshelf — Magnetic Resonance Imaging — Mathematics and Physics of Emerging Biomedical Imaging