Magnetic Resonance Imaging

Magnetic resonance imaging (MRI) forms cross-sectional images from the nuclear magnetic resonance signal of hydrogen nuclei in the body. Placed in a strong magnetic field and excited by radiofrequency pulses, protons emit a signal whose strength depends on proton density and on tissue relaxation properties; spatial encoding with magnetic field gradients turns this signal into an image. MRI gives excellent soft-tissue contrast without ionising radiation.

Definition

Magnetic resonance imaging is a tomographic technique that maps the spatially encoded nuclear magnetic resonance signal of tissue hydrogen nuclei, with contrast governed primarily by proton density and the T1 and T2 relaxation times.

Scope

The topic covers the physical basis of the magnetic resonance signal, the roles of proton density and the T1 and T2 relaxation times in generating tissue contrast, the use of field gradients for spatial encoding, and the way different pulse sequences weight an image. It is a reference on how MRI depicts anatomy, not clinical guidance.

Core questions

• How does the nuclear magnetic resonance signal of protons arise in a magnetic field?
• How do proton density and the T1 and T2 relaxation times generate tissue contrast?
• How do magnetic field gradients encode spatial position into the signal?
• How do pulse sequences determine whether an image is T1- or T2-weighted?

Key concepts

• Nuclear magnetic resonance of hydrogen nuclei
• Proton density
• T1 (longitudinal) relaxation
• T2 (transverse) relaxation
• Magnetic field gradients and spatial encoding
• Pulse sequences and image weighting
• Non-ionising radiation

Mechanisms

When the body is placed in a strong static magnetic field, hydrogen nuclei align with the field and can be tipped by a radiofrequency pulse; as they relax, they emit a radiofrequency signal. The signal amplitude reflects local proton density, while the rates of recovery (T1, longitudinal relaxation) and decay (T2, transverse relaxation) differ between tissues and provide the dominant source of contrast (Pykett et al., 1982). Magnetic field gradients superimposed on the main field make the resonant frequency and phase depend on position, which allows the signal to be spatially encoded and reconstructed into an image (Lauterbur, 1973). By varying pulse timing, sequences can be made T1-weighted, T2-weighted, or proton-density-weighted, emphasising different tissue properties. The detailed physics is covered in standard references (Bushberg et al., 2012).

Clinical relevance

MRI provides superior soft-tissue contrast for displaying neural, musculoskeletal, and visceral anatomy without ionising radiation, and the relationship between sequence weighting and tissue appearance is fundamental to reading these images (Pykett et al., 1982). This entry describes how MRI depicts anatomy and is not a basis for individual diagnostic or treatment decisions.

History

MRI grew from the nuclear magnetic resonance spectroscopy of the mid-twentieth century. In 1973 Paul Lauterbur showed that magnetic field gradients could spatially encode the NMR signal to form images, and Peter Mansfield contributed methods for fast spatial encoding and reconstruction; the two shared the 2003 Nobel Prize in Physiology or Medicine. Early clinical principles were consolidated in the following decade (Pykett et al., 1982), after which higher field strengths and faster sequences progressively expanded the technique's anatomical applications.

Key figures

• Paul Lauterbur
• Peter Mansfield

Seminal works

• lauterbur-1973
• pykett-1982

Frequently asked questions

Why does MRI not use ionising radiation?

MRI generates its signal from hydrogen nuclei responding to a strong magnetic field and radiofrequency pulses rather than from X-rays, so it does not expose the patient to ionising radiation.

What determines whether an image is T1- or T2-weighted?

The timing of the pulse sequence determines which relaxation property dominates the contrast: appropriate parameters make the image emphasise T1 (longitudinal) or T2 (transverse) relaxation, changing how tissues appear.