MRI

Nobel Prize 2003

Medicine — diagnostic imaging without ionising radiation

A scanning technique that builds images of the inside of the body from the behaviour of hydrogen nuclei in water and fat. The patient lies in a powerful magnet, typically 1.5 or 3 tesla; radio pulses tip the protons out of alignment, and as they relax they emit a faint radio signal whose position is encoded by superimposed field gradients. Paul Lauterbur supplied the gradient idea in 1973 and Peter Mansfield the fast mathematics and echo-planar technique, and the two shared the 2003 Nobel Prize in Physiology or Medicine. Unlike a CT scan it uses no ionising radiation.

How to spot them

▸Built on nuclear magnetic resonance, but the word 'nuclear' was dropped to reassure patients
▸Images hydrogen nuclei in water and fat; field strengths measured in tesla
▸Lauterbur and Mansfield, Nobel Prize in Physiology or Medicine 2003
▸T1- and T2-weighted images; gadolinium as the contrast agent
▸No ionising radiation, but metal implants are a serious hazard

Key facts

Paul Lauterbur's magnetic field gradients of 1973, which he called zeugmatography (1973)

The insight that turned a chemist's spectrometer into a scanner

Peter Mansfield's echo-planar technique, developed at Nottingham

It cut acquisition from hours to a fraction of a second

The word 'nuclear' quietly dropped from the name, to avoid alarming patients

The underlying physics is still nuclear magnetic resonance

Hydrogen nuclei tipped by radio pulses in a field of 1.5 or 3 tesla

Protons in water and fat are what is actually being imaged

T1 and T2 relaxation times, which give the tissue contrast

Longitudinal and transverse relaxation; the weighting is chosen by the operator

John Mallard's team in Aberdeen, who scanned the first patient in 1980 (1980)

The Aberdeen Mark 1, the first clinically useful whole-body machine

The BOLD signal, and Seiji Ogawa's functional variant of 1990 (1990)

Blood-oxygen-level-dependent contrast, which made brain mapping possible

Why a pacemaker, a cochlear implant or a metal splinter can rule the scan out

The magnet is always on and will turn ferromagnetic objects into projectiles

Key ideas

Position is encoded by making the magnetic field vary across the body, so that protons in different places resonate at slightly different frequencies — that gradient trick is the whole basis of imaging.
Contrast comes from how quickly protons in different tissues return to equilibrium, which is why the technique is so much better than X-rays at distinguishing one kind of soft tissue from another.

Quick-fire facts

•Derived from nuclear magnetic resonance: Isidor Rabi won the 1944 Nobel Prize in Physics for the resonance method, and Felix Bloch and Edward Purcell the 1952 prize
•Paul Lauterbur published the gradient method in Nature in 1973, naming it zeugmatography
•Peter Mansfield, at the University of Nottingham, developed the mathematics and echo-planar imaging, and was knighted in 1993
•Lauterbur and Mansfield shared the 2003 Nobel Prize in Physiology or Medicine
•Raymond Damadian, who showed in 1971 that tumours have different relaxation times and built an early scanner, took out newspaper advertisements protesting his exclusion from the prize
•John Mallard's group at the University of Aberdeen obtained the first clinically useful whole-body patient image in 1980
•Clinical scanners typically operate at 1.5 or 3 tesla; research machines reach 7 tesla and above
•The technique images hydrogen nuclei, overwhelmingly those in water and fat
•Image contrast depends on T1 (spin–lattice) and T2 (spin–spin) relaxation times
•Gadolinium chelates are the standard contrast agents
•Functional imaging uses the BOLD effect, described by Seiji Ogawa in 1990, to track changes in blood oxygenation
•It uses no ionising radiation, but the loud knocking noise comes from the gradient coils vibrating

Drill it — 9 cards

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Sources

The Nobel Foundation — The Nobel Prize in Physiology or Medicine 2003
NHS — MRI scan