- Person
- Felix Bloch
- Lived
- 1905–1983
- Field
- Physics
- From
- Swiss-American
The story
Within months of returning to Stanford in the autumn of 1945, Felix Bloch and his Stanford colleagues William Hansen and Martin Packard succeeded in detecting radio signals from atomic nuclei, a method Bloch called nuclear induction. Their first sample was ordinary water, and the signal came from the protons in its hydrogen atoms. Detecting the signal was only half the challenge. Physicists also needed a way to describe how the nuclear magnetisation of a sample behaves over time once it is disturbed.
In a theoretical paper that accompanied the experimental demonstration in 1946, Bloch provided that description. His Bloch equations are a set of equations of motion for nuclear magnetisation. They describe how the tiny nuclear magnets precess in a magnetic field and how they relax back toward equilibrium afterwards.
The key to the equations was two parameters that Bloch introduced. The longitudinal relaxation time, T1, describes how quickly the magnetisation realigns with the main magnetic field. The transverse relaxation time, T2, describes how quickly the nuclei fall out of step with each other as they precess. Both depend on how each nucleus interacts with the atoms and molecules around it, so they reflect chemical bonding and biological processes in the material being studied. Between 1950 and 1957 Bloch and R. K. Wangsness worked out a microscopic explanation of why these two parameters behave as they do.
Decades later Paul Lauterbur and others developed a way to turn Bloch's technique into pictures. By applying a magnetic field that varies across space, they linked each resonance frequency to a position, so the signals could be mapped into an image. The result was magnetic resonance imaging, or MRI. Writing in 1994, Robert Hofstadter judged it probably the greatest advance in medical imaging since Röntgen discovered X-rays in 1895.
Bloch's T1 and T2 are much of what gives MRI its power. Because relaxation times differ from one kind of tissue to another, and change with physiological and metabolic processes, scanners can be set to bring out contrasts that X-rays cannot show. Radiologists speak of T1-weighted and T2-weighted images, and the Bloch equations remain the starting point for understanding and designing scanning methods.
Why it matters
MRI lets doctors see soft tissue inside the body in fine detail without ionising radiation, aiding the diagnosis of cancers, strokes, injuries and many other conditions.
About Felix Bloch
Swiss-born physicist who laid the quantum foundations of solid-state physics, discovered nuclear magnetic resonance – the basis of MRI – and became CERN’s first director-general.
More about Felix Bloch
- Felix Bloch helped explain how particles slow in matterIn Copenhagen in the early 1930s, Felix Bloch refined Hans Bethe’s theory of how charged particles lose energy in matter, work remembered in the Bethe–Bloch formula.
- Felix Bloch was CERN's first director-generalIn 1954 Felix Bloch became the first director-general of CERN, the new European laboratory for particle physics, as construction began on its site at Meyrin near Geneva.
- Felix Bloch measured the neutron's magnetismIn 1939, working with Luis Alvarez at the Berkeley cyclotron, Felix Bloch measured the neutron's magnetic moment to about one percent accuracy, a theorist turned experimenter.
- Felix Bloch's method became chemistry's key toolChemical shift experiments in Felix Bloch's own laboratory helped turn nuclear magnetic resonance into the most important spectroscopic tool in chemistry.
- Felix Bloch discovered how to listen to atomic nucleiIn 1946 Felix Bloch, William Hansen and Martin Packard at Stanford developed nuclear induction, one of the two founding methods of nuclear magnetic resonance.
- Felix Bloch described magnetic domain walls and spin wavesFelix Bloch described the boundaries between magnetic domains, now called Bloch walls, and proposed spin waves, partly in a paper written in hospital after a climbing accident.
- Felix Bloch explained how electrons move through metalsIn his 1928 thesis as Werner Heisenberg's first graduate student, Felix Bloch showed how electrons travel as waves through a crystal, founding the quantum theory of solids.
Sources
- Felix Bloch 1905–1983: A Biographical Memoir by Robert Hofstadter National Academy of Sciences
- Felix Bloch Wikipedia
- Bloch equations Wikipedia
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