Today I had my first ever diffusion-weighted MRI scan, Some people dislike the noises from MRI machines, but all the beeping and buzzing sounded quite hypnotic to me, like a kind of minimalist ambient techno music. Maybe the devices are actually silent, and the manufacturers secretly hired Brian Eno to compose a soundtrack that would make them seem more mysterious and impressive.

But MRI is impressive enough on its own. The hand-wavy explanation is that nuclear spins lined up in a magnetic field are made to precess by applying an oscillating, orthogonal magnetic field, and then as the excited spins relax they emit radio waves at a frequency that’s highly sensitive to the magnetic field at the nucleus, which includes contributions from its particular chemical environment.

The details are quite subtle; the spins do not relax by spontaneous emission, but from interactions between all the nuclei.

[See https://en.wikipedia.org/wiki/Spin–lattice_relaxation and https://en.wikipedia.org/wiki/Spin–spin_relaxation ]

Gradients in the imposed magnetic field make it possible to identify where in the body the signal originates. But if a water molecule moves through this gradient while its spin is relaxing, that modifies the signal. Diffusion-weighted MRI exploits this, to discriminate between, say, water inside a cell and in various extracellular environments.

The fanciest version, diffusion tensor imaging, can determine the *direction* water is diffusing along neurons, mapping connectivity between brain regions!

But the purpose of my scan was to measure the “cellularity” of my bone marrow, to see where, and to what degree, the intracellular vs extracellular water ratio has been elevated by a pesky clone of over-reproducing plasma cells.