The morning I wrote this I had a brain MRI, and lying in the tube I heard rhythms, intervals, a whole symphony. So we went digging into how scanners actually make sound, and built the piece out of what we found. It's a string quartet that walks through a routine brain scan in the order the scans really happen.
Inside the magnet, coils of wire called gradient coils get switched on and off thousands of times a second to locate signal in space. Each switch makes the coils jolt against the huge magnetic field, so the whole tube acts like a loudspeaker. When the switching is fast (every few milliseconds) you hear a pitch. When the slow repeat of a scan (a few times a second or slower) is slow enough to count, you hear rhythm. Every scan type has its own timing, so every scan has its own song, and each one starts and stops instantly with no fade. That hard on-and-off is the first thing the quartet copies.
Even between scans, the magnet's helium cooling pump keeps going with a soft thump about once a second, around 72 beats per minute, like a slow heartbeat. In the piece that's the cello's quiet pizzicato on E, under every gap, until the very end.
Localizer: a few seconds of low buzzing, one burst per direction. Here it's three low tremolo buzzes. The step between them is the first fourth in the piece.
T1: steady, even, mechanical knocking, roughly 100 to 170 knocks a minute. Here it's the wood of the bow tapping the strings, dead even.
T2 and FLAIR: a chugging, galloping rattle, bursts of fast notes four or five times a second. FLAIR is the same idea, softer, with more space. Here they're scratchy near-the-bridge sixteenths with snapped pizzicato hits.
Diffusion: the iconic MRI sound and the loudest, sometimes over 110 decibels. It's a high metallic beep chopped up ten to thirteen times a second. Measured scanners put that beep around 880 to 920 hertz, almost exactly the A an octave and a half above middle C. Here the violins play it as fierce sixteenths, with the second violin tuned a shade sharp so the two beat against each other like metal.
SWI: several minutes of low, unchanging buzz. Here it's a low tremolo drone that never moves, then cuts off dead.
I remembered a clear perfect fourth in the scanner, like "Here Comes the Bride." No paper we found documents a fourth, but scanners shift pitch from one scan to the next, and the different coils ring at different notes, so a fourth is well within what a real scan can do. The piece puts it where the beep lives: E up to A, in the Bride's long, dotted-short, long rhythm.
The tune is the patient breathing: in on the rise, hold on the long note, out on the fall. It's heard in the quiet gaps, sung through the rattle, and floating over the drone. Breath and machine share the same fourth, and the pump only settles on A when the scanning stops.