For as long as I can remember, I’ve felt a compulsion to harmonize with machines. I mean this in a literal sense. Lawnmowers, microwave ovens, humming air conditioners… they all produce a steady drone, like the base note of a bagpipe, that presents a key and begs for melody. The pitch and timbre of the drone restrict the tune, and so it’s an amusing subconscious game, to find the song to fit the sound.
Lawnmowers were perhaps my best source of drones, for many years. When someone is mowing a lawn, they’re likely to keep at it for a long time, and the pitch is surprisingly constant. Doppler effects are minimal. Lawnmowers, though, have recently been surpassed.
My new best accompanist is an MRI machine. MRI machines build up images by successively acquiring data representative of each location in the imaging volume. Each acquisition requires a new position encoding, which is performed by the “gradient coils”, three oddly shaped electromagnets that live inside the MRI bore. Different image types require these coils to be pulsed in different ways.
A pulsed electromagnet, combined with a strong permanent magnet, is exactly the design of any speaker, like the one in a telephone or a movie theater. Pulsing the electromagnet inside the main static magnetic field produces a force, which creates a vibration. The resulting sound in an MRI machine is tremendously loud, because the magnetic fields and electric currents are both designed to be as large as possible. In MRI, stronger magnets mean faster images, and speed is critical in MRI.
Patients being scanned typically wear earplugs to prevent hearing damage, and the scanner room is mostly soundproofed, perhaps as a convenient side effect of the electromagnetic shielding. However, my experiments require me to run motors outside the room, and so I have been running long scans with the door open.
My experiment on Friday was using GE’s FIESTA sequence, a gradient-echo single-shot free precession sequence. It makes an image about once every two seconds, providing both a drone and a metronome.
It makes three hours alone with a scanner a lot more palatable.