My project’s first baby step succeeded today. I was able to hook up a motor to a phantom (a dummy object used to study MRI or ultrasound) and move it back and forth inside the bore of the magnet in order to mimic the motion of the patient due to breathing.
In the abstract, this should be beyond simple. Just attach a dowel to a disk attached to the driveshaft, like the rods visible on a steam locomotive. Unfortunately, there’s a catch: nothing ferrous or magnetizable can be allowed anywhere near the MRI, and no metal of any kind can stay inside the bore. A DC motor is even worse, since it actually contains fixed permanent magnets. If brought anywhere near the MRI, it will tear itself out of your hands, or free of its supports, and fly through the air into the middle of the magnet, possibly causing millions of dollars worth of damage or injury in the process. To be safe, therefore, we cannot allow a motor into the room.
The room has extensive electromagnetic shielding, and the only hole is a “waveguide”: a copper pipe a few inches in diameter through which cables and such may be routed. Therefore, it seemed logical enough to me to place the motor just outside this waveguide, and transmit the motion through a long flexible coupling. Specifically, I took one of our standard vinyl hoses, typically used to fill our fishtanks, and threaded about 15 feet of thin nylon rope through it. The hose is attached to the motor-mount with velcro, and the motor pulls on the string. You can’t push a string, so I set up a primitive counterweight on the far side to maintain tension. Since it’s inside the bore, the counterweight can’t be metal, so it’s a plastic bottle full of water.
In one of the magnets, the path from the waveguide into the bore of the magnet is a straight shot, but that magnet is rarely available. At the machine that is more often available, the path is rather tortuous, especially now that another research group has taken to storing enormous immovable electronic devices right in front of the waveguide. The path turned out to have so many bends that the friction of the string rubbing against the inside of the tube exceeded the strength of the motor.
Today we tried a different approach: leave the door open. Many MRI machines won’t run with the door open, but this one will. Running with the door open, and a heavy steel DC motor right outside, does inspire a certain amount of nervousness, but in fact our setup is perfectly safe. Safety standards require that the magnetic field be negligible outside of the magnet room. With the door open, the path from the motor to the bore is nearly straight, and so it was with great joy, but not surprise, that I watched my phantom move back and forth, like breathing.
We also sent off the National Instruments digitizer board for repair. Once it comes back, I’ll be using it to receive ultrasound data for the next stage of this experiment.
Things are moving.