A magnetic resonance imaging system has a electromagnet coil assembly for exciting nuclei of an animal placed within the assembly and a mechanism to receive magnetic resonance signals from the nuclei and reconstruct an image of the animal from those signals. A plethysmograph is provided to sense the peripheral arterial pulse of the animal and gate the MRI system. A sensor for the plethysmograph includes a fiber optic cable having a pair of optical fibers which extend into the magnet assembly. A probe is connected to one end of the cable and is adapted to be attached to a digit of the animal so that ends of the optical fibers are adjacent the animal's skin. The other end of the cable couples to a circuit which sends light through one of the optical fibers to the probe, and which receives light returned from the probe through the other optical fiber. The circuit produces an electrical signal corresponding to the intensity of light received from the other optical fiber.