Using haptic interfaces in combination with functional magnetic resonance imaging (fMRI) could lead to important insights into the brain mechanisms of human motor control and related dysfunctions. However, in addition to the usual requirements for haptic interfaces (e.g. smooth force control, back-drivability, low friction and inertia) these devices must also be MR safe and MR compatible. Previous MR-compatible actuation methods for force-feedback present drawbacks with respect to conventional haptic interfaces. Here, we present a novel MR-compatible actuator designed especially for impedance control and to meet the requirements for haptic interfaces. It consists of an ultrasonic motor controlled in speed combined with an electrorheological fluid brake which modulates the output torque over a differential gear. The entire system is integrated into a compact housing with an encoder at the output.