A stepper motor suitable for use in a medical imaging environment has (a) a cylindrical central gear having an external surface with circumferentially distributed and radially directed teeth, (b) a shaft for mounting the central gear such that it is constrained to move in rotational motion about its centerline, (c) a cylindrical hoop gear having a bore with an internal surface having circumferentially distributed and radially directed teeth, (d) level arm crank mechanisms for mounting the hoop gear such that it is constrained to move in translational-circular motion about the central gear's centerline, wherein this central gear is further configured to fit within the hoop gear's bore in such a manner that a plurality of the central gear and hoop gear teeth intermesh and cooperate so that the planetary movement of the hoop gear causes the central gear to rotate, and (e) piston mechanisms for applying a fluid pressure driven force to specified points on the hoop gear so as to cause its movement.