The invention relates to a system and the use, in a medical treatment or imaging apparatus, or a dental apparatus, having a rotary element 2 which when the apparatus is in use rotates about an axis. A contra-rotating flywheel 12 driven relative to and coaxially with the rotary element, compensates for torque reaction forces when the rotary element is accelerated or decelerated. The rotary element may be a C-arm apparatus. The rotary element and the flywheel may be driven by motors 16 and 14 respectively. Both the rotary element, which may be a drum or an annulus, and the flywheel may have brakes to decelerate the rotation.
