Variable aperture and actuator assemblies are provided for a radiation detector housing in an imaging system. The variable aperture assembly includes a ring mounted over the housing's window, a plate having a first aperture disposed over the window, an aperture blade operatively coupled to the ring so the blade moves laterally relative to the first aperture, and an aperture drive mechanism having a body and an actuator coupling member extending at an angle from the body. The drive mechanism is adapted to drive the blade laterally away from the first aperture when the member is moved in a first lateral direction, and over the first aperture to define a second aperture disposed over the window when the member is moved in a second lateral direction. The actuator assembly has an actuator and an actuator arm adapted to engage the actuator coupling member to control the lateral movement of the member.