Ring artifact in all fields.

First I would clean the Mylar ring and the detector cover to make sure there is no contrast or other objects on it. Then run your air cals make sure nothing it in the field. You should be able to clean the detector cover without opening the gantry. Just lift the Mylar ring out of the way. If that doesn't take care of it you may have a bad QV2 or ADC card in the detector. If that's the case you can use the DCA program in engineering to determine the channel that's causing the ring