1. A method for scanning a bag to determine if the bag poses a threat, the method comprising: performing a CT scan of the bag to produce CT scan data; processing the CT scan data to obtain lineogram data; determining measured values for x-ray attenuation and size of a first object in the bag based on the lineogram data; comparing the measured values to predetermined values; and if the measured value is below the predetermined value for at least one of x-ray attenuation and size then determining that the object does not pose a threat and clearing the bag.