relationship between the recorded tube input value and the measured reference value is not linear that is the value of q changes with the change of x ray tube current therefore q c is a tube input dependent function the function can be determined experimentally or theoretically for example experiments have shown that the following function holds for a wide range of x ray tube current where q 0 is the conversion factor q c at a particular known x ray tube current value e g q 0 8 5 s is a scaling parameter e g s 1 0 u00d7 10 u2212 5 and c 0 is a tube input reference value e g c 0 9915 alternatively s can be made a function of c 0 it should be noted that the conversion factor q also changes with the das sampling rate this simple linear relationship will not be discussed here note that here the tube input value is in some measurement unit and not the x ray tube current milli amp the measurement unit can be converted to the x ray tube current in ma by a simple scaling next at step 304 the average measured flux or average reference channel reading for both the left side and the right side is calculated the average left reference channel reading r l t may be expressed as r l u2061 t 1 m u2062 u2211 i 1.