Parameters affecting radiographic contrast

Kilovoltage, in most cases a variable readily controlled by the operator, and half-value layer have come to be regarded as factors defining image contrast. This has led to the assumption that comparable levels of image contrast may be obtained using X-ray units operated at the same kilovoltage and with the same half-value layer. To ascertain the validity of this view X-ray beams generated by seven X-ray units were compared with each other and with those from a Gendex Model 1000 operated at the same kilovoltage. The basis for comparison was the level of image contrast the X-ray beams were capable of producing after passage through selected thicknesses of aluminium absorber. While results showed no correlation between the nominal (stated) operating kilovoltage of any X-ray generator studied and the level of image contrast produced, a correlation (-0.98) was found between the measured half-value layer and image contrast when the units tested were compared with the Model 1000. This information should significantly affect the criteria used for the selection and/or operation of X-ray generators.