Physics of contrast mechanism and averaging effect of linear attenuation coefficients in a computerized transverse axial tomography CTAT) transmission scanner.

Detailed studies of the basic contrast mechanisms in computerized transverse axial tomography scanners have been carried out. Contrast is related to the effective atomic numbers and electron densities of materials and the resultant linear attenuation coefficients. We have therefore quantitatively evaluated various samples defined by these parameters. A multienergetic X-ray source causes resolution degrading problems arising from the averaging effect of the linear attenuation coefficients. The controversy regarding the use of fixed length water bath as a reference to compensate the spectral shift (hardening) effect of the multienergetic X-ray source is also analysed and reported. Computer simulations demonstrating the sensitivities of the linear attenuation coefficient measurements and errors, as functions of the energy spectrum, were made for representative cases. Simulation results indicate that by using a full water bath, artifacts stemming from the multienergetic X-ray souce can be significantly reduced. An alternative approach using a count rate equalizer, considered to be another way of reducing the wide dynamic range in count rate when a water bath is not used, is also studied and the results reported.