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

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Abstract
Detailed studies of the basic contrast mechanisms in computerized transverse axial tomography scanners were carried out. Contrast is related to the effective atomic numbers and electron densities of materials and the resultant linear attenuation coefficents. Various samples were quantitatively evaluated and defined by these parameters. A multienergetic X-ray source causes resolution degrading problems arising from the averaging effect of the lelinear attenuation coefficients. The controversy regarding the use of a fixed length water bath as a reference to compensate the spectral shift (hardening) effect of the multienergetic X-ray source is also analyzed 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, artifcats stemming from the multienergetic X-ray source 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.