Effects of tomographic motion, slice thickness, and object thickness on film density.

The experiment used the computer-aided CommCat model IS 2000 tomographic machine (Imaging Sciences International, Roebling, N.J.). The objective of the experiment was to study the influence of tomographic tube motion, tomographic slice thickness, and object thickness on film density. The experiment was conducted by x-radiating an aluminum step-wedge placed along the x-axis. Exposures were made for different tube motions and for different slice thicknesses. In linear horizontal and linear vertical motions, an increased slice thickness decreased film density. Slice thickness had a stronger effect on film density when the object to be x-radiated was thinner. In circular, elliptical, spiral, and hypocycloidal motions, changes in slice thickness had no noticeable effect on film density because the manufacturer had programmed the machine to produce approximately similar exposure times by increasing the x-ray tube velocity thickness had a greater effect on film density for circular, elliptical, spiral, and hypocycloidal tube motions than for linear horizontal and linear vertical tube motions. Clinical observation showed that except for the linear vertical motion (motion that was oriented in the same direction as that of the tube travel) all other motions produced a zone of diffusion along the edges of the steps of the step-wedge. An increase in slice thickness had an effect on film density. Slice thickness had a noticeable effect on film density in linear but not in multidirectional tomography. Object thickness had a greater effect on film density in multidirectional than in linear tomography.