Bradford C D, Peppler W W, Waidelich J M
Department of Medical Physics, The University of Wisconsin, Madison 53706, USA.
Med Phys. 1999 Nov;26(11):2286-94. doi: 10.1118/1.598742.
The slit camera was analyzed in order to establish its utility and limitations as an MTF measurement tool for characterizing radiographic imaging systems. Commercial slit cameras are attractive for MTF measurements because the beveled edges significantly reduce their alignment sensitivity as compared to the conventional parallel jaw slit. Radiation passing through the beveled edges increases the effective width of the slit camera so that a correction based on the nominal slit width would leave residual error in the MTF measurement. Experimental and Monte Carlo simulated MTF measurements were made on a slit camera (10 microm nominal slit width) in order to estimate its sensitivity in alignment, quantify the error in MTF due to transmission through the beveled jaws, and provide a correction factor. The alignment tolerances of the slit camera were found to be about 12 times larger than for the parallel jaw slit at small HVLs (approximately 1.3 mm Al) of the incident beam and 9 times larger at higher HVLs (approximately 7 mm Al). The magnitude of the residual error in MTF was dependent on the quality of the incident spectrum. For incident spectra with high kVp and HVL (> or = 120 kVp, > or =5 mm Al HVL), transmission through the beveled edges produced errors in MTF up to 15% at 5 cycles/mm and 30% at 10 cycles/mm. By assuming a rectangular slit profile with an effective width based on the kVp, HVL, and filtration material of the incident beam, an MTF correction factor was determined. Application of this correction factor reduced the errors to less than 4% up to 10 cycles/mm. At low beam energies and spatial frequencies, the correction is less critical. Ease of alignment and greater availability make a commercial slit camera useful for MTF measurements. Accurate MTF measurements can be made if appropriate correction factors are applied.
