Measurement And Optimalization Of The Mtf's Of The Microradiographic Method And Its Subsystems

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Abstract
Contactmicroradiography produces an X-ray absorbance image of mineralized tissue slices. We use it to calculate the mineral content in tissue from the absorbance data of thin tooth sections as a function of depth in the tissue. This paper gives a complete analysis of the loss of detail caused by the microradiographic subsystems, describing this loss with the Modulation Transfer Function (MTF). Images (CuKaradiation) of thin test objects with a sharp edge were made on holographic film; an aluminium stepwedge was used to calibrate the film. Densitometric tracings of the images were fed into a microcomputer. The dose-density relation of the film was approximated by a 3rd degree polynomial using the stepwedge data. The resulting curve was used to convert the densitometric data of the slice into X-ray absorbance data. To filter out the film grain noise, 20 microradiograms were made of the same object and their edge scans were averaged. The resulting Edge Spread Function (ESF) was transformed into the MTF using standard techniques. Thus the MTF of the total method was obtained, using a 100 pm thick Al slab as object. The combined MTF of the densitometer together with the film was found with a 27 pm thick Cu slab as object, closely pressed to the film. The densitometer MTF was obtained using a knife edge as object in the densitometer. The results show that the total loss of spatial detail depends on the contributions of the X-ray projection geometry (projection subsystem), the film emulsion (film subsystem) and densitometer window width (densitometer subsystem). The MTF's of the individual subsystems were calculated from the measured combined MTF's assuming that the imaging subsystems are linear. After optimalization of the MTF's of the subsystems the maximum transmitted spatial frequency of the method was found to be 330 lines per mm.

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