This paper discribes a comparative study of measurement methods of modulation transfer functions ( MTF ) of X-ray CT-images. They are 1) a contrast method which uses the response to a square wave chart of Lucite in water phantom,2) an edge method which uses the response to an interface of Lucite and water, and 3) a wire method which uses the response to a thin metal wire inserted perpendicularly to a tomographic slice. An experiment with Pfizer AS&E CT model-0450 showed that three methods gave practically the same MTFs for the same scan conditions. But because the contrast method ( method 1) ) showed the smallest measurement error and did not need the additional software which was necessary for the other methods, it was the most practical for measuring MTF of X-ray CT-images.

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A random sampling (Monte Carlo) program which simulates the behaviour of photons in a germanium detector has been written in BASIC, using an interpreter type personal computer.
Since the d ifficulty for Monte Carlo calculations using a computer of this type lies in the processing speed, the author devised the program so as to reduce the processing time taking account of the characteristics of the interpreter. Especially high speed calculating techniques were adopted in the processing of coherent scatter and in the determination of the passage of photons.
Using this program, it has been able to trace the histories of ten thousands photons within two hours and a half. This processing speed was enough to check the functions of the experimental system and to determine the experimental conditions. Furthermore, these Monte Carlo calculations using a personal computer were very useful for planning of the preliminary experiments and to back up the experimental results.

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The Monte Carlo simulation has come to occupy an important position in various aspects. The authors investigated on the necessity to take into account the effects of the coherent and incoherent scattering in the simulation of photon scattering process in the matter. The present report deals with the related theories, data, computer algolism and some numerical results.

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