MODULATION TRANSFER FUNCTIONS OF THE IONIZATION CHAMBER

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Abstract
The blur in the measurement system is analyzed by bringing into the dosimetric system the concept of Modulation Transfer Function (MTF) which has recently been introduced into the radiographic system with fruitful results in the analysis of images. In this paper, the dosimetry, in air and in depth, of 190 kvp. x rays and Co60 γ rays is treated, respectively. First, the values of calculations and experiments MTF of an ionization chamber are obtained. Then the MTFs of the dose distribution of the collimated field measured by the ionization chamber are obtained. After some treatments are given to these MTFs in the spatial frequency domain, the real dose distribution of the field is evaluated by inverse Fourier transforms. The results of experiments obtained are discussed in the light of the assumptions for calculations. Thus by the method of Fourier analysis, it has become possible to know the real dose distribution by correcting the errors of blurring caused by the chamber size, although this has long been regarded as difficult. Therefore, the application of Fourier analysis is tried not only to obtain the expression of MTF of the ionization chamber, but, to go a step farther, and to also measure the real dose distribution.

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