Recently there has been a great increase in the uses of radioisotopes in medicine for the detection of lesions in the internal organs of the body. Many suggestions have been made concerning different methods of characterisation of the imaging systems used, and their analysis, design and intercomparison. Modulation transfer function characterisation of such systems is a powerful, efficient technique for these purposes. In Chapter one the basic concepts of sensitivity, point spread function (PSF) , line spread function (LSF) and modulation transfer function are introduced. In Chapter two the MTSs for different types of collimators assuming some ideal PSFs have been calculated. In Chapter three an experiment is described to determine the MTF as a function of spatial frequency for an imaging device based on scattering of highly collimated γ-rays from 60Co. It is found that one of the theoretical forms is a close approximation to the observed spread function. It is shown that it is not worthwhile to construct collimators with square shaped holes. Circular holes perform with no perceptible loss of image quality.