The aim of image restoration is to correct imperfections introduced by the imaging system. Knowledge of the point spread function (PSF) of an imaging system is useful because of the potential for removing blurring by way of an inverse convolution operation, or deconvolution, between the measured image and the PSF. The Fourier transform of the PSF is called the modulation transfer function (MTF). The MTF is used in the deconvolution process, but it is also useful to be aware of this function for another reason. The characteristic blurring function is called the point spread function. The concept of the modulation transfer function, a plot used to indicate the degree of blurring of an imaging system in terms of the spatial resolution, is introduced. Geometrical distortion is possible in all imaging modalities, which is why clinical imaging equipment undergoes regular quality assurance testing, so that the users can be sure that the system is performing within its prescribed limits.