MTF means Modulation Transfer Function, and it means generally Modulation Function(Fourier Space) affecting image by AD Convert by Input device. This meaning is almost the same as CTF, but it is sometimes regarded that CTF means influence by input device before AD convert(e.g. Numerical aperture), and MTF means influence with AD convert.
For example, noise which is called quantum noise depending on physical process (Poisson Process) expresses influence by AD convert as MTF. This noise is originally White Noise(It has uniform intensity on whole spatial frequency.), but it has color(It has different intensity for each spatial frequency.) by influence of MTF.