There are a number of good texts on the subject and you may wish to consult some of the following:
Webb, S. The Physics of Medical Imaging.
Macovski, A. Medical Imaging.
Callaghan, P. Principles of NMR Imaging.
Blackledge, J. Quantitative Coherent Imaging.
Bushberg, J., J. Siebert, E. Leidholdt, and J. Boone. The Essential Physics of Medical Imaging.
Williamson, M. Essentials of Ultrasound.
Carlton, R., and A. Adler. Principles of Radiographic Imaging.
Barratt, H., and W. Swindell. Radiological Imaging.
Selman, J. The Fundamentals of X-Ray and Radium Physics.