In this chapter we consider further the principle of generating contrast in medical images. Having already explored physical principles that we can use to generate contrast between body tissues in earlier chapters, in this chapter we meet ways in which we can enhance the contrast and thus tailor our medical imaging devices to particular applications. We consider ways to enhance the inherent contrast mechanisms available in different medical imaging techniques both through changes in the way the signals are acquired and through the use of externally introduced contrast agents. We will also consider how we can make our images quantitative in terms of the physical processes that have given rise to the contrast.