The present invention is directed to detectable compositions and methods for making and using such compositions. Detectable compositions comprise detectable constructs comprising a detectable agent, such as MRI contrast agents, comprising a paramagnetic metal ion including, but not limited to, Gd(III), Fe(III), Mn(II), Yt(III), Cr(III) and Dy(III). Due to the actions of a specific bioactivity in vivo or in vitro, the detectable construct is altered in some manner so that the detectable agent is detected. The present invention provides diagnostic imaging agents such as for MRI and optical imaging, which are used for sensitive detection of a specific bioactivity within a tissue. In the presence of the bioactivity, or because of actions on the detectable composition, there is a change that increases the signal (or image) contrast between tissues which contain the targeted bioactivity and those which do not, which thus reflects the presence of the targeted bioactivity. The present invention comprises micellar compositions and methods of making and using such micellar compositions for delivery of active agents, such as pharmaceuticals, imaging agents, immunogens or nucleic acids, to specific or target sites in a body. Micellar compositions comprise micelles comprising amphiphilic polymeric molecules comprising pendant side chains capable of bonding with cross-linking molecules. Cross-linking molecules function to stabilize the micelle and may also function to target the micelle to a particular site, provide a reaction region or be an active agent.