Effect of molecular architecture on the performance of F-19 NMR imaging agents

Traditional magnetic resonance medical imaging agents based on paramagnetic ions or particles have a number of critical limitations, namely image contrast is often limited at low concentrations and sensitivity of the agents becomes diminished at higher magnetic field strengths. In this chapter we describe the development of a new class of polymeric imaging agent which relies on direct detection of the NMR signal of fluorine nuclei and hence circumvents these potential problems. The imaging performance of the new materials relies on the maintenance of librational motion of the fluorinated segments, necessary for partial averaging of the otherwise strong dipole-dipole interactions and large chemical shift interactions of the 19F nuclei. The performance of partially-fluorinated block copolymers assembled into particles is described and related to the interactions of the polymer chains with the surrounding solvent medium. A second generation of materials based on a hyperbranched structure was developed based on the learning obtained from the earlier experiments and these demonstrated excellent in vitro and in vivo imaging performance.