Microgel-Based Thermosensitive MRI Contrast Agent

Monitoring subtle temperature changes noninvasively remains a challenge for magnetic resonance imaging (MRI). A temperature-sensitive contrast agent based on thermosensitive microgel is proposed and synthesized using a manganese tetra(3-vinylphenyl) porphyrin core reacting with N-isopropylacrylamide (NIPAM) or N-isopropylmethacrylamide (NIPMAM) monomers and N,N'-methylenebis(acrylamide) (MBA) cross-linkers. The volume of the NIPAM-incorporated microgel (M-1) decreased sharply around its lower critical solution temperature (LCST, 29-33 degrees C), whereas the volume of the NIPMAM-incorporated microgel (M-2) decreased gradually. MR longitudinal relaxivity (r(1)) enhancement (44%) was obtained for M-1, while the corresponding change for M-2 was much smaller. M-1 was further optimized in synthesis without an MBA cross-linker to obtain M-3 which showed a 67% increase in r(1) around its LCST. Our results suggested that the longitudinal relaxivity is strongly modulated by microgel volume change around the LCST, leading to a significant increase in r(1). This novel thermally sensitive microgel could potentially be applied to monitor small temperature changes using MRI methods.