Microgel-Based Thermosensitive MRI Contrast Agent

Abstract

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 <i>N</i>-isopropylacrylamide (NIPAM) or <i>N</i>-isopropylmethacrylamide (NIPMAM) monomers and <i>N</i>,<i>N</i>′-methylenebis­(acrylamide) (MBA) cross-linkers. The volume of the NIPAM-incorporated microgel (<b>M-1</b>) decreased sharply around its lower critical solution temperature (LCST, 29–33 °C), whereas the volume of the NIPMAM-incorporated microgel (<b>M-2</b>) decreased gradually. MR longitudinal relaxivity (<i>r</i>₁) enhancement (44%) was obtained for <b>M-1</b>, while the corresponding change for <b>M-2</b> was much smaller. <b>M-1</b> was further optimized in synthesis without an MBA cross-linker to obtain <b>M-3</b> which showed a 67% increase in <i>r</i>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 <i>r</i>₁. This novel thermally sensitive microgel could potentially be applied to monitor small temperature changes using MRI methods