Thermally-triggered ‘off–on–off’ response of gadolinium-hydrogel–lipid hybrid nanoparticles defines a customizable temperature window for non-invasive magnetic resonance imaging thermometry

For effective and safe thermotherapy, real-time, accurate, three-dimensional tissue thermometry is required. Magnetic resonance imaging (MRI)-based thermometry in combination with current temperature responsive contrast agents only provides an ‘off–on’ signal at a certain temperature, not indicating temperature increases beyond the desired therapeutic levels. To overcome this limitation, a novel Gd-chelated hydrogel–lipid hybrid nanoparticle (HLN) formulation was developed that provides an ‘off–on–off’ signal defining a thermometric window for MR thermometry. Novel thermally responsive poly(N-isopropylacrylamide-co-acrylamide) (NIPAM–co-AM) hydrogel nanoparticles (< 15 nm) with bisallylamidodiethylenetriaminetriacetic acid, a novel crosslinker with Gd3+ chelation functionality, were synthesized. The Gd-hydrogel nanoparticles were encapsulated in a solid lipid nanoparticle matrix that prevented T1-weighted contrast signal enhancement. Melting of the matrix lipid freed the Gd-hydrogel nanoparticles into the bulk water and an ‘off–on’ contrast signal enhancement occurred. As the temperature was further increased to temperatures greater than, the volume phase transition temperature of the hydrogel nanoparticles, they collapsed and provided an ‘on-off’ signal diminution. Both the ‘off–on’ and the ‘on–off’ transition temperature could be tailored by changing the lipid matrix and altering the NIPAM/AM ratio in the hydrogel, respectively. This allowed MRI thermometry of different temperature windows using the Gd-HLN system.

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