Magnetic resonance imaging (MRI) is a noninvasive imaging technique that is widely used to diagnose diseases and guide therapies. MRI is often improved with respect to sensitivity through the use of chemicals known as contrast agents. The combination of MRI with temperature-responsive contrast agents has the potential to increase the ability to accurately map real-time temperatures. Lanthanide-based contrast agents for MRI have been encapsulated in macromolecular hydrogel systems or thermosensitive liposomes to increase contrast upon increasing temperature. However, there is a lack of small-molecule MRI thermometers that transition from dark to bright with increasing temperature. We drew inspiration from a previously reported redox-responsive fluorinated EuII-containing complex that exhibited a temperature-dependent increase in relaxivity. We hypothesized that replacing EuII with GdIII would isolate temperature control from redox activity if the coordination chemistry remained similar. In their solid-state, fluorinated complex possesses a cage-like structure that was formed by the fluorinated tetra-amide arms. However, methylated tetra-amide arms are extending away from the GdIII ion. Relaxivity of the fluorinated complex was remained constant at 1.9 mM–1 s–1 from 293 to 318 K at 9.4 T with an increase in innersphere relaxivity over the same range at both 1.4 and 9.4 T. Further, we observed a steady decrease in the relaxivity, both observed and innersphere, of the methylated complex with increasing temperature. The calculated water-exchange rate of fluorinated complex is an order of magnitude slower than the water-exchange rate of methylated analog over the entire temperature range that we studied. These studies are a critical step toward the design of temperature-sensitive contrast agents.

Magnetic resonance thermometry using a GdIII-based contrast agent

ACS Spring 2022

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