Highly Sensitive and High-Throughput Magnetic Resonance Thermometry of Fluids Using Superparamagnetic Nanoparticles

Abstract

Magnetic resonance imaging (MRI) enables noninvasive three-dimensional thermometry, which has potential applications in biological tissues and engineering systems. In biological tissues, where MRI is routinely used to monitor temperature during thermal therapies, ${T}_{1}$ or ${T}_{2}$ contrast in water are relatively insensitive to temperature, and techniques with greater temperature sensitivity, such as chemical shift or diffusion imaging, suffer from motional artifacts and long scan times. MR thermometry is not well developed for nonbiological or engineering systems. We describe an approach for highly sensitive and high-throughput MR thermometry that is not susceptible to motional artifacts and could be applied to various biological systems and engineering fluids. We use superparamagnetic iron-oxide nanoparticles (SPIONs) to spoil ${T}_{2}$ of water protons. Motional narrowing results in proportionality between ${T}_{2}$ and the diffusion constant, dependent only on the temperature in a specific environment. Our results show, for pure water, the nuclear magnetic resonance linewidth and ${T}_{2}$ follow the same temperature dependence as the self-diffusion constant of water. Thus, a ${T}_{2}$ mapping is a diffusion mapping in the presence of SPIONs, and ${T}_{2}$ is a thermometer. For pure water, a ${T}_{2}$ mapping of a 64 \ifmmode\times\else\texttimes\fi{} 64 image (voxel size = 0.5 mm \ifmmode\times\else\texttimes\fi{} 0.5 mm \ifmmode\times\else\texttimes\fi{} 3 mm) in a 9.4 T MRI scanner resulted in a temperature resolution of 0.5 K for a scan time of 2 min. This indicates a highly sensitive and high-throughput MR thermometry technique that potentially has a range of applications from thermal management fluids to biological tissues.