Determination of blood circulation times of superparamagnetic iron oxide nanoparticles by T2* relaxometry using ultrashort echo time (UTE) MRI

Abstract

Blood circulation is an important determinant of the biodistribution of superparamagnetic iron oxide nanoparticles. Here we present a magnetic resonance imaging (MRI) technique based on the use of ultrafast echo times (UTE) for the noninvasive determination of blood half-lives at high particle concentrations, when conventional pulse sequences fail to produce a useful MR signal. Four differently coated iron oxide nanoparticles were administered intravenously at a dose of 500 μmol Fe/kg bodyweight and UTE images of C57BL/6 mice were acquired on a 1-T ICON scanner (Bruker). T2* relaxometry was done by acquiring UTE images with echo times of 0.1, 0.8 and 1.6 ms. Blood circulation time was then determined by fitting an exponential curve to the time course of the measured relaxation rates. Circulation time was shortest for particles coated with malic acid (t1/2=23 min) and longest for particles coated with tartaric acid (t1/2=63 min). UTE-based T2* relaxometry allows noninvasive determination of blood circulation time and is especially useful when high particle concentrations are present.

Keywords: Blood half-life; Ferumoxytol; Iron oxide particles; UTE; VSOP.