Relaxivity of Ferumoxytol at 1.5 T and 3.0 T

Objectives: The aim of this study was to determine the relaxation properties of ferumoxytol, an off-label alternative to gadolinium-based contrast agents, under physiological conditions at 1.5 T and 3.0 T. Materials and Methods: Ferumoxytol was diluted in gradually increasing concentrations (0.26-4.2 mM) in saline, human plasma, and human whole blood. Magnetic resonance relaxometry was performed at 37 degrees C at 1.5 T and 3.0 T. Longitudinal and transverse relaxation rate constants (R1, R2, R2*) were measured as a function of ferumoxytol concentration, and relaxivities (r1, r2, r2*) were calculated. Results: A linear dependence of R1, R2, and R2* on ferumoxytol concentration was found in saline and plasma with lower R1 values at 3.0 T and similar R2 and R2* values at 1.5 T and 3.0 T (1.5 T: r1(saline) = 19.9 2.3 s(-1)mM(-1); r1(plasma) = 19.0 +/- 1.7 s(-1)mM(-1); r2(saline) = 60.8 +/- 3.8 s(-1)mM(-1); r2(plasma) = 64.9 +/- 1.8 s(-1)mM(-1); r2*(saline) = 60.4 +/- 4.7 s(-1)mM(-1); r2*(plasma) = 64.4 +/- 2.5 s(-1)mM(-1); 3.0 T: r1(saline) = 10.0 +/- 0.3 s(-1)mM(-1); r1(plasma) = 9.5 +/- 0.2 s(-1)mM(-1); r2(saline) = 62.3 +/- 3.7 s(-1)mM(-1); r2(plasma) = 65.2 +/- 1.8 s(-1)mM(-1); r2*(saline) = 57.0 +/- 4.7 s(-1)mM(-1); r2*(plasma) = 55.7 +/- 4.4 s(-1)mM(-1)). The dependence of relaxation rates on concentration in blood was nonlinear. Formulas from second-order polynomial fittings of the relaxation rates were calculated to characterize the relationship between R1(blood) and R2 (blood) with ferumoxytol. Conclusions: Ferumoxytol demonstrates strong longitudinal and transverse relaxivities. Awareness of the nonlinear relaxation behavior of ferumoxytol in blood is important for ferumoxytol-enhanced magnetic resonance imaging applications and for protocol optimization.