Aqueous paramagnetic solutions for MRI phantoms at 3 T: a detailed study on relaxivities

Phantoms with known T1 and T2 values that are prepared using solutions of easily accessible paramagnetic agents are commonly used in MRI imaging centers, especially with the goal of validating the accuracy of quantitative imaging protocols. The relaxivity parameters of several agents were comprehensively examined at lower B0 field strengths, but studies at 3 T remain limited. The main goal of this study is to measure r1 and r2 relaxivities of three common paramagnetic agents (CuSO4, MnCl2, and NiCl2) at room temperature at 3 T. Separate phantoms were prepared at various concentrations of 0.05-0.5 mM for MnCl2 and 1-6 mM for CuSO4 and NiCl2. For assessment of T1 relaxation times, inversion recovery turbo spin echo images were acquired at 15 inversion times ranging between 24 and 2500 ms. For assessment of T2 relaxation times, spin-echo images were acquired at 15 echo times ranging between 8.5 and 255 ms. Voxel-wise T1 and T2 relaxation times at each concentration were separately determined from the respective signal recovery curves (inversion recovery for T1 and spin echo decay for T2). Relaxivities r 1 and r2 for all three agents that were derived from these relaxation time measurements are reported: r1 = 0.602 mM-1 s-1 and r2 = 0.730 mM-1 s-1 for CuSO4, r1 = 6.397 mM-1 s-1 and r2 = 108.266 mM-1 s-1 for MnCl2, r1 = 0.620 mM-1 s-1 and r2 = 0.848 mM-1 s-1 for NiCl2. These results will serve as a practical reference to design phantoms of target T1 and T2 values at 3 T, in particular phantoms with relaxation times equivalent to specific human tissues.