Diffusion Coefficient Measurements of T1-Enhanced Contrast Agents in Water Using 0.3 T Spin Echo Proton MRI

The diffusion coefficients of four contrast agents (CAs, compounds of paramagnetic ions), Gd-DTPA, Gd-HP-DO3A, MnCℓ2, and albumin-(Gd-DTPA), in water were measured using 0.3 T proton MRI. The pixel size, number of signal averages (NSA), and scan time were 312 µm, four, and 6 min, respectively. Eight contrast solutions with initial paramagnetic ion concentrations (PCs) from 0.25 to 4.0 mmol/ℓ were added to eight rectangular grooves, 4 mm (the phase axis) × 1 mm (the frequency axis), in an agar gel electrophoresis plate. The diffusion motions of the CAs were revealed by the Gaussian concentration profiles along the frequency axis. The peaks of these profiles decreased with time, and their widths increased. The MRI signal profiles associated with the contrast solutions were proportional to and saturated with the PCs when the PCs were lower and higher than the saturation concentration of approximately 1 mmol/ℓ (just above the clinical dose), respectively. Thus, one hour after the start of diffusion, the signals whose initial concentration was neither too low nor too high exhibited normal Gaussian profiles that were not distorted by background noise or signal saturation. The diffusion coefficients of the CAs were determined by fitting the Gaussian concentration profiles to the normal Gaussian signal profiles. Although the sampling average of five image lines, summed along the phase axis, reduced irregularities in the signal profile, the coefficients determined reflected a measurement error of 10% because of the remaining irregularities produced by background noise. Increasing the NSA and decreasing the division along the phase axis are expected to reduce the measurement error without increasing the scan time, which is limited by the diffusion motion of the CA crossing the pixel size.