Temporal changes in the T1 and T2 relaxation rates (ΔR1 and ΔR2) in the rat brain are consistent with the tissue‐clearance rates of elemental manganese

Temporal changes in the T1 and T2 relaxation rates (ΔR1 and ΔR2) in rat olfactory bulb (OB) and cortex were compared with the absolute manganese (Mn) concentrations from the corresponding excised tissue samples. In vivo T1 and T2 relaxation times were measured before, and at 1, 7, 28, and 35 d after intravenous infusion of 176 mg/kg MnCl2. The values of ΔR1, ΔR2, and absolute Mn concentration peaked at day 1 and then declined to near control levels after 28 to 35 d. The Mn bioelimination rate from the rat brain was significantly faster than that reported using radioisotope techniques. The R1 and R2 relaxation rates were linearly proportional to the underlying tissue Mn concentration and reflect the total absolute amount of Mn present in the tissue. The in vivo Mn r1 and r2 tissue relaxivities were comparable to the in vitro values for aqueous Mn2+. These results demonstrate that loss of manganese‐enhanced MRI (MEMRI) contrast after systemic Mn2+ administration is due to elimination of Mn2+ from the brain. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.