The role of magnetization transfer in the observed contrast in T₁ weighted imaging under clinical setups

In T₁ weighted magnetic resonance imaging of brain and spinal cord in the clinical setting, the white matter (WM) appears with greater intensity than the gray matter (GM). This contrast has been assigned to differences in T₁ values. In these experiments the RF pulses are too long to excite both the water and the species with restricted motion of the protons (SRMP). In in vitro studies using short RF pulses, the contrast is reversed, with greater intensity for the GM. These results raise the question of whether magnetization transfer (MT) plays a role in the contrast observed in the T₁ weighting experiments. In the present work we implemented selective saturation recovery alone and together with the conventional magnetization transfer contrast (MTC) method. The results confirm that a major factor that determines the characteristic WM/GM averaged intensity ratio observed in T₁ weighted imaging under clinical conditions is MT between the SRMP and water. When selective saturation recovery is combined with MTC, the SRMP yields spectral widths ranging from a few to tens of kilohertz, indicating that more than one type of SRMP is involved in the MT. The z-spectrum obtained with this combination is free of the effect of direct saturation of the water peak. Selective saturation recovery enables an independent measurement of the exchange time and T₁, while the combination with MTC with complete saturation of the SRMP enables measurement of T₁ without the effect of MT. The latter measurement can be carried out on a timescale much shorter than T_1.