In vitro Gd-DTPA relaxometry studies in oxygenated venous human blood and aqueous solution at 3 and 7T

In vitro T₁ and T₂^* relaxivities (r₁ and r₂^*) of Gd-DTPA (GaD) in oxygenated human venous blood (OVB) and aqueous solution (AS) at 3 and 7T were calculated. GaD concentrations ([GaD]) in OVB and AS were prepared in the range 0-5mM. All measurements were acquired at 37±2°C. At both 3 and 7T, a linear relationship was observed between [GaD] and R₁ in both AS and OVB. At 7T, r₁ in AS decreased by 7.5% (p=0.045) while there was a negligible change in OVB. With respect to R₂^*, a linear relationship with [GaD] was only observed in AS, while a more complex relationship was observed in OVB; quadratic below and linear above 2mM at both field strengths. There was a significant increase of over 4-fold in r₂^* with GaD in OVB at 7T (for [GaD] above 2mM, p<<0.01) as compared with 3T. Furthermore, in comparison to r₁, r₂^* in AS was less than 2-fold higher at both field strengths while in OVB it was ~20-fold and ~90-fold higher at 3 and 7T, respectively. This observation emphasizes the importance of r₂^* knowledge at high magnetic fields, ≥3T. The comparison between r₁ and r₂^* presented in this work is crucial in the design and optimization of high-field MRI studies making use of paramagnetic contrast agents. This is especially true in multiple compartment systems such as blood, where r₂^* dramatically increases while r₁ remains relatively constant with increasing magnetic field strength.