Effect of r₁ and r₂ relaxivity of gadolinium-based contrast agents on the T₁-weighted MR signal at increasing magnetic field strengths

Most contrast agents for magnetic resonance imaging (MRI) are gadolinium-based T₁ shortening agents. At increasing magnetic field strengths their r1 relaxivity tends to decrease while the r₂ relaxivity increases. In parallel, at high fields the tissue T1 times increase and may mitigate the loss in contrast enhancement in T₁-weighted images owing to improved background suppression. In the present work we explored the MR signal for T₁-weighted spoiled gradient echo MRI sequences by simulations at three magnetic field strengths: 3, 7 and 9.4 T. The maximal available contrast enhancement (maxCE) was evaluated in absolute terms with the purpose of assessing how much of the full, underlying magnetization can be exploited, for a wide range of compound properties (r₁, 2-45 mM⁻¹ s⁻¹ ; r₂ /r₁, 1.2-30). Despite the theoretically predicted loss in r1 relaxivity at high fields, the same maxCE can be obtained as at low fields if the r₂ /r₁ ratio remains unchanged, albeit at the cost of a longer sequence repetition time and 1.5-2 times higher administered doses. For a fixed maximum tissue concentration, there is an optimum field-dependent value for the r1 relaxivity that yields the greatest maxCE. If the upper bound for the gadolinium concentration is 2 mM, the greatest maxCE is found for compounds with a r₂/r₁ ratio of 1.2 and an r₁ relaxivity of 20.5 mM⁻¹ s⁻¹ at 3 T, 18 mM⁻¹ s⁻¹ at 7 T and 16.5 mM⁻¹ s⁻¹ at 9.4 T. For compounds that do not change their r1 relaxivity or r₂ /r₁ ratios, the necessary dose can be reduced by 10-15% owing to the improved background suppression at higher fields.