Accelerating relaxivity measurements by exploiting bulk magnetic susceptibility effects of paramagnetic metal ions

Paramagnetic metal chelates are routinely used in magnetic resonance imaging (MRI) as contrast agents. They enhance image contrast by reducing the relaxation times of the nearby water protons. Their efficiency, termed relaxivity, is thus defined as the change in the relaxation rates of the water protons induced by the addition of 1 mM contrast agent [1]. For any new potential paramagnetic MRI contrast agent, it is imperative to experimentally determine its 1 H relaxivities, often at multiple magnetic field strengths and temperatures. Traditionally this is performed by first measuring T1 and T2 of three or more dilutions of a contrast agent at a particular magnetic field strength and temperature. The exact concentration of the paramagnetic metal ion is then determined typically through inductively coupled plasma mass spectrometry or by measuring the bulk magnetic susceptibility (BMS) shift induced by the paramagnetic metal ions (i.e., the Evans method) [2,3]. Relaxivities are then determined from the slope of the plot of 1/Ti vs concentration of the contrast agent. In this presentation a significantly quicker method of characterising relaxivities of MRI contrast agents is demonstrated. Since paramagnetic chelates have both relaxation and shift altering properties, it will be shown that the relaxivities of paramagnetic metal chelates can be determined from a single experiment by correlating the relative relaxation rates and the BMS shifts of their dilutions, without knowing the metal ion concentrations [4]. An update to the Evans method of calculating paramagnetic metal ions concentrations will also be provided.