The ionic radius of Gd+3 (108 pm) is very close to that of Ca+2 (114 pm). Free gadolinium is therefore a competitive inhibitor of physiologic processes that depend on Ca+2 influx, including voltage-gated calcium channels and the activity of some enzymes. All Gd-based MR contrast agents therefore use some sort of ligand to chelate (contain) the free ion and reduce its high toxicity.
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Bellin M-F, Van Der Molen AJ. Extracellular gadolinium-based contrast media: an overview. Eur J Radiol 2008; 66:160-167.
de Haën C. Conception of the first magnetic resonance imaging contrast agents: a brief history. Top Magn Reson Imag 2001; 12:221-230.
Rohrer M, Bauer H, Mintorovitch J et al. Comparison of magnetic properties of MRI contrast media solutions a different magnetic field strengths. Invest Radiol 2005; 40:715-724.
Are non-ionic gadolinium contrast agents better than ionic ones?
What are the important physical properties to consider when choosing a contrast agent?