Theoretical MRI contrast model for exogenous T2 agents

The rational development of new generations of MRI contrast agents (CAs) requires a scheme for predicting contrast enhancement. Previous contrast predictions have been based largely on empirical results in specific systems. Here we present a general theoretical model for evaluating the minimum concentration of T-2 CA required for satisfactory image contrast. This analytic contrast model is applicable to a wide range of T-2-type agents and delivery scenarios, and requires only a few readily evaluated parameters. We demonstrated the model by predicting contrast produced by superparamagnetic ferumoxide and the iron storage protein, ferritin. We then experimentally verified the predictions using suspensions of Feridex (R) and ferritin in phantoms. The model was also used to compare the contrast efficacy of the metal ions in two clinically approved T, and T2-type CAs. In the Appendix we present a numerical formalism that is useful for relating image contrast and agent concentration when gradient-echo (GRE) T-2*-weighted (T-2*W) pulse sequences are used.