Quantitative assessment of magnetic agent tagged cells in a subject comprises: acquiring a series of T2 weighted images of the subject acquiring a series of T2* weighted images of the subject and generating a value indicative of quantitative assessment of magnetic agent tagged cells in the subject based on both the T2 weighted images of the subject and the T2* weighted images of the subject. The generating may be further based on predetermined relationships (26) between (i) R2 and intracellular magnetic agent concentration, (ii) R2* and intracellular magnetic agent concentration, (iii) R2 and extracellular magnetic agent concentration, and (iv) R2* and extracellular magnetic agent concentration. Said predetermined relationships may be generated based on R2 and R2* measurements of a plurality of calibration phantoms having different concentrations of substantially purely intracellular magnetic agent and having different concentrations of substantially purely extracellular magnetic agent.
