Structure activity relationship of magnetic particles as MR contrast agents.

Structure activity relationship (SAR) of superparamagnetic MR contrast agents is discussed based on physicochemical properties and relaxivity data of 16 different particles. All the magnetic particles reduce both relaxation times, T1 and T2. The effect on T2 is stronger than the effect on T1. The relaxation efficacy varies over a wide range. Minor modifications in the preparation of the magnetic particles result in products with different susceptibility properties. The T2 relaxivity is dependent upon the magnetic susceptibility as well as particle size. Small particles reduce the relaxation times to a larger extent than the larger particles. No significant difference in relaxivity is observed between compact and porous particles. Magnetic particles coated with nonmagnet polymer are effective relaxation agents, while nonmagnetic monodisperse particles show no effect on the relaxivity.