Relaxation efficacy of paramagnetic and superparamagnetic microspheres in liver and spleen.

A distinct knowledge of the relationship between physiochemical properties, cellular distribution and relaxation efficacy of particulate MR contrast media is needed for the development of tissue specific contrast compounds. To study these relations paramagnetic gadolinium labelled microspheres and superparamagnetic iron oxide microspheres (MSM) were injected intravenously to rats. The T1 and T2 relaxation times of the liver and spleen were recorded and the gadolinium tissue content quantified. A clear relationship between the gadolinium dose and the gadolinium concentration of the liver and spleen was observed while the T1 of the tissues remained unchanged. After injection of MSM, T2 of liver and both T1 and T2 of spleen decreased dose-dependently. The splenic relaxation efficacy of MSM was higher compared with that of liver, probably due to the morphology of the spleen allowing a scattered cellular sequestration of MSM. To mimic a uniform tissue distribution of the contrast agents, the liver and spleen samples were homogenized and a marked increase in the intrinsic relaxation efficacy of both the paramagnetic and superparamagnetic microspheres was observed.