Investigation of lanthanide-based starch particles as a model system for liver contrast agents.

Gadolinium and dysprosium diethylenetriamine pentaacetic acid-labeled starch microparticles (Gd-DTPA-SP and Dy-DTPA-SP) were investigated as model liver contrast agents. The liver contrast efficacy of particles with low and high metal contents was compared in two imaging models: in vivo rat liver and ex vivo perfused rat liver. The biodistribution of intravenously injected particles was also assessed by ex vivo relaxometry and inductively coupled plasma atomic emission spectrophotometry of tissues. All particles reduced the liver signal intensity on T2-weighted spin-echo and gradient-recalled echo images as a result of susceptibility effects. Because of their higher magnetic susceptibility, the Dy-DTPA-SP were more effective negative contrast enhancers than the Gd-DTPA-SP. On T1-weighted spin-echo images, only the Gd-DTPA-SP with low metal content significantly increased the liver signal intensity. In addition, these low-loading Gd-DTPA-SP markedly reduced the blood T1. The two latter observations were not consistent with the anticipated blood circulation time of microparticles, but were a result of the lower stability of these particles in blood compared with Gd-DTPA-SP, which has a high metal content. Regardless of stability or imaging conditions, the paramagnetic starch particles investigated showed potential as negative liver contrast enhancers. However, the observed accumulation of particles in the lungs represented a biological limitation for their use as contrast agents.