Contrast agents: X-ray contrast agents and molecular imaging--a contradiction?

It is the purpose of this article to discuss whether and how X-ray contrast media may contribute to molecular imaging. X-ray contrast media are small molecules containing heavy elements, preferentially iodine. Modern CT allows precise, fast and reliable quantification of contrast media concentrations in large volumes with excellent spatial resolution throughout the body. The main disadvantage is the low contrast sensitivity requiring iodine concentrations of > or =0.5 mg/ml. Various approaches of the past and the present to specific contrast agents reflecting physiological, cellular or molecular processes are presented and options for the future are discussed.