Why do Tc-99m chelates work for cholescintigraphy?

There are four structural requirements for substances that are excreted by the liver: 1. Their molecular weight is between 300 and 1,000. 2. They are organic anions. 3. They contain at least two ring systems in the molecule. 4. They are bound to serum albumin. All the Tc-99m chelates suggested for cholescintigraphy fulfill these requirements. For the first time a rational for their efficacy is provided.