Total body opacification.

Total body opacification is the phenomenon whereby introduction of iodinated contrast material 
into the blood stream renders structures differentially radiopaque according to their 
blood supply. The term implies the physiology: it is the whole 
body which is opacified, in proportion to the iodine in its 
plasma and extravascular extracellular fluid at the time of the radiograph. 
To the density inherent in the tissue before injection is added 
the opacity of the iodine circulating through and transuding into it. 
Just as tissues, even those of "water density," have minor intrinsic 
differences in x-ray absorption, so they may also have minor differences 
in blood flow and transudation into the extracellular fluid; the combination 
of these minor differences often makes the invisible visible. 