Extravascular contrast material: the major component of contrast enhancement.

The distribution volume of 125l-diatrizoate in rat tissues was studied 5 seconds to 5 minutes after intravenous injection and 5 seconds to 2 minutes after intra-arterial injection. Immediately after injection, diatrizoate was distributed into a larger volume than plasma. Marked temporal changes in the distribution volume of individual tissues occurred during the first 2 minutes. At 5 minutes, the diatrizoate space approached the total extracellular fluid volume, with more than 80% of the contrast agent remaining outside the blood vessels. Thus the relative magnitude of contrast enhancement of a tissue appears to be related to the volume of the rapidly equilibrating extracellular space. Intra-arterial administration can significantly overload muscle with contrast material for about 2 minutes. This and other temporal changes in contrast distribution may prove useful in contrast-enhanced computed tomography using short scanning times.