Detection of pulmonary emboli by using MR angiography with MPEG-PL-GdDTPA: an experimental study in rabbits. 
A macromolecular MR contrast agent (MPEG-PL-GdDTPA), which consists of Gd-DTPA covalently attached to a polyamino acid (PL) derivatized by monomethoxy ether of polyethylene glycol (MPEG), has recently been developed.  This contrast agent exhibits long intravascular retention, which makes it suitable for MR angiography.  The current study was performed to test whether this agent would improve visualization of the pulmonary vasculature during MR angiography and whether it aids in the detection of pulmonary emboli in an animal model.  All experiments were performed in rabbits (n = 21), six of which were healthy and 15 of which had induced pulmonary emboli.  Contrast-enhanced MR angiograms (0.02 mmol Gd/kg) were obtained at 1.5 T by using time-of-flight and phase-contrast sequences.  MR images were ultimately compared with polymer casts of pulmonary arteries and/or pathologic sectioning.  MR angiograms obtained before the administration of a contrast agent were of low quality primarily because of low vessel/background signal-to-noise ratio, presumably due to slow and complex flow in the pulmonary vasculature.  After IV administration of the contrast agent, pulmonary arteries could be visualized up to the third-order branches.  Of the 18 pathologically or angiographically proved emboli, 16 could be detected with contrast-enhanced MR angiography.  Our results indicate that macromolecular contrast agents with long blood half-lives, such as MPEG-PL-GdDTPA, increase signal intensity of pulmonary vessels, improve the quality of MR angiography, and increase the detectability of pulmonary emboli.