The T1-shortening effects of gadolinium within circulating blood results in a transient increased signal intensity in vascular structures. This T1-shortening effect has had a dramatic effect on MR angiography of the arterial system and a similar technique can be used to image the venous system. Because this approach relies only on the decreased T1 of enhanced venous blood and does not rely on ?ow-related enhancement , like the TOF techniques, large ?eld-of-view, time-efficient coronal “in-plane” imaging can be performed without saturation effects. This volumetric approach, without imaging gaps, provides high spatial resolution, high signal-to-noise studies resulting in near isotropic three-dimensional (3D) data sets that can be constructed in any plane.
When performing dynamic gadolinium- enhanced 3D GRE studies, several acquisitions are acquired sequentially. A 3D T1-weighted sequence with fat saturation, such as volumetric interpolated breath-hold examination (VIBE), can function as an efficient and robust technique for obtaining contrast-enhanced MR venography. The initial acquisition is timed with the gadolinium's ?rst pass through the arterial system but prior to substantial venous enhancement. Multiple acquisitions are performed following the arterial phase; these images usually have both arterial and venous enhancement. A selective venous study can be generated by subtracting the arterial-phase study from a mixed venous-arterial phase study. The arterial signal is nulli?ed, while the subtracted data set contains only venous signal. Because this is a recirculation technique (vein to artery to vein), a dose of up to 0.2 mmol/kg of gadolinium may be required.
Direct venography is a new technique similar to conventional catheter venography in which very dilute gadolinium (5 mL in 250 mL of saline) is injected directly into the distal extremity of expected pathology and imaged with a 3D GRE sequence. This can be incorporated with a moving table technique to image from the feet to the inferior vena cava (IVC). However, this technique requires venous cannulation of the affected extremity and cannot demonstrate alternative sites of intravenous access if thrombus or obstruction is identified.
