Vascular Application Tips: Magnetic Resonance Venography

Magnetic resonance venography (MRV) has emerged as an effective imaging tool for the evaluation of diseases of the veins of the abdomen, pelvis, thorax, and extremities. Although duplex sonography is often the first modality for the evaluation of the venous system, it is hindered by acoustic access, especially in the evaluation of the deep veins of the pelvis, thorax, and the calf. MRV plays an important clinical role in the evaluation of venous disease, and in some clinical instances, such as evaluation of the pelvic veins, it has become the test of choice. Advances in local coils, gradient systems, and other system hardware and software have enabled faster scanning, reduced artifacts, increased signal-to-noise, and reduced examination times. Comprehensive examinations of the entire venous system can be performed in < 30 minutes. The clinical applications of MRV include diagnosis of deep venous thrombosis, for which MRV is the “new gold standard,” evaluation of chest and upper extremity veins for venous access, differentiating bland from tumor thrombus, diagnosis of superior vena caval syndrome, identification of superior vena caval invasion or encasement by lung or mediastinal tumors, diagnosis of the Budd-Chiari syndrome, diagnosis of caval anomalies such as persistent left superior vena cava and interrupted inferior vena cava, and identification of the presence and cause of obstruction or occlusion of the brachiocephalic, subclavian, and jugular veins.

Two MR techniques used for venous imaging include two-dimensional (2D) time-of-flight (TOF) MR angiography and three-dimensional (3D) gadolinium enhanced gradient-recalled (GRE) imaging. TOF is a widely used, noninvasive technique used for the evaluation of the venous system. However, due to saturation and flow effects, which may result in non-diagnostic studies, three-dimensional (3D) gadolinium enhanced gradient-recalled (GRE) imaging may be used for problem solving. Alternatively, may centers, including our institution, now forgo TOF and go directly to a 3D contrast-enhanced approach.

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.