1. To review various options for MR venography contrast agents

2. To review optimized MR venography protocols

3. To review alternative venous imaging modalities

4. To review emergency applications of MR venography

- Acute iliocaval / lower extremity DVT

- Acute SVC syndrome

While MR angiography has historically focused on arterial imaging, MR venography has evolved substantially over the past few decades. The advent of a blood-pool contrast imaging agent, gadofosveset, has greatly improved our capability for depicting venous structures with high conspicuity, and comprises the optimal venous imaging contrast agent. Ferumoxytol, an iron-based agent, also has blood-pool properties; although it is currently approved for use as an intravenous iron supplement for patients with renal impairment, its ferromagnetic properties are easily exploited as an excellent MR imaging agent. Thus, in patients with renal impairment, contrast-enhanced MR venography can be readily performed with ferumoxytol. Because of the prolonged intravascular retention time for these blood-pool agents, MR venography protocols are not time-sensitive, and imaging can be performed as long as an hour after administration without significant loss in signal intensity, thus allowing high spatial resolution imaging. Time-resolved techniques are also crucial for venous imaging, since visualization of collateral venous flow is highly optimized. The lack of well-developed collateral veins in the presence of occlusive thrombus suggests an acute process, whereas well-developed collateral veins are characteristic of a chronic occlusion. The acuity of venous occlusion has a significant impact on treatment options and outcomes. The presence of perivascular T2 signal is also important for grading acuity. Although conventional venography is the gold standard venous imaging modality, there are a number of limitations, including central hemodilution, mixing artifact, solely imaging of direct outflow veins, and two-dimensionality. Perhaps the most important limitation is the logistic difficulty in obtaining IV access in a swollen extremity. While CT venography can also be performed, the venous opacification conspicuity is suboptimal, which can render diagnosis of DVT challenging.

One of the most common indications for MR venographic imaging in the emergency setting is for diagnosis of acute iliocaval / lower extremity DVT. Although ultrasound is the gold standard for diagnosis of lower extremity DVT, there are a number of circumstances where it cannot be performed or is inadequate. In these cases, MR venography of the lower extremities is an excellent imaging modality. Ultrasound is poor for imaging the iliac veins and IVC, so MR venography can play a crucial role in the comprehensive work up of patients with suspected DVT. In patients with suspected embolic stroke, emergency imaging of the pelvic veins and IVC plays an important role in the etiologic workup. In patients with acute DVT and severe leg swelling, determination of the extent of thrombosis is important to assess the feasibility and risk of thrombolysis and thrombectomy strategies. This is particularly important in cases of acute on chronic DVT, where recognition of the correct acuity level is important for treatment decisions. Recognition and diagnosis of May-Thurner syndrome, which is the compression of the left common iliac vein by the right common iliac artery, is especially important because it has a particularly good prognosis with endovascular stent deployment being the first-line therapy.

Another pathology that may be encountered in the emergency setting is acute SVC syndrome. Acute SVC syndrome may be caused by acute thrombosis related to catheters or other intravascular trauma, malignant compression, or hypercoaguability. Determination of the extent of thrombosis and the underlying etiology is important for determining the optimal treatment. Catheter-directed thrombolysis, percutaneous thrombectomy, and stent deployment may be considered depending on the anatomic location and extent of the thrombosis. Acute on chronic SVC syndrome is also important to diagnose, since treatment methods will vary significantly.