Imaging techniques

Following the development of gradient echo techniques, TOF MR venography quickly evolved as a clinically reliable method for detecting DVT of the pelvic and lower extremity veins (Fig. 6) [16-18]. Although TOF venography can detect venous thrombosis in the femoral and trifurcation veins [ 19], lengthy acquisition times have limited its use mainly to the pelvis. Due to in-plane flow saturation preventing reliable depiction of perforating veins which run in the horizontal plane, and the technique's lack of sensitivity to slow or retrograde flow, TOF MR venography has not been employed for assessing varicose veins or post-thrombotic changes. Contrast-enhanced 3D MR venography overcomes the limitations inherent to TOF venog-raphy. Specifically, direct MR venography with unilateral or bilateral injection of diluted paramagnetic contrast agent allows the display of all vessels, regardless of the underlying flow characteristics and the orientation of the vessel (Fig. 7, 8). Thus in-plane saturation is eliminated and imaging along the vessel axis is possible. Perforating and superficial veins containing slow or even retrograde flowing blood are fully depicted. The underlying 3D data sets provide high spatial resolution, which permits delineation of very small vessels.

In the pelvic veins, dilution from draining venous tributaries can cause a reduction of the very bright signal. Thus for display of the pelvic veins and IVC by means of the direct MR venography technique, it is of advantage to use a slightly lower dilution of the contrast agent, e.g. a dilution of 1:10. Whereas the indirect "equilibrium" MR venogra-phy approach is commonly sufficient for the diagnostic display of pelvic veins and IVC (Fig. 9), the direct imaging approach usually results in superior CNR which translates into better image quality and a more detailed depiction of the more peripheral venous anatomy.

Vein Imaging Techniques

Fig. 7. Direct contrast-enhanced MR venography of the veins of the upper thigh, and pelvis as well as of the inferior vena cava. A filling defect in the left proximal iliac vein (arrow) is present in the region where the right common iliac artery crosses anteriorly. Pelvic suprapubic and retroperitoneal collaterals are visualized. Normal drainage of the right pelvic veins into the inferior vena cava is visualized

Contrast Venogram
Fig. 8. Direct contrast-enhanced MR venography following bilateral injection of diluted contrast agent into a dorsal pedal vein showing marked postthrombotic changes on the right side with collateralization (arrows) via superficial veins. The left leg shows normal deep venous anatomy

Fig. 7. Direct contrast-enhanced MR venography of the veins of the upper thigh, and pelvis as well as of the inferior vena cava. A filling defect in the left proximal iliac vein (arrow) is present in the region where the right common iliac artery crosses anteriorly. Pelvic suprapubic and retroperitoneal collaterals are visualized. Normal drainage of the right pelvic veins into the inferior vena cava is visualized

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