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PERIPHERAL VASCULAR ULTRASOUND
in 25% of healthy volunteers. This could lead to potential diagnostic errors if one half of the system is occluded and the other is patent, as it is possible to miss the occluded system during the examination. Careful scrutiny of the transverse sectional image should demonstrate any bifid vein systems. The sonographer should also be highly suspicious of veins that appear small in caliber or that are located in abnormal positions with respect to their corre- sponding arteries. Another potentially difficult sit- uation occurs when there is a large deep femoral vein running between the popliteal vein and pro- funda femoris vein, as the superficial femoral vein may be unusually small. Both the superficial femoral vein and the deeper vein should be carefully exam- ined for defects. In addition, it is possible to misiden- tify veins in the deep venous system and even confuse them with superficial veins. This occurs most com- monly in the popliteal fossa and upper calf. The gastrocnemius vein can be mistaken for the popliteal vein or for the short saphenous vein. It is impor- tant to be able to identify the fascial layer that sep- arates the superficial and deep venous systems to avoid this type of error (see Figs 12.1 and 12.2).
Investigation of the iliac veins can be extremely difficult, especially in situations in which the vein may be under compression by structures in the pelvis, or by tumors, as this can be misinterpreted as a partially occluding thrombus. Compression of the iliac vein can also occur during pregnancy and is observed more frequently on the left side. This may lead to unilateral limb swelling and a reduc- tion in the normal venous flow pattern in the femoral vein.
ACCURACY OF DUPLEX SCANNING
FOR THE DETECTION OF DVT
Many studies have been performed to compare the accuracy of duplex scanning with venography. The results of these studies are variable. Baxter et al (1992) reported 100% sensitivity and specificity for the femoropopliteal veins and 95% specificity and 100% sensitivity for calf veins. Miller et al (1996) achieved sensitivities and specificities of 98.7% and 100%, respectively, at above-knee level, and corresponding values of 85.2% and 99.2% at below-knee level. In contrast, a study by Jongbloets et al (1994) that involved the screening
of asymptomatic postoperative patients at high risk of developing DVT demonstrated sensitivities as low as 38% and 50% for thigh and calf veins, respectively.
These variable results may reflect factors such as patient population, operator experience or equip- ment availability. To implement a high-quality ser- vice, it is essential that staff are properly trained and a patient management protocol defined. In-house comparisons, or audit of ultrasound against other imaging techniques and outcomes, should also be performed to ensure the accuracy of the service.
NATURAL HISTORY OF DVT
The natural history of a DVT is variable and is dependent on the position and extent of the thrombi (O’Shaughnessy & Fitzgerald 2001). In addition, the patient’s age and physical condition will have a significant bearing on the final outcome. The throm- bus can:
● spontaneously lyse
● propagate or embolize
● recanalize over time
● permanently occlude the vein.
Complete lysis of smaller thrombi can occur over a relatively short period of time due to fibrinolytic activity. Full recanalization of the vein will be seen, and the lumen will appear normal on the ultra- sound image. Valve function can be preserved in these circumstances. If there is a large thrombus load, the process of recanalization can take several weeks. The thrombus becomes more echogenic over time as it becomes organized (Fig. 13.12A). The vein frequently diminishes in size due to retraction of the thrombus. As the process of recanalization begins, the developing venous flow channel within the vein lumen may be tortuous due to irregularity of lysis in the thrombus. It is even possible to see multiple flow channels within the vessel. In cases of partial recanalization, old residual thrombus can be seen along the vein wall, producing a scarred appear- ance (Fig. 13.12B). It is sometimes possible to see fibrosed valve cusps, which appear immobile and echogenic on the B-mode image. Deep venous insuf- ficiency is frequently the long-term outcome of slow or partial recanalization.