In normal lower-extremity veins, blood flow exhibits a steady forward pattern, with superimposed variations related to respiration. In the presence of acute venous obstruction, these respiratory variations disappear. When obstruction is chronic, however, the flow pattern may appear normal because of collateralization. Indeed, the presence of collateral vessels may be interpreted as evidence of old thrombosis. The most specific criterion for diagnosing deep vein thrombosis is the presence of echogenic intraluminal material, as represented by a series of white dots on the sonogram.
Older thrombi tend to be more echogenic than fresh ones.
Some thrombi are not echogenic at all. Loss of normal vein compressibility is the most helpful criterion for diagnosing venous thrombosis. If the vein wall does not completely collapse under compression, thrombosis should be suspected. Whether reflux arises in the superficial or the deep venous system, its location, degree, and extent can be determined with duplex scanning. Because of increased anatomic variability, saphenopopliteal incompetence is more challenging to assess than saphenofemoral incompetence.