 Unlike arterial ultrasonography , venous ultrasonography is carried out with the probe in a transversal position , ( perpendicular to the vein axis ) . All collateral veins are better detected this way , including perforator veins , but of most importance is the detection of venous thrombosis . The most reliable sign of thrombosis ( even when a good image and color is present ) is the absence of compressibility . However , if the probe is parallel to the vein axis , when the examiner compresses it , the probe can slide to the right or to the left giving a false negative for thrombosis as the probe has moved away and the vein will not then be evident . Nevertheless , when the examiner needs to show the head thrombus in a printout , the probe will be presented parallel to the vein axis . 
