The normal deep veins of the lower limb- the femoral and popliteal veins are usually studied to rule out deep venous thrombosis. One of the various methods of ruling out clot formation (thrombosis) in the deep veins is the application of mild pressure with the ultrasound probe on the deep vein. This method is called compression and the method is called testing the compressibility of the deep vein.
This ultrasound video clip (above) shows the femoral vein (FEM V.) and the popliteal vein (POPL V.), after probe pressure is applied to determine whether complete collapse of the veins is possible. The end stage shows total apposition of the walls of the vein as pressure is applied. This confirms the absence of thrombus (clot) within the vein. This testing of the normal compressibility of the femoral and popliteal veins must be done along the full extent of the deep veins. As one goes downwards along the course of the femoral vein to the mid thigh, this may be difficult due to the femoral vein entering the adductor canal.
At this stage the sonologist may try looking for normal augmentation of flow by pressure on the calf muscles. A sharp augmentation spike on spectral doppler trace means there is no thrombus along the course of the deep veins till the calf.
For more on this topic visit: color Doppler and ultrasound study of the deep veins of lower limb.