Hi Linh,
I have a scanning bed that the back can sit up.
I have them sitting up in the bed with their hip external rotated and I ask them to move right over to my side of the bed.
This creates enough gravitational pressure to fill the calf veins nicely.
I use a convex 10C3 transducer which is lovely for these examinations.
When I get to the popliteal vein I ask them to bend their knee a little and roll slightly towards me allowing access to the pop vein. I do my PTV with knee gently flexed using a medial approach and I do the peroneals with the knee slightly more flexed using a poster-lateral approach.
It works well for me,
I am not a fan of sitting the patient on the edge of the bed. It does work, but you get their feet in your lap and they are less well supported and I worry about them falling backwards.
Steve.