The procedure of Selective Venography (SV) inserts a catheter (usually via femoral vein) which is then moved into the vein to be investigated. A dye (usually iodine based) is released into the vein and an X-ray image is taken.
A clear image of the vein is usually available and any stenosis (twisting, web, membrane etc) will be visable.
SV is invasive because a catheter is inserted into a vein. There are risks of X-rays and an iodine based dye. They have been used millions of times before so unless you are pregnant or have an iodine allergy are considered a reasonably low risk.
There is a good paper on SV verses MRI in CCSVI, in the publication library, if you are research minded.
Mark Walker - Oxfordshire, England. Retired Pharmacist. 12 years of study around MS.
Mark's CCSVI Report 7-Mar-11: