Using image-guidance, a catheter (a long, thin, hollow plastic tube) is inserted through the skin into the femoral vein, a large blood vessel in the groin, and maneuvered to the treatment site.

Normally, oxygenated blood flows to the testicles via an artery, and deoxygenated blood flows out via a network of small veins that drain into another vein, and is eventually carried back to the heart. During this process, backflow to the testicles is prevented by a series of one-way valves in the vein. In some individuals, however, these valves do not work well. As a result, the blood flows in reverse and causes "back pressure" to stretch and enlarge the network of small veins around the testes, forming a varicocele.