A system marks and measures a Vessel lumen, heart valve or body cavity using a circumference measuring catheter. A catheter system comprises a catheter device including a loop having an adjustable circumference that is adjustable within a vessel. The catheter device includes a means for adjusting the circumference of the loop and a means for measuring adjusted loop circumference. An output unit sends data indicating the measured loop (vessel) circumference or cross-sectional area derived using the measured loop circumference for presentation to a user.