The Use and Reproducibility of Duplex Ultrasound to Provide Indices of Left Common Iliac Vein Diameter

The goal of this feasibility/pilot study is to investigate whether or not vascular ultrasound can be used to aid the diagnosis of May Thurner Syndrome, in adult patients presenting to the vascular department with unexplained left leg swelling. The main research objective it aims to answer are: * The diameter variation of the left common iliac vein based on posture, using duplex ultrasound. * To determine inter- and intra-observer variation of the diameter measurement of the left common iliac vein using duplex ultrasound. Participants will have an ultrasound scan performed on their abdomen by three different clinical vascular scientists, and will have each scan performed whilst lying supine on an examination couch and again when the couch is tilted to 45 degrees. Researchers will compare the results of symptomatic patients to asymptomatic patients, to see if there is a difference in the diameter of the left common iliac vein between these two groups.