Being tested for may thurner syndrom. one abd/pelvic ct in march showed ikiac vein compressed. follow up ct venogram showed it wasn't. how can this be?

CT angiogram is more specific than a CT of the abdomen and pelvis with IV contrast so the images from the first study are likely inaccurate.

I wish I had an easy answer. It's caused by the overlying right iliac artery compressing the underlying left iliac vein (usually against the spine) and causing partial block of blood flow. This can lead to left leg DVT's and possibly PE. Sometimes factors such as "dehydration" can lead to a false positive study on ct. Some claim the "gold standard" is IVUS (intravasc. ultrasound) if available.