IVUS — intravascular ultrasound — is an ultrasound probe small enough to ride a catheter inside the vein, imaging the vessel from within as it's pulled through. Where external scans estimate, IVUS measures: the true cross-section of the vein, exactly how much a compression narrows it, and what the vein wall looks like.
It matters because compression diagnoses are easy to over- and under-call from pictures alone — many people have some iliac vein squeeze with no symptoms, and CT taken lying flat can under- or overstate it. IVUS during a venogram is how specialists separate "anatomical curiosity" from "the cause of your symptoms", and it guides stent sizing when stenting is the answer.
For you, the word's main use is as a quality marker: if compression is being diagnosed or ruled out in your work-up, "was it assessed with IVUS or pressure measurements?" is a fair and revealing question — see the treatment-order guide.