Can a stent have stenosis if the us says it's patent but the velocities are abnormally high ? is catheter angiogram next step?

Yes: Yes, it is often the case that high velocities through a vessel is suggestive of a higher grade stenosis Within.. and narrowing in a stent does occur by overgrowth of tissue called intimal hyperplasia. Angiogram through a catheter placed within the blood vessel can usually give more exact detail. Although you didn’t state which artery was stented, the general concepts are often similar