If you are into the medical side of thrombosis, the Veterinary Radiology blog has an entry on ultrasound of aterial and venous thrombosis. It's quite technical but worth a look if you are interested.
In the acute phase, thrombi in the arterial or venous systems are typically anechoic. These can be caused by migration of a fragment of thrombus from the left atrium to the terminal aorta (such as cats with cardiomyopathy), or a portal vein thrombus that causes portal hypertension and ascites. Acutely formed thrombi are anechoic. You may see some faint echogenicity within the vessel, but these are usually diagnosed using Doppler ultrasound. The color flows around the filling defect in the vessel.
If you are into the medical side of thrombosis, the Veterinary Radiology blog has an entry on ultrasound of aterial and venous thrombosis. It's quite technical but worth a look if you are interested.