Inferior vena caval filter thrombi: evaluation with intravascular US. A 20-MHz intravascular ultrasound (US) transducer inside a percutaneously inserted catheter was used to evaluate inferior vena caval (IVC) filters for thrombi in vitro and in vivo. Six different IVC filters were studied with intravascular US in a saline-filled model. Each filter had a characteristic, recognizable US pattern. Experimental thrombi as small as 0.5 cm3 were easily detected. Intravascular US was used clinically 25 times to evaluate the IVC in 23 patients with 24 IVC filters. Positive-contrast cavograms were available for comparison in all 25 cases. In 13 cases, no thrombi were identified in the filter or IVC with either intravascular US or cavography; in five of 12 cases with thrombi, intravascular US and cavography demonstrated the thrombi equally well. In six cases, intravascular US was superior to cavography in detection or delineation of thrombus in the IVC or filter. Intravascular US was considered superior to external duplex US in evaluation of caval thrombi in all 21 cases available for comparison. No complications from intravascular US were noted. 