Hepatic tumors: signal enhancement at Doppler US after intravenous injection of a contrast agent. Experiments were carried out to detect and establish the origin of Doppler and echo signal enhancement in small vessels of the systemic circulation and of tumors after intravenous injection of a contrast agent. Eight woodchucks (Marmota monax) were studied; each woodchuck had naturally occurring hepatocellular carcinoma. Injections of 0.1-4.0 mL of air-filled human serum albumin microspheres were administered into a hind limb or jugular vein. Ultrasound (US) examination included transabdominal duplex scanning, color Doppler imaging, placement of a Doppler transducer on the exposed tumor, and surgical implantation of pulsed Doppler cuffs. Doppler signals were assessed by recording color Doppler images and results of spectral analysis. While subjective echo level was unaffected within the tumors, dramatic Doppler signal enhancement was recorded in both normal and tumor vessels. At optimum dose levels, a signal gain of approximately 10 dB was recorded from the tumor. Analysis showed that the echo enhancement was due to the presence of the contrast agent in branches of the hepatic artery. These results suggest that tumor detection may be enhanced by intravenous administration of a US contrast agent. 