Ultrasonography is a highly sensitive test in detecting ascites of small volume; (Branney and Wolfe 1995) for example, 5–10 ml ascites are clearly detectable. However, ascites is common in many benign conditions such as liver cirrhosis, portal hypertension, nephrotic syndrome, cardiac insufficiency, etc. (Runyon et al. 1988). Even if ultrasonography is often unable to distinguish between benign and malignant ascites, its presence in patients with known neoplasia is highly suggestive for neoplastic involvement of the peritoneum, and its presence in the lesser omentum (in absence of pancreatitis) is also highly suggestive for malignancy (Dodds et al. 1985). Some ultrasonographic characteristics may be helpful to differentiate benign from malignant ascites.

Frequently, in malignant ascites the fluid is echogenic (due to the presence of blood and/or neoplastic cells) (Fig. 1), septa are often present (Fig. 2), bowel loops are fixed and smashed for mesenteric involvement (Fig. 3), serosal and parietal peritoneal implants are often visible.

Indeed, abundant ascites facilitates visualization of small nodules infiltrating parietal peritoneum, since it acts as a means of contrast (Fig. 4; Goerg and Schwerk 1991).

Table 2 summarizes differential ultrasonographic characteristics between benign and malignant ascites; however, significant ascites is present in about 50 % of patients with peritoneal metastasis (Rioux and Michaud 1995).

Diagnosis of peritoneal metastatic involvement is often based on fluid analysis and ultrasound-guided aspiration of ascites is frequently used in patients with small amounts of liquid (Fig. 5).

Combined with repeated cytological examination of fluid, ultrasound offers 92 % of sensitivity in the diagnosis of peritoneal carcinomatosis thus avoiding laparotomy or laparoscopy (Gerbes 1991; Runyon et al. 1988).

Stein (1977) clearly described the omental band as a new sign of peritoneal metastasis. Neoplastic infiltration of the great omentum is visualized by ultrasound as a uniformly thick, hyposonic band-shaped structure adjacent to the anterior and lateral walls of the abdomen, following the contour of the abdominal convexity and containing low-level non-structural internal echos (omental-cake) (Fig. 6). From this finding, a diffuse metastatic involvement of the omentum can probably be inferred with a high degree of reliability.