Indications for Ultrasonography
The most common indication to evaluate the peritoneal cavity with ultrasonography is to search for intraperitoneal fluid collections such as ascites, abscesses, or hemorrhage. Ultrasonography is useful not only to identify intraperitoneal fluid collections but also to guide percutaneous needle aspiration for definitive diagnosis. Solid peritoneal masses representing either primary or metastatic tumors may also be detected on occasion with ultrasonography. However, CT is the preferred imaging modality in patients with a clinical suspicion for peritoneal metastases.
Patient Preparation and Technique
Scanning of the pelvic peritoneal spaces is easier when the bladder is full. The distended bladder provides an excellent acoustic window to evaluate the cul-de-sac or retrovesical recess. However, the remainder of the peritoneal cavity can readily be imaged with little or no patient preparation. In general, a curved 3.5- or 5-MHz transducer provides adequate penetration for a general abdominal survey to detect intraperitoneal fluid in the upper peritoneal compartments. Where access is difficult, sector transducers may be useful. High-resolution linear arrays (5 to 7.5 MHz) are best for evaluation of superficial peritoneal implants and omental abnormalities.
Intraperitoneal Fluid Collections
Ultrasonography excels in demonstrating even minute quantities of intraperitoneal fluid (Fig. 153-20
). With endovaginal scanning, small amounts of intraperitoneal fluid may routinely be demonstrated in the cul-de-sac as a normal finding in ovulating women. In the upper abdomen, free intraperitoneal fluid first collects in the hepatorenal fossa, which is the most dependent portion of the supramesocolic peritoneal cavity.188
As fluid increases within the peritoneal cavity, it involves the subphrenic, subhepatic, and intraloop compartments.189
Massive ascites typically displaces bowel loops centrally unless tethered by adhesions.190
FIGURE 153-20. Peritoneal implants from mesothelioma. Transverse scan of right lower quadrant demonstrates ascites (A) and plaquelike peritoneal implants (arrows). K, right kidney.
Uncomplicated serous fluid collections are typically anechoic. On ultrasonography, complex fluid characteristically demonstrates internal debris, septations, and low-level echoes. The differential diagnosis of complex intraperitoneal fluid includes hemorrhage, infection, and carcinomatosis. On occasion, long-standing bland ascites may contain internal septations and be loculated. However, ascites that is not infected generally has no mass effect and passively conforms to its intraperitoneal components. Intraperitoneal abscesses, on the other hand, typically exhibit mass effect and may displace adjacent bowel or adjacent solid organs. It is not always possible to differentiate loculated bland ascites from an abscess by ultrasonographic criteria alone. Therefore, diagnostic needle aspiration is often required.
CT and ultrasonography have greatly facilitated the diagnosis of intra-abdominal abscesses.191
With both modalities, the predominant finding is a localized fluid collection with mass effect. Gas bubbles and an air-fluid level may be present in some patients but are not identified in most intra-abdominal abscesses. Ultrasonography is the imaging method of choice for hepatic, pericholecystic, and pelvic abscesses. CT is superior to ultrasonography in patients with pancreatic, retroperitoneal, paraspinous, intraloop, or gas-forming abscess.192
In the postoperative setting, CT is often preferred to ultrasonography owing to frequent problems with access as a result of open wounds and surgical drains.
The ultrasonographic appearance of intra-abdominal abscesses is variable and depends on the presence or absence of septations and solid debris.194
Also, there is considerable overlap with the ultrasonographic appearance of other complex intraperitoneal fluid collections, such as lympho- celes, bilomas, pancreatic pseudocysts, postoperative seromas, and hematomas. Because of the attenuation of sound by proteinaceous debris, many intra-abdominal abscesses demonstrate little if any enhanced through-transmission of sound. Thus, an intra-abdominal abscess may ultrasonographically mimic a solid hypoechoic mass.194
This pitfall should always be kept in mind in a febrile patient. Color Doppler may be of value in resolving this dilemma because liquefied abscesses lack internal vascularity. Intra-abdominal enteric abscesses are often walled off by adjacent mesenteric or omental fat. In some patients, it may be possible with ultrasonography to identify adjacent abnormal bowel to identify the cause of the abscess (i.e., appendicitis or diverticulitis). Both CT and ultrasonography may be valuable for guidance of percutaneous needle aspiration for confirmation of an abscess and subsequent catheter insertion for definitive therapy.191
Tumors of the Peritoneum, Mesentery, and Omentum
The most common intraperitoneal neoplasms are metastatic carcinomas. Most of these lesions are tiny, plaquelike implants on the surface of the bowel or mesentery that cannot be imaged with any current diagnostic techniques. In the presence of ascites, peritoneal implants may be detected by ultrasonography as solid masses arising from the peritoneum or mesentery surrounded by peritoneal fluid (see Fig. 153-20
). Thickened septations may also be demonstrated ultrasonographically. Diffuse infiltration of the omentum is a characteristic finding in patients with ovarian metastases, carcinomatosis from GI tract malignances, and primary omental tumors. Primary neoplasms involving the omentum, such as mesotheliomas, can be imaged with graded compression ultrasonography using high-frequency linear array transducers.195
Diffuse omental infiltration (“omental caking”) typically results in a broad, bandlike mass anterior to the transverse colon. Because of its superficial location, these abnormalities are often best imaged with a high-resolution linear array transducer.
Pseudomyxoma peritonei, an unusual form of peritoneal carcinomatosis caused by mucinous adenocarcinomas of the appendix, ovary, or colon,196
results in gelatinous intraperitoneal masses that cause subtle scalloping of solid viscera, such as the liver and spleen. Echogenic intraperitoneal masses containing debris may be detected ultrasonographically.
Mesenteric masses may often be best imaged with ultrasonography using a graded compression technique and color Doppler imaging. Adenopathy with the dorsal and ventral leaves of the mesentery may be noted to encase the mesenteric vessels highlighted by color Doppler. Primary mesenteric and peritoneal tumors such as mesothelioma cannot be distinguished from the more common metastatic peritoneal tumors.