Diagnostic ultrasound ( PDFDrive )

CHAPTER 14 The Peritoneum 505
he small bowel mesentery is a specialized, fan-shaped,
peritoneal fold extending from the second lumbar vertebra to
the right iliac fossa. It connects the jejunum and ileum to the
posterior abdominal wall. It is composed of a double layer of
peritoneum, blood vessels, nerves, lacteals (lymphatic capillaries
in villi), lymph nodes, and a variable amount of fat. Normal
bowel mesentery is best assessed with ultrasound in the presence
of ascites; it appears as freely loating, smooth leaves separated
by luid, directed toward the center of the abdomen, away from
small bowel loops (Fig. 14.1). In the absence of ascites, the
mesentery is more diicult to appreciate but has been described
FIG. 14.1 Normal Mesentery With Gross Ascites. Oblique sagittal
ultrasound image of the midabdomen shows the normal small bowel
mesenteric leaves (arrows) outlined by luid.
as a series of elongated, aperistaltic structures separated from
each other by specular echoes, best appreciated in the let lower
quadrant. 9 It is frequently diicult to localize a disease process
to the mesentery, and the relationship to other anatomic landmarks
may be helpful. For example, lymphoma may be correctly
localized to the mesentery if a mass is seen that encases the
mesenteric vessels.
he omenta are also specialized peritoneal folds. hey are
composed of a double layer of peritoneum, blood vessels, lymphatics,
and a variable amount of fat. he lesser omentum connects
the lesser curvature of the stomach and proximal duodenum
with the liver. he greater omentum descends from the greater
curvature of the stomach, anterior to the abdominal contents,
oten as low as the pelvis, then relects back on itself to form a
four-layered structure that ascends and separates to enclose the
transverse colon. A potential space exists between the two layers
of the greater omentum, which is continuous with the lesser sac.
In the normal state, the omenta may be extremely diicult
or impossible to distinguish with ultrasound. In the presence of
ascites, the free inferior edge of the normal greater omentum
may be visible loating in the luid with variable thickness,
depending on the fat content. In patients with disease the greater
omentum may become iniltrated, thickened, and nodular (Fig.
14.2, Video 14.1). Its supericial location allows for careful
sonographic evaluation with high-frequency transducers, and
disease processes may oten be correctly identiied and localized
to the greater omentum even in the absence of ascites.
SONOGRAPHIC TECHNIQUE
Sonographic assessment of the peritoneum requires the motivation
to evaluate, as far as possible, the parietal and visceral peritoneum,
mesentery, omentum, and peritoneal cavity. he initial survey
of the peritoneum and peritoneal cavity is performed with a
standard-frequency, 3.5-MHz or 5-MHz transducer (Fig. 14.3A).
FIG. 14.2 Tumor Iniltration of the Omentum. Sagittal ultrasound image of the midabdomen shows thickening and hypoechoic nodularity of
the omentum (arrows) immediately posterior to free luid deep to the anterior abdominal wall. See also Video 14.1.