Diagnostic ultrasound ( PDFDrive )

218 PART II Abdominal and Pelvic Sonography
4 WEEKS 5 TO 6 WEEKS
L
G
V
D
G
8 WEEKS
G
V
D
FIG. 7.16 Embryologic Development of Pancreas. At 4 weeks the embryologic precursors of the adult pancreas develop as two outpouchings
(“buds”), called the dorsal (D, blue) and ventral (V, brown) pancreatic anlage. The ventral bud is also the embryologic origin of the gallbladder (G),
bile duct, and liver (L). The embryonic pancreatic buds arise from opposite sides of the junction of the primitive foregut and midgut. At 5 to 6
weeks the two pancreatic anlagen rotate into proximity. At 6 to 8 weeks the anlagen typically fuse. The dorsal pancreatic bud becomes the body
and tail of the pancreas.
through the minor papilla. 19 he minor duodenal papilla is several
centimeters proximal to the major papilla.
Usually, only an insigniicant amount of the pancreatic secretions
drain through the accessory duct. An exception occurs
when the accessory duct is the only duct entering the duodenum;
the duct from the pancreatic head empties into the dorsal duct,
not the duodenum (Fig. 7.18A). his anatomic variant is found
in 10% of individuals who have an accessory duct. Another
exception is pancreatic divisum, in which the ventral and dorsal
pancreatic ducts do not fuse (Fig. 7.18B). his results in most
of the pancreatic secretions (those secreted by dorsal pancreas)
entering the duodenum through the accessory duct via the minor
papilla.
he descriptive terminology of the pancreatic ducts is confusing.
It is most clear to use the functional description shown in
Fig. 7.17B, that is, the main pancreatic duct and accessory duct.
Most authors use the terms accessory duct and the duct of
Santorini synonymously. Some deine the duct of Santorini as
the entire dorsal duct, including the accessory duct. he main
pancreatic duct is sometimes called the duct of Wirsung, whereas
others reserve that name for the ventral duct only.
In normal individuals the pancreatic duct diameter is usually
3 mm or less. Hadidi 20 found that the mean duct diameter was
3 mm in the head, 2.1 mm in the body, and 1.6 mm in the tail. he
diameter of the duct can vary signiicantly. In fasting individuals,
the duct is oten seen as a linear structure in the pancreatic body
(Fig. 7.19). he diameter increases with age, although 3 mm is
still an appropriate upper limit of normal in older patients. 6
Using a 2.5-mm upper limit of normal, Wachsberg 21 showed
that inspiration could increase duct size to exceed that limit in
12% of patients without pancreatic disease. Secretin injection
increases duct size, presumably because of increased pancreatic
secretion, 22 which likely explains the observation that pancreatic
duct size may increase postprandially in some normal individuals. 8
Imaging Anatomic Variants
Transabdominal ultrasound plays little role in the diagnosis of
anatomic variants of the pancreas, which are generally discovered
and evaluated with CT, endoscopic retrograde cholangiopancreatography
(ERCP), and magnetic resonance cholangiopancreatography
(MRCP). 17,23 Endoscopic ultrasound has been found
to be useful in diagnosing pancreas divisum, especially in patients
with unexplained pancreatitis. 24 Congenital variants of the
pancreas are common, occurring in about 10% of the population.
Variants include pancreas divisum, annular pancreas, and partial
agenesis. Most pancreatic variants are of no clinical signiicance
and are found incidentally with imaging, at surgery, or on autopsy.
Pancreas divisum, the most common variant, may predispose
to pancreatitis, although the literature is unclear about this
association. 25 he vast majority of patients (95%) with divisum
do not develop pancreatitis. 26
Peripancreatic Structures
he intraperitoneal stomach is generally located immediately
ventral to the retroperitoneal pancreatic body, with the collapsed
potential space of the lesser peritoneal sac between the two organs