Diagnostic Ultrasound - Abdomen and Pelvis

Approach to Pancreatic Sonography
Diagnoses: Pancreas
Imaging Anatomy
The pancreas resides in the anterior pararenal space of the
retroperitoneum, which also includes the second-fourth
segments of the duodenum and the ascending and
descending segments of the colon. The gland is an elongated
structure situated in the transverse plane, with the head to
the right of the midline, surrounded by the c-loop of the
duodenum, and the body/tail extending laterally and slightly
cranially to the splenic hilum. The head, neck (isthmus), and
body are almost always visible via transabdominal ultrasound;
the tail and uncinate are variably obscured by bowel gas. The
gland is typically isoechoic or slightly hyperechoic to the liver,
often increasing in echogenicity with age, which may in part
be secondary to increasing lipomatosis.
In patients with good sonographic visualization, the pancreatic
duct can be identified as a thin curvilinear structure situated
within the center of the gland, oriented along the long axis,
although when normal in caliber it may not always be visible. It
can be seen as two thin echogenic lines, representing the
epithelial walls of the duct, separated by a thin hypoechoic
layer of fluid within the duct itself. Other readily visible
anatomic landmarks include the superior mesenteric vein
between the uncinate and pancreatic neck; in the head, the
gastroduodenal artery anteriorly and common bile duct
posteriorly; and in the body, the splenic vein along the
posterior margin.
Anatomy-Based Imaging Issues
Frequently, the pancreatic tail, and often parts of the distal
body, are not visible secondary to the presence of gas within
the stomach, colon, and small bowel. Obesity is another
common limitation in scanning of the pancreas. Related fatty
infiltration of the liver may alter the relative echogenicity of
the pancreas, which may then appear as hypoechoic relative
to the steatotic liver, potentially mimicking a pathologic
process such as pancreatitis.
Pathologic Issues
The pancreas can be affected by acute and chronic
inflammatory processes, benign and malignant cystic and solid
neoplasms, and autoimmune processes.
Imaging Protocols
Transabdominal ultrasound imaging can be facilitated by
fasting prior to the exam, preferentially for at least six hours
or overnight, in order to reduce the amount of gas within the
stomach and bowel. Imaging is obtained with a curved
transducer with the highest possible frequency, typically up to
five MHz, although technological advances on modern
scanners may allow for imaging at up to nine MHz without loss
of acoustic penetration. Tissue harmonic imaging is used to
improve image quality, particularly of fluid-filled structures
such as cystic lesions, pancreatic duct, and the vasculature
system. Compound imaging is used to improve tissue contrast
and spatial resolution. Doppler ultrasound is essential to
evaluate the vascular structures, as well as internal vascularity
of tumors.
The gland should be evaluated in both the transverse and
longitudinal planes. Imaging in different orientations such as
in decubitus or erect positions, or with suspended respiration
(inspiration or expiration), may improve visualization of
structures not visible in the usual supine position. Graded
continual transducer pressure on the abdomen can improve
visualization by collapsing and mobilizing bowel; however, this
may be limited by focal tenderness depending upon the
clinical setting. Although not routinely utilized, a moderate
amount (100-300 mL) of degassed water or oral contrast
administered prior to imaging can improve visualization of the
tail; however, this can also introduce air bubbles leading to
additional artifacts. Overdistention of the stomach should be
avoided, as it is less compressible and may make the exam
uncomfortable for the patient. The spleen can be used as an
acoustic window to visualize the pancreatic tail.
Contrast-enhanced ultrasound can be obtained using second
generation microbubble contrast agents, following a
conventional ultrasound in which focal or diffuse pancreatic
pathology has been detected. As microbubble contrast
remains entirely intravascular, the distinction between solid
and cystic masses is improved. Parenchymal enhancement can
also be evaluated, which can potentially aid in distinguishing
focal pancreatitis from neoplasm. Imaging requires specialized
software, most commonly pulse inversion, to suppress
background tissues and allow visualization of only vascularized
structures. Imaging acquisition occurs immediately after
intravenous administration in order to evaluate the arterial
inflow to the pancreas and early parenchymal enhancement.
Usage is limited in the United States, as there are no contrast
agents approved by the Food and Drug Administration for
noncardiac use.
Clinical Implications
The major role of sonography in imaging of the pancreas is in
the evaluation of acute pancreatitis and pancreatic
malignancy.
Acute Pancreatitis
Acute pancreatitis is diagnosed by a combination of clinical
presentation and laboratory abnormalities, with imaging
acquired to evaluate atypical presentations and for
complications. Ultrasound is the primary imaging test
obtained within the first 48-72 hours in a patient presenting
for the first time with classic pancreatitis, in order to assess for
the presence of gallstones. Transabdominal ultrasound is
limited in evaluating the pancreas in the acute inflammatory
phase, and findings may be subtle or absent in mild cases.
Grayscale assessment includes evaluation of the pancreatic
parenchyma for signs of hemorrhage or necrosis, and
peripancreatic tissues for the presence of fluid and fluid
collections. The duct is visualized for signs of obstruction,
either from stones in the common bile duct or secondary to
pancreatic edema. Color Doppler can demonstrate the
presence of splenic vein thrombosis.
Chronic Pancreatitis
Chronic pancreatitis results from progressive destruction of
the gland secondary to multiple episodes of mild or even
subclinical pancreatitis, with development of fibrosis and
atrophy. Ultrasound is not sensitive for the diagnosis;
however, the presence of ductal dilatation with ductal and
parenchymal calcifications is highly suggestive. The location of
stones, i.e., intraductal vs. parenchymal, may be better
demonstrated with ultrasound than with CT.
Diffuse or focal enlargement of the gland is common, and the
appearance can mimic neoplasm, particularly when focal in
the pancreatic head. Contrast-enhanced MR and endoscopic
ultrasound (EUS) are useful for distinguishing the two.
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