ENDOSCOPIC ULTRASOUND IMAGING
Category: Diagnostic Radiology
Abstract : endoscopic ultrasound imaging : Some of the limitations of conventional
ultrasonography in biliary and pancreatic imaging can be overcome by the use of
endoscopic probes and miniprobes. Endoscopic probes are either radial or linear
arrays which are incorporated into the end of an endoscope. They have a
frequency of 7.5–12 MHz and are used to image the pancreas, biliary tract,
po
endoscopic ultrasound imaging :
Some of the limitations of conventional
ultrasonography in biliary and pancreatic imaging can be overcome by the use of
endoscopic probes and miniprobes. Endoscopic probes are either radial or linear
arrays which are incorporated into the end of an endoscope. They have a
frequency of 7.
5–12 MHz and are used to image the pancreas, biliary tract,
portal vein and adjacent structures within 5–6 cm of the probe. Radial probes
may be used in the preoperative staging of a number of diseases, including
oesophageal, gastric, pancreatic and lung cancer, whilst linear array probes are
used for interventional procedures such as fineneedle aspiration analysis of
mediastinal lymph nodes, solid organ assessment, for example pancreas,
occasionally liver, adrenals, pseudocyst drainage and coeliac plexus neurolysis.
Endoscopic ultrasound is more sensitive and specific than spiral CT, MRI or
transabdominal ultrasound in the detection of small pancreatic masses and its
diagnostic ability can be further enhanced by the use of endoscopic
ultrasonically guided fineneedle aspiration cytology and biopsy.
It may
also detect early changes of pancreatitis which are not visible on endoscopic
netrograde cholangiopancreatography (ERCP), and one of its main uses is in
staging pancreatic tumours, predicting their resectability, identifying small
lymph node metastases and assessing vascular invasion. It is particularly
accurate in identifying small pancreatic insulinomas, often difficult or
impossible to identify on conventional cross-sectional imaging despite a
documented biochemical abnormality, and thus guiding subsequent surgical
procedures. Endoscopic ultrasound is also used in the detection of biliary
calculi, particularly in the normal-calibre common bile duct, with a much higher
accuracy than other imaging techniques and without the potential additional
risks of ERCP.
Further, less-established uses of endoscopic ultrasound
include gastrointestinal examinations, in which invasion of gastric lesions into
and through the wall of the stomach can be assessed, anal ultrasound, which is
used to visualize the sphincter muscles in cases of sphincter dysfunction, the
staging of colorectal carcinomas and the demonstration of bowel wall changes in
inflammatory bowel conditions.
The miniprobe has a higher frequency
(20–30 MHz) and may be passed down a conventional endoscope. It therefore has
the advantage of a onestage gastrointestinal tract endoscopy/ERCP, rather than
requiring a separate procedure. It may be inserted into the common duct of the
biliary tree to assess local tumour invasion and to clarify the extent and/or
nature of small lesions already identified by other imaging methods. It shows
remarkable accuracy in the detection of common bile duct tumours and other
biliary tract disease when compared with other imaging modalities. It may be
used in the staging of oesophageal and gastric cancer, and is especially useful
when a tight oesophageal stricture prevents the passage of the endoscope. The
layers of the oesophageal or gastric wall and the extent of tumour invasion can
be accurately assessed.
The miniprobe is also used in patients with
suspected pancreatic carcinoma, for example in patients with a negative CT but
who have irregularity of the pancreatic duct on contrast examination. The probe
can be passed into the pancreatic duct during ERCP to detect small lesions,
assess the extent of the tumour and predict resectability. It is superior to
conventional endoscopic ultrasound in the detection of the smaller, branch
tumour nodules, and can also detect local retroperitoneal or vascular invasion
in areas adjacent to the probe.
The use of endoscopic ultrasound is
currently limited to a few specialist centres. A steep learning curve together
with the expense of the equipment is likely to restrict its widespread use;
however, as its applications expand and its value becomes proven, it is likely
to become a more routine investigation at many centres.
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