Diagnostic Ultrasound - Abdomen and Pelvis

Approach to Bowel Sonography
Given the relatively fixed position of the colon, the
examination begins with identifying the cecum in the right
lower quadrant. The characteristic blind ending arc of large
bowel gas can be identified, which leads into the terminal
ileum. The colon is first assessed systematically by performing
a sweep along the entire path of the colon down to the
sigmoid colon and rectum in the pelvis. The rectum can be
visualized with a full bladder, which serves as an acoustic
window. In certain circumstances, the transvaginal probe can
be used to assess the rectum and pelvic loops of bowel that
are obscured by bowel gas or adipose tissue on
transabdominal scan. This process is repeated in a retrograde
manner. When indicated, a focused assessment is performed
of the appendix and terminal ileum. Then the small bowel is
visualized with a generalized sweep over the four quadrants
of the abdominal cavity.
Slow-graded sonographic compression is applied when
scanning the bowel. Normal bowel is readily compressible,
shows peristaltic activity, and dispels intraluminal gas. This is a
crucial observation in contrast to abnormal bowel, which is
usually thick walled, rigid, demonstrates reduced peristalsis,
and remains in a relatively fixed position. This technique also
allows for the demonstration of localized peritonism, which
increases the index of suspicion for underlying focal
pathology.
Careful attention is required for the detection of thickened
segments looking for alterations in the gut signature or focal
mass lesions, in particular at sites of tenderness. Bowel
presets that offer higher contrast images or adjustment of
gain allow for the clearer demonstration of the gut signature
and changes related to pathology. Any thickened segment
should be assessed in two orthogonal planes. The effect on
the gut wall layers should be scrutinized and the length of
involved bowel documented.
The adjacent fat planes in thickened segments should be
assessed for useful secondary signs, such as presence of
extramural spread of disease, tracts, collection, enlarged
lymph nodes, and inflamed hyperechoic fat. The inflamed
hyperechoic fat is a useful clue and draws attention to the
underlying thickened loop of bowel. These regions of interest
should be reassessed more thoroughly using a high-frequency
7-12 MHz linear probe. While there is increased resolution,
note that there is limited depth of penetration as a trade off
determined by the body habitus.
Doppler assessment provides additional information
regarding the vascularity of the bowel wall and adjacent fat;
however, this technique has a limited sensitivity due to
artifact. Increased vascularity is typically seen in infectious or
inflammatory thickening and absence of flow can potentially
indicate bowel ischemia. These findings require correlation
with the underlying clinical context and baseline ultrasound
images.
Contrast-enhanced ultrasound (CEUS) is now is being used to
provide a more detailed and dynamic assessment of
vascularity of thickened segments of bowel. CEUS is the
application of intravenous contrast agent to conventional
grayscale ultrasound imaging. The agent used is highly
reflective, gas-filled microbubbles. The microbubbles enhance
intravascular contrast by increasing ultrasound backscatter.
The contrast agent can demonstrate intravascular blood flow
as well as different phases of organ and bowel enhancement
similar to CT and MR, hence disease activity and perfusion can
be assessed. This is a quick and safe technique, which can be
performed at the bedside with no risk of nephrotoxicity.
The motility of bowel loops should be assessed. Ultrasound is
a dynamic study and offers real-time imaging. US has a
significant advantage over CT and MR, which can be
hampered by artifact from spasm that may mimic a stricture.
The examination is completed with assessment of the
stomach. The stomach and small bowel may be reassessed
after providing the patient with oral fluid intake. Dedicated
bowel ultrasound following luminal distension with water (US
enterography) provides contrast, which allows for
characterization of the gut wall and lumen. Routine use of this
technique may not be feasible due to time constraints.
Limitations
Limited visualization when large amount of subcutaneous
adipose tissue is present
• Hinders penetration of ultrasound waves, resulting in
poor image quality
• Graded compression challenging
Overlying intraluminal bowel gas:
• May mask deep-seated pathology, such as retro-cecal
appendicitis
Pathology-Based Imaging Issues
Meticulous assessment of the bowel wall thickness, effect on
the gut signature and motility, in conjunction with the clinical
history, forms the basis of ultrasound evaluation of the gut.
Benign disease: Inflammatory conditions
• Long segment involvement
• Uniform thickening
• Preservation and exaggeration of gut signature
Neoplastic disease; bowel carcinoma
• Short segment involvement
• Eccentric, asymmetric, and irregular mural thickening
These are general guidelines. Some conditions may overlap;
for instance, in chronic sigmoid diverticulitis, the findings may
mimic a cancer. With ischemia, there may be disruption of the
layers due to bowel infarction.
Motility is typically reduced in both inflammatory and
neoplastic conditions.
Ultrasound is useful in identifying fluid-filled, dilated loops of
bowel in underlying bowel obstruction. The dilated loops can
be traced downstream to identify the site and cause of
obstruction at the transition point. Motility of the dilated
loops is useful to distinguish ileus from a mechanical
obstruction.
• Pitfall: Dilated loops can be adynamic in prolonged
obstruction
Using these fundamental principles to approach bowel
assessment, together with the clinical picture, an accurate
diagnosis can be made.
Selected References
1. Muradali D et al: US of gastrointestinal tract disease. Radiographics. 35(1):50-
68, 2015
2. O'Malley ME et al: US of gastrointestinal tract abnormalities with CT
correlation. Radiographics. 23(1):59-72, 2003
3. Puylaert JB: Acute appendicitis: US evaluation using graded compression.
Radiology. 158(2):355-60, 1986
Diagnoses: Bowel
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