Diagnostic Ultrasound - Abdomen and Pelvis

Approach to Bowel Sonography
642
Diagnoses: Bowel
Introduction
Assessment of the bowel with ultrasound has been
undervalued due to the commonly held perception that
artifact (notably from gas in the lumen of the bowel) will limit
visibility. The lack of awareness of normal and diseased bowel,
limited technical expertise, and prolonged time of study are
some compounding factors that have lead to a significant
inconsistency in radiology departments for the application of
ultrasound in the assessment of the bowel. US is safe,
noninvasive, inexpensive, and does not use ionizing radiation.
Real-time images can be obtained to provide a dynamic
assessment. With the appropriate skills and knowledge,
ultrasound has a valuable role in the assessment of patients
with suspected and known bowel diseases. US remains the
first-line imaging modality in acute abdominal conditions,
particularly in Europe, hence a systematic approach and
familiarity of the sonographic appearances of the spectrum of
diseases affecting the bowel is crucial.
Anatomy-Based Imaging Issues
The gut is a long, tortuous, confluent hollow viscus that has a
reproducible stratified layered pattern on ultrasound referred
to as the gut signature. This alternating layered pattern
corresponds to distinct histological layers of the normal bowel
wall.
Five sonographic layers are identified as alternating
hyperechoic and hypoechoic layers. These consecutive layers
from innermost (layer 1) to outermost (layer 5) are listed
below.
• Layer 1: Interface of lumen and mucosa
• Layer 2: Muscularis mucosa
• Layer 3: Submucosa
• Layer 4: Muscularis propria
• Layer 5: Serosa
There are typically three hyperechoic layers (layer 1, 3, and 5).
These three hyperechoic layers are the interface of the lumen
and mucosa, submucosa and serosa. The hypoechoic layers
(layers 2 and 4) represent the two respective muscular layers;
the muscularis mucosa (inner) and muscularis propria (outer),
respectively.
The number of visible layers depends on:
• Frequency of transducer
• Proximity of transducer to gut wall
The trade off between the improved resolution with a highfrequency
probe is the depth of penetration of the sound
beam. It is ideal to maximize the frequency to obtain the best
permissible resolution; however, this is reliant on the body
habitus of the patient.
On a routine 5 MHz curvilinear probe, the transverse section
of the bowel demonstrates a stratified target-like appearance
with an inner hyperechoic layer representing the submucosa
and a hypoechoic outer rim of the muscularis propria.
Using high-resolution linear transducer (12-18 MHz), the five
distinct layers can be readily appreciated. On axial imaging, a
pattern of concentric rings of alternating bands of hyper and
hypoechoic rings can be seen.
The technological advances in transducers, combined with
endoscopy, have lead to the evolution of endoscopic
ultrasound technique. This represents the combined
endoscopic and ultrasound assessment of the gut wall. The
direct contact of the ultrasound transducer with the gut wall
and high-frequency transducer allows for a focused
assessment of a lesion, depiction of the depth of penetration
in relation to the gut wall layers and lumen. In addition,
histological assessment with needle biopsy is now also
possible using this minimally invasive technique.
Normal bowel has an average maximal single wall thickness of
3-5 mm dependent on the degree of distension.
Fixed points of the bowel are easier to assess with
transabdominal ultrasound: C loop of the duodenum, ileocecal
junction, rectum, and recto sigmoid.
Features of small bowel loops are the central location,
presence of valvulae conniventes, and active bowel peristalsis.
In contrast, the colon has a heterogenous haustral pattern
associated with a prominent linear arc of gas and posterior
reverberation artefact.
Clinical Implications
US of the bowel:
• Can be part of routine abdominal scan for nonspecific
broad range of abdominal symptoms
• Has an established focused role in suspected acute GI
conditions, such as acute appendicitis, pyloric stenosis,
and intussusception, especially in children and young
adults
• Can be used for dedicated assessment of chronic gutrelated
disorders
In patients with Crohn disease, ultrasound has a role in:
• Detection of disease
• Evaluation of disease extent
• Detection of complication such as formation of abscess,
fistula, or obstruction
• Assessment of activity and follow-up of disease
Given the increasing use of ultrasound as the first-line
modality, in addition to the standard assessment of the solid
abdominal viscera, it is crucial for the operator to have an
understanding of the principles of bowel ultrasound to avoid
overlooking a GI tract-related diagnosis. The value is
significantly felt in the younger population where ultrasound
is much better tolerated and can significantly reduce the
radiation burden of unjustified CT scans.
One of the inherent strengths of performing ultrasound is the
instant clinical history and specific detail that is directly
available from the patient. Ultrasound becomes an extension
of the clinical examination and facilitates the diagnostic
process. With accurate history and clinical signs, the operator
can formulate a tailored approach to the ultrasound
examination. Note that sonographic findings often overlap,
hence the clinical history is crucial to determine the accurate
diagnosis.
Imaging Technique
Routine transabdominal assessment of the bowel is typically
performed after assessment of the solid abdominal viscera.
The assessment of the bowel starts by using a standard 5 MHz
curvilinear transducer. While scanning, it is important to
optimize the settings, in particular the focal point and dynamic
contrast, to enhance image quality. Recognizing useful
landmarks such as the interface of the peritoneum with the
abdominal wall and the psoas muscle provides an indicator of
the depth of assessment and is helpful to avoid any confusion
at the outset.