Diagnostic Ultrasound - Abdomen and Pelvis
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Diagnostic Ultrasound - Abdomen and Pelvis

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09.07.2019 Views

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 643

Approach to Bowel Sonography Diagnoses: Bowel (Left) US depicts suboptimal resolution of bowel . This is due to a careless wide field of view and poor depth control, hence there is loss of detail of the bowel near the abdominal wall and poor orientation. Image optimization is a key step in bowel ultrasound. (Right) US in the same patient following adjustment of the depth and field of view denotes normal cecum and characteristic bowel gas pattern . There is normal compression of the terminal ileum between the abdominal wall and psoas muscle. (Left) Axial US in the right lower quadrant denotes useful landmarks of the peritoneal cavity. Note abdominal wall interface with the peritoneum and the margin with the psoas muscle . Terminal ileum (TI) is also shown. It is important to identify landmarks for correct anatomic orientation. (Right) Ileocecal junction ſt is shown in the same patient using a 12 MHz linear transducer. TI and cecum are noted. Highresolution transducer allows demonstration of the gut signature. (Left) Short axis US using linear high-resolution 12 MHz shows Crohn colitis. Calipers denote the single wall thickness of the colon. Note the exaggerated stratified layers of the colonic wall, a hallmark of benign inflammatory disease. (Right) Corresponding long axis US in same patient outlines the full thickness of the colon . This demonstrates preservation of the gut signature, a feature of benign inflammatory thickening. It is important to assess thickened segments in 2 orthogonal planes. 644

Approach to Bowel Sonography<br />

Diagnoses: Bowel<br />

(Left) US depicts suboptimal<br />

resolution of bowel . This is<br />

due to a careless wide field of<br />

view <strong>and</strong> poor depth control,<br />

hence there is loss of detail of<br />

the bowel near the abdominal<br />

wall <strong>and</strong> poor orientation.<br />

Image optimization is a key<br />

step in bowel ultrasound.<br />

(Right) US in the same patient<br />

following adjustment of the<br />

depth <strong>and</strong> field of view<br />

denotes normal cecum <strong>and</strong><br />

characteristic bowel gas<br />

pattern . There is normal<br />

compression of the terminal<br />

ileum between the abdominal<br />

wall <strong>and</strong> psoas muscle.<br />

(Left) Axial US in the right<br />

lower quadrant denotes useful<br />

l<strong>and</strong>marks of the peritoneal<br />

cavity. Note abdominal wall<br />

interface with the peritoneum<br />

<strong>and</strong> the margin with the<br />

psoas muscle . Terminal<br />

ileum (TI) is also shown. It<br />

is important to identify<br />

l<strong>and</strong>marks for correct<br />

anatomic orientation. (Right)<br />

Ileocecal junction ſt is shown<br />

in the same patient using a 12<br />

MHz linear transducer. TI <br />

<strong>and</strong> cecum are noted. Highresolution<br />

transducer allows<br />

demonstration of the gut<br />

signature.<br />

(Left) Short axis US using<br />

linear high-resolution 12 MHz<br />

shows Crohn colitis. Calipers<br />

denote the single wall<br />

thickness of the colon. Note<br />

the exaggerated stratified<br />

layers of the colonic wall, a<br />

hallmark of benign<br />

inflammatory disease. (Right)<br />

Corresponding long axis US in<br />

same patient outlines the full<br />

thickness of the colon . This<br />

demonstrates preservation of<br />

the gut signature, a feature of<br />

benign inflammatory<br />

thickening. It is important to<br />

assess thickened segments in 2<br />

orthogonal planes.<br />

644

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