Some studies include a required length of dilated bowel. Some studies use a lower threshold for the ileum (15 mm). The small bowel is considered dilated when greater than 25 mm in diameter ( Fig. The hallmark finding of small bowel obstruction (SBO) is fluid-filled, dilated loops of bowel. The air-filled transverse colon is demonstrated here. The large bowel has periodic indentations in the wall, creating segments known as haustra ( arrows ). The appendix will be discussed in the next section. These sonographic findings should be combined with the anatomic location of the bowel to determine the bowel loop under examination. At the terminal ileum, a hypoechoic “halo” may be seen representing the muscular lips of the ileocecal valve. The ileum has fewer of these mucosal folds and may have a smooth appearance. This can create a “feather-like” or “herring-bone” appearance ( Fig. The jejunum contains plicae circulares seen 3 to 5 mm apart. The duodenum is difficult to see on ultrasound, but a transverse segment can be seen on the caudal edge of the liver ( Fig. However, bowel can be visualized when it is filled with liquid, semisolid, or solid material. This shadow is typically of mixed echogenicities (“dirty shadow”), as opposed to the typically anechoic shadows (“clean shadow”) of bone or calcium stones. This gas creates a shadow artifact that prevents visualization of any structures posterior to this artifact. This technique, known as graded compression, is performed in an attempt to disperse bowel gas and to bring the probe closer to the abdominal contents by compressing abdominal wall soft tissue toward muscular structures of the back. The bowel is examined in a systemic fashion, which is sometimes described as a “lawn mower pattern.”Īs the sonographer moves the probe across the abdomen, he or she will hold steady, consistent, posteriorly directed pressure into the abdomen at 1- to 2-cm intervals. Projected path of the probe when evaluating bowel. This pattern is repeated until the whole abdominal surface has been covered. The probe is then moved slightly medially and now scanned inferiorly, parallel to the original path. In this method, begin in the right lower quadrant with the probe held in a transverse orientation (indicator directed to the patient’s right) and scan superiorly toward the right upper quadrant. One technique is described as the “lawn mower” pattern ( Fig. The bowel should be evaluated in a systematic fashion with the patient supine. The higher-frequency linear array can be used to evaluate thin or pediatric patients and to examine bowel adjacent to the abdominal wall in greater detail. Imaging of the bowels is performed using a low-frequency transducer, such as a curvilinear probe (typically 3- to 6-MHz frequency), which allows for good penetration and a large field of view. Ultrasound Techniques and Normal Findings Reprinted from Netter Anatomy Illustration Collection.
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