Published in IVIS with the permission of the editor Close window to return to IVIS Ultrasonography of the small intestine in small animals Examination techniques Ideally, the patient should be fasted overnight to reduce the interference with gastric contents and intraluminal gas. However, non-fasted dogs may show adequate image quality. The intra-luminal gas causes imaging artifacts, such as reverberation, comet tail and acoustic shadowing. Sedation is not usually necessary, but when needed, xylazine Amalia Agut, DVM, PhD, Dipl. ECVDI should be avoided because it causes gastric stasis The Veterinary Faculty, Department of Medicine leading to massive gaseous distension (1). and Animal Surgery, University of Murcia, Spain In 1981 Dr. Agut graduated from the University of The animal is typically placed in dorsal recumb- Zaragoza in Spain and obtained her PhD in 1984. ency, although the position may depend on the She joined the University of Murcia in 1985 and is patient’s restlessness, discomfort, or on the operat- currently Lecturer in Radiology. She received her Diploma in the European College of Veterinary or’s preferences. The examination with the patient Diagnostic Imaging (ECVD) in 1998. Dr. Agut’s main standing, or via a hole in the supporting table, the interests lie in the fields of ultrasound in small recumbent side is useful. This makes use of the animals. intraluminal fluid gravitating to the dependent wall of the stomach or intestinal tract where it serves as an acoustic window (2). KEY POINTS Five MHz, 7.5 MHz or higher frequency transducers ± Ultrasound examination of the small intestine has are used, with higher frequency transducers offer- become routine in investigation of the intestinal ing the best resolution of bowel wall layers (1,2). diseases Transducers with small footprint are useful to ± The principal limitation of intestinal tract ultrasound is evaluate the proximal duodenum when the probe the presence of luminal gas must be placed below the rib cage or between ribs (2). ± Radiographs should precede the ultrasound examination to evaluate the amount, location and pattern of intestinal gas Ultrasonographic anatomy ± Ultrasound examination can provide information of of the small intestine bowel wall thickness, layering of the wall, peristalsis, The duodenum is located in the right side of the and luminal contents abdomen, beginning from between the last ribs and then followed distally along the right body wall. The other portions of small bowel are assessed moving the transducer from right to left and left to 20 / / Veterinary Focus / / Vol 19 No 1 / / 2009 Published in IVIS with the permission of the editor Close window to return to IVIS Figure 1. Figure 2. Ultrasonographic layers of the intestinal wall. Ultrasound image of the ileocolic junction of a cat. s (submucosa), m (musculature). right, and then from cranial to caudal to image the Wall thickness entire small intestinal tract. Sections of small The thickness of the bowel wall is measured intestine will be viewed sagittally, transversely and between the outer echogenic serosal surface and in various oblique images, depending on the trans- the mucosal-luminal interface (Figure 1). In dogs, ducer and intestinal tract position (2). The ileum the intestinal wall is between 2-6 mm thick depend- can be identified by its location in the right mid-to ing on the size of the dog and part of the small cranial abdomen and its relationship with the bowel (Table 1 and 2) (5), while in cats a mean ascending colon and cecum. In the intestinal tract of 2 mm thick has been evaluated (Table 1) (6). wall layers, wall thickness, peristalsis, and luminal contents must be assessed (1). Luminal patterns The ultrasonographic appearance of the small Layers of the bowel wall intestine depends on the type and amount of lumin- Five ultrasonographic layers can be identified al content (Figure 3). When empty, a “mucous in the small bowel, corresponding from the pattern” is present, the bowel lumen appears as a lumen outward, to the luminal/mucosal interface, hyperechoic core ("mucosal stripe") surrounded mucosa, submucosa, muscular, and the serosa layer by a hypoechoic halo of the bowel wall. This (Figure 1). The mucosa and muscular layers are hyperechoic core represents mucus and small air hypoechoic, whereas mucosal surface, submucosa bubbles trapped at the mucosal-luminal interface. and serosa are hyperechoic (3). The mucosal layer When fluid is present in the bowel lumen (“fluid is the thickest layer of the intestinal wall. The ileum pattern”), an anechoic area is seen between the in cats can be identified by a thicker echogenic walls of the bowel that appears tubular in long axis and irregular submucosal layer (Figure 2) (4). views, and circular in short axis views. Gas-filled Table 1. Table 2. Normal range of wall thickness for different Normal range of wall thickness (mm) for the segments of the intestinal tract in cats (4) and different segments of the intestinal tract in dogs dogs (2) based on body weight (5) Wall thickness Cats Dogs Body weight Duodenum Body Jejunum (mm) (kg) weight (kg) Duodenum 2.0 - 2.4 3 - 6 < 20 < 5.1 < 20 < 4.1 Jejunum 2.1 - 2.5 2 - 5 20 - 29.9 < 5.3 20 - 39.9 < 4.4 Ileum 2.5 - 3.2 2 - 4 > 30 < 6 > 40 < 4.7 Vol 19 No 1 / / 2009 / / Veterinary Focus / / 21 Published in IVIS with the permission of the editor Close window to return to IVIS A A B B Figure 3. Figure 4. Luminal Patterns: A.- Longitudinal sonogram of a normal duo- A.- Transverse and B.- Longitudinal sonogram of a jejunal denum. The mucous (m) and gas (g) patterns can be observed. intussusception in a 5-year old German Shepherd. The hypo- B.- Transverse sonogram of jejunum segments. Fluid pattern is echoic and hyperechoic rings with hyperechoic center (Fat) is noted. consistent with intussusceptions. small intestinal loops cause a highly echogenic within the jejunum, ileocolic or ileocecal junctions interface with distal acoustic shadowing or reverb- or within the colon (colocolic) and rarely do they eration (“gas pattern”) (7). involve the stomach or duodenum. They often occur in puppies and kittens secondary to primary Peristalsis intestinal disease such as enteritis from intestinal The mean number of peristaltic contractions parasites, bacterial or viral infections. In older observed in the proximal duodenum is four to patients, the intussusceptions can occur close to five per minute and two contractions per minute pseudocysts, enlarged lymph nodes, foreign bodies in the jejunum and ileum (7). or tumoral mass (2). Ultrasonographic patterns of Intussusceptions have a characteristic ultrasound small intestinal diseases appearance that in most cases allows a definitive Intussusception diagnosis to be made with confidence (8). Intussusception is the invagination of a portion of the intestine, called intussusceptum, into the The most common sonographic pattern observed in lumen of an adjacent segment of intestine, called transverse sections of the bowel is a target-like mass intussuscipiens. Intussusceptions usually occur consisting of multiple hyperechoic and hypoechoic 22 / / Veterinary Focus / / Vol 19 No 1 / / 2009 Published in IVIS with the permission of the editor Close window to return to IVIS ULTRASONOGRAPHY OF THE SMALL INTESTINE IN SMALL ANIMALS A Figure 6. The linear foreign body appears as a bright linear interface in the bowel lumen (arrow). with irreducible intussusceptions (10). However, the recognition of blood flow in the intussuscepted bowel using color flow Doppler ultrasonography appears to be the most valuable factor for predict- B ing bowel reducibility (10). Figure 5. Foreign bodies Intestinal foreign bodies. A.- A rubber teat is in the lumen of a In small animals, foreign bodies are the main cause bowel loop as two ovoid echogenic lines (arrows). B.- A peach pit of mechanical obstruction. A distension of the small is in the lumen of the bowel, as curved interface with small intestine with fluid, gas, or a combination of both, is protuberances associated with strong acoustic shadowing. an indicator of mechanical ileus (obstruction). The degree of bowel distension depends on whether the obstruction is partial or complete, on the duration and the location of the obstruction. When the entire concentric rings around a hyperechoic center that small intestine is dilated, it is important to different- represents the entrapped mesentery (Figures 4A iate diffuse intestinal disease (e.g., parvovirus and 4B). In longitudinal sections, multiple hyper- infection) from a distal small bowel obstruction. echoic and hypoechoic parallel lines are usually Visualizing a small portion of normal, non-distend- visible. Ultrasonographic patterns may vary with ed small bowel (distal to the obstruction) is a clue the length of bowel involved, the duration of the that a distal obstruction is present (1). process, and the orientation of the scan plane in relation to the axis of the intussusception (9). So, Some foreign bodies like balls or rocks are easily in some instances the concentric or layered appear- identified by ultrasonography because of their ance is distorted and not as easily recognized characteristic shape and the presence of acoustic because of inflammation and edema. The presence shadowing. However, the balls may vary in echo- of a thin, external hypoechoic ring of the target-like genicity, depending on their physical properties mass is usually associated with reducible intus- (Figure 5A). Most of foreign bodies produce a susceptions. The appearance of fluid within the bright interface associated with strong shadowing. apex of the intussusceptions, absence of peristaltic Occasionally, the contour of the interface can activity of the bowel, and enlarged lymph nodes help identifying the type of foreign body in the in the intussuscepted intestine are compatible intestine (Figure 5B) (2). Vol 19 No 1 / / 2009 / / Veterinary Focus / / 23 Published in IVIS with the permission of the editor Close window to return to IVIS The ultrasonographic appearance of the perforated The presence of gastrointestinal parasites can mimic bowel wall by foreign bodies, such as sticks, is a the appearance of a linear foreign body (7,12).