Diagnostic imaging of the intestines

RADIOLOGY

Small bowel_¹: the duodenum is often identifiable in plain X-rays of the abdomen as a straight part of bowel in a characteristic position (near the right abdominal wall in a sagittal view and in the centre of the abdomen in lateral projections); it is easier to see if distended by gas. The loops of the jejunum and ileum, although often central, vary in position depending, in part, on the volume of the other abdominal organs.

In the dog, the normal diameter of the loops of the small intestine, measured radiologically, should be less than the height of the central part of the second lumbar vertebra (L2), less than 1.6 times the height of L5, or twice the width of a rib. In the cat, the diameter should be less than double the height of the central part of L4.

Large bowel_²: the caecum in dogs can often be identified in radiographs of the abdomen thanks to its semi-circular shape and the gas that it frequently contains; it is located on the right of the sagittal plane, near to the second and third lumbar vertebrae. In the cat it is cone-shaped and almost never visible radiographically. In sagittal radiographic views, the colon has the shape of a question mark. The ascending colon is found on the right of the midline sagittal plane, the transverse colon runs from right to left, cranial to the mesenteric root. The proximal part of the descending colon is located left of the sagittal midline plane, with the proximal part running along the midline. The rectum originates at the level of the entry of the bowel into the pelvis and terminates at the proximal part of the anal canal. Contrast radiography of the large intestine can be carried out using a radio-opaque enema or double contrast, although this is now a rarely used procedure³.

Abnormal radiographic findings

Dimensions:

Ileus_¹: this is a block of normal intestinal transit, with consequent dilatation of the lumen and the presence of gas, liquids or food in the lumen. X-rays of the abdomen are useful for differentiating between the two types of ileus: “mechanical” and “functional”.

Mechanical ileus is the result of a physical obstruction of the intestinal lumen (possible causes are masses, ingested foreign bodies, adhesions, volvulus or intestinal invagination). The radiographic appearance depends on the duration and the degree of the obstruction. The dilatation often appears localised in mechanical ileus. If the occlusion is complete, the part cranial to the obstruction becomes dilated, whereas the part distal remains empty or a normal size (Fig. 1). The dilated part, cranial to the occlusion, contains fluid or, more frequently, gas, creating the characteristic “barrier” appearance; dilatation of the small intestine is considered significant when the normal diameter has doubled. The finding of bowel loops with a diameter more than 1.6 times the height of L5 is strongly indicative of an intestinal obstruction⁴.

If the occlusion is partial, the part of the gastrointestinal tract cranial to the narrowing usually contains gas or undigested food which often has a radio-opaque, granular appearance (the so-called gravel sign). The loops cranial to the partial occlusion may be dilated, but to a lesser extent than in cases of complete occlusion.

Depending on the foreign body or tumour that causes the partial or complete occlusion, there are “characteristic” radiographic images that can help to reach the diagnosis; for example, an “accordion” appearance should raise the suspicion of a linear foreign body (string, rope). Radio-opaque foreign bodies (metal objects, bone, etc.) can be seen easily in plain X-rays. If the foreign body is radiolucent_⁵ and occlusion or sub-occlusion is suspected, contrast radiography can be useful; the contrast agent is blocked, thus demonstrating the site of the obstruction.

As far as concerns the large bowel, a change in size may be due to megacolon_², a pathological condition characterized by a notable increase in the dimensions of the colon together with loss of motility, consequent constipation, and accumulation of dry faeces.

Functional ileus is caused by a dilation of the intestines without an obvious cause of obstruction. It can be the result of an autonomous nervous system disorder_⁶, spinal cord trauma, chronic mechanical obstruction, volvulus, viral enteritis, chronic enteritis, peritonitis, or pancreatitis. It is often a transient phenomenon that is reversible without surgery being necessary. There is a total absence of peristalsis and uniform dilatation of all the loops involved; from a radiographic point of view, there are no signs able to differentiate unequivocally a functional ileus from a mechanical one; however, in the former case, the loops are mildly to moderately dilated and the involvement is generalised_¹ (Fig. 2).

Position_^1,2: given the natural mobility of the bowel within the abdominal cavity, displacement of the intestinal loops is often a radiographic sign of disorders of other abdominal or retroperitoneal organs (pancreatic, hepatic, splenic, renal, lymph node, peritoneal, retroperitoneal tumours). In the case of diaphragmatic hernia, the intestinal loops can be displaced into the chest. Evaluation of the thickness and radio-opacity of the intestinal wall is very difficult because of the superimposition of other organs and the mixture of fluids and gas present within the bowel.

ULTRASONOGRAPHY

Ultrasound examination is the method of choice for studying the intestine: it is cheap, can be carried out quickly, with the animal conscious, and enables both the small and the large bowel to be evaluated accurately. It is always advisable to keep the animal fasted for at least 12 hours before the examination, since intraluminal gas and food can hamper a study of the intestinal loops. The examination should always be carried out in both lateral decubitus positions and in the dorsal view, in order to move the gas within the loops and visualise the whole intestine better.

Measured by ultrasound, the thickness of the wall of the duodenum is considered normal when it is between 3 and 6 mm, whereas the normal thickness of the walls of the rest of the small intestine is from 2 to 4 mm; the wall of the colon (2-3 mm) is thinner than that of the small bowel_^7-9,12. The corresponding normal values reported in the literature for the cat are about 2.4 mm, 2.1 mm and 1.67 mm_^10-12. The walls are formed of five layers (Fig. 3). The small bowel is characterized by peristalsis with about four or five contractions a minute in the duodenum and fewer in the other parts of the small intestine_⁷.

Abnormal ultrasound findings

Obstruction by foreign bodies: in most cases ultrasonography can detect both radio-opaque and radiolucent foreign bodies. The ultrasound signs found most frequently are presented in the table below.

Invagination_^17,18: the characteristic ultrasound image of intestinal invagination in longitudinal scans is alternation of hypoechoic and hyperechoic layers whereas in transverse sections there is alternation of hypoechoic and hyperechoic concentric rings, corresponding to the walls of the intestine overlapping each other. The central part is often hyperechoic because the invaginated loop carries some mesenteric adipose tissue with it (Fig. 5). In young animals, invagination is secondary to the presence of a foreign body, a viral infection (for example, parvovirus infection) or severe inflammation. In elderly animals it is often secondary to intestinal tumours..

Infectious and inflammatory diseases_^19-22: the ultrasound examination may be normal in the case of chronic enteritis and some forms of acute enteritis, so if the clinical signs are clearly indicative of enteritis, endoscopy should be performed and biopsies taken when appropriate^23,24. In the case of acute viral enteritis, such as parvovirus infection in the puppy, there is generalised dilatation of the intestinal loops, thickening of the wall and, sometimes, partial alterations in the normal wall layers, with irregularity of the mucosa, a corrugated appearance of the loops, enlargement of the mesenteric and colonic lymph nodes, effusion and sometimes hyperechogenicity and thickening of the peritoneum^20,25,26 (Fig. 6). There may be more or less marked alterations in the ultrasound picture of the intestines in the case of massive parasitic infestation or toxicosis. In some forms of granulomatous enteritis it may be possible to find focal lesions on the wall, as can occur in some cats with infectious peritonitis. Care must be taken when interpreting changes in the ultrasound appearance of the intestinal wall following enterotomy or enterectomy_²⁷. There is a published case report of a spontaneous haematoma in the intestinal wall of a dog_²⁸.

Neoplasms: The most frequent ultrasound findings in the case of bowel tumours are reported in the following table.

HIGHER LEVEL DIAGNOSTIC IMAGING

Computed tomography is a very useful technique in the case of suspected intestinal neoplasia for correct staging of the disease. In veterinary medicine there are no specific publications on the use of  magnetic resonance imaging or scintigraphy  in the study of the intestines.

References