Intestinal obstruction is a state in which the intestinal contents cannot function normally or the passage is obstructed. According to the cause of obstruction, it can be divided into mechanical intestinal obstruction, dynamic intestinal obstruction and hemodynamic intestinal obstruction; according to the presence or absence of blood flow obstruction of the intestinal wall, it can be divided into simple intestinal obstruction and strangulated intestinal obstruction; according to the height of the obstruction site, it can be divided into high intestinal obstruction and low intestinal obstruction.
When simple intestinal obstruction occurs, the peristalsis above the obstruction increases to overcome the obstruction of intestinal content passage. At the same time, the intestinal cavity is dilated by accumulation of fluid and gas, and the fluid mainly comes from the retained gastrointestinal secretion; the air swallowed, the gas diffused by blood into the intestinal cavity, and the gas produced by bacterial decomposition of intestinal contents make a large amount of gas accumulation in the intestinal cavity. The lower the site of intestinal obstruction and the longer the time, the more obvious the dilatation of the intestinal canal. Below the obstruction, the intestinal canal is atrophied, empty or only a small amount of feces is accumulated.
In acute intestinal obstruction, the intestinal canal dilates rapidly, the intestinal wall thins, and the pressure in the intestinal lumen rises, which can lead to impaired blood flow in the intestinal wall at a certain degree. The initial manifestation is obstruction of venous reflux, depression of blood in capillaries and small veins of the intestinal wall, and congestion, edema and thickening of the intestinal wall. As a result of hypoxia, capillary permeability increases, and there are bleeding spots on the intestinal wall, and bloody exudate leaks into the intestinal and abdominal cavities. With the development of blood flow disorders, arterial blood flow is subsequently blocked and thrombosis occurs, leading to ischemic necrosis of the intestinal tube.
The main clinical manifestations of intestinal obstruction include abdominal pain, vomiting, abdominal distension, and cessation of defecation and exhaustion, and generally the higher the obstruction site, the earlier vomiting appears. Depending on the complications, more complex clinical manifestations may occur.
With the wide application of CT and the progress of CT technology, especially the application of spiral CT, CT plays an increasingly important role in the diagnosis of intestinal obstruction, which can show the thickening of the intestinal wall and abnormal blood supply to the intestinal wall that cannot be shown by abdominal plain film and barium enema, and whether there are pathological changes in the intestinal mesentery and abdominal interstitial space, etc. CT has many advantages in defining the cause of obstruction, the site of obstruction and judging strangulation, and is important for observing the changes of the disease and guiding the treatment. It is important to observe the changes of the disease and guide the treatment. Therefore, CT can be the first choice of examination method for intestinal obstruction in hospitals with conditions.
The timing of CT examination is best chosen to be performed before gastrointestinal decompression, which is conducive to the correct determination of the site and degree of obstruction. For cases where it is not clinically clear whether there is intestinal obstruction or incomplete obstruction, the method of oral administration of 2-4% iodine-containing contrast agent can be adopted. Enhancement scan has a very important value for diagnosis and should be done as a routine examination of intestinal obstruction.
(I) Basic signs of intestinal obstruction
X-ray manifestations.
The dilated intestinal curvature is arch-shaped in the standing abdominal plain film. When the tension of intestinal curvature is low, the liquid plane in the arch-shaped intestinal curvature is wide and long, the gas column above the liquid plane is low and flat, and the liquid plane submerges the top of the lower wall of the arch-shaped intestinal curvature, which is called long liquid flat sign; if the liquid plane in the intestinal curvature is narrow and the gas column is high, it is short liquid flat sign, or there are two liquid planes in the vicinity, and the inflatable intestinal curvature above it is continuously inverted “U” shaped, which means that the intestinal lumen When the amount of gas in the intestinal flexure is small, the gas can be gathered below the edematous and thickened mucosal folds at the edge of the intestinal cavity, and a series of small liquid planes can be seen on the standing plain film, which are diagonally arranged at the edge of the intestinal cavity, called the string of beads sign. In the standing position, the fluid planes in the colon are mostly located in the long descending colon, and if more fluid accumulates, it can submerge the colon liver and spleen flexure and form a wide fluid plane in the transverse colon, or small parallel-arranged fluid planes can be seen on both edges of the ascending or descending colon, which is caused by a small amount of gas accumulating below the edge of the semilunar folds.
Ultrasound findings.
The intestinal canal above the obstruction is significantly dilated, and the intestinal lumen is filled with a large amount of fluid. Proximal intestinal peristalsis of obstruction is obvious, accompanied by high-speed flow of fluid, reverse flow and “gas over water sign. The peristalsis of paralytic intestinal obstruction is reduced or disappeared. The mucosal folds in the longitudinal section of intestinal collaterals are clear and may be accompanied by edema and thickening, which is manifested as “piano key sign” or “fish spine sign”. The bending and twisting of intestinal collaterals can form the “coffee bean sign”.
CT manifestation
After the occurrence of intestinal obstruction, the lumen of the intestine is widened with the accumulation of fluid and gas, and the lower and longer the site of obstruction, the more obvious the dilatation of the lumen, and the lumen below the obstruction is atrophied, empty or only a small amount of feces is stored. It is worth noting that the atrophied intestinal canal, especially the mesenteric canal, may be displaced by the extrusion of the dilated intestine; in low-level obstruction, the upper jejunum may also not appear dilated (especially after gastrointestinal decompression).
The intestinal canal is dilated, the canal diameter is significantly enlarged, the plane of gas and liquid is visible, or it may be completely filled with liquid, and the intestinal wall is thinned. The marked collapse of the intestinal canal distal to the obstruction and the marked difference in the diameter of the canal distal and proximal to the obstruction are very valuable signs for the diagnosis of intestinal obstruction.
The dilated ileocecal valve can have more obvious enhancement and the illusion of limited thickening of the intestinal wall on the enhancement scan, which can be easily mistaken for a mass, and the symmetry of its morphology and the presence of dilatation of the ascending colon and ileum at the same time can help to differentiate it. Obstruction of the right hemicolectum can also cause dilatation of the appendix with fluid, which appears as a small tubular structure with a thin and smooth wall attached to the lower wall of the dilated cecum.
(B) Closed-collar intestinal obstruction
Closed-collar intestinal obstruction is mostly caused by intestinal torsion caused by the rotation of intestinal collaterals along the long axis of the mesentery, and can also be formed by the adhesion of the fibrous band that constricts and gathers the two ends of a section of intestinal tube. Intestinal torsion can be seen in part of the small intestine, all of the small intestine and the sigmoid colon.
X-Ray
The small span of curled intestinal collaterals formed by the mesentery as the axis, due to the edema contracture of the mesentery becomes shorter, so that the intestinal collaterals form a special arrangement, such as “C” shape, “8” shape, flower petal shape, banana string shape, etc., which is easier to show in the supine position.
When a large amount of gas and liquid in the proximal intestinal canal of the closed collaterals enters the closed collaterals, the closed collaterals expand to form an oval-shaped collaterals with smooth edges in the central part of the closed collaterals, which looks like coffee beans, so it is called coffee bean sign. When the closed collaterals are completely filled with fluid, it appears as a soft tissue mass with a clearer outline, which is called the pseudotumor sign. Under normal conditions, the jejunum is located in the left upper abdomen and the ileum is located in the right lower abdomen. When the small intestine is twisted, the degree of twisting is 180° or its odd times, the ileum is shifted to the left upper abdomen and the jejunum is shifted to the right lower abdomen.
In closed-collar sigmoid torsion, the sigmoid colon is significantly dilated, and the transverse diameter can exceed 10 cm. when standing, two wider fluid planes are visible. The dilated sigmoid colon is horseshoe shaped, and the rounded top of the horseshoe can reach the mid and upper abdomen. On barium enema examination, the barium fills the lower part of the sigmoid colon and gradually thins upward, with the tip pointing to one side in a bird’s beak shape.
CT manifestations
When the scanned level passes through the closed collaterals, it may appear as two dilated intestinal loops, and as the level gradually approaches the root of the closed collaterals, the distance between two adjacent intestinal loops is seen to gradually approach, and when the closed collaterals are parallel to the scanned level, it appears as a dilated U-shaped intestinal collaterals. When the scanned level passes through the root of the closed collaterals, deformation of the intestinal canal is seen, and when the intestine is twisted, it appears as a triangular soft tissue density shadow. When the scan passes through the input and output of the closed collaterals, two adjacent atrophic loops of intestine are seen. When the long axis of the input or output segment of the closed collaterals is parallel to the CT scan level, the input end gradually becomes thinner and the output segment becomes thicker due to the torsion, which is expressed as the “beak sign” on the CT image.
The CT manifestation of the mesenteric vascular bundle in closed collar intestinal obstruction also has certain characteristics, which shows that the mesenteric vessels of dilated intestinal collaterals converge radially toward the root of the closed collar, and the converged mesenteric vessels may form a “vortex sign” during intestinal torsion, and the soft tissue density shadow in the center is the superior mesenteric artery, surrounded by small extended and dilated vessels.
Closed collar intestinal obstruction can develop into strangulated intestinal obstruction, which is described below.
(C) Narrow intestinal obstruction
CT manifestation
When intestinal obstruction causes blood flow disorders in the intestinal wall, CT may be accompanied by the following CT manifestations in addition to the basic signs of intestinal obstruction.
(1) The intestinal wall is circularly and symmetrically thickened, with a thickness of about 0.5~1.0 cm, which may be segmentally distributed. The intestinal wall appears to be stratified, which is called “target sign” or “double halo sign”, and is a sign of thickened submucosal edema. In the jejunum, the disappearance of the annular folds of the dilated intestine can be seen.
(2) On enhancement scan, the intestinal wall at the lesion does not strengthen or the strengthening is significantly weakened. When delayed scanning, the normal intestinal wall strengthening phenomenon has disappeared, while the intestinal wall at the lesion appears to strengthen, and the strengthening level of the normal intestinal wall can be reached with time extension.
(3) The smooth bird’s beak sign during intestinal torsion changes to a jagged bird’s beak sign due to edema thickening of the intestinal wall at the obstruction and congestion and edema of the mesentery.
(4) The mesenteric density is increased, blurred and cloudy, and the CT value increases up to -40~-60 Hu. The mesenteric vessels lose their normal structure and gradually become thicker and radiate outward from the obstruction.
(5) The appearance of ascites. It starts as a small amount and gathers in the peritoneal space, gradually becomes a large amount and diffusely distributed, causing the density of the peritoneal cavity and the mesentery to increase.
(6) In the presence of infarction of the intestinal wall, pneumatization is seen in the intestinal wall. Gas shadows can also be seen in the mesenteric vein and portal vein, and mesenteric arterial and venous thrombosis can be found on enhanced scan.
(D) Localization diagnosis
Based on the morphological characteristics of dilated intestinal collaterals and the migrated areas of dilated and atrophied intestinal tubes, the site of obstruction can be determined. If the number of dilated intestinal collaterals is small and most of them are located in the upper abdomen, the site of obstruction is located in the jejunum, and the jejunal circumferential folds of dilated intestinal canal can be seen. If most of the dilated ileal collaterals cover the whole abdomen with more gas and fluid planes, and there is no gas or only a small amount of gas in the colon, but no dilatation and fluid planes, the site of obstruction is in the distal ileum. Colonic obstruction is manifested by dilatation of the proximal end of the obstruction with gas-fluid planes, and the dilated colon is visible as a colonic pouch and semilunar folds. The small intestine is mostly undilated or less dilated.
The power type intestinal obstruction is mostly manifested as diffuse inflation and dilatation of small intestine and large intestine, which is more obvious in the colon, and the gas and liquid planes are mostly seen in it, and a large amount of gas is also seen in the stomach.
(V) Etiological diagnosis
The etiology of intestinal obstruction is complex and diverse, such as intestinal adhesions, primary or secondary tumors, Crohn’s disease, vascular lesions, parasites, bold stones, fecal masses, abdominal hernias, chronic colonic diverticulitis, intussusception, intestinal torsion, and so on. CT can play a greater role in the pathological diagnosis of obstruction compared to abdominal plain films.
Intestinal adhesions account for about 1/3 of obstruction cases, and the use of the window-width window technique can well show the cords of adhesions, their location and relationship with the surrounding intestinal canal and abdominal wall. For intestinal obstruction caused by tumor, CT can generally accurately show the site of tumor and its invasion range to the surrounding tissues and organs. Enhancement scan is very important for diagnosis, and attention should be paid to look for other signs of tumor, such as liver metastasis, lymph node enlargement, infiltration of surrounding intestinal canal and organs, etc.
In intestinal obstruction caused by inflammatory stenosis, the stenosis of intestinal lumen is more obvious than that caused by tumor on CT, and the lumen of the stenotic segment may be in the shape of thin lines, but the contour of intestinal lumen is relatively smooth; the thickening of intestinal wall is mostly mild to moderate, and the contour of intestinal wall is more regular; inflammatory stenosis may form multiple stenoses and dilated intestinal segments, while tumor is often a single stenosis. Gallstone intestinal obstruction is less common, and its CT manifestation has certain characteristics, called Rigler’s triad: pneumatization and dilatation of intestinal collaterals; ectopic calcified gallstones in the lower abdomen; and a small amount of gas in the gallbladder or bile duct.
When diagnosing intestinal obstruction by CT, the following issues should also be considered: the presence or absence of intra- and extra-abdominal hernias; the presence of more than two etiologies (such as adhesions with intestinal torsion or abdominal hernia); obstruction of the intestinal canal at more than two sites (such as multiple stenosis of the intestinal canal due to tuberculosis, extensive adhesive intestinal obstruction, etc.); and whether congenital intestinal malformations are combined.