In recent years, new techniques such as ultrasonography, computerized tomography (CT), and magnetic resonance (MR) imaging have been used widely and have altered the use of plain films of the abdomen in the evaluation of abdominal diseases. Plain films of the abdomen are still used primarily to assess intestinal perforation (intraperitoneal air) or bowel obstruction or assessment for catheter placement. The plain radiograph is commonly used as a preliminary radiograph before other studies such as CT and barium enema. The yield of plain radiographs is higher in patients with moderate or severe abdominal symptoms and signs than in those with minor symptoms.
The most common plain radiograph of the abdomen is an anteroposterior (AP) view with the patient in supine position. The AP view of the abdomen is also called by the acronym KUB film because it includes the kidneys, ureters, and bladder. When acute abdominal disease is suspected clinically, an erect film of the abdomen and a posteroanterior (PA) view of the chest are also required. Digital imaging is becoming more common, and abdominal images may be viewed on a computer monitor rather than on films.
The abdomen is composed primarily of soft tissue. The density of soft tissue is similar to the density of water, and the difference in density between solid and liquid is not distinguishable on a plain radiograph. The liver is a homogeneous structure located in the right upper quadrant; the hepatic angle delineates the lower margin of the posterior portion of the liver (Figure 8-1). In the left upper quadrant, a similar angular structure, the splenic angle, can be identified by the fat shadow around the spleen (see Figure 8-1).
(A) Normal plain film of the abdomen. The lower margins of the posterior portion of the liver, the hepatic angle (H), and the lower part of the spleen (S) are delineated by a fat shadow. Both kidneys (K) and the psoas muscle shadows (arrowheads) are outlined by a fat shadow. The properitoneal fat stripe is also shown bilaterally (arrows). (B) Diagram of normal abdominal plain film.
Organ enlargement can be recognized by the effect of displacement on nearby bowel loops or by obliteration of the adjacent normal fat or gas pattern. Hepatomegaly may compress the proximal transverse colon below the right kidney. Splenomegaly may push the splenic flexure of the colon downward. A large fused renal shadow across the psoas muscle and lumbar spine suggests a horseshoe kidney.
Fat density, which is between that of soft tissue and that of gas, outlines the contour of solid organs or muscles. ...