The Plain Abdominal Radiograph

For non-acute situations, the relatively small yield of information from a plain radiograph does not justify the radiation dose that it entails. There are exceptions, such as in the follow-up of renal calculi and in assessment of bowel transit times in constipation.

In the situation of the acute abdomen, a supine abdomen radiograph and an erect chest radiograph (CXR) can be regarded as the basic standard radiographs. The erect CXR is invaluable in diagnosing visceral perforation since the X-ray beam is passing through any free gas under the diaphragm at an optimum angle, and at the correct exposure for visualisation. For this investigation the patient should ideally remain in the erect position for 10 min before radiography to allow time for any free gas to rise to the highest point, although this may not be achieved in practice. The supine radiograph should ideally be taken with an empty bladder, and should include the area from the diaphragm to the hernial orifices, which in practice means that the obturator foramina should be included.

Historically, an erect abdominal radiograph was performed additionally for the acute abdomen in order to assess the number and length of any fluid levels within bowel. This was thought to distinguish between obstruction and ileus. However, this distinction is highly unreliable, and evidence shows that the erect abdominal radiograph can be misleading.¹ The erect CXR appearances can be confusing when looking for free peritoneal gas, as discussed later in this chapter. Occasionally a left lateral decubitus radiograph can clarify, since small amounts of gas can be seen over the liver if there has been a perforation.

In the acute abdominal setting, plain radiography should only be requested in situations where it is likely to yield useful and not misleading information. Plain radiography is useful in diagnosing perforation of a viscus, and for assessing bowel dilatation (the main valid indications; see Table 25-1). Plain abdominal radiography is not helpful in diagnosing the common inflammatory conditions of the abdomen such as appendicitis, diverticulitis, cholecystitis and pancreatitis. For this reason, plain radiography should be avoided in these situations unless there is the suspicion of perforation or bowel dilatation. Clinical assessment, US and CT all play a role in these inflammatory conditions.

The presence of free intra-abdominal gas almost always indicates perforation of a viscus, for instance a peptic ulcer. Other causes include obstruction, inflammatory conditions, bowel ischaemia and colonoscopy. About 70% of perforated peptic ulcers will demonstrate free gas, a phenomenon rarely seen in the case of a perforated appendix. Small amounts of gas are detectable under the right hemidiaphragm on an erect CXR, outlining the smooth surface of the liver clearly (Fig. 25-1), but on the left it can be difficult to distinguish free gas from stomach and colonic gas. The diaphragm is seen as a relatively thin structure outlined by gas below, and lung above. The gas under the right and left hemidiaphragms may join across the middle (cupola sign). There are many circumstances when interpretation of an erect CXR is difficult. Table 25-2 lists the most frequent situations when the radiologist or clinician may be fooled into thinking that there is a perforation (pseudo-pneumoperitoneum). A lateral decubitus radiograph can resolve the problem by demonstrating gas between the liver and the abdominal wall, but nowadays CT is usually requested when there is doubt. Since interpretation of the plain AXR can be unreliable, it is important to interpret the plain radiographic findings in the light of the clinical findings.

It is also important to be able to recognise the signs of pneumoperitoneum on supine AXRs, for example in patients being monitored for colitis. In many patients, particularly those who are unconscious, have suffered trauma, are old or are critically ill, perforation may be clinically silent as it is overshadowed by other serious medical or surgical problems. In around half of patients with a pneumoperitoneum, gas may be detectable on the supine radiograph.² Almost half will have gas in the right upper quadrant adjacent to the liver, lying mainly in the subhepatic space and the hepatorenal fossa (Morrison’s pouch). Visualisation of both the outer and inner walls of a bowel loop is known as Rigler’s sign (Fig. 25-2). The bowel loops then take on a ‘ghost-like’ appearance. This sign can be difficult if several loops of bowel lie close together. The falciform or medial umbilical ligaments may be demonstrated by free gas lying on either side (Fig. 25-3). Air can be seen in the fissure for the ligamentum teres.³ The signs are listed in Table 25-3.

CT is the most sensitive investigation for the detection of free peritoneal gas. Small volumes of free peritoneal gas can be seen over the liver and anteriorly in the central abdomen. Tiny pockets of free gas can also be seen in the peritoneal recesses. In order not to miss small amounts of free gas, the images should be reviewed on broad window settings (Fig. 25-4).

There are a number of causes of pneumoperitoneum without peritonitis. These are listed in Table 25-4.

Gas in Bowel Wall

If linear gas streaks are seen in the bowel wall, intestinal infarction should be suspected. Intestinal infarction is a most serious condition caused by thrombosis or embolism of the superior mesenteric artery. Other radiological signs are non-specific, consisting of slightly dilated loops of small bowel that may be gas-filled, or poorly seen when mainly fluid filled. The walls of the small-bowel loops may be thickened due to submucosal haemorrhage and oedema (Fig. 25-5). Free gas may be present if perforation has occurred. Intraluminal gas in the mesenteric veins or portal vein may be seen in advanced cases. These features can be readily demonstrated on CT.

Pneumatosis cystoides intestinalis is an uncommon condition consisting of cyst-like collections of gas in the walls of hollow viscera. The aetiology is unknown, but there is an association with obstructive airways disease. The left hemicolon is the most common site, and the term pneumatosis coli is used to describe it. Most patients are past middle age; symptoms include vague abdominal pain, diarrhoea, constipation and mucus discharge. The cysts vary in size from 1 to 3 cm and lie both in the subserosa and submucosa. The plain radiographic appearances are characteristic, the gas-containing cysts giving rise to an appearance quite different from normal gas shadows (Fig. 25-6). Intermittently and subclinically these cysts rupture, producing a pneumoperitoneum without evidence of peritonitis. It is important to recognise pneumatosis as the cause of the pneumoperitoneum in such cases so as to avoid an unnecessary laparotomy.

Gas in Retroperitoneum

Gas can escape into the retroperitoneum, and the causes for this include perforation of a posterior peptic ulcer, perforated sigmoid diverticular disease, colonoscopy and other iatrogenic causes. The gas may be best visible in the layers of the abdominal wall in the flanks, or around the kidneys (Fig. 25-7). Retroperitoneal gas can track superiorly into the mediastinum, and inferiorly into the buttock and thigh. Gas in the soft tissues of the left thigh is a classical site for gas from a diverticular perforation.

Gas in Other Organs

Gas may also be identified in the biliary tree, portal veins, renal collecting systems (emphysematous pyelonephritis, Fig. 25-8), pancreas (infected necrosis or abscess), gall bladder wall and urinary bladder.