Recognizing the Normal Abdomen and Pelvis: Conventional Radiographs

Normal Bowel Gas Pattern

• Virtually all gas in the bowel comes from swallowed air. Only a fraction comes from the bacterial fermentation of food. For the purposes of this chapter, the terms gas and air are used interchangeably to refer to the contents of the bowel.

• Loops of bowel that contain a sufficient amount of air to fill the lumen completely are said to be distended. Distension of bowel is normal.

• Loops of bowel that are filled beyond their normal size are said to be dilated. Dilatation of the bowel is abnormal.

• Stomach

  • There is almost always air in the stomach , unless:

    • The patient has recently vomited, or

    • There is a nasogastric tube in the stomach and the tube is attached to suction.

• Small bowel

  • There is usually a small amount of air in about two or three loops of nondilated small bowel ( Fig. 12.1 ).

    

  • The normal diameter of small bowel is less than 2.5 cm, which is about 1 inch or the diameter of one United States quarter.

• Large bowel

  • There is almost always air in the rectum or sigmoid. There may be varying amounts of gas in the remainder of the colon ( Fig. 12.2 ).

    

  • Use this rule to decide whether the large bowel is dilated or not:

    • The large bowel can normally distend to about the same size as it does on a barium enema examination. To give you an idea of how large that is, look at Fig. 12.3 .

      

    • Stool is recognizable by the multiple, small bubbles of gas present within a semisolid appearing soft-tissue mass. Recognizing the appearance of stool will help in localizing the large bowel ( Fig. 12.4 ).

      

Important Points

• Individuals who swallow excessive quantities of air may develop aerophagia, accompanied by belching, bloating, and abdominal distension. A gas pattern characterized by numerous polygonal-shaped , air-containing loops of bowel, none of which is dilated, may be seen ( Fig. 12.5 ).

  

Normal Gastrointestinal Tract Air-Fluid Levels

• Stomach

  • There is almost always air and fluid in the stomach so there is almost always an air-fluid level visible in the stomach on either an upright abdomen, a study done with the patient in the decubitus position, or an upright chest radiograph.

    • To see an air-fluid level , the path of the x-ray beam must be directed horizontally —parallel to the floor.

• Small bowel

  • Two or three air-fluid levels in small bowel may be seen normally on an upright or decubitus view of the abdomen.

• Large bowel

  • The large bowel functions, in part, to remove fluid so there are usually no or very few air-fluid levels in the colon ( Fig. 12.6 ).

    

Important Points

• There may be many air-fluid levels present in the colon if the patient has had a recent enema or if the patient is taking medication with a strong anticholinergic, antiperistaltic effect.

• The normal distribution of bowel gas and fluid is summarized in Table 12.1 .

  Table 12.1

  Normal Distribution of Gas and Fluid in the Abdomen

  Organ	Normally Contains Gas	Normally Has Air/Fluid Levels
  Stomach	Yes	Yes
  Small bowel	Yes, 2–3 loops	Yes
  Large bowel	Yes, especially rectosigmoid	No

Differentiating Large from Small Bowel

• Recognizing large bowel

  • Large bowel is peripherally placed around the perimeter of the abdominal cavity except for the right upper quadrant, which is occupied by the liver ( Fig. 12.7 ).

    

  • Haustral markings usually do not extend completely across the large bowel from one wall to the other. If they should connect one wall with another, haustral markings are spaced more widely apart than the valvulae conniventes of the small bowel ( Fig. 12.8 ).

    

• Recognizing small bowel

  • Small bowel is centrally placed in the abdomen. Valvulae markings typically extend across the lumen of small bowel from one wall to the other. The valvulae are spaced much closer together than the haustra of the large bowel ( Fig. 12.9 ).

    

  • Small bowel can achieve a maximally dilated diameter of about 5 cm. Large bowel can dilate to several times that size.

Supine View (Scout Film) (See Fig. 12.1 )

• Why it is important

  • For the overall appearance of the bowel gas pattern , its general location, volume, and configuration is more important than identifying every small bubble of air on the radiograph.

  • In identifying the presence or absence of calcifications.

  • For identifying the presence of soft-tissue masses.

• How it is acquired

  • The patient lies on their back on the x-ray table or stretcher, and the x-ray beam is directed vertically downward ( Fig. 12.10 ).

    

• Substitute view

  • There is really no other view that substitutes for a supine view of the abdomen. Virtually all patients, regardless of their condition, can tolerate this part of the examination.