DOI: 10.1148/radiol.2283020302
(Radiology 2003;228:706-707.)
© RSNA, 2003
The Rigler Sign1
Justin Q. Ly, MD
1 From the Department of Radiology, Wilford Hall Medical Center, 759th MDTS/MTRD, 2200 Bergquist Dr, Suite 1, Lackland AFB, TX 78236-5300. Received March 20, 2002; revision requested June 4; revision received October 9; accepted December 19. Address correspondence to the author (e-mail: jly15544@hotmail.com).
Index terms: Intestines, radiography, 74.11, 75.11 • Pneumoperitoneum, 791.71 • Signs in Imaging
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APPEARANCE
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In the Rigler sign, which is also known as the double-wall sign or the bas-relief sign (1,2), both sides of the bowel wall can be visualized on a radiograph of the abdomen obtained with the patient in the supine position (Figure).
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Anteroposterior abdominal radiograph obtained with the patient in the supine position. Extensive free intraperitoneal air is seen, which outlines the outer wall of multiple loops of air-filled bowel. Notice the discernible white stripe (arrows) of bowel wall between the intraluminal air and the free intraperitoneal air. This appearance is known as the Rigler sign.
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EXPLANATION
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Gas normally outlines only the luminal surface of the bowel wall and not the serosal surface, which has a degree of opacity similar to that of adjacent peritoneal contents. When at least a moderate amount of free intraperitoneal air exists, however, this free air is more likely to accumulate between bowel loops, thus permitting visualization of the outer walls of the bowel. This is the classic appearance of the Rigler sign. A variant of the Rigler sign occurs when only the outside of the bowel wall is visible because the lumen is filled with fluid (3).
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DISCUSSION
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Pneumoperitoneum can be an indicator of an underlying benign or serious process. If pneumoperitoneum is present, it may require emergent surgical attention and therefore should be recognized on routine abdominal radiographs. There are four etiologic categories of pneumoperitoneum: iatrogenic, spontaneous, traumatic, and miscellaneous. Iatrogenic causes include surgery, peritoneal dialysis, feeding tube placement, recent endoscopy, use of gynecologic instruments, and vigorous respiratory resuscitation. Spontaneous causes include peptic perforation, ischemia, bowel obstruction (benign or malignant), toxic megacolon, and inflammatory conditions (appendicitis, tuberculosis, necrotizing enterocolitis). Traumatic causes can be blunt or penetrating, either of which can result in bowel perforation. Miscellaneous causes include drugs (steroidal drugs, nonsteroidal anti-inflammatory drugs) and pneumatosis coli or intestinalis. Miscellaneous causes may also be female genital tract–related (douching, sexual intercourse, insufflation). A wide range of clinical manifestations of pneumoperitoneum are possible; there may be no symptoms at all, or there may be marked peritoneal signs. Often, a careful history can elucidate the cause.
At supine abdominal radiography, which is often performed in infants or in patients in the intensive care unit, the presence of at least a moderate if not a large amount of free intraperitoneal air is typically required before detection is possible (4,5). A study that is more sensitive for depicting smaller amounts of free intraperitoneal air is upright chest radiography, which enables a quick assessment of the hemidiaphragms for subjacent free air (6). In many institutions, upright chest radiography is routinely performed as part of an acute abdominal series. For the patient unable to achieve an upright position, an alternative is the left lateral decubitus position (7); published results of experimental data (8) suggest that a properly collimated and exposed decubitus radiograph is as sensitive as an upright chest radiograph in enabling the detection of as little as 1 mL of free intraperitoneal air. A cross-table horizontal-beam study, in which the film is adjacent to the patient’s left side, can be performed in patients who are unable to assume the decubitus position, or it may be useful as a means to confirm suspected free air on another view. Like the supine anteroposterior view, this horizontal view requires at least a moderate amount of free air before pneumoperitoneum can be detected. Computed tomography (CT), which is able to depict as little as 1 mL of free air (9,10), can help confirm radiographically suspected pneumoperitoneum.
There are many possible radiographic appearances of pneumoperitoneum. The most common finding is the presence of right upper quadrant subdiaphragmatic free air (3), which is typically well delineated from the liver because of differences in opacity. The Rigler sign is the second most common sign of pneumoperitoneum on supine radiographs, with a prevalence of 32% in the studied cases of pneumoperitoneum (3). Less common radiographic signs of free air include triangular or rhomboid collections of air, air in the Morison pouch (11), air outlining the falciform ligament, air outlining the peritoneum in the shape of a football (football sign), and the inverted V sign, in which the lateral umbilical ligaments are visible on the supine radiograph (12). Aside from right upper quadrant air and the Rigler sign, these radiographic appearances of free air are infrequently seen.
As appearances mimicking the Rigler sign are not uncommon, it is necessary to be able to differentiate a true-positive Rigler sign from a false-positive finding in a patient without pneumoperitoneum. The Rigler sign can sometimes be simulated by contiguous loops of bowel, whereby intraluminal air in one loop of bowel may appear to outline the wall of an adjacent loop, which results in a misdiagnosis of free air (3). In a patient who has recently undergone an abdominal CT study, a small amount of residual contrast material coating the luminal surface of bowel may increase the apparent bowel-wall attenuation and create a pseudo-Rigler sign (1). A false-positive Rigler sign may also result from Mach bands because there is the perception of a line at the interface of two differing radiographic densities (13,14). Unlike the Rigler sign, these false-positive signs rarely depict a discernible wall. Thus, radiographic criteria can be useful in differentiating true-positive from false-positive Rigler signs. In unclear cases, a left lateral decubitus radiograph or upright abdominal radiograph can be obtained to confirm the presence of pneumoperitoneum.
In conclusion, pneumoperitoneum can be an indicator of potentially serious intraperitoneal disease. Often, a sick patient can undergo imaging only in the supine position. The detection of free air on the supine radiograph by recognition of the Rigler sign can provide important patient care information.
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FOOTNOTES
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A trainee (resident or fellow) wishing to submit a manuscript for Signs in Imaging should first write to the Editor for approval of the sign to be prepared, to avoid duplicate preparation of the same sign.
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REFERENCES
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- Rigler LG. Spontaneous pneumoperitoneum: a roentgenologic sign found in the supine position. Radiology 1941; 37:604-607.
- Levine MS, Scheiner JD, Rubesin SE, Laufer I, Herlinger H. Diagnosis of pneumoperitoneum on supine abdominal radiographs. AJR Am J Roentgenol 1991; 156:731-735.[Abstract/Free Full Text]
- Berdon WE, Baker DH, Leonidas J. Advantages of prone positioning in gastrointestinal and genitourinary roentgenologic studies in infants and children. Am J Roentgenol Radium Ther Nucl Med 1968; 103:444-455.[Medline]
- Markowitz SK, Ziter FM. The lateral chest film and pneumoperitoneum. Ann Emerg Med 1986; 15:425-427.[CrossRef][Medline]
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- West OC, Tamm EP, Kawashima A, Jarolimek AM. Abdomen: nontraumatic emergencies. In: Harris JH, Harris WH, eds. The radiology of emergency medicine. 4th ed. Philadelphia, Pa: Lippincott, 2000; 598-687.
- Miller R, Becker G, Slabaugh R. Detection of pneumoperitoneum: optimum body position and respiratory phase. AJR Am J Roentgenol 1980; 135:487-490.[Abstract]
- Stapakis JC, Thickman D. Diagnosis of pneumoperitoneum: abdominal CT vs upright chest film. J Comput Assist Tomogr 1992; 16:713-716.[Medline]
- Earls JP, Dachman AH, Colon E, Garrett MG, Molloy M. Prevalence and duration of postoperative pneumoperitoneum: sensitivity of CT vs. left lateral decubitus radiography. AJR Am J Roentgenol 1993; 161:781-785.[Abstract/Free Full Text]
- Brill PW, Olson SR, Winchester P. Neonatal necrotizing enterocolitis: air in Morrison pouch. Radiology 1990; 174:469-471.[Abstract/Free Full Text]
- Bray JF. The "inverted V" sign of pneumoperitoneum. Radiology 1984; 151:45-46.[Abstract/Free Full Text]
- Edholm P. Boundaries in the radiologic image. I. General principles for perception of boundaries and their application to the image. Acta Radiol 1981; 22:457-473.
- Chasen MH. Practical applications of mach band theory in thoracic analysis. Radiology 2001; 219:596-610.[Abstract/Free Full Text]
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