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Accuracy of Abdominal Radiography in Acute Small-Bowel Obstruction: Does Reviewer Experience Matter?

¹ All authors: Department of Radiology, Duke University Medical Center, Box 3808, Durham, NC 27710.

Received June 26, 2006; accepted after revision August 18, 2006.

 
Address correspondence to W. M. Thompson (thomp132{at}mc.duke.edu).

Presented at the 2006 annual meeting of the American Roentgen Ray Society, Vancouver, BC, Canada.

WEB This is a Web exclusive article.

Abstract

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OBJECTIVE. The purposes of this study were to determine the accuracy of abdominal radiography in the detection of acute small-bowel obstruction (SBO), to assess the role of reviewer experience, and to evaluate individual radiographic signs of SBO.

MATERIALS AND METHODS. A retrospective study was performed in which the subjects were 90 patients with suspected SBO who underwent CT and abdominal radiography within 48 hours of each other. The patients were enrolled from June 1, 2003, to February 2004. Twentynine of the patients had proven SBO. Hard-copy radiographs were reviewed by three groups of radiologists: senior staff, junior staff, and second-year radiology residents. Each reviewer evaluated the quality of the radiographs, patient position for acquisition of the radiographs, and whether SBO was present. The reviewers rated their confidence on a five-point scale and recorded the presence or absence of specific radiographic signs of SBO. Chi-square tests were used to compare the three groups. A statistically significant finding was considered p < 0.05. Receiver operating characteristic (ROC) curves were fit with a 10-point confidence scale.

RESULTS. The sensitivity for SBO among the six reviewers ranged from 59% to 93%. The senior staff members were significantly more accurate. The mean sensitivity, specificity, and accuracy for all six reviewers were 82%, 83%, and 83%, respectively. Three radiographic signs were highly significant (p < 0.001): two or more air-fluid levels, air-fluid levels wider than 2.5 cm, and air-fluid levels differing more than 5 mm from one another in the same loop of small bowel. ROC analysis showed that senior staff is significantly more accurate than the other groups in the detection of acute SBO.

CONCLUSION. Our results confirmed that abdominal radiographs are accurate in the detection of acute SBO, that more-experienced radiologists are more accurate than less-experienced reviewers in the evaluation of abdominal radiographs, and that three types of air-fluid levels are highly predictive of the presence of SBO.

Keywords: abdominal imaging • radiography • small bowel

Introduction

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CT has become the preferred imaging technique for evaluation of patients with suspected small-bowel obstruction (SBO) [1-21]. Despite reports of success with CT, some authors [22-25] find that conventional abdominal radiography is the preferred initial examination of patients with symptoms of acute SBO. There is concern, however, about the accuracy of abdominal radiography in patients with proven SBO, the reported accuracy ranging from 50% to 60% [11, 26-29]. A recent article from our institution [21] reported that CT scout digital radiography alone had a sensitivity of 88% and a specificity of 86%, similar to the results reported for axial CT and axial plus coronal CT. These findings suggest that in the clinical setting of suspected acute SBO, abdominal radiography may be both highly sensitive and highly specific. To our knowledge, there are no reports in the literature on evaluations of the level of experience of the radiologists, particularly residents, interpreting abdominal radiographs of patients with suspected acute SBO.

A number of signs on abdominal radiographs have been reported to be sensitive and specific for differentiating high-grade SBO from low-grade SBO and normal intestine [30]. The best radiographic signs are more than two air-fluid levels, air-fluid levels wider than 2.5 cm, and air-fluid levels differing more than 5 mm from one another in the same small-bowel loop [30]. Other findings, such as gasless abdomen, the string-of-pearls sign, distended stomach, the stretch sign, and collapsed colon, have not been shown to be as predictive as the other three signs of the presence of high-grade SBO [30]. The purposes of this study were to determine the sensitivity, specificity, and accuracy of abdominal radiography for acute SBO, to assess the role of reviewer experience, and to evaluate the individual radiographic signs reported to be sensitive in the diagnosis of SBO.

View larger version (154K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1 —38-year-old woman with closed-loop small-bowel obstruction found at surgery. Highly predictive air-fluid levels are visible. Upright anteroposterior abdominal radiograph shows all three highly predictive types of air-fluid levels: two or more levels (thick arrows), levels with a width of 2.5 cm or more (long line), and levels differing 5 mm or more from one another in same loop (thin arrow, short lines).

Top Abstract Introduction Materials and Methods Radiographic Evaluation Statistical Analysis Results Discussion References

Radiographic Evaluation

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Hard-copy radiographs were loaded on several viewers and interpreted by three groups of radiologists with wide variation in training: group 1, two senior staff members with 15 (S1) and 30 (S2) years of experience after radiology residency; group 2, two junior radiology staff members with 4 (J1) and 5 (J2) years of experience after training; and group 3, two second-year radiology residents (R1 and R2).

The reviewers were asked to evaluate the following: quality of study (adequate or inadequate) and patient position for radiography (flat, upright, or left lateral decubitus). The reviewers then were asked whether there was radiographic evidence of SBO and to rate on a five-point scale (1-5, low to high) their confidence with the interpretation. Reviewers were asked to document the presence or absence of the following: more than two air-fluid levels, air-fluid levels measuring 2.5 cm or wider, air-fluid levels of different heights in the same loop of small bowel (Fig. 1), string-of-pearls sign, gasless abdomen, dilated colon, dilated stomach or duodenum, nasogastric tube, free air, portal venous gas, biliary gas, ascites, and abdominal mass. The number of patients without either an upright or a decubitus radiograph was tabulated for the true-positive, true-negative, false-positive, and false-negative findings for each of the six reviewers.

Statistical Analysis

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Sensitivity, specificity, and combined sensitivity and specificity (a measure of accuracy) were tabulated for each reviewer and combined for comparisons among the three groups. The groups were compared with use of chi-square tests. Wilcoxon's rank sum test was used to compare the six reviewers' results between patients with and those without upright or decubitus radiographs. The association between the outcome variable proven SBO and the other variables was assessed by means of a Wilcoxon and Mann-Whitney test for continuous variables and Pearson's chi-square test for binary-value variables. Significance was considered p < 0.05 in all statistical tests. Comparison of sensitivity and specificity estimates among groups were based on across-reviewer average scores. Receiver operating characteristic (ROC) curves from Student's t tests were fit in a semiparametric binormal model [31] with a 10-point confidence scale. Areas under the ROC curves were assessed by means of the Wilcoxon and Mann-Whitney test as applied with methods of Delong et al. [32].

Results

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A mean of 8.5 (10%) of the 90 studies (range, 1 senior staff member study to 20 resident studies) were rated inadequate, most because of the lack of an upright or decubitus radiograph and others mostly because of motion. However, only one case of proven SBO in this group was misclassified by all six reviewers. The patient had a gasless abdomen that could be visualized only on a supine radiograph with only minimal motion. Supine abdominal radiographs were obtained for all 90 patients; either upright or decubitus images were not obtained in the cases of 23 (26%) of the 90 patients. Either upright (n = 15) or left lateral decubitus radiographs (n = 7) were obtained in the cases of 22 (76%) of the 29 patients with proven SBO (Figs. 1, 2A, 2B, 3A, 3B, 4A, 4B). Either upright (n = 31) or left lateral decubitus radiographs (n = 14) were obtained in the cases of 45 (74%) of the 61 patients without SBO (Table 1).

View larger version (161K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 2A —42-year-old woman with abdominal pain. True-negative findings were reported. Supine (A) and upright (B) abdominal radiographs show multiple nondilated air-containing loops (arrows) of small bowel in left side of abdomen. B shows no significant air-fluid levels. CT on same day as radiography (not shown) did not reveal small-bowel obstruction, and symptoms resolved.

View larger version (148K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 2B —42-year-old woman with abdominal pain. True-negative findings were reported. Supine (A) and upright (B) abdominal radiographs show multiple nondilated air-containing loops (arrows) of small bowel in left side of abdomen. B shows no significant air-fluid levels. CT on same day as radiography (not shown) did not reveal small-bowel obstruction, and symptoms resolved.

View larger version (140K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 3A —78-year-old woman with abdominal pain, nausea, and vomiting. True-positive findings were reported. Supine abdominal radiograph shows multiple dilated loops (arrows) of small bowel in right lower quadrant.

View larger version (137K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 3B —78-year-old woman with abdominal pain, nausea, and vomiting. True-positive findings were reported. Upright radiograph shows multiple air-fluid levels in right lower quadrant, some of which are wider than 2.5 cm, and air-fluid levels (arrows) of unequal height. Patient was treated conservatively, and obstruction resolved.

View larger version (143K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 4A —83-year-old man with abdominal pain. True-positive findings were reported. Upright radiograph shows dilated small bowel (arrows) in left lower quadrant.

View larger version (129K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 4B —83-year-old man with abdominal pain. True-positive findings were reported. Left lateral decubitus radiograph shows multiple air-fluid levels (arrows), some of which are wider than 2.5 cm and of unequal heights. Patient was treated conservatively, and obstruction resolved.

Sensitivity for the detection of SBO ranged from 59% to 93% for the six radiologists (Table 2). In comparisons of the three groups of radiologists, senior staff had a statistically significantly higher rate of detection of SBO than either junior staff (p < 0.001) or residents (p < 0.004). Residents had statistically significantly better sensitivity than junior staff (p < 0.02). There was less variation of specificities and there were no statistically significant differences in specificity among the three groups of radiologists. For the combination of sensitivity and specificity, which is a measure of accuracy, senior staff were statistically significantly more accurate than either of the other two groups, but there was no difference between junior staff and residents (Table 2). The mean sensitivity, specificity, and combined sensitivity and specificity for all six reviewers were 82%, 83%, and 83%, respectively (Table 2).

There were 174 possible positive responses for SBO (six reviewers x 29 proven cases of SBO) and 366 possible true-negative responses (six reviewers x 61 no SBO). The residents had more false-positive findings (mean, 12.5) (Fig. 5A, 5B) than the senior (mean, 10) and junior staff (mean, nine). The six reviewers collectively had 33 (19%) false-negative findings, and 12 (36%) of the patients in these cases did not undergo upright or decubitus radiography (Fig. 6), whereas 29 (21%) of the patients with true-positive findings did (Figs. 1, 3A, 3B, 4A, 4B, and 7A, 7B). Nine (14%) of the patients with false-positive findings and 86 (28%) of those with true-negative findings did not undergo either upright or decubitus radiography. There were no significant differences (p > 0.05) among these four groups for the six reviewers. The lack of upright or decubitus images did not influence the detection or exclusion of SBO. ROC analysis of confidence showed a statistically significant difference between senior staff and the other two groups (p < 0.005 and p < 0.05) but no difference between junior staff and residents (p > 0.52).

View larger version (137K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 5A —74-year-old man with abdominal distention. False-positive findings were reported. Supine (A) and upright (B) abdominal radiographs show dilated small bowel out of proportion to colon; condition causes air-fluid levels (arrows, B). CT (not shown) performed later same day revealed only ascites and no evidence of small-bowel obstruction.

View larger version (145K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 5B —74-year-old man with abdominal distention. False-positive findings were reported. Supine (A) and upright (B) abdominal radiographs show dilated small bowel out of proportion to colon; condition causes air-fluid levels (arrows, B). CT (not shown) performed later same day revealed only ascites and no evidence of small-bowel obstruction.

View larger version (130K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 6 —91-year-old woman with abdominal pain. False-negative findings were reported. Supine radiograph shows little small-bowel gas. Five of six reviewers rated study inadequate owing to motion. All reviewers interpreted findings as no obstruction. CT (not shown) and surgery on same day revealed incarcerated right femoral hernia, which produced marked small-bowel obstruction with fluid-filled loops of dilated small bowel.

View larger version (151K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 7A —48-year-old woman with nausea and vomiting. True-positive findings were reported. Gasless abdomen and string-of-pearls sign were seen. Supine abdominal radiograph shows paucity of small-bowel gas and suggestion of stretch sign valvulae conniventes in dilated small-bowel loop outlined by air (arrow). Stretch sign is defined as abnormal distention of predominantly fluid-filled small-bowel loops in which luminal gas has striped appearance running perpendicular to long axis of bowel [26].

View larger version (162K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 7B —48-year-old woman with nausea and vomiting. True-positive findings were reported. Gasless abdomen and string-of-pearls sign were seen. Upright radiograph shows multiple tiny air-fluid levels (arrows) in small bowel (string-of-pearls sign). Small-bowel obstruction due to adhesions was found at surgery.

Three radiographic signs were highly significant in prediction of the presence of SBO: more than two air-fluid levels (Figs. 1, 3A, 3B, and 4A, 4B), air-fluid levels wider than 2.5 cm (Figs. 1, 3A, 3B, and 4A, 4B), and air-fluid levels differing more than 5 mm from one another in the same loop (Table 3) (Figs. 1, 3A, 3B, and 4A, 4B). Nineteen (42%) of the 45 patients without SBO had multiple air-fluid levels compared with 21 (95%) of the 22 with SBO. Air-fluid levels measuring 2.5 cm or wider and air-fluid levels of unequal heights in the same loop were present in 15 (68%) of 22 and 14 (64%) of 22, respectively, of the patients with acute SBO but only in eight (18%) of 45 and two (4%) of 45, respectively, of the patients without SBO.

Although dilated colon (p < 0.05) and the string-of-pearls sign (p < 0.001) were statistically significant discriminating signs in exclusion or detection of SBO, these signs rarely occurred in the patients. One (3%) of the 29 patients with SBO had dilated colon, and only eight (14%) of the 61 patients without SBO had this sign. The string-of-pearls sign occurred in only three (10%) of 29 SBO patients (Fig. 7A, 7B) and one (1.7%) of 61 patients without SBO. None of the other findings was helpful in excluding or detecting SBO.

Discussion

Top Abstract Introduction Materials and Methods Radiographic Evaluation Statistical Analysis Results Discussion References

 
The overall sensitivity, specificity, and accuracy of abdominal radiography in the detection of acute SBO were higher in our series than in most previous reports. Frager et al. [10] reported only 19% sensitivity of radiography, compared with 100% sensitivity of CT, in examinations of 36 patients with suspected SBO. Maglinte et al. [11] in 1996 reported a sensitivity of 67% for both abdominal radiography and CT in patients with low-grade and those with high-grade SBO. With classification into high-grade obstruction, the sensitivity increased to 86%, a finding similar to ours. Shrake et al. [22], however, described 117 consecutively examined patients with clinically suspected SBO and found the abdominal radiographic findings were misleading. Twenty-two percent of patients with normal findings had proven obstruction, and 42% of patients with abnormal findings did not have obstruction. The overall sensitivity was 66%. A number of authors have reported only 50-60% accuracy of abdominal radiography in the detection of SBO [26-29].

Our results suggest that abdominal radiography is accurate in the evaluation of patients with suspected acute SBO. The findings can be used to screen patients for whom a physician may consider ordering CT. Radiography also is a relatively inexpensive way of following patients who have been treated for SBO. This form of management has become more important with the increasing emphasis on nonsurgical treatment of patients who have undergone abdominal surgery and are likely to have an adhesion causing obstruction. Abdominal radiography also is less expensive and requires less radiation than CT.

To our knowledge, no studies have been conducted to examine the role of reviewer experience in the evaluation of abdominal radiographs of patients with suspected SBO. In 1989 Markus et al. [33] studied interobserver variation among four radiologists in the interpretation of abdominal radiographs but did not examine experience per se. Those authors found fair to good agreement for SBO. In our comparison of three groups of radiologists whose levels of experience differed, we found a statistically significant difference in sensitivity and accuracy among the most experienced radiologists compared with those with less than 5 years of experience. There was variation in the two senior staff: S1 had a slightly high sensitivity (93% vs 90%) but a lower specificity (77%) than S2 (90%).

Although there was no statistically significant difference in specificity for the two junior staff members, there was a difference in sensitivity. J1 had a sensitivity (82%) similar to that of the residents, but J2 had a much lower sensitivity (59%). J2, however, had a specificity of 89%, which was higher than that of any of the reviewers, except for senior staff member S2. We have no good explanation for these variations. Most abdominal radiographs are interpreted by members of the gastrointestinal fluoroscopic service, which is usually covered by senior staff. Our junior staff rarely work on this service and therefore may lack the experience of the senior staff.

Additional evidence that experience plays a role in interpretation of abdominal radiographs of patients with suspected SBO was found in the ROC analysis. Senior staff had significantly more confidence than either junior staff or residents. There was no difference between junior staff and residents. Senior staff members were consistently more confident in detecting both the presence and the absence of SBO.

Our findings corroborate those of Lappas et al. [30], who found that the two most significant (p < 0.0003) radiographic signs of SBO were air-fluid levels of different heights or an air-fluid level width of 2.5 cm or more. Lappas et al. also found that when both of these findings were present, the degree of SBO was likely high grade or complete. The findings of Lappas et al. contrast to those of other investigators [33-38], who have not found the presence of multiple air-fluid levels and air-fluid levels of differing heights helpful in the diagnosis of SBO. Mirvis et al. [38] found that findings on upright radiographs did not add to those on supine radiographs of patients with proven SBO. In our study, three patients who had SBO had the string-of-pearls sign, which has been shown to be highly sensitive in the diagnosis of SBO [26, 31], on upright or decubitus images. Although the role of upright and decubitus radiography in the detection of highly predictive air-fluid levels is still in question, we believe these images do add additional information. This point was emphasized by Mindelzun and McCort [39]. None of the other signs evaluated in our study were predictive of SBO. Dilated colon, however, was highly suggestive that a patient did not have SBO.

There were a number of limitations to our study. First, it was retrospective, and the reviewers had only two choices: SBO or no SBO. Second, 26% of our patients did not undergo upright or decubitus radiography. Third, 16 (55%) of 29 patients with SBO did not undergo surgery. Their diagnosis was based on clinical improvement with nasogastric suction. Fourth, there was a wide range in what was considered an adequate or an inadequate study (one inadequate study for a senior staff member to 20 for a resident) because we did not provide instruction about what was considered an adequate examination. This factor, however, did not affect any of the six reviewers' results because lack of upright or decubitus radiographs was the most common cause of inadequate studies. In addition, there were no differences among the six reviewers' results in comparison of patients with and those without upright or decubitus radiographs.

The results of this study confirm that abdominal radiography is sensitive and specific in the detection of acute SBO. Radiologists with a greater degree of experience are likely to be more accurate in the evaluation of abdominal radiographs. Three specific types of air-fluid levels are highly predictive of SBO.

References

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