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Radiologists' Confidence in Interpretation of Sonography and CT in Suspected Pediatric Appendicitis

¹ Department of Medicine, Division of Emergency Medicine, Children's Hospital, Harvard Medical School, 300 Longwood Ave., Boston, MA 02215.
² Department of Radiology, Children's Hospital, Harvard Medical School, Boston, MA 02215.

Received October 25, 1999; accepted after revision December 7, 1999.

 
Address correspondence to B. M. G. Peña.

Abstract

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OBJECTIVE. We sought to determine radiologists' confidence in interpretation of sonography and CT performed using rectal contrast material for diagnosing pediatric appendicitis.

SUBJECTS AND METHODS. We prospectively examined 139 children and young adults with equivocal clinical findings for appendicitis who were seen in the emergency department of an urban pediatric teaching hospital between July and December, 1998. Patients were initially examined with pelvic sonography. If the sonographic results were equivocal or if the appendix was not visualized, CT was performed. Radiologists recorded their level of confidence in interpretation of the sonograms and CT examinations.

RESULTS. Sonography was interpreted with very low, low, or medium confidence in 59 (42.4%) of 139 patients and with high or very high confidence in 80 (57.6%) of 139. CT was interpreted with very low, low, or medium confidence in nine (8.3%) of 108 and with high or very high confidence in 99 (91.7%) of 108 children. Radiologists were more confident in their interpretation of CT than sonography (p < 0.001). If the sonography was a true-positive examination, radiologists were more confident in its interpretation than if it was a false-positive study (p = 0.003). Radiologists were more confident in sonographic interpretation of a true-negative examination than of a false-negative study (p = 0.03). Patient age and sex and the radiologists' level of experience did not make a significant difference in the confidence of interpretation of sonography or CT.

CONCLUSION. In the evaluation of childhood appendicitis, radiologists' confidence in interpretation is influenced by the choice of imaging technique as well as by the results of the study.

Introduction

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The use of imaging in the diagnosis of pediatric appendicitis has been increasing steadily over the past decade. Sonography has been described as widely available, quick, relatively simple to perform, well tolerated, and noninvasive [1,2,3,4,5]. In addition, sonography does not involve ionizing radiation, an important attribute when imaging pediatric patients [6]. Conversely, sonography is highly operator-dependent, is technically difficult in the obese patient, and rarely visualizes the normal or retrocecal appendix [7,8,9,10]. Hence, negative findings on a sonographic examination do not exclude appendicitis if the normal appendix is not visualized [8, 10].

Recent advances in CT have yielded high predictive accuracies in the diagnosis of acute appendicitis in both the adult and pediatric populations. Rao et al. [7, 11,12,13] found the highest accuracy (98%) in adults when CT with contrast material administered only though the rectum was performed. We have previously evaluated this technique in the pediatric population after negative or indeterminate sonographic results and found an accuracy of 94% for the diagnosis of acute appendicitis. These data have been published in part elsewhere [14]. CT with rectal contrast material is quick to perform, easy to interpret, and minimally invasive. The normal appendix is visualized in most patients, and the average radiation exposure is one third of the average exposure of a standard abdominopelvic CT [7, 11,12,13,14].

The usefulness of appendiceal imaging depends strongly on the confidence in interpretation of the examination by the radiologist, for when the radiologist is confident about the interpretation, definitive clinical decisions can be made more easily. To our knowledge, no prior studies have evaluated the confidence of the radiologist in the interpretation of imaging studies performed for diagnosing pediatric appendicitis. The purpose of this study was to evaluate radiologists' confidence in their interpretation of sonography and CT with rectal contrast material in the evaluation of the pediatric appendix.

Subjects and Methods

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Subjects and Setting
Patients between the ages of 3 and 21 years old with a clinical presentation suggestive of acute appendicitis who presented to the emergency department from July 1998 to December 1998 were prospectively identified. Pregnant patients and patients with a history of previous appendectomy or a contraindication to rectal contrast material were ineligible. The study was approved by the hospital's institutional review board.

Study Protocol
The consulting senior surgical resident examined all children or young adults with suspected appendicitis in the emergency department under the supervision of an attending pediatric surgeon. Children with equivocal clinical findings for appendicitis were initially examined with pelvic sonography. If the sonogram was definitive for appendicitis, appendectomy was performed. If the sonographic findings were normal but the appendix was not visualized, or if the findings were equivocal for appendicitis, limited CT of the pelvis was performed with rectal contrast material. Results of both the sonography and CT were immediately made known to the treating physicians. In addition, the primary investigator was informed of the sonographic results and the radiologists' confidence in interpretation immediately after the examination was completed. Data regarding the measures of test performance and changes in patient management have been published elsewhere [14].

Performance and Interpretation of Imaging Studies
All pelvic sonographic studies were performed by one of five attending staff between 8:00 A.M. and 5:00 P.M. or by one of six pediatric radiology fellows between 5:00 P.M. and 8:00 A.M. The graded compression technique [15] was performed using 5.0- or 7.5-MHz linear array transducers (Model XP10; Acuson, Mountain View, CA). The sonographic diagnosis of appendicitis was made by detecting a fluid-filled, noncompressible, distended structure (6 mm in diameter) with or without an appendicolith, which showed no peristaltic activity, appeared constant in shape and position, and was located either anterior to the psoas muscle or in a retrocecal position. The presence of pericecal inflammatory changes in the absence of visualizing an abnormal appendix was strongly suggestive but not specific for the diagnosis of acute appendicitis.

CT was performed with 9800 HiLight scanners (General Electric Medical Systems, Milwaukee, WI) using the helical technique. Children received between 200 and 1000 ml of 3% diatrizoate meglumine (Gastrografin; Bristol-Meyers-Squibb, Princeton, NJ) saline solution through a rectal catheter by slow controlled drip immediately before scanning. Oral or IV contrast material was not used. Thincollimation helical scanning was performed from the top of L3 to the acetabular roof with a pitch of 1.5 and 3-mm collimation for children 3-10 years old and 5-mm collimation for patients older than 10 years.

Each CT examination was immediately interpreted by one of six pediatric radiology fellows or an attending pediatric radiologist. The CT diagnosis of appendicitis was made by visualizing an abnormal appendix and pericecal inflammation or abscess with or without the presence of an appendicolith. An abnormal appendix was defined as a fluid-filled tubular structure measuring greater than 6 mm in its maximum diameter with adjacent inflammatory changes. Highly suggestive signs included the presence of an appendicolith, focal cecal apical thickening, the arrowhead sign, and the cecal bar in cases of nonvisualization of the appendix [7, 11,12,13]. Examinations performed with CT were interpreted as negative for appendicitis if a normal appendix was visualized. In addition, if the normal appendix was not visualized, the scan was interpreted as negative if no associated periappendiceal inflammatory changes were seen.

Final Diagnoses
The final diagnosis for patients who underwent appendectomy was determined at pathologic examination. Patients who were treated nonoperatively had clinical follow-up by telephone at 2 weeks after the emergency department visit. In addition, the medical records of all patients were reviewed at 4-6 months after study completion.

Radiologists' Confidence
Radiologists were asked to record their level of confidence in their interpretation of the sonograms and CT scans immediately after each examination. The five levels of confidence were very low, low, medium, high, and very high. The radiologists' level of confidence in sonographic interpretation was reported to the primary investigator immediately after the examination.

Statistical Analysis
Descriptive statistics on the cohort were obtained and have been partially described elsewhere [14]. Radiologists' confidence level was initially treated as an ordinal variable and was subsequently reduced to a dichotomous variable (very low, low, and medium confidence versus high and very high confidence). Confidence in interpretation of sonography and CT was evaluated using the chi-square test with Yates correction. Interpretation confidence based on the results of the sonography and CT and on whether the results were true-positive or true-negative was evaluated using Fisher's exact test. Interpretation confidence was also evaluated according to the child's age and sex and whether the radiologist was a fellow or staff member using the chi-square and Fisher's exact tests. Logistic regression models were constructed with backward selection to evaluate possible interactions between these variables. All analyses were performed with SPSS for Windows, version 7.5 (Statistical Package for the Social Sciences, Chicago, IL).

Results

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One hundred seventy-seven children and young adults with suspected appendicitis were examined in the emergency department during the 6-month study period. Thirty-four patients (19.2%), 30 of whom had pathologically proven appendicitis, went directly for appendectomy without imaging studies. Four children were discharged without imaging; none had appendicitis. One hundred thirty-nine patients (78.5%) had equivocal clinical findings and constituted the study cohort.

Study Cohort
The mean age of the children was 11.1 ± 4.25 years (range, 3-20 years; median, 11 years). Two patients were older than 18 years. Seventy patients (50%) were male. Appendicitis was proven at pathologic examination in 50 patients (36%). Eleven patients (22%) had perforated appendicitis and three (6%) had a gangrenous appendix.

Thirty-one patients were imaged solely with sonography. Of these, 19 patients underwent appendectomy immediately after sonography and all had pathologically proven appendicitis. Eleven children who had negative sonographic examinations did not undergo CT because of resolved symptoms (64%), established alternative diagnoses (9%), visualization of a normal appendix (18%), or patient inability to retain the rectal contrast material (9%). One 5-year-old boy had an equivocal sonographic examination and did not undergo CT. His symptoms progressed and he underwent appendectomy 8 hr after admission.

One hundred eight patients underwent CT after negative or equivocal findings on sonography. The mean age of these children was 11.34 ± 4.28 years (range, 4-20 years; median, 11 years). Fifty-six patients (52%) were female. Thirty-one patients underwent appendectomy immediately after CT. Of these, 28 patients had pathologically proven appendicitis. Twenty-five patients were hospitalized for observation, 24 of whom did not have appendicitis. Fifty-two children were discharged directly from the emergency department; none had appendicitis.

Radiologic Confidence
The radiologists' levels of confidence in the interpretation of sonography and CT are listed in Table 1. Sonography was interpreted with very low, low, or medium confidence in 59 (42.4%) of 139 children and with high or very high confidence in 80 (57.6%) of 139 patients. Radiologists interpreted CT with a very low, low, or medium level of confidence in nine (8.3%) of 108 patients and with a high or very high level of confidence in 99 (91.7%) of 108 patients. Radiologists were more confident in their interpretations of CT than in their interpretations of the sonographic examinations (p < 0.001).

Table 2 summarizes the radiologists' levels of confidence based on whether the CT and sonographic findings were interpreted as positive, negative, or equivocal. Radiologists were more confident in their interpretation of positive sonographic results than in negative or equivocal sonographic results (p < 0.001). However, radiologists were equally confident in their interpretation of positive versus negative or equivocal CT results (p = 0.55).

The radiologists' levels of confidence based on whether the examinations were interpreted as false- or true-negative and false-or true-positive are shown in Tables 3 and 4. Radiologists were more confident in their interpretations of true-negative than false-negative sonograms (p = 0.03) and in their interpretations of true-positive than false-positive sonograms (p = 0.003). The false-negative rate was 11.2% and the false-positive rate was 2.5% for sonographic examinations interpreted with high confidence. Regarding CT, radiologists were equally as confident in the interpretations of false-negative and true-negative studies (p = 1.00), although there was only one false-negative scan. Conversely, the radiologists were more confident in their interpretations of true-positive than of false-positive CT scans (p = 0.02). For CT examinations interpreted with high confidence, the false-negative rate was 1% and the false-positive rate was 2%.

The sex of the patients did not make a significant difference on the radiologists' level of confidence in interpretations of sonography (p = 0.84) or CT (p = 0.35). No difference was seen in the level of interpretation confidence for sonography among the different age groups (p = 0.58). Similarly, for CT, radiologists' confidence in interpretation was similar among the different age groups, although approaching significance (p = 0.06).

Finally, interpretation confidence was evaluated according to whether the radiologists were fellows or staff members. Fellows interpreted 97 (69.8%) of 139 sonograms and 74 (68.5%) of 108 CT scans. No difference was seen in the level of confidence in interpretation of sonography (p =0.71) or CT (p = 0.27) according to whether the radiologist interpreting the studies was a fellow or staff member.

Age group, sex of the child, and whether the radiologist was a fellow or staff member were entered into logistic regression models using backward selection. None of the variables was found to be associated with sonography or CT interpretation confidence when controlling for the other variables.

Discussion

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The morbidity and mortality rates of acute appendicitis remain high in the pediatric population, mostly because of the complications associated with delayed diagnosis [16,17,18,19]. Many children with suspected appendicitis are often admitted to the hospital for an observation period. This period of observation is expensive, and there remains the risk of perforation of the appendix if surgery is delayed in children with appendicitis [15, 20]. Hence, rapid diagnosis when the child is first evaluated is essential in the management of these patients.

The use of sonography and CT to image the pediatric appendix has increasingly become the standard of care at many institutions. We have previously shown that a protocol using sonography followed by CT with rectal contrast material was 94% accurate for the diagnosis of acute appendicitis in children [14]. In addition, the protocol saved children without appendicitis from undergoing laparotomy and, more important, it expedited the time to appendectomy for children with appendicitis.

The confidence of the radiologist when interpreting a scan is of great importance to clinicians in making definitive management decisions. Clinical decisions can be more assuredly made when the radiographic interpretation is made with confidence. We have shown that radiologists are more confident when interpreting CT with rectal contrast material than when interpreting sonography in the diagnosis of pediatric appendicitis. A low level of confidence further suggests that sonographic interpretation is more likely to be in error. However, when the results of the sonogram were positive for appendicitis, the radiologists were more confident in their interpretations than when the sonographic results were negative or equivocal. This is in keeping with previous studies in the literature that have shown that negative sonographic findings do not affect initial patient management and, in fact, may lengthen hospital stays and increase costs [8, 10, 14]. However, the radiologists' levels of confidence were higher with true-negative results than with false-negative interpretations. This may be a helpful finding in that clinicians may be more apt to make definitive management decisions on the basis of negative sonographic findings if the radiologist is confident in his interpretation. However, sonographic examinations that were interpreted with high confidence had an 11.2% false-negative rate and a 2.5% false-positive rate. Although the numbers are relatively small, they add validity to the use of CT with rectal contrast material in the examination of children and young adults with appendicitis.

Our study had several limitations. First, the radiologists were aware that if the sonographic findings were negative or equivocal, CT would be performed. This knowledge may have led the radiologists to report a low interpretation confidence for sonography so that they would not be seen as overly confident of an interpretation that was subsequently shown to be erroneous. Although this may be true, the positive sonographic results were typically interpreted with a high or very high degree of confidence. In addition, the true-negative sonographic examinations were also interpreted with a high or very high degree of confidence more often than the false-negative sonograms. Second, the radiologist interpreting the CT was almost always the same one who interpreted the sonogram and thus, that radiologist was not unaware of the sonographic results when interpreting the CT. However, the sonographic interpretation confidence was immediately reported to the primary investigator after completion of the sonography so that the confidence would not be influenced by the CT results. Last, there may have been bias in the CT interpretation, because if the sonographic findings were negative, it was more likely that the CT findings would also be negative. However, in the 40% of sonograms interpreted as negative, the findings were positive for appendicitis on CT.

Although we did not evaluate CT without a preceding sonogram, the possibility of eliminating the sonogram as part of the diagnostic workup arises. However, sonography enabled the radiologist to make the diagnosis of appendicitis without radiation and noninvasively in 40% of children who had the disease. Further studies are needed to identify the clinical characteristics of those children for whom CT may be justified without preliminary sonographic examination.

Positive sonographic findings exhibit high interpretation confidence by radiologists in the diagnosis of pediatric appendicitis. Conversely, radiologists' interpretation confidence declines with negative sonographic findings. However, if the radiologist is confident about a negative interpretation, the examination results are more likely to be true-negative than false-negative. CT was interpreted with a high or very high degree of confidence in 92% of all scans obtained. Therefore, after negative sonographic findings with a low interpretation confidence by the radiologist, we believe that CT can be a useful adjunct to making a timely diagnosis of acute appendicitis in children.

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