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Gastrointestinal Imaging |
1 From the Sunnybrook and Women’s College Health Sciences Centre (O.B., M.A., P.H.), Mount Sinai Hospital (D.R.), and University Health Network (D.R.), University of Toronto, 2075 Bayview Ave, Toronto, Ontario, Canada M4N 3M5. Received September 18, 2001; revision requested October 25; final revision received March 18, 2002; accepted March 25. Address correspondence to M.A. (e-mail: mostafa.atri@swchsc.on.ca).
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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MATERIALS AND METHODS: Three radiologists blinded to patient surgical history retrospectively reviewed CT scans obtained for renal colic assessment in 187 consecutive patients. No contrast material was administered. The frequency of visualization and the two-wall thickness of normal appendices were recorded. Interobserver agreement and effect of adequacy of intraperitoneal fat on identification of the appendix were assessed.
RESULTS: The prevalence of appendectomy was 10.7% (20 of 187 patients). The means for the three reviewers’ sensitivity, specificity, positive and negative predictive values, and accuracy of visualization of normal appendix were 79% (CI: 73%, 84%), 90% (CI: 78%, 96%), 98% (CI: 97%, 99%), 34% (CI: 22%, 47%), and 80% (CI: 74%, 86%), respectively. There was no significant difference among the three reviewers (P > .05) according to conditional logistic regression and exact McNemar test results. For all reviewers, the frequency of appendix visualization was significantly lower in patients with less intraperitoneal fat (P = .01–.001, 
2 test). The mean thickness of normal appendix if no intraluminal content was visualized was 6.6 mm ± 1.0 (SD), and the mean thickness, excluding visualized intraluminal content, was 3.6 mm ± 0.8. The nonweighted 
value for interobserver agreement for normal appendix visualization was 0.69–0.75 among the three reviewers, which indicated good to excellent agreement.
CONCLUSION: Most normal appendices are seen at nonenhanced helical CT. The thickness of normal appendix, when the content is not recognizable, overlaps the values currently used to diagnose appendicitis at CT.
© RSNA, 2002
Index terms: Appendicitis, 751.291 • Appendix, 751.92 • Appendix, CT, 751.12111, 751.12115
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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Different CT techniques have been proposed for imaging appendicitis. These techniques include nonenhanced CT (ie, CT without orally, rectally, or intravenously administered contrast material) (1,6), CT with rectally administered contrast material with or without orally administered contrast material (3,4,21), and CT with orally administered contrast material with or without intravenously administered contrast material (5,14,17). Reported accuracies (1,3–6,14,17) of these techniques are comparable, and this explains the gaining popularity of techniques without intravenously and/or rectally administered contrast material.
Despite the widespread use of CT to diagnose appendicitis, our literature review failed to reveal studies in which a large number of normal appendices were systematically evaluated. The reported thickness of a normal appendix at CT is based on US results (7–9,22), with use of a 6-mm short-axis thickness as the upper limit of normal (1–4,17,21). This extrapolation of US findings of a normal appendiceal thickness is based on the size of a compressed collapsed appendix without measurement of the luminal content and consequently does not apply to CT. Visible luminal content is, therefore, necessary to measure the true thickness of the appendix at CT without intravenously administered contrast material. If the luminal content is not recognizable, it is not possible to differentiate a collapsed appendix from one with content of the same attenuation as the wall.
Data concerning the location of the appendix are acquired from either autopsy series or surgery literature (23,24), and no data are available from imaging techniques. The visualization of the ileocecal valve and its relationship to the base of the appendix provide important information to help identify the appendix on CT scans. The purpose of our study was to evaluate the frequency of visualization, thickness, and features of the normal appendix at nonenhanced helical CT.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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Therefore, our study consisted of 187 CT examinations performed in 187 patients (116 male patients, 71 female patients; age range, 17–92 years; mean age, 47 years ± 15 [SD]). These patients included those who had undergone appendectomy. Eighty-six examinations were performed with a multi–detector row CT scanner and 101 with a single-detector CT scanner.
The final diagnoses in these 187 patients included right ureteric calculi in 43, left ureteric calculi in 45, left-sided diverticulitis in three, right-sided diverticulitis in two, and left-sided appendagitis in one. The cause of pain was not identified in the remaining 93 patients. Final diagnoses were obtained by means of emergency department chart review. The standard of reference for the final diagnosis was the diagnosis at discharge based on the CT, clinical, and laboratory results. This cohort did not include any patient with a diagnosis of colitis or inflammatory bowel disease or with a discharge diagnosis of spontaneously resolved appendicitis. No patient underwent surgery after CT.
At the time this study was conducted, our institution (Sunnybrook and Women’s College Health Sciences Centre, Toronto, Ontario, Canada) did not require either institutional review board approval for retrospective study of the medical records or images or patient informed consent.
CT Examinations and Review Process
CT scans were obtained with a helical scanner (CTI or Light Speed; GE Medical Systems, Milwaukee, Wis) by using 5-mm collimation with reconstruction every 2.5 mm. A pitch of 1.6:1.0 was used for single-detector CT, and a high-speed table speed of 30 mm per rotation and a pitch of 6:1 were used for multi–detector row CT. No contrast material was administered orally, rectally, or intravenously. Images were reviewed in a cine format at standard electronic workstations.
Three experienced abdominal radiologists (M.A., P.H., D.R.—reviewers 1, 2, and 3, respectively), each with more than 6 years of experience, retrospectively and independently reviewed CT images at a workstation. They were blinded to patients’ surgical histories but aware of the clinical histories of renal colic.
The appendix was interpreted as either visualized or not visualized. The maximum full thickness of the appendix was measured (Fig 1). In a normal appendix, the content may or may not be recognizable, depending on the attenuation of the content. Therefore, it is not possible to differentiate a collapsed appendix from a distended appendix with content of the same attenuation as the wall. The reviewers were asked to measure the maximum thickness of the two walls of the appendix on either side of the content, with exclusion of the content—regardless of whether it was fluid or gas—if it was recognizable (Fig 1a). If the content could not be recognized, the maximum full thickness was measured (Fig 1b).
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Reviewer 1 identified the location of the tip of the appendix and described it as paracolic, or adjacent and along the ascending colon; retrocecal, or behind the colon; pelvic, or extending to the pelvis; or midline, or extending to the midline (Fig 2). The same reviewer judged the position of the cecal pole relative to the lowest portion of the ascending colon (identified according to the position of the ileocecal valve) to be cephalic, the same, or caudal. The reviewer also described the location of the base of the appendix relative to the ileocecal valve as caudal, cephalic, medial, lateral, anterior, or posterior.
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We documented the patients’ history of appendectomy from their medical charts, if it was specified in the chart, or by means of telephone contact, if the history was not specified in the chart. Data were collected by one of the authors (O.B.) who was not involved in the image review process.
Statistical Analyses
To determine the mean and range of appendiceal thickness, we used measurements of the appendices that were seen by all three reviewers. To calculate the between-reader and between-subject SDs in appendiceal thickness, we used a random-effects model (25). This same model was used to compute the overall mean thickness. We evaluated the potential effect of ureteric stones on appendix thickness by using analysis of variance to compare the thicknesses of the appendix when there was a right-sided ureteric stone, a left-sided ureteric stone (located anywhere in the right or left ureter), and no ureteric stone.
Sensitivity, specificity, negative predictive value, positive predictive value, and accuracy for the visualization of the appendix were determined for each reviewer, and the mean values for the three reviewers were calculated. The standard of reference for the presence or absence of appendix was data from the chart review (history of appendectomy) or information obtained by means of telephone contact. Ninety-five percent CIs for these means were calculated by using the bootstrap method (26).
Sensitivity of the three reviewers was compared by using conditional logistic regression (27). Because of sample size limitations, specificity values for the three reviewers were compared pairwise by using an exact McNemar test (27).
The statistic was used to assess interobserver agreement in the detection of appendix. A statistic of less than 0.40 indicated poor agreement; 0.40–0.59, moderate agreement; 0.60–0.74, good agreement; and 0.75 or greater, excellent agreement (28).
For other analyses, independent proportions were compared by using the Fisher exact or 2 test, and paired proportions were compared by using a McNemar test. A P value of less than .05 was considered to indicate statistical significance. Statistical analysis was performed by using commercially available software (SPSS Base 10.0 for Windows, SPSS, Chicago, Ill, and S-Plus 2000, 1998-2000, Mathsoft, Cambridge, Mass).
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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Table 1 lists the frequencies of visualization of a normal appendix in patients who had their appendix (sensitivity), the frequency of recognition of an absent appendix in the patients with a history of appendectomy (specificity), the predictive value of lack of visualization of an appendix (negative predictive value), the predictive value of visualization of an appendix (positive predictive value), the accuracy of visualization of the normal appendix for each reviewer, and the mean values for the three reviewers in each category. There was no significant statistical difference in these values among the three reviewers (P > .05).
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value was 0.75 (CI: 0.65, 0.86), which indicated excellent agreement, for the detection of appendix between reviewers 1 and 2; 0.74 (CI: 0.63, 0.85), which indicated good agreement, for the detection of appendix between reviewers 1 and 3; and 0.69 (CI: 0.57, 0.80), which indicated good agreement, for the detection of appendix between reviewers 2 and 3. The multirater value was 0.73 (CI: 0.65, 0.81). According to the three reviewers, the ileocecal valve was visualized in a mean of 88% of the patients. Table 2 shows the mean and the range of thickness of the visualized appendices for each reviewer and the mean and the range for those 117 appendices visualized by all three reviewers. When the luminal content was not visualized and the full thickness was measured (Fig 1b), the mean full thickness of the normal appendix was 6.6 mm ± 1.0 (SD) (range, 4.0–11.0 mm). In 40% of the cases, the mean full thickness of the appendix was 7.0 mm or greater. When the luminal content was visualized and therefore the wall itself could be measured (Fig 1a), the mean thickness of normal appendix was 3.6 mm ± 0.8 (range, 2.0–6.0 mm).
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The reviewer assigned to determine the location of the appendiceal tip identified 135 of 167 appendices. The most common location of the appendiceal tip, as identified by this reviewer, was paracolic in 84 (62%) of 135 appendices (Fig 2a). The appendiceal tip was pelvic in 25 (19%) (Fig 2b), midline in 12 (9%) (Fig 2c), and retrocecal in 14 (10%) of 135 (Fig 2d) appendices. The tip of one of the retrocecal appendices was located in the subhepatic space, and one of the pelvic appendices was on the left side in a patient with situs inversus viscerum.
Both the ileocecal valve and the appendix were visualized in 130 patients. The relationship of the base of the appendix to the ileocecal valve was cephalic in five (4%) of these 130 patients and caudal in 125 (96%). The appendiceal base was medial to the ileocecal valve in 11 (8%), lateral in one (1%), posterior in 118 (91%), and anterior in zero (0%) of the 130 patients. Cecal poles were higher than the lowest portion of the ascending colon in only 3.5% (six of 170) of patients whose ileocecal valves were seen.
In the 117 appendices that were seen by all three reviewers, at least one appendicolith was seen by reviewer 1 in 15 of 117, by reviewer 2 in 12 of 117, and by reviewer 3 in 13 of 117 appendices, and all three reviewers saw at least one appendicolith in nine (7.7%) of 117 of the normal appendices. The mean appendicolith size was 4.3 mm ± 2.5 in the latter group.
Reviewer 1 saw 13 (52%) of 25 normal appendices when there was not enough pericecal fat and 122 (86%) of 142 normal appendices when there was adequate fat (P = .001). Reviewer 2 saw 14 (56%) of 25 normal appendices when there was not enough pericecal fat and 113 (80%) of 142 normal appendices when there was adequate fat (P = .01). Reviewer 3 saw 15 (60%) of 25 normal appendices when there was not enough pericecal fat and 118 (83%) of 142 normal appendices when there was adequate fat (P = .008).
Periappendiceal stranding was suggested in nine patients—four by reviewer 2, five by reviewer 3, and none by reviewer 1. All cases of stranding were indicated in the appendices that were visualized by all three reviewers. However, stranding was not identified in the same patient by more than one reviewer. The mean full-thickness diameter of each of these nine appendices, as measured by the three reviewers, was less than 6.6 mm. None of these nine patients had a diagnosis of appendicitis before or after CT.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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CT evaluation of appendicitis without the use of intravenously administered contrast material is a growing trend. Some authors (1–4) have advocated the use of examinations with both orally and rectally administered contrast material, as well as the use of examinations without contrast material. However, in the absence of intravenously administered contrast material, the true wall thickness can be measured only if the luminal content of the appendix can be recognized. Since all normal appendices do not fill with rectally or orally introduced contrast material and the content is not always recognizable from the wall, it is important to determine the range of the thickness of the normal appendix at nonenhanced CT.
The main CT criteria for the diagnosis of acute appendicitis published in the literature include identification of a thickened appendix with a two-wall diameter greater than 6.0–7.0 mm, periappendiceal inflammatory changes, and a calcified appendicolith (1–4,6,14). The size criterion to diagnose appendicitis is especially important in the absence of periappendiceal stranding. In a recent study by Jacobs et al (30), in 22% of cases of appendicitis there was no periappendiceal stranding at nonenhanced CT. The values for normal and abnormal thicknesses of the appendix are derived from US studies (7–9). However, at US the appendix can be compressed and, therefore, the true wall thickness excluding the content can be measured. In our study, CT performed for renal colic assessment without orally, rectally, or intravenously administered contrast material was considered the ideal method to assess the range of appendiceal thickness in those cases in which the appendix was not filled with contrast material.
The mean full thickness of the normal appendix in our series was 6.6 mm ± 1.0 (range, 4.0–11.0 mm) when the luminal content was not visible. When the content was recognizable, the mean thickness of the normal appendix was 3.6 mm ± 0.8 (range, 2.0–6.0 mm), which is comparable with the reported US values (7–9). Consequently, we can infer that an upper limit of 6.0 mm for normal appendiceal thickness can be used reliably at CT only if the luminal content is visualized. If the content is not visualized, and in the absence of extraappendiceal inflammatory changes, it is not possible to differentiate a noncollapsed appendix filled with fluid of the same attenuation as the wall from a thick inflamed appendix.
We suggest 10.0 mm (mean ± 3 SDs) as the upper limit of normal if the content is not visualized and in the absence of extraappendiceal inflammatory changes; among the 117 appendices seen by all three reviewers, only one was thicker than 10.0 mm according to two reviewers. Therefore, the diagnosis of appendicitis is indeterminate at CT in the latter group when appendiceal thickness is 6.0–10.0 mm, and these patients should undergo further examination with rectally or intravenously administered contrast material or with US to visualize the wall and thus prevent a false-positive diagnosis of appendicitis. Since the cohort in our study was not referred for exclusion of appendicitis, we cannot determine in what percentage of patients with documented acute appendicitis the appendiceal thickness overlaps with the normal values seen in the cohort in our study.
The ease of identification of the normal appendix at CT (3,4,7–9,18–20) accounts for the high negative predictive value of CT in the evaluation of acute appendicitis. In our study, the mean frequency of visualization of a normal appendix by the three reviewers at nonenhanced CT was 79% (CI: 73%, 84%), and the positive predictive value of the existence of an appendix was 98% (CI: 97%, 99%). These results are in agreement with those of a study performed by Lane et al (2) in which an appendix was identified in 77% of normal appendices examined with nonenhanced CT.
In other studies performed with orally and rectally administered contrast material (3,21) and with rectally administered contrast material alone (4), a normal appendix was visualized in 90%–100% of patients. In studies (14,16–18,31) in which intravenously administered contrast material was used, a normal appendix was visualized in 43%–68% of patients. However, the reviewers were not blinded to surgical history in these studies: Only patients with appendices were included, and, consequently, the positive predictive value of identification of a normal appendix could not be determined. Moreover, only one reviewer was involved, and interobserver agreement was not evaluated.
In our study, there was good to excellent interobserver agreement in the detection of the appendix (nonweighted , 0.69–0.75). At US, the normal appendix is less frequently identified and the results are variable, between 0% and 82%, reflecting the operator dependency of US (7–9,19,20).
Our study results demonstrated increased identification of the appendix for all three reviewers when an adequate amount of fat was present (P = .01–.001). In a recent pediatric study (32), the authors also observed a statistically significant correlation between the visualization of the appendix and the degree of peritoneal fat.
Familiarity with the normal anatomy of the appendix in relation to the cecum and the ileocecal valve is helpful for identifying the appendix. We found it useful to identify the appendix by initially locating the ileocecal valve, which was visualized in 88% of this study population. The location of the ileocecal valve helps define the contour of the cecum and consequently facilitates the search for the base of the appendix.
We found the cecal pole to be higher than the lowest portion of the ascending colon in only 3.5% of patients. The base of the appendix was posterior to the ileocecal valve in 91% of patients and medial or lateral in 9% of patients, and no bases were anterior to the ileocecal valve. The normal development and migration of the appendix from an anterior position during fetal life and childhood to a more posterior location during adulthood explain this finding. The migration of the base is caused by the faster growth of the frontal and lateral cecal walls compared with the growth of the other walls (23) and can be further explained by an incomplete rotation of the cecum or an early adhesion to the abdominal back wall prior to the rotation process (24).
The presence of a calcified appendicolith associated with periappendiceal inflammation is one of the CT criteria used to diagnose acute appendicitis. Our frequency of visualizing at least one appendicolith in 7.7% of patients is higher than that in previous articles (1–3), which documented 0%–2% in the healthy population. We believe the explanation for the higher frequency in our study is our definition of appendicolith. We defined an appendicolith as any focal hyperattenuating material in the appendix, although some could have been bowel content. This difficulty in differentiating a small appendicolith from hyperattenuating bowel content is a potential limitation of CT.
Our study had a number of limitations. The main limitation was the lack of a reference standard as proof for a normal appendix. However, it is not possible to design a study of normal appendix with pathologic confirmation. Although it is theoretically possible that some of the cases in our study were spontaneously resolved appendicitis, it is unlikely, considering that 40% of the appendices that were seen by all three reviewers were 7.0 mm or greater in full thickness. It is unlikely that a high percentage of individuals in a cohort of patients who present with renal colic will have spontaneously resolving appendicitis. Moreover, no patient in our cohort had a diagnosis of appendicitis at discharge.
There was no case of colitis or inflammatory bowel disease in this group to explain a thick appendix. The two cases of right-sided diverticulitis in this cohort involved 6.0- and 7.3-mm-thick appendices. Although one of the reviewers saw periappendiceal stranding in nine patients, the lack of identification of this finding by the other two reviewers and the small size of these appendices (maximum diameter of 6.6 mm) made the diagnosis of appendicitis unlikely.
Another limitation of our study was the potential for verification and recall biases, since we relied on chart review or telephone contact to obtain history of appendectomy. It is possible that some patients had their normal appendix removed during a pelvic surgery and either were not aware of or could not remember this information. It was not possible for us to review all surgery information in the patients who had undergone previous surgeries at other institutions, some of which were performed in another city or country. However, it is routine clinical practice to rely on the patient’s knowledge of previous appendectomy to exclude the possibility of appendicitis.
It is also theoretically possible that some of these patients had congenital absence of appendix. However, considering that the reported estimated incidence of appendix agenesis is one in 100,000 laparotomies for patients suspected of having appendicitis (33), agenesis of appendix was an unlikely occurrence in the cohort in our study.
We designed our study to reduce the potential effect of these biases. We collected data from the 117 appendices that were identified by all three reviewers to calculate normal appendiceal thickness. There were no false-positive cases of an appendix (ie, history of appendectomy) among these 117 appendices. Moreover, the interobserver agreement for recognition of an appendix varied from good to excellent in this study.
In conclusion, our study results showed that most normal appendices are seen at nonenhanced helical CT. The full thickness of the normal appendix, when the content is not recognizable, overlaps the values used to diagnose appendicitis at CT but is similar to US values when the content is visualized and subtracted from the total thickness.
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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