Pediatr Radiol DOI 10.1007/s00247-015-3491-9
ORIGINAL ARTICLE
Appendiceal diameter: CT versus sonographic measurements Emily S. Orscheln 1 & Andrew T. Trout 2
Received: 14 May 2015 / Revised: 20 August 2015 / Accepted: 22 October 2015 # Springer-Verlag Berlin Heidelberg 2015
Abstract Background Ultrasound and CT are the dominant imaging modalities for assessment of suspected pediatric appendicitis, and the most commonly applied diagnostic criterion for both modalities is appendiceal diameter. The classically described cut-off diameter for the diagnosis of appendicitis is 6 mm when using either imaging modality. Objective To demonstrate the fallacy of using the same cut-off diameter for both CT and US in the diagnosis of appendicitis. Materials and methods We conducted a retrospective review of patients younger than 18 years who underwent both US and CT of the appendix within 24 h. The shortest transverse dimension of the appendix was measured at the level of the proximal, mid and distal appendix on US and CT images. We compared mean absolute difference in appendiceal diameter between US and CT, using the paired t-test. Results We reviewed exams of 155 children (58.7% female) with a mean age of 11.3±4.2 years; 38 of the children (24.5%) were diagnosed with appendicitis. The average time interval between US and CT was 7.0±5.4 h. Mean appendiceal diameter measured by CT was significantly larger than that measured by US in cases without appendicitis (5.3±1.0 mm vs. 4.7±1.1 mm, P<0.0001) and in cases with appendicitis (8.3± 2.2 mm vs. 7.0±2.0 mm, P<0.0001). Mean absolute diameter
* Andrew T. Trout
[email protected] 1
Department of Radiology, University of Cincinnati Medical Center, Cincinnati, OH, USA
2
Department of Radiology, Cincinnati Children’s Hospital Medical Center, 3333 Burnet Ave., Cincinnati, OH 45229, USA
difference at any location along the appendix was 1.3–1.4 mm in normal appendices and 2 mm in cases of appendicitis. Conclusion Measured appendiceal diameter differs between US and CT by 1–2 mm, calling into question use of the same diameter cut-off (6 mm) for both modalities for the diagnosis of appendicitis. Keywords Appendicitis . Appendix . Children . Computed tomography . Diameter . Ultrasound
Introduction Ultrasound and CT are the two dominant imaging modalities for the assessment of suspected pediatric appendicitis. Although US is the preferred examination for evaluation of right lower quadrant pain and suspected appendicitis, most children with suspected appendicitis are evaluated at facilities with limited pediatric US capability [1]. Therefore, CT is often used for primary evaluation of suspected appendicitis [2–4]. CT also plays a role in the workup of suspected appendicitis when sonography is equivocal or when there remains a clinical concern for appendicitis despite a negative US examination [5–7]. For both US and CT, appendiceal diameter is one of the most commonly applied diagnostic criteria for acute appendicitis [5, 6, 8–10]. Specific cut-off diameters have been extensively debated in the literature, but the classically described cut-off diameters for US and CT are the same, both 6 mm, a cut-off that was initially described for US but has subsequently been extrapolated to CT [6, 8, 9, 11, 12]. It is counterintuitive that the diameter cut-off for a normal appendix would be the same for CT and US given that graded compression is a key component of the US examination of the appendix. One would expect that the absence of compression during CT would result in overall larger appendiceal diameter, especially
Pediatr Radiol
Our institutional review board approved this study, for which we retrospectively reviewed imaging and clinical data for all patients younger than 18 years who underwent both right lower quadrant US and abdomen and pelvis CT within 24 h of each other between January 2010 and February 2014 at Cincinnati Children’s Hospital Medical Center. Cincinnati Children’s is a large academic pediatric medical center with 539 inpatient beds and more than 90,000 pediatric emergency visits annually. US examination for suspected appendicitis at our institution during the study period consisted of a graded-compression examination of the right lower quadrant using Toshiba Aplio XG machines (Toshiba America Medical Systems, Tustin, CA) with 9- to 6-MHz linear transducers. All clinical examinations included static images and cine clips of the right lower quadrant and appendix; all of these images were available for review. CT examinations performed at our institution for acute abdominal pain were performed with intravenous contrast media. At the discretion of the protocolling radiologist, oral contrast media with a 1-h preparation was administered. Exams were performed on either a Toshiba Aquilion One 320-slice or a Toshiba Aquilion 64-slice CT scanner (both Toshiba America, Tustin, CA) with adaptive iterative dose reduction (AIDR) and size-based scan technique (weight-based kVp 80–120). Images were reformatted at 5-mm slice thickness in the axial plane and 3-mm slice thickness in the coronal plane for clinical review (sagittal reconstructions are not performed at our
institution for routine examinations). We only included cases in which the appendix was at least partly visualized on both modalities (CT and US). Children were included regardless of clinical diagnosis (e.g., normal, appendicitis, cystic fibrosis, inflammatory bowel disease). Clinically acquired US and CT images were reviewed by a 4th-year diagnostic-radiology resident (E.O.). In each case, the appendix was identified and measurements of the outer diameter (serosa to serosa) were performed at the level of the proximal one-third, mid one-third and distal one-third of the appendix on US images and on axial and coronal CT images (Fig. 1). If the appendix was partly visualized, measurements were performed of the visualized segments. For US, cine clips were used to identify the appendix using the following structural criteria: blind-ending, aperistaltic, hollow viscus, originating from the cecal tip. On CT, the appendix was identified as a blind-ending tubular structure arising from the cecal tip. In any case where the appendix was partly identified or if there was uncertainty regarding visualization of the appendix or accuracy of measurement, the images and location of measurements were reviewed by a pediatric radiologist (A.T.) with 1 year of post-fellowship experience. In addition to difficult cases, an additional 10% of cases measured by the primary reviewer were reviewed for accuracy of appendiceal identification and location of appendiceal measurement. Measurements performed for this study were done in the same manner as clinical work because this study directly relates to measurement of the appendix in the routine clinical context. All measurements were performed on the clinical PACS (v6.4.7.103271; Merge Healthcare, Chicago, IL) and recorded to the 10th of a millimeter. For both modalities, at each measurement location and in each plane, the shortest transverse dimension of the appendix was measured to limit the likelihood of oblique measurements. It is our experience that axial CT images are most commonly used for clinical interpretation of CT for suspected appendicitis, with coronal and sagittal reformats variably performed and primarily used for locating the appendix and for problem-solving. As such, no effort was made to precisely measure the transverse
Fig. 1 Sample measurements of the proximal appendix on US (a), axially reformatted CT (b) and coronally reformatted CT (c) images in a 14-year-old girl without appendicitis. The appendix measured 2.6 mm
in diameter by US, 4.7 mm on axial CT and 5.7 mm on coronal CT. Similar measurements were performed in the mid and distal appendix on both modalities
when there is air, stool or free-flowing fluid within the appendiceal lumen. To demonstrate the fallacy of using the same diameter cutoff for both CT and US in the diagnosis of appendicitis, we compared appendiceal diameters measured on abdominopelvic CTs and right lower quadrant US exams performed in the same children within a 24-h period. We hypothesized that there would be demonstrable differences in the measured diameter of the appendix by US versus CT.
Materials and methods
Pediatr Radiol
diameter of the appendix by CT through the use of curved reformats. Instead, we primarily used axial CT measurements (denoted axial) to compare to US measurements. To account, however, for the potentially tortuous course of the appendix, CT measurements are also presented for the smallest transverse appendiceal diameter in either the axial or coronal plane (denoted smallest), because this measurement might be a closer reflection of true appendiceal diameter. For all CT measurements, the appendix was measured at 400% magnification on the 5-mm axial and 3-mm coronal reconstructed images, except when not available. We also reviewed each child’s clinical chart to identify those who underwent appendectomy within 2 weeks of imaging and to determine the resultant pathological diagnosis. The absolute difference in appendiceal diameter between US and CT (absolute value of CT diameter minus US diameter) was calculated for the proximal, mid, and distal appendix for each child. Absolute values were used because there is no gold standard for true appendiceal diameter in this study. Overall difference in diameter was expressed as a mean of the absolute differences at the three measurement locations. We used mean actual differences in diameter to construct a Bland–Altman plot and descriptive statistics to report population characteristics. A paired t-test served to compare diameter measurements by CT and US in the same children and an unpaired t-test served to compare diameter measurements between children stratified by the presence of appendicitis. We used Microsoft Excel (2010; Microsoft Corp., Redmond, WA) for all statistical calculations.
Results Figure 2 illustrates the study population. The study population consisted of 155 children with a mean age of 11.3±4.2 years. Of these children, 41.3% (64/155) were male with a mean age of 10.6±4.2 years, and 58.7% (91/155) were female with a mean age of 11.8±4.1 years. Thirty-eight children (24.5%) were ultimately diagnosed with acute appendicitis, 13 of whom were diagnosed with perforated appendicitis. Thirtyfive of the 38 cases underwent appendectomy and had confirmed acute appendiceal inflammation on surgical pathology. The other three children were managed expectantly for perforated appendicitis; two of these underwent drainage catheter placement. The appendix was completely visualized on 149/155 (96.1%) CT examinations and 133/155 (85.8%) US examinations, with partial visualization on the remaining studies. Eight of 155 CT examinations were reformatted in axial slices of thickness other than 5 mm, including: 4-mm slices in one child, 3-mm slices in six children, and 2.5-mm slices in one child. Sixty-eight (43.8%) of the 155 examinations, including both the CT and US, were reviewed by the second reviewer,
Fig. 2 Flow diagram of study population
with confirmation of identification of the appendix and location of measurement in all cases. The average interval between US and CT examination in all children was 7.0±5.4 h (range 1–23 h). In children with appendicitis, the average interval was 5.2±3.3 h. In children without appendicitis, the average interval was 7.6±5.8 h. US preceded CT in 152/155 (98.1%) children. Graphical comparison of CT and sonographic measurements, subdivided based on the presence of appendicitis, is shown in Fig. 3. In 115 of 155 (74.2%) cases the appendix measured larger on the axial CT images than on the US images. In 106 of 155 (68.4%) cases the smallest appendiceal diameter as measured by CT (axial or coronal) was greater than the smallest appendiceal diameter as measured by US.
Cases without appendicitis In children without appendicitis the mean appendiceal diameter as measured by CT — whether taken from the axial images (5.3 ±1 mm, P<0.0001) or the smaller of the axial and coronal measurements (5.1±1 mm, P<0.005) — was significantly larger than the mean appendiceal diameter as measured by US (4.7 ±1.1 mm). Based on comparison between the axial CT and the US images, mean absolute diameter difference at any location along the appendix was 1.2±0.7 mm, with relatively constant absolute differences in diameter at any of the three measurement points (proximal: 1.3±0.9 mm, mid: 1.1±0.9 mm, distal: 1.0±0.8 mm). Absolute diameter differences were similar when the smallest transverse diameter measurement by CT was compared to the US measurement (mean: 1.4±1.1, proximal: 1.6± 1.5 mm, mid: 1.2±1.4 mm, distal: 1.3±1.6 mm).
Pediatr Radiol
significantly greater in cases of appendicitis than in normal appendices (P<0.005).
Discussion
Fig. 3 Bland–Altman plots of CT versus sonographic measurements of the appendix in patients without (a) and with (b) pathologically proven appendicitis. Compared values are the average of the three measurements obtained of each appendix (proximal, mid, distal) by each modality. CT values are the smallest measured diameter (axial vs. coronal) of each segment of the appendix. Black dashed line indicates the mean difference in diameter between CT and sonographic measurements (CT minus US); gray dashed lines indicate the bounds of the 95% confidence interval (±2 standard deviations [SD] of the mean)
Cases with appendicitis In cases with appendicitis, the mean appendiceal diameter as measured by CT — whether taken from the axial images (8.3±2.2 mm, P<0.0001) or the smaller of the axial and coronal measurements (8.1± 2.0 mm, P <0.0001) — was significantly larger than mean appendiceal diameter as measured by US (7.0±2.0 mm). Based on comparison between the axial CT and the US images, mean absolute diameter difference at any location along the appendix was 2.0±1.3 mm, with relatively constant absolute differences in diameter at any of the three measurement points (proximal: 1.8±1.3 mm, mid: 1.8±1.6 mm, distal: 2.1±2.8 mm). Absolute diameter differences were similar when the smallest transverse diameter measurement by CT was compared to the US measurement (mean: 2.0±1.5 mm, proximal: 1.9±1.5 mm, mid: 2.0±2.2 mm, distal: 2.1±2.4 mm). Of note, the disparity in diameter measurements was
Our data demonstrate that in the majority of cases the same appendix measures larger on CT than on US, with CT and sonographic measurements of the appendix differing, on average, by greater than 1 mm in the absence of appendicitis and by 2 mm in cases of appendicitis. Although 1- to 2-mm differences in measured diameter are small, such a difference has potential implications if diameter cut-offs are rigidly used for diagnostic purposes. These data demonstrate the fallacy of using the same diameter cut-off for both CT and US diagnosis of appendicitis. Normative cut-offs for CT and US should instead be derived from specific studies of those modalities in children. To date, some studies have tried to identify optimal cutoffs for sonography. Results in those studies have been highly variable, with Je et al. [13] suggesting optimal performance of a 5.7-mm cut-off, Goldin et al. [9] suggesting 7 mm as the optimal cut-off, and Trout et al. [14] suggesting 8 mm. A more recent study by Trout et al. [15] suggests two diameter cut-offs, with appendices measuring <6 mm considered normal and >8 mm considered compatible with appendicitis. The authors suggest that appendices measuring 6–8 mm in diameter should be considered equivocal [15]. For CT, there has been no directed study of diameter cut-offs for the diagnosis of appendicitis in children, and therefore such cut-offs must be extrapolated from studies of normal children. Two such studies exist, the first by Grayson et al. [16], who described normal appendices measuring up to 10 mm with a mean of 6 mm [16]. A second study, by Trout et al. [17], showed that normal appendices measure up to 8.7 mm with a mean of 5.6 mm and that the appendix increases in diameter through about 6.5 years of age. It is not surprising that measured appendiceal diameter differs between US and CT, and yet the classically quoted diameter cut-offs are the same (6 mm) [6, 8, 9, 11, 12]. The most simplistic explanation for the observed difference in diameter is that right lower quadrant US exams are performed with compression, which would be expected to both compress the appendix and to displace appendiceal content. Such compression is not applied during CT, which would explain the larger measured diameter with that modality. This explanation, however, does not fit with the 25–30% of cases in our study in which the appendix measured larger by US nor does it fit with the finding of greater disparity in measured diameter in cases of acute appendicitis. For the latter, given that non-compressibility is a diagnostic criterion for
Pediatr Radiol
appendicitis by US, one would expect less difference in measured diameter between US and CT in cases of appendicitis. As such, the observed disparity is likely multifactorial, with compression during sonography and technical factors contributing. Potential technical factors that might be contributing include improved definition of the appendix by one modality over the other, particularly in the setting of right lower quadrant inflammation; CT slice thickness and reconstruction algorithms; and image pixel size, which limits the accuracy of measurement. In cases of acute appendicitis, some of the observed difference in appendiceal diameter could reflect progressive dilatation of the appendix over the interval between studies. While this is a simple study designed to demonstrate the fallacy of using the same diameter cut-offs for a diagnosis of appendicitis by both US and CT, it does have limitations. Chief among those limitations is the retrospective nature of the study, which limits the study population to cases in which both imaging exams were performed as part of routine clinical care. At our institution, US is the initial imaging modality in suspected appendicitis, and the 155 cases that form the study population represent only a fraction (2%) of the 7,865 cases of suspected appendicitis imaged during the study period. As such, these children are likely a selected population, which might have implications for the results. Our study population is likely composed of three main groups of patients: children with acute appendicitis and concern for complication such as fluid collection, children in whom there is persistent clinical concern for appendicitis despite a normal or equivocal US, and children who require further workup of their acute abdominal pain after appendicitis has been ruled out by US. The interval between CT and US allowed in our study (up to 24 h) might also have implications for the presented data, potentially allowing for progression of appendiceal pathology and change in appendiceal content, that latter of which was not assessed in this study. The other limitation of our study is technical in that the study was designed to mimic clinical practice with measurements on CT performed on standard axial and coronal reconstructed images (rather than on curved reformats) and within the clinical PACS. As such, the measured diameter may not accurately reflect the true transverse appendiceal diameter and the measurements are limited by pixel size. We attempted to mitigate this to some degree by not only comparing axial measurements but also by measuring the appendix in two planes and using the smaller measurement for comparison to US. One important issue not addressed by our study is the importance of secondary findings in making a diagnosis of appendicitis by both US and CT. Although this study is focused on the single diagnostic criterion of appendiceal diameter, it is our assertion that diameter should not be used to the exclusion of other criteria.
Conclusion The results of this study confirm our hypothesis that measured appendiceal diameter differs between US and CT. The results thereby confirm that the application of the same diameter cutoff (6 mm) to both modalities for the diagnosis of appendicitis is inappropriate. The classic teaching of a 6-mm cut-off diameter for both modalities should be abandoned. Instead, cut-off diameters should be defined based on specific studies of each modality in a pediatric population, some of which exist in the literature (albeit with some disagreement among them). Compliance with ethical standards Conflicts of interest None
References 1.
2.
3.
4.
5. 6.
7.
8.
9.
10.
11. 12.
13.
14.
Saito JM, Yan Y, Evashwick TW et al (2013) Use and accuracy of diagnostic imaging by hospital type in pediatric appendicitis. Pediatrics 131:e37–44 Axelrod DA, Sonnad SS, Hirschl RB (2000) An economic evaluation of sonographic examination of children with suspected appendicitis. J Pediatr Surg 35:1236–1241 Kim ME, Orth RC, Fallon SC et al (2015) Performance of CT examinations in children with suspected acute appendicitis in the community setting: a need for more education. AJR Am J Roentgenol 204:857–860 Rosen MP, Ding A, Blake MA et al (2011) ACR Appropriateness Criteria® right lower quadrant pain — suspected appendicitis. J Am Coll Radiol 8:749–755 Friedland JA, Siegel MJ (1997) CT appearance of acute appendicitis in childhood. AJR Am J Roentgenol 168:439–442 Sivit CJ (2004) Imaging the child with right lower quadrant pain and suspected appendicitis: current concepts. Pediatr Radiol 34: 447–453 Trout AT, Sanchez R, Ladino-Torres MF et al (2012) A critical evaluation of US for the diagnosis of pediatric acute appendicitis in a real-life setting: how can we improve the diagnostic value of sonography? Pediatr Radiol 42:813–823 Applegate KE, Sivit CJ, Myers MT et al (2001) Using helical CT to diagnosis acute appendicitis in children: spectrum of findings. AJR Am J Roentgenol 176:501–505 Goldin AB, Khanna P, Thapa M et al (2011) Revised ultrasound criteria for appendicitis in children improve diagnostic accuracy. Pediatr Radiol 41:993–999 Peletti AB, Baldisserotto M (2006) Optimizing US examination to detect the normal and abnormal appendix in children. Pediatr Radiol 36:1171–1176 Birnbaum BA, Wilson SR (2000) Appendicitis at the millennium. Radiology 215:337–348 Rettenbacher T, Hollerweger A, Macheiner P et al (2001) Outer diameter of the vermiform appendix as a sign of acute appendicitis: evaluation at US. Radiology 218:757–762 Je BK, Kim SB, Lee SH et al (2009) Diagnostic value of maximalouter-diameter and maximal-mural-thickness in use of ultrasound for acute appendicitis in children. World J Gastroenterol 15:2900– 2903 Trout AT, Sanchez R, Ladino-Torres MF (2012) Reevaluating the sonographic criteria for acute appendicitis in children: a review of
Pediatr Radiol
15.
the literature and a retrospective analysis of 246 cases. Acad Radiol 19:1382–1394 Trout AT, Towbin AJ, Fierke SR et al (2015) Appendiceal diameter as a predictor of appendicitis in children: improved diagnosis with three diagnostic categories derived from a logistic predictive model. Eur Radiol 25:2231–2238
16.
17.
Grayson DE, Wettlaufer JR, Dalrymple NC et al (2001) Appendiceal CT in pediatric patients: relationship of visualization to amount of peritoneal fat. AJR Am J Roentgenol 176: 497–500 Trout AT, Towbin AJ, Zhang B (2014) Journal club: the pediatric appendix: defining normal. AJR Am J Roentgenol 202:936–945