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 Table of Contents  
ORIGINAL ARTICLE
Year : 2018  |  Volume : 32  |  Issue : 2  |  Page : 111-117

Low-dose noncontrast computed tomography in adults with acute appendicitis


Department of Radiodiagnosis, RIMS, Imphal, Manipur, India

Date of Web Publication25-Oct-2018

Correspondence Address:
Dr. Nongthombam Roshan Singh
Department of Radiodiagnosis, RIMS, Imphal, Manipur
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/jms.jms_35_17

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  Abstract 

Background: Computed tomography (CT) is now considered the imaging modality of choice for acute abdomen. However, with the increasing concern for exposure to ionizing radiation and its related risk of cancer in long run, low-dose CT has come up in recent studies with the attempt to minimize the effective radiation dose, yet giving the similar diagnostic yield.
Aim: The aim of this study was to evaluate the diagnostic performance of low-dose noncontrast multidetector CT in adult patients with suspected acute appendicitis in relation to patients' body mass index.
Materials and Methods: Eighty-three adult patients with suspected acute appendicitis were subjected to both standard and low-dose abdominal noncontrast CT (NCCT). The images were interpreted independently by two radiologists (blinded). Data collected were summarized and statistically analyzed. P < 0.05 was considered statistically significant.
Results: With low-dose NCCT abdomen, appendix was visualized in 80 patients (96.4%), with nonvisualization in 3 patients (3.6%), which was similar to standard dose. Low-dose NCCT has an overall sensitivity of 98.2% and a specificity of 100% in the diagnosis of acute appendicitis. Only one false-negative case and zero false-positive case was found, considering 6 mm as the cutoff value in appendiceal diameter with associated fat stranding. Alternative diagnoses were found in 22 (10 females and 12 males) patients.
Conclusion: Low-dose NCCT abdomen was found highly accurate in diagnosing acute appendicitis with achievement of significant radiation dose reduction (about 88%) and thus can be recommended as a fast, safe, and cost-effective initial imaging technique in patients with right lower quadrant pain.

Keywords: Acute appendicitis, body mass index, effective radiation dose, low-dose noncontrast computed tomography


How to cite this article:
Singh NR, Luwang NT, Priyabarta Y, Singh CG, Singh WJ. Low-dose noncontrast computed tomography in adults with acute appendicitis. J Med Soc 2018;32:111-7

How to cite this URL:
Singh NR, Luwang NT, Priyabarta Y, Singh CG, Singh WJ. Low-dose noncontrast computed tomography in adults with acute appendicitis. J Med Soc [serial online] 2018 [cited 2018 Dec 9];32:111-7. Available from: http://www.jmedsoc.org/text.asp?2018/32/2/111/244130


  Introduction Top


Acute appendicitis is one of the most common diagnoses suspected in patients presenting in emergency rooms with acute abdominal pain and is the most common indication for an urgent abdominal intervention.[1] Surgical removal (open or laparoscopic) of the inflamed appendix remains the mainstay as the curative therapy. However, a mean negative appendectomy rate of 26% (16%–47%) has been reported when the diagnosis is based only on clinical and laboratory findings, dropping to 6%–10% when imaging is performed.[2] Therefore, the use of imaging modalities is critical to confirm the diagnosis, when facing a clinical suspicion of appendicitis.[1] Computed tomography (CT) is now regarded as the most accurate test to diagnose appendicitis.[3] Among the various prevailing CT techniques, use of both unenhanced and helical CT scans appears to have some merits.[3] A key strength of CT is its ability to make alternative diagnosis in right iliac fossa pain.[3],[4],[5],[6] The routine use of CT in patients suspected of having appendicitis has been reported to be cost-effective since it prevents delayed or inaccurate diagnoses.

Increasing consciousness of the exposure to ionizing radiation and its hazards leads to considerable research and industry drive to reduce radiation exposure during CT and adhere to As-Low-As-Reasonably-Achievable principles while preserving image quality and diagnostic yield.[6] Dose reduction in CT is possible by decreasing tube voltage or tube current or by increasing pitch and it has already been successfully used in conditions characterized by intrinsic high contrast between structures, that is, in screening of pulmonary nodule, in the diagnosis of nephrolithiasis and ureterolithiasis, and with CT colonography.[7] Various recent reports suggest that low-dose CT protocols using very low tube current–time product of 30 mA (instead of 180 mA or above) giving low-effective dose of about 2 mSv (nearly 80% less than that of standard-dose CT) may achieve similar diagnostic performances as standard-dose CT in patients presenting with acute abdominal conditions, such as appendicitis or renal colic. Unenhanced low-dose helical CT is particularly appealing because there is no delay caused by waiting for oral contrast transit, no risk of contrast-induced nephropathy, and no risk of allergic reaction.[8] It significantly improves the rate of identification of normal appendix as well as inflamed appendix in low-dose CT protocols. As intraperitoneal infectious and inflammatory conditions manifest with abnormalities of the fat adjacent to the inflamed organ, in patients with high body mass index (BMI), it is presumed that with a larger percentage of adipose tissue surrounding a given organ, visualization of the inflammatory changes would be more readily apparent even without oral or intravenous (IV) contrast.[9]

This study determines the diagnostic performance of low-dose noncontrast helical multidetector CT (MDCT) in preoperative evaluation of adult patients with suspected acute appendicitis, without the risk of exposure to IV, oral, or rectal contrast.


  Materials and Methods Top


This is a cross-sectional study carried out at the department of radiodiagnosis in a tertiary care center, in collaboration with the department of surgery. A total of 83 patients above 18 years of age presenting with acute right iliac fossa pain, suspected of acute appendicitis, were enrolled in the study which was commenced from October 2014 till September 2016. Patients with distended abdomen or having abdominal mass, patients with recent abdominal surgery, and pregnant women were excluded from the study.

Informed consent was obtained from all the patients before the study. Body weight in kilograms and height in centimeters were recorded for every patient. Low-dose helical MDCT scan starting from lung bases to pubic symphysis (planed from a surview) was done for every patient with single breath hold, immediately followed by standard-dose helical MDCT scan using the following parameters:

  1. Surview test with preset parameters (view angle – 180°/anteroposterior view)
  2. Low-dose helical CT parameters: Tube voltage 120 kVp, tube current–time product 30 mA/slice, collimation 64 × 0.625, pitch 0.797, rotation time 0.75 s, field of view (FOV) 350 mm, and acquisition slice thickness of 5 mm without oral, rectal, or IV contrast
  3. Standard-dose/routine helical CT parameters: Tube voltage 120 kVp, tube current–time product 250 mA/slice, collimation 64 × 0.625, pitch 0.797, rotation time 0.75 s, FOV 350 mm, and acquisition slice thickness of 5 mm, without oral, rectal, or IV contrast.


This study was carried out using Philips Brilliance 64-Slice CT Machine, manufactured in the Netherlands on September 2007 with model serial number 10311. No specific patient preparation was done. Imaging findings of the standard-dose and low-dose CT were independently interpreted by two radiologists, keeping their imaging findings blinded from one another. The images were interpreted using postprocessing techniques such as thick and thin slice reconstructions, multiplanar reformation (MPR), maximum intensity projection, and image smoothening.

The presence for any alternative diagnosis was sought, when the appendix was found normal. Subsequent contrast-enhanced CT was advised only if needed and done for a case of right psoas abscess.

The parameters measured and recorded include the following:

  1. Visualization or nonvisualization of appendix.
  2. Position of appendix.
  3. Diameter and length of appendix if visualized.
  4. Presence of appendicoliths/fecoliths in the appendiceal lumen.
  5. Presence/absence of air in the appendiceal lumen.
  6. Presence of periappendicular inflammatory changes such as surrounding fat stranding, cecal wall thickening, extraluminal fluid collection, abscess or phlegmon, extraluminal air, and lymph nodes.
  7. Presence of any alternative diagnosis was also recorded if identified.
  8. Data on dose length product (DLP) in mGy.cm from the radiation dose information generated at the end of scanning.
  9. Details on operative procedure/s, intraoperative findings, clinical follow-ups, and histopathological reports were also recorded.


The definitive diagnosis was based on standard-dose noncontrast CT (NCCT) findings, surgical findings (n = 56), and/or histopathological reports. For all patients who did not undergo surgery, information from the clinical follow-up was obtained by reviewing the medical records and telephone calls 1 month after the acute episode.

The effective radiation dose delivered by the standard-dose and low-dose noncontrast MDCT scans was calculated using one practical formula as follows:

Effective dose (mSv) = DLP × conversion factor

where conversion factor is 0.015 mSv/mGy.cm for abdomen and pelvis; as per recommendation in the International Commission on Radiological Protection Publication 103.[10]

BMI was calculated for each patient using the following formula: BMI = weight (kg)/height (m2). Patients were then categorized as underweight (BMI <18.5), normal/optimal weight (BMI 18.5–25), overweight (BMI 25–30), and obese (BMI >30). Image quality and visualization or nonvisualization of normal or inflamed appendix in low-dose NCCT images are analyzed subjectively in different bodyweight categories and documented in the study.

The collected data were checked for consistency and completeness. Statistical analysis was carried out using descriptive statistics with the help of IBM SPSS software version 20 (Armonk, New York, United States). P <0.05 was considered statistically significant.


  Results Top


Out of the 83 patients, 44 were male (53%) and 39 were female (47%). Patients' age ranged from 18 to 85 years, with a mean age of 37.36 years and a standard deviation of 14. In the entire study group, the mean BMI was 22.97 kg/m2 ± 3.06 (range, 16.44–31.53 kg/m2).

Low-dose NCCT images are found to provide sufficient information to diagnose disease conditions of appendix despite increased image noise [Figure 1]. In low-dose NCCT, appendix could be visualized in 80 out of the 83 patients (96.4%) [Table 1]. Delineation of appendix was poor in 3 patients (3.8%) due to increased image noise and artifacts in 2 obese patients (BMI >30) [Figure 2] and appendix was adhered close to the cecal wall in the remaining one patient (BMI 18.87). Appendix could not be traced in three other patients (3.6%) and was labeled as “not visible” in both low-dose NCCT and standard-dose NCCT. Delineation of appendix was better with standard-dose NCCT abdomen in the remaining eighty patients due to higher signal-to-noise ratio. The position of appendix was found to be pelvic in 30 patients, retrocecal in 21 patients, subcecal in 16 patients, paracecal in 8 patients, postileal in 3 patients, and preileal in 2 patients.
Figure 1: Oblique coronal reformatted low-dose computed tomography image (a) and standard-dose computed tomography image (b) in a 22-year-old female (body mass index – 19.56) for comparison of image quality. A fluid-distended paracecal appendix (white arrows) with greatest diameter of 16.5 mm is seen in this patient with the presence of periappendiceal fat stranding. There is no significant degradation of image quality in low-dose computed tomography (30 mA) when compared to standard-dose computed tomography (250 mA). The image noise in low-dose computed tomography image is still acceptable and does not impede visualization of appendix and periappendicular fat stranding

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Table 1: Frequency of signs and overall diagnosis in 56 patients with definite appendicitis

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Figure 2: Axial-section low-dose noncontrast computed tomography (a) and standard-dose noncontrast computed tomography (b) images of a 50-year-old female with body mass index – 30.44 showing a normal pelvic appendix (green arrows). At high body mass index, there was less clarity of appendicular margins in low-dose computed tomography, but identification of the appendix was not a difficulty

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The greatest diameter (outer to outer wall) of the visualized appendix ranges from 4.9 to 60 mm with a mean diameter of 10.5 mm and a standard deviation of 6.8. Out of 56 patients with definite appendicitis, 55 (98.2%) patients had diameter above or equal to 6.7 mm. Only 1 patient (1.8%) had appendicular diameter of 4.9 mm as seen on low-dose NCCT abdomen, later found to have distal appendicitis on standard-dose NCCT. The tip was not delineated on low-dose NCCT image due to paucity of mesenteric fat (BMI 18.73). The mean of the measured diameter of appendix in patients with definite appendicitis was 12.3 mm ± 7.4 and, in patients with no appendicitis, it was 6.1 mm ± 1.5. There were few patients in whom there was underestimation of the length in low-dose NCCT. Periappendiceal fat stranding with or without focal peritoneal thickening was appreciable on low-dose NCCT in 83.9% of patients (47 out of 56) with appendicitis. Pericolic fat stranding around the colonic diverticula was also well appreciated on low-dose NCCT in three cases of colonic diverticulitis. Cecal wall thickening was seen in four patients (4.8%) only. Luminal air was absent in most of the cases (40 of 56 patients) diagnosed as appendicitis. On low-dose NCCT abdomen, luminal air was visible in 70% of patients with appendicolith, mostly distal to the site of appendicolith. Ten of 56 patients (17.8%) with definite appendicitis had regional lymphadenopathy. Presence of extraluminal air in periappendiceal region along with the presence of appendicolith was seen in one case and is highly suggestive of appendiceal perforation. Wall discontinuity was also seen in this patient [Figure 3].
Figure 3: Low-dose noncontrast sagittal computed tomography image in an 85-year-old female patient (body mass index – 19.98) showing appendix perforation with appendicolith (green arrow) and luminal air (blue arrow). The wall discontinuity (white arrow) is visible just distal to the site of appendicolith. Presence of extraluminal air and extensive surrounding fat stranding is noted. Mesenteric lymph nodes are also seen

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Fifty-six (25 females and 31 males) of the 83 patients were classified as definitely having appendicitis. Intraoperative sign of appendicitis was found in all these 56 patients. The diagnoses were confirmed on histopathology examination of the surgical specimens. Two patients (2 of 25 females) were found having mucocele with diameters of 24.6 mm and 60 mm, respectively, associated with inflammatory changes which were confirmed on histopathology. There was not even a single case of negative appendectomy. Low-dose NCCT has an overall sensitivity of 98.2% (55 of 56 patients) and an overall specificity of 100%. There was only one false-negative case and 0 false-positive case when we considered a cutoff value in appendiceal diameter as 6 mm in the presence of periappendiceal fat stranding, appendicolith, luminal fluid distension, cecal wall thickening, or surrounding collection/abscess. In patients with no definite appendicitis, alternative diagnosis was found on low-dose NCCT in 22 (10 females and 12 males) of the 27 patients. Two patients (both male) underwent surgical interventions: one for psoas abscess (ultrasonography-guided drainage) and another for hollow viscus perforation (laparotomy) [Figure 4]. Five patients (4 females and 1 male) were considered as having undetermined/nonspecific abdominal pain because their symptoms were not elucidated with any diagnostic technique and had resolved with conservative management. Surgical procedures were avoided and continued with conservative management in 25 (30%) of 83 patients. Low-dose NCCT of 12 (7 males and 5 females) patients with definite appendicitis also showed additional findings over and above the features of appendicitis.
Figure 4: Axial-section low-dose noncontrast computed tomography images (a and b) of a 25-year-old male (body mass index – 22.03) with hollow viscus perforation. Low-dose noncontrast computed tomography is very sensitive in detecting free intraperitoneal air (green stars). There was also minimal collection in the pelvic region. Laparotomy confirmed duodenal ulcer perforation

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The mean DLP generated by the CT machine with low-dose NCCT and standard-dose NCCT protocols for abdomen (from dome of diaphragm to pubic symphysis) was 98.3 mGy.cm ± 6.2 and 820.8 mGy.cm ± 55.1, respectively. In low-dose NCCT, the calculated mean effective radiation dose was 1.43 mSv ± 0.08 for female and 1.50 mSv ± 0.09 for male patients. In standard-dose NCCT, the calculated mean effective radiation dose was 11.95 mSv ± 0.61 for female and 12.62 mSv ± 0.87 for male patients. On an average, there was overall 88% reduction in effective radiation dose when tube current of 30 mA is used (low-dose NCCT) in the place of 250 mA (standard-dose NCCT).


  Discussion Top


Appendicitis is one of the most common causes of emergent abdominal surgery in children and young adults. Delay in treatment increases the risk of perforation and its complications. Imaging plays a critical role in early diagnosis, thereby reducing the negative appendectomy rate. An ideal imaging diagnostic test should be safe, fast, noninvasive, highly accurate, inexpensive, and readily available. Several studies have demonstrated that higher sensitivity can be achieved when using helical CT compared to ultrasound (US), and it has been recommended as the method of first choice by several authors for diagnosing acute appendicitis. The modality is reproducible and also offers high accuracy and sensitivity in depicting unsuspected alternative diagnoses.[11] In recent years, however, concern over the risks of ionizing radiation generated by CT has increased, especially in the pediatric population and young adults. Carcinogenesis is the primary concern with proven stochastic effect of ionizing radiation. Currently, there is a considerable research and industry drive to reduce radiation exposure during CT while preserving image quality and diagnostic yield. Results from several studies have suggested that reducing the radiation dose by 50%–80% does not significantly impair the diagnosis of appendicitis.[12],[13],[14],[15],[16]

In the present study, low-dose NCCT using 30 mA as the tube current–time product, while other parameters remaining the same as that of standard-dose CT (i.e., tube voltage 120 kVp, collimation 64 × 0.625, pitch 0.797, rotation time 0.75 s, FOV 350 mm, and acquisition slice thickness of 5 mm), delivers about 88% less effective radiation dose as compared to the standard-dose CT using tube current–time product of 250 mA. The mean effective radiation dose from the low-dose NCCT in this study was 1.47 mSv ± 0.9 only as against 12.30 mSv ± 0.82 from standard-dose NCCT. This result is comparable to that of studies by various authors who have reported low-dose MDCT protocols to have delivered effective radiation dose ranging from 1.2 to 2 mSv.[12],[13],[14],[15],[16] Low-dose MDCT delivers approximately the same dose as three plain radiographic views of the abdomen traditionally advised in acute abdomen. The only drawback of low-dose NCCT was slight degradation in the image quality due to increased noise and streak artifacts.

In various studies, many authors recommended elimination of IV, oral, and rectal contrast in CT scan for suspected acute abdominal process. Administering oral and rectal contrast for CT scanning of suspected appendicitis is associated with substantial disadvantages. Their elimination can improve patient comfort, decrease patient risk, save valuable time, and minimize financial cost. In the present study, there was no specific patient preparation or administration of any contrast.

The main CT criteria for acute appendicitis are appendicular diameter more than 6 mm, presence of appendicolith, periappendiceal inflammatory changes (periappendiceal fat stranding, fluid collections, phlegmon, or abscess formation), focal cecal wall thickening, extraluminal air, and adjacent lymphadenopathy. However, we often encounter a normal appendix with a diameter of more than 6 mm and sometimes encounter a normal appendix with a wall thickness of more than 3 mm. The three most predictive signs in the present study were appendiceal diameter >6 mm, luminal distension with low attenuating fluid, and periappendiceal fat strandings (P < 0.001). Presence of appendicolith with distal luminal distension by either fluid or air was also a definite sign of appendicitis (P < 0.001). Keyzer et al.[7] also reported fat stranding, appendicolith, and diameter as the most predictive signs, regardless of dose, yielding approximately 90% of correct diagnoses. A paucity of peri-appendiceal fat in patients with low BMI may give rise to false-negative result in low-dose CT. The length of appendix has no significant role in predicting acute appendicitis from low-dose NCCT. However, few cases of underestimation of appendicular length were encountered with low-dose NCCT in patients with low BMI.

Various reported alternative diagnoses that can be detected by low-dose NCCT while scanning patients with suspected acute appendicitis include acute pyelonephritis, ovarian cystic mass, colonic diverticulitis, epiploic appendagitis, colonic malignancy, pelvic inflammatory disease (PID), acute cholecystitis, ureteric calculus, enteritis, acute pancreatitis, and duodenal ulcer perforation.[7],[12],[17]

In the present study, 22 out of 83 participants (26.5%) were found having alternative diagnosis with low-dose NCCT study of abdomen. There were 6 cases of ureteric calculus, one with features of acute right-sided pyelonephritis. There were also five cases of mesenteric lymphadenitis, three cases of colonic diverticulitis, three cases of liver parenchymal disease/hepatomegaly, and one case each of hollow viscus perforation, carcinoma colon, hemorrhagic ovarian cyst, PID, and psoas abscess. Colonoscopy with biopsy confirmed the case of carcinoma colon (adenocarcinoma involving transverse colon). Laparotomy was done for the hollow viscus perforation and found to be duodenal ulcer perforation.

Udayasankar et al.[13] reported a high sensitivity (100%), specificity (98.5%), and positive predictive value (91.7%) of low-dose NCCT in cases of acute abdominal pain. Various other studies further reported that low-dose CT was noninferior to standard-dose CT and more accurate than USG in the diagnosis of acute appendicitis. The sensitivity and specificity for the low-dose CT were 94.5% and 93.3%, respectively, as compared to 95% and 93.8%, respectively, for standard-dose CT in a study conducted by Kim et al.[15] When clinical and surgical findings were used as the reference standard, low-dose CT and standard-dose CT had the same sensitivity of 100% and specificity of 98% to diagnose appendicitis in patients with BMI >18.5 as reported by Platon et al.[18] In a very recent study, Karabulut et al.[19] reported sensitivity, specificity, and accuracy for US versus low-dose CT as 82.5% versus 92.5%, 83% versus 89%, and 82.7% versus 90.4%, respectively. In the present study, we achieved a sensitivity of 98.2% and specificity of 100% with 1.8% false-negative study.

Various reports suggest that an increased peritoneal fat (BMI >18.5) significantly improves the rate of identification of normal as well as inflamed appendix. In the present study, out of the 83 patients, 2 were underweight (BMI <18.5), 60 were having normal/optimal weight (BMI 18.5–25), 18 were overweight (BMI 25–30), and only 3 were classified as obese (BMI >30). Paucity of intra-abdominal fat in underweight patients had little impact in the delineation of appendix. However, identification of periappendiceal inflammatory changes was a difficulty in these patients with BMI <18.5. Some increase in image noise and streak artifacts in pelvic region were found in patients with BMI >30. This was expected with the choice of low tube–current product (30 mA) in this study. The clarity of appendiceal margins in these patients was slightly degraded. However, visibility of appendix and periappendiceal inflammation was not hindered by increased intra-abdominal fat, despite the image noise. The presence of periappendiceal fat brings out a good tissue contrast in patients with BMI >18.5 and <30. Adjusting the tube–current product (a slight increase in mA) according to the BMI might give a satisfactory result in those obese patients too. Thick slab MPR from the thin section low-dose CT images brings about a significant improvement in the image quality.

The study did have some limitations. First, in patients who were not treated surgically, we had no absolute confirmation that they truly had no acute appendicitis. Second, we inferred the final diagnosis for some patients with alternative findings from noncontrast standard-dose CT only with no additional contrast-enhanced scan or other procedure. Third, the prevailing CT machine is using an old image reconstruction algorithm which is filtered back projection instead of iterative reconstruction technique which is available with newer machines. Published studies indicate that this iterative reconstruction technique yields images with reduced image noise and artifacts in low-dose CT.


  Conclusion Top


Low-dose NCCT abdomen was found to be highly accurate in diagnosing acute appendicitis (98.2% sensitivity and 100% specificity) and also in proposing a correct alternative diagnosis in patients with normal appendix. Using tube current as low as 30 mA in the CT protocol, significant reduction in the effective radiation dose by about 88% was achieved compared to the routine standard-dose CT, with no significant degradation in the image quality. Low mesenteric fat in underweight patients (BMI <18.5) and the increased image noise and streaking artifact in obese patients (BMI >30) were the only average challenges in using low mA in the CT protocol for detecting acute appendicitis. The increased image noise can be suppressed to some extent through postprocessing techniques such as using thick slice reformatted images and application of iterative reconstruction software, thereby improving the image quality. Low-dose noncontrast MDCT abdomen can thus be recommended as a fast, safe, and cost-effective initial imaging technique in patients with right lower quadrant pain.

Declaration of patient consent

The authors certify that they have obtained all appropriate patient consent forms. In the form the patient(s) has/have given his/her/their consent for his/her/their images and other clinical information to be reported in the journal. The patients understand that their names and initials will not be published and due efforts will be made to conceal their identity, but anonymity cannot be guaranteed.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

 
  References Top

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