AJR 2003; 180:201-205
© American Roentgen Ray Society
Value of Bone Window Settings on CT for Revealing Appendicoliths in Patients with Appendicitis
Mohammad Alobaidi1 and
Ali Shirkhoda
1 Both authors: Department of Diagnostic Radiology, William Beaumont Hospital,
3601 W. 13 Mile Rd., Royal Oak, MI 48073.
Received January 28, 2002;
accepted after revision June 25, 2002.
Address correspondence to M. Alobaidi.
Abstract
OBJECTIVE. Our retrospective study was designed to determine whether
the use of bone window settings increases sensitivity of CT for diagnosing
appendicitis and for detecting an appendicolith in patients with
pathologically confirmed appendicitis.
CONCLUSION. The use of bone window settings is helpful for detecting
appendicoliths when evaluating patients for acute appendicitis, particularly
patients in whom evidence of appendicitis is equivocal. In this era of PACS
(picture archiving and communication systems), bone window settings should be
used routinely.
Introduction
The diagnosis of appendicitis in the emergency department is a challenging
one because not all patients present with classic symptoms or with classic
imaging findings. Many diseases can mimic appendicitis and present with right
lower quadrant abdominal pain. The use of multidetector CT (MDCT) in the
emergency department to evaluate abdominal pain has greatly increased
diagnostic accuracy and reduced the number of unnecessary surgical procedures.
In addition, MDCT has increased the physician's level of confidence, decreased
hospital admission rates, and reduced delayed intervention
[1]. Yet surgical and
radiologic discrepancies remain in the diagnosis of acute appendicitis.
Occasionally, a patient must undergo laparotomy with appendectomy even though
imaging findings are negative. A relatively high rate of false-positive
outcomes in appendectomies (from 20% before the introduction of CT to 7% after
[2]) has been viewed as
acceptable. In contrast, a radiology report with falsely negative findings for
appendicitis has more serious ramifications for a patient. The refinement of
CT techniques tailored to diagnosing appendicitis—including the use of
thinner sections, selective contrast enhancement, and focused
examinations—has proven to be useful
[3,4,5,6].
Several secondary CT findings in the right lower quadrant are helpful in
identifying an inflamed or obstructed appendix, including the presence of an
appendicolith. In the proper clinical setting, an appendicolith revealed on CT
is known to be highly specific for the diagnosis of appendicitis, ranging from
86% to 100% [7,
8]. However, the presence of an
appendicolith may be difficult to recognize when reviewing CT scans obtained
after administration of gastrointestinal contrast material using routine
soft-tissue window settings.
Our study was designed to assess the diagnostic value of CT bone window
settings in the detection of appendicoliths in all patients with
pathologically proven appendicitis, particularly in those patients with
equivocal findings.
Materials and Methods
Using the keyword "appendix," we generated a computerized
patient list from our pathology department's database. During the 7-month
period between November 1, 2000 and May 28, 2001, 232 appendectomies were
performed. Patients presented with a variety of symptoms including right lower
quadrant pain with leukocytosis (n = 179), right lower quadrant pain
without leukocytosis (n = 24), generalized abdominal pain with
leukocytosis (n = 18), right and left lower quadrant abdominal pain
with leukocytosis (n = 7), and right and left lower quadrant pain
without leukocytosis (n = 4). Thirty-one patients with appendicoliths
were identified at surgery and pathology, of which 16 underwent MDCT before
appendectomy. These 16 patients composed our study population—seven
males and nine females whose ages ranged from 11 to 79 years (mean age, 47.5
years; one pediatric patient, an 11-year-old girl). Leukocytosis was defined
as a WBC greater than 10.1 x 109/L for men and greater than
10.9 x 109/L for women. The 11-year-old girl also had
leukocytosis, which was defined as a WBC greater than 14.0 x
109/L.
Patients who were evaluated in our emergency department for appendicitis
with abdominal and pelvic CT during the specified time period routinely
received between 120 and 180 mL of IV nonionic contrast material (Omnipaque
[iohexol]; Nycomed, Princeton, NJ), depending on patient weight, and 800 mL of
oral contrast material (Gastrografin [3.7% diatrizoate meglumine and
diatrizoate sodium solution]; Bracco Diagnostics, Princeton, NJ). A dedicated
emergency department CT scanner (Somatom Plus 4; Siemens Medical Solutions,
Iselin, NJ) was used to scan the abdomen and pelvis from the lung bases to the
proximal femurs in a single breath-hold, usually with 5-mm collimation and a
pitch of 1.0. The CT slice sections ranged from 3 to 8 mm, with the median
slice section being 7.5 mm. Because data collection was retrospective, a fixed
protocol was not used for all patients, which resulted in the variation in
slice thickness.
The original CT reports were generated by an attending radiologist. Five
major objective descriptors were identified in these reports: inflammatory
changes, appendix lumen dilated more than 6 mm, wall thickening, free fluid,
and the presence of an appendicolith. These initial reports were then
objectively graded on a 5-point scale, using a hierarchial system from 0 to 4.
Four points were assigned for an appendicolith, independent of other findings.
If no evidence of an appendicolith was found, one point each was given for
inflammatory changes, dilated appendix lumen, wall thickening, and free fluid.
A score of 0 was considered to indicate completely normal findings, 1 was
considered weak or equivocal findings, and 2 or higher was considered to be
strong or unequivocal findings for appendicitis.
The terminology used by the attending radiologist in the final impression
section of the original CT reports was also taken into consideration. We
decided that if we found cases in which the objective score was 1 but the
original reviewer had unequivocally recorded a final impression of acute
appendicitis, the terminology used in the impression would be deemed more
important than the objective score. However, we found no such cases in any of
the 16 patients. The mention of an appendicolith in the original report and
unequivocal terminology in the final impression or summary were considered
strongly positive findings for appendicitis, and a score of 4 was assigned.
The diagnoses were correlated with surgical and pathologic findings. Patients
who had pathologic specimens with semisolid fecal material and presence of
appendicitis were not included in the study. Semisolid fecal material is not
considered an appendicolith by pathologic or surgical criteria.
The CT scans were then reviewed on a PACS (picture archiving and
communication system) viewing station and reevaluated for CT-perceptible
appendicoliths by the attending body imaging radiologist, who was unaware of
the surgical and pathologic outcomes. The original CT scans of the 16 patients
were examined using soft-tissue window settings (380 x 40). The same
images were then set to standard bone window parameters (1250 x 250) and
reexamined for the presence of appendicoliths. Criteria used to identify
appendicoliths were the presence of single or multiple round or oval
calcifications in the lumen of the appendix or single or multiple
rim-calcified calcifications in the appendix or both. Intraluminal
calcifications that were visualized on CT in the appropriate location of the
appendix were identified. The variable slice sections of the CT scans
precluded the use of size criteria. Use of the wider settings of the bone
windows made distinguishing well-formed calcified appendicoliths from
intraluminal barium much easier because of the differences in densities.
Patients with appendicoliths revealed on CT showed a single round or oval
stone in the lumen of the appendix (n = 6), two separate stones
(n = 1), or a single rim-calcified stone (n = 3).
For a control group, we randomly selected 50 cases from the 176 patients
(207 patients with pathologically confirmed appendicitis minus 31 patients
with appendicoliths confirmed at surgery) who had pathologically confirmed
appendicitis but no appendicoliths. The cases of these 50 patients were
combined with those of 16 patients in our study group and presented (with no
distinction made between the two sets of cases) for review by the same
attending radiologist, who was unaware of the pathologic and surgical outcomes
and original CT findings. In total, the observer interpreted 66 cases. The
sensitivity of CT using bone window settings for detection of appendicolith
associated with appendicitis was then calculated.
Results
Patients presented with symptoms such as right lower quadrant pain with
leukocytosis (179/232 patients, 77.2%), right lower quadrant pain without
leukocytosis (24/232 patients, 10.3%), generalized abdominal pain with
leukocytosis (18/232 patients, 7.8%), right and left lower quadrant abdominal
pain with leukocytosis (7/232 patients, 3.0%), and right and left lower
quadrant pain without leukocytosis (4/232 patients, 1.7%). The histopathologic
results from the 232 appendectomies performed were as follows: 207 specimens
(89.2%) were positive for acute appendicitis, 21 specimens (9.1%) were normal,
three specimens (1.3%) were positive for follicular hyperplasia, and one
specimen (0.4%) showed endometriosis with an incidental appendicolith.
Before the appendectomy, 154 patients underwent MDCT, the findings of which
were unequivocally positive for appendicitis in 123 patients (120
true-positive and three false-positive). In 31 patients, findings for
appendicitis were equivocal or negative (12 true-negative and 19
false-negative findings). Seventy-eight patients did not undergo MDCT before
appendectomy. Pathologic findings for specimens from 68 patients were positive
for appendicitis, and those from 10 patients were negative for
appendicitis.
Among the 207 patients with confirmed appendicitis, an appendicolith was
present in 30 patients (14.5%), 16 of whom had a preoperative MDCT
examination. Using the 5-point scale based on the original CT report from the
emergency department, five patients were retrospectively assigned a score of
4, one was assigned a score of 3, seven were assigned a score of 1, and three
were assigned a score of 0. No patients were assigned a score of 2.
Among this group of 16 patients, five (31.3%) had an initial CT report with
an unequivocally positive finding of acute appendicitis with the presence of
an intraluminal appendicolith. The original reports of 11 patients (68.8%) had
no description of an appendicolith, although the entity was found to be
present at pathology. Of these 11 patients, seven (7/16, 43.8%) had an
equivocal CT diagnosis of appendicitis, three (3/16, 18.8%) had a completely
negative CT finding and only one (1/16, 6.3%) had an unequivocally positive
finding. Sensitivity of CT using soft-tissue window settings for detection of
an appendicolith was calculated to be 31.3% in the setting of acute
appendicitis.
Retrospectively, on CT with bone windows set at standard parameters, 10
(62.5%) of the 16 patients actually had an appendicolith that was perceptible
on CT compared with five (31.3%) of the 16 patients who had an appendicolith
described in the original reports, an increase of 100%. Six (37.5%) of the 16
patients did not have a CT-perceptible appendicolith, even in retrospective
examinations using bone window settings. Thus, CT identification of an
appendicolith using bone window settings was calculated to have a sensitivity
of 62.5% compared with 31.3% for CT using soft-tissue window settings. In
addition, the cases of five of the 16 patients were important because those
patients were assigned an initial score of 0 or 1 for appendicitis. In fact,
all five had a pathologically confirmed intraluminal appendicolith that was
visualized retrospectively on CT with bone window settings (Figs.
1A,1B,2A,2B,3A,3B,4A,4B).
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Fig. 1A. —33-year-old man who presented with 2-day history of right
lower quadrant pain. CT scan obtained at presentation using soft-tissue window
setting was interpreted as revealing only tubular structure with wall
thickening. Appendicolith (arrow) was easily mistaken for
intraluminal barium. Diagnosis of appendicitis was equivocal.
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Fig. 2A. —22-year-old man with pain in and guarding of right lower
quadrant. CT scan obtained with soft-tissue window setting shows nonspecific
inflammatory changes in right side of pelvis. Appendicolith was misinterpreted
as intraluminal barium, and tubular structure (arrows) was thought to
represent unopacified bowel loop.
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Fig. 2B. —22-year-old man with pain in and guarding of right lower
quadrant. CT scan obtained with bone window setting reveals appendicolith
(arrow) obstructing dilated appendix, which was source of patient's
pain.
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Fig. 3A. —68-year-old woman who presented to emergency department with
abdominal pain. CT scan obtained at presentation using soft-tissue window
setting reveals only inflammatory changes (arrows) in right upper
pelvis with no appendicolith.
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Fig. 3B. —68-year-old woman who presented to emergency department with
abdominal pain. CT scan obtained using bone window setting reveals
appendicolith with rim calcification (arrow) clearly distinguishable
from barium and surrounding inflammation.
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Fig. 4A. —52-year-old woman who presented to emergency department with
abdominal pain. CT scan obtained at presentation using soft-tissue window
setting reveals fluid- and barium-filled loops of small bowel with no findings
to support clinical diagnosis of appendicitis.
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Fig. 4B. —52-year-old woman who presented to emergency department with
abdominal pain. Same CT image as in A obtained with bone window setting
reveals dilated appendix with small proximal appendicolith (arrow)
that was mistaken for barium in small-bowel loop.
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In the 50 patients of our control group, CT evaluation for the presence of
an appendicolith using same soft-tissue and bone window parameters with which
we evaluated the study group revealed no evidence of an appendicolith. Other
secondary signs of appendicitis—wall thickening, dilatation of the
lumen, inflammatory changes, or abscess formation—were present. We
correlated these findings with the original CT reports, and this correlation
revealed similar findings of appendicitis with no mention of
appendicoliths.
Discussion
The presence of an appendicolith in a patient with acute abdominal pain is
widely accepted as highly specific for the diagnosis of appendicitis
[7,
8]. In our study of 232
patients who underwent appendectomies, 31 had an appendicolith. The incidence
of appendicoliths in patients with acute appendicitis was 14.5% (30
appendicoliths in 207 patients with acute appendicitis). Thirty (96.8%) of 31
patients with appendicoliths had acute appendicitis, whereas one patient had
appendiceal endometriosis with an incidental appendicolith. According to our
results, detection of an appendicolith would therefore positively predict
acute appendicitis in 96.8% of the cases. Hence, in evaluating a patient with
abdominal pain and suspected acute appendicitis, specifically evaluating for
the presence of an appendicolith should be integrated into the CT examination.
This technique may be similarly helpful in detection of ureteral calculi in
stented ureters. The small calculus that is easily lost on CT using
soft-tissue window settings because of its proximity to the equally
high-attenuating stent may be easier to detect with bone window settings if
the stent and calculus have distinctly different densities.
We found relatively little information in the radiology literature
regarding the CT detection of an appendicolith in the setting of acute
appendicitis. Lowe et al. [7]
found the specificity of an appendicolith seen in children with appendicitis
to be 86%. Although the mechanism of appendicitis in children may be
different, the underlying pathophysiology is similar: obstruction of the lumen
of the appendix by a fecalith or purulent exudate, dilatation of the appendix,
and wall edema leading to inflammation. Our study suggests that in cases of
equivocal findings for appendicitis, CT with bone window settings is an
important tool with which to actively search for an appendicolith. Twice as
many cases of unequivocal appendicitis were diagnosed on CT with bone window
settings as on CT with soft-tissue window settings. The scans of patients with
these newly identified cases had initially been interpreted as showing either
equivocal or negative findings for appendicitis. Although each secondary sign
of appendicitis has its own diagnostic value
[8], the detection of an
appendicolith adds strength to the radiologic interpretation in diagnosing
appendicitis. Secondary signs such as a dilated lumen, wall thickening,
inflammatory changes, free fluid, and a thickened cecal tip (the
"arrowhead" sign) are sensitive but not specific for the diagnosis
of appendicitis
[8,9,10].
In the literature, focused contrast-enhanced CT examinations for equivocal
cases of appendicitis have been described as improving detection
[3,4,5].
Using bone window settings can be considered part of a focused CT examination
to increase accuracy, and the versatility of PACS has made this step
easier.
The diagnosis of acute appendicitis on CT can be limited by several factors
and may indeed account for the occasional radiologic and surgical
discrepancies, adding to the rate false-negative findings. In our study,
although the soft-tissue window settings, bone window settings, and contrast
enhancement were comparable for all patients, several other technical factors
were considered. Six patients (37.5%) of the 16 did not have an appendicolith
that was visible on CT, even in retrospect. The use of thin-section CT has
been shown to significantly improve radiologists' ability to diagnose
appendicitis [9], and
therefore, such technique may also improve rates of detection of small
appendicoliths that the interpreter may otherwise not see. A 3-mm
appendicolith cannot easily be seen on CT if 7.5-mm sections are used.
Another potential source of error is the degree of calcification of an
appendicolith. Although large obstructing appendicoliths tend to be more
heavily calcified than small ones, they may not necessarily be easy to
identify in the presence of barium, even using bone window settings. The
degree of solidification and calcification varies at different stages of
fecalith maturation. Several patients had pathologic specimens with presence
of appendicitis and semisolid fecal material. We did not include these
patients in our study because semisolid material obviously cannot be
identified as calcifications on CT, nor is it considered calcifications
accordong to pathologic or surgical criteria.
Another source of error is misinterpretation of precipitated barium
collections as an appendicolith. We had no patients with such findings at
surgery or pathology, but such misinterpretation could conceivably increase
the rate of false-positive findings of appendicoliths on CT using bone window
settings. Assuming that precipitation of barium in the lumen does not occur
only focally in the appendix but also in multiple areas of the colon and small
bowel, we suggest one way to avoid this possible source of error would be to
look for other areas of precipitation and compare similarities in densities
among these precipitated collections.
In summary, we believe on the basis of the findings in our patient sample
that adding an evaluation with bone window settings to the CT examinations of
patients with suspected acute appendicitis is a useful and quick tool to
increase sensitivity in diagnosing the presence of an appendicolith. By
documenting the presence of an appendicolith, the CT report is strengthened,
and the rate of false-negative CT findings of appendicitis can be reduced
because the presence of an appendicolith is widely accepted as highly specific
for acute appendicitis. For the 16 patients studied, the sensitivity of CT
with bone window settings for detection of an appendicolith was 62.5% compared
with 31.3% for CT with soft-tissue window settings. This finding, however,
needs further verification with a larger prospective study group. Nonetheless,
the presence of an appendicolith is significantly associated with appendicitis
and can be detected more easily on CT using bone window parameters,
particularly in patients with equivocal findings and in whom other secondary
signs are minimal or absent.
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Abstract
Introduction
