In most situations, the diagnosis of acute appendicitis will be apparent clinically. However, in equivocal cases, either computed tomography or ultrasound may be used to assist in definitive diagnosis or establish an alternative etiology for the patient's right lower quadrant pain. The author presents the techniques and imaging findings of each of these modalities in cases of possible appendicitis
Dr. McGahan is a Professor of Radiology and the Director of
Abdominal Imaging and Ultrasound at the University of
California Davis Medical Center, Davis, CA. He is also a member
of the Editorial Advisory Board of this journal.
Appendicitis is one of the most common conditions in children
In most situations, signs and symptoms can establish the diagnosis
reliably. Patients usually present with periumbilical pain that
later localizes to the right lower quadrant (RLQ) at McBurney's
point. In most cases, pain is accompanied by mild anorexia and
nausea or vomiting. In a recent series, the combination of
abdominal pain and tenderness accompanied by leukocytosis
had the highest predictive value for acute appendicitis.
However, in some patients there is a delay in diagnosis resulting
in increased risk of perforation, sepsis, and even death. In a
recent series of childhood appendicitis, all 10 patients younger
than 10 years of age had a perforated appendix at the time of
operation. In 35 children between 1 and 5 years of age with
appendicitis, there was a perforation rate of 69%.
Recent series have demonstrated a 19.8% morbidity rate among
patients with perforated appendicitis.
The surgical literature has accepted that up to 25% of patients
undergoing appendectomy will have a normal appendix.
This is the so-called negative appendectomy rate. Therefore, the
goal of any further studies would be threefold. The first goal
would be to diagnose appendicitis promptly in hope of reducing the
rate of perforation and associated complications. The second goal
would be to reduce the number of patients undergoing unnecessary
removal of a normal appendix. After exclusion of appendicitis, the
final goal would be to establish the etiology of the patient's RLQ
Computed tomography versus ultrasound
Acute appendicitis and its complications may be imaged with
either computed tomography (CT) or ultrasound (US). Several
questions remain concerning the use of these imaging modalities.
The first question is "Should any imaging be used?" The second
question is "If imaging is performed, should it be CT, US, or any
other imaging modality?"
These questions have no clear answer. A recent controversial
report by Lee
indicated that neither the use of CT nor US led to improved
diagnostic accuracy for appendicitis, but, in fact, these
procedures actually might delay surgical consultation and necessary
appendectomy. However, many individuals do not agree with this
view. Most institutions have reached a general consensus that for
patients with suspicion of acute appendicitis, selective, rather
than routine, use of imaging studies is recommended.
In many, if not most, patients, the diagnosis or exclusion of
appendicitis may be made clinically. However, when imaging is used,
it may reduce the negative appendectomy rate.
In a report by Schuler et al,
the use of CT reduced the negative appendectomy rate from 21%
without the use of preoperative CT, to 6% with the use of CT.
Therefore, most would utilize imaging, either CT or US, in cases in
which the diagnosis of appendicitis is equivocal.
In those cases in which selective use of imaging is performed,
should it be CT or US? Original publications comparing CT to US
usually showed that CT had a higher sensitivity than US. For
instance, in 1994, Balthazar et al
published data showing US sensitivity of 76% and CT sensitivity of
96%, with US accuracy of 83% compared with the CT accuracy of 94%.
Similar comparative sensitivities have been reported with spiral CT
compared with compression US, with CT sensitivity ranging from 85%
to 100% and US sensitivity ranging from 74% to 100%.
Computed tomography is usually associated with a greater decrease
in the negative appendectomy rate than is US. In most studies, use
of both CT and US led to a decrease in the negative appendectomy
rate compared with clinical evaluation.
However, in patients for whom CT is used as the imaging
modality, a higher rate of perforation is often reported than is
reported in patients for whom US is used. Appendiceal perforation
at appendectomy was found in 23% when patients were examined by US
alone, 54% for CT alone, and 71% when both examinations were used.
This result is similar to a report presented at the 2001 RSNA
meeting that reported a perforation rate of 23% in patients
examined by US and 55% in those in which CT was utilized to
To further complicate the issue, some such as Lee
have shown an average delay in diagnosis of 7.8 hours when CT was
utilized for evaluation of patients with RLQ pain. This delay may
be a cause for higher perforation rates when using preoperative CT
imaging. However, Lee did not believe that this delay was
necessarily associated with the increased perforation rate. It is
possible that the more complicated or equivocal cases are more
diagnostically difficult and therefore require imaging studies. The
extended workup delays diagnosis and thus may lead to an increased
perforation rate. More straightforward cases may go directly to
surgery, and thus exhibit a lower perforation rate at
Currently, most individuals advocate CT as the most adequate
method for imaging appendicitis. However, there are certain
situations in which US may be especially helpful. These typically
would include many women and children.
Within the literature there are multiple combinations of CT
techniques that have been used. Some of these techniques advocate
use of intravenous (IV) contrast, while others do not use IV
contrast. Some include a complete examination of the abdomen and
pelvis, while others include only a focused examination of the
abdomen or pelvis. Some advocate use of oral contrast and others
advocate use of rectal contrast. Some of the advantages and
disadvantages of each of these techniques are listed in Table
There are proponents of virtually every variation in technique.
However, in reviewing the literature, it appears that 5-mm, rather
than 10-mm, sections have resulted in identifying 94% versus 69% of
Second, it seems that using CT of the entire abdomen and pelvis has
higher sensitivity in diagnosing appendicitis and perhaps, more
importantly, other causes of abdominal pain, rather than a more
focused examination. For instance, a complete study of the abdomen
attained a 99% sensitivity compared with 88% sensitivity for a
focused study for diagnosing appendicitis.
There are a number of advocates of oral or rectal contrast.
Wise et al
take the approach of using rectal contrast only in those cases in
which a CT without contrast is not definitive. They reserve rectal
contrast for problem cases. The author's preference for CT is, in
general, to administer IV contrast and scan the entire abdomen and
pelvis. However, the approach can vary depending on the patient's
particular situation. If the patient has a possible bowel
obstruction, then oral contrast is given. For those patients who
may require emergency surgery, no oral or rectal contrast is used
initially. Rectal contrast is reserved for equivocal cases,
although some pediatric radiologists feel it should be used in all
children suspected of acute appendicitis.
CT is very useful to visualize the bowel and localize the normal
appendix once the ileocecal valve is identified. The appendix is
usually located arising from the cecum approximately 3 cm below the
ileocecal valve. The appendix itself may be up to 10 cm in length.
It may be collapsed, air-filled, or contrast-filled if oral or
rectal contrast is used (figures 1 through 3). There are a number
of anatomic variations and locations of the cecum; the cecum itself
typically may lie in the right upper quadrant or deep within the
pelvis. To compound this problem, there are various anatomic
possibilities for the location of the appendix. The appendix itself
may lie, among other locations, deep within the pelvis,
paraumbilically in the RLQ, or retrocecally. Computed tomography is
very helpful to identify the location of the appendix. Typically on
CT, the normal appendix is 1 to 2 mm in thickness with an overall
diameter of 6 mm. The lumen is usually collapsed and the mesentery
around the appendix exhibits a uniform fat density with crossing
linear vessels. The abnormal appendix will show a thickened wall
>3 mm and a distended lumen >= 6 to 7 mm. There may be an
associated appendolith, mesenteric fat infiltration or fluid, and
there may be associated adjacent adenopathy (figure 4). More
complex appendicitis may be associated with abscess formation and
cecal thickening, the so-called "arrowhead sign" (figure 5). In a
recent report on the CT diagnosis of appendicitis, the most common
signs of acute appendicitis included fat stranding (100%), an
enlarged appendix >6 mm in lumen width (97%), adenopathy (63%),
and an appendolith (43%).
There is little doubt that technique and experience of the
examiner plays a key role in the US diagnosis of appendicitis. When
dedicated individuals perform sonography, the sensitivity and
overall accuracy of US remain quite high. Dedicated radiologists
available 24 hours a day to perform appendiceal US had a
sensitivity of 89%, a specificity of 95%, a positive predictive
value of 86%, and a negative predictive value of 96% in a recent
report by Dilley.
When surgeons performed both the US and the clinical
examination, they attained a sensitivity of 98%.
Can such high sensitivities be obtained in routine practice? In
many instances, the answer is "yes," and in other instances the
answer is "no." Much depends on the expertise of the individual
performing the study and a number of other factors, such as patient
selection. Thin patients are better candidates for US than are
obese individuals. Pediatric patients may be better candidates for
US since there is a substantial radiation dose associated with
thin-cut CT. This is an important consideration in young patients.
If possible, this increased radiation should be avoided, but not at
the expense of a missed diagnosis.
The author finds several points helpful when performing US for
RLQ pain. The technique of the abdomen examination is modified
slightly from that described by Baldisserotto et al.
Using their technique, they reported US sensitivity of 98.5%,
specificity of 98.2%, positive predictive value of 98%, and
negative predictive value of 98%.
First, the entire abdomen is examined with a 4- to 5-MHz curved
sector scanner. There are a number of other etiologies of abdominal
or RLQ pain, ranging from mesenteric adenitis, to obstructive
uropathy, to ovarian torsion. Thus, an examination of the entire
abdomen is clearly warranted. In women, one must examine the pelvis
through a distended bladder with a sector or curved ray scanner. Do
not fail to perform an endovaginal examination in female patients
if possible pelvic disease exists or if possible abnormalities are
identified on the transabdominal scan.
Second, the compression examination is performed with a
high-resolution (>= 7.5 MHz) linear array transducer (figure 6).
Patient localization can be added to the study. It has been shown
that if the patient points to the location of the pain, this
sonographic "self localization" helps reduce examination time and
is a valuable addition to the standard graded compression US of the
When performing the graded compression, the common femoral
artery and vein is identified in order to orient to the RLQ. The
patient is then scanned cephalad from this position. The transducer
is then used to compress the RLQ, including the terminal ileum and
the cecum, to identify any possible appendix. Better compression is
obtained if the left hand is placed behind the patient's flank.
The normal appendix may be commonly identified.
When visualized, the normal appendix should be a blind-ended,
tubular structure with a maximum wall thickness of ¾ 2 mm with an
outer diameter of ¾ 6 mm, have no peristalsis, and originate from
the base of the cecum. Normal bowel will have peristalsis and thus
change lumen size (figure 7). In most instances, the
inner-echogenic submucosal ring and the outer hypoechoic ring may
also be helpful to identify the normal appendix, although this
pattern may be seen with other segments of the bowel.
Ultrasound findings of acute appendicitis are listed in Table 2.
These include a blind-ended, noncompressible fluid-filled structure
with a wall thickness of >= 3 mm and an outer diameter of >=
7 mm identified around the appendix (figure 8). There may be a
circumferential color-flow identified around the appendix (figure
9). There may also be free fluid noted in the RLQ of the abdomen
and the pelvis, and there may be echogenic mesenteric fat (figure
10). If appendiceal perforation has occurred, an ill-defined and/or
fluid-filled abscess may be identified. While 6 mm is usually
identified as the cutoff between normal and abnormal appendix, in
some instances, 5 mm is used as the upper limits of normal, while 7
mm is considered to be positive for acute appendicitis. Thus, in
some instances, a measurement between 5 and 7 mm is considered to
There are several pitfalls in diagnosing appendicitis. These can
include the following: tip appendicitis; spontaneous resolution of
appendicitis; chronic appendicitis; abnormal appendix location; and
other etiologies of RLQ pain.
Appendicitis localizing to the tip of the appendix in an early
inflammatory response. Thus, if only the base of the appendix is
examined, appendicitis may be missed.
Spontaneous resolution of appendicitis--
It is estimated that appendicitis will spontaneously resolve in
approximately 10% of cases.
While in most instances acute appendicitis will eventually result
in perforation and abscess formation, there are documented cases in
which appendicitis has been chronic with a patient's having
persistent, and/or intermittent RLQ pain.
Abnormal appendix location--
This is perhaps the most common pitfall in diagnosing appendicitis.
The appendix may be in locations that are not readily accessible by
US. These may include a retrocecal, mid-abdominal, deep pelvic, or
even right upper quadrant location. As such, it is important to
examine the patient initially with a curved or sector scanner, and
if necessary, use an endovaginal scan to diagnose appendicitis.
Other etiologies of RLQ pain--
There are a number of other etiologies of RLQ pain, including other
bowel disease (figures 11 and 12), disease of the mesentery, or
other processes involving the appendix, including appendiceal
tumors. In addition, organs that surround the appendix may be
etiologies of RLQ pain. This could include gynecological disease
(figure 13), gallbladder disease, renal disease (figure 14), or
disease of the retroperitoneum.
Complications of appendicitis
Appendicitis may have a variable course. Acute appendicitis may
resolve spontaneously in ¾ 10% of cases. Appendicitis may be a
chronic process with the patient having indolent, long-standing RLQ
pain. However, in most cases of acute appendicitis, if left
untreated, the condition will progress to gangrenous appendicitis,
resulting in periappendiceal phlegmon or a periappendiceal abscess.
In most situations, an appendiceal abscess will be walled off
(figure 15). However, in some cases, an appendiceal abscess may
localize to the dependent portion of the pelvis within the
cul-de-sac. In other situations, appendiceal abscesses may result
in widespread abscesses throughout the abdomen.
Treatment of appendiceal abscesses
Either US and/or CT guidance may be used for treating
periappendiceal or pelvic abscesses. In some situations, a combined
use of CT and US may be required. This is usually performed using a
guidewire exchange technique. In many circumstances, percutaneous
drainage alone may be curative if the abscess is well localized in
the periappendiceal area and the appendix has been resorbed in the
inflammatory response. In other situations, the abscess may
localize in the pelvis.
Previously, the authors found that CT, using the transgluteal
approach, was helpful in draining these abscesses. However, more
recently, we have drained these abscesses through an endorectal
route (figure 16). This is best performed using a trocar technique
and a single step method, although this procedure also may be
performed under sonographic guidance using a guidewire exchange
technique. Ultrasound is very useful in guiding drainage of deep
pelvic abscesses associated with appendicitis.
In most situations, the diagnosis or exclusion of acute
appendicitis will be apparent clinically. However, in equivocal
cases, either CT or US may be utilized to assist in definitive
diagnosis or establish an alternative etiology for the patients'
RLQ pain. In most adult patients, when CT is performed at the
authors' institution, the entire abdomen is examined using 5-mm
slice increments with intravenous contrast enhancement. However, in
women and children, especially those who are thin, US may prove
very useful. Ultrasound is best performed using a 4- to 5-MHz
curved sector scanner examining the entire abdomen. After
performing this portion of the study, graded compression US using a
7.5-MHz linear probe is performed in the right lower abdomen. If a
gynecological process is suspected or cannot be excluded,
endovaginal scanning may be performed. By a meticulous examination
performed by well-trained personnel, US will have a fairly high
accuracy in diagnosing appendicitis. Either sonography or CT may be
used to drain periappendiceal or pelvic abscesses associated with