Appendicitis Imaging

Abdominal radiographs are normal in many patients with acute appendicitis. Therefore, plain radiographic films rarely add to the diagnosis. In one study, plain-film findings consistent with acute appendicitis were observed in only 20% of patients with appendicitis.[29]

The presence of a calcified appendiceal fecalith, which is the most specific plain-film sign, occurs in fewer than 10% of patients.

Other plain-film findings include convex lumbar scoliosis, obliteration of the right psoas margin, RLQ air-fluid levels, air in the appendix, and pneumoperitoneum.

Barium enema can be performed on an unprepared bowel quickly and safely by using the single-column technique. Complete filling of a normal appendix effectively excludes the diagnosis of appendicitis; however, nonfilling or incomplete filling of the appendix, coupled with pressure effect or spasm in the cecum, suggests appendicitis (see the image below).

Degree of confidence

The consensus in the literature is that plain radiography is insensitive and nonspecific. Plain radiographs should not be obtained routinely, except to evaluate for obstruction of free air.

Appendicoliths may be found in individuals without appendicitis, and other plain-film signs of appendicitis can be observed in other abdominal diseases.

Although the diagnostic accuracy of a barium enema is reported to be 80-100%, this technique has several drawbacks, such as nonfilling of the appendix, which can be observed in 15-20% of patients without appendicitis.

CT scanning has the advantage of direct visualization of the appendix, as well as the periappendiceal and other intra-abdominal structures. CT scanning can be used to evaluate an abnormal appendix and the nature, severity, and extent of the associated inflammatory process.[30]

In a 22-year retrospective study of 2108 patients with appendicitis, CT use increased more than 20-fold, but there was no statistically significant trend for increased use of ultrasound or MRI.[31]

Advantages of CT scanning include its superior sensitivity and accuracy compared to those of other imaging techniques, ready availability, noninvasiveness, and potential to reveal alternative diagnoses. In a study by Pickhardt et al, CT scanning is confirmed as a valuable test for confirming acute appendicitis in adults.[32] During this study of 2871 patients, 675 had acute appendicitis. Multidetector CT (MDCT) scanning had a sensitivity value of 98.5%, a specificity value of 98%, a negative predictive value of 99.5%, and a positive predictive value of 93.9%. MDCT scanning also suggested an alternative diagnosis in 893 patients without appendicitis or appendectomy. Disadvantages include radiation exposure, potential for anaphylactic reaction if intravenous contrast agent is used, lengthy acquisition time if oral contrast is used, and patient discomfort if rectal contrast is used.[33, 34]

The typical findings are a nonfilling appendix with distention and thickened walls of the appendix and the cecum, enlarged mesenteric nodes, and periappendiceal inflammation or fluid (see the image below).

A meta-analysis of CT findings from 23 studies found 10 features were informative for complicated appendicitis. There was a general tendency for these features to have a relatively high specificity but low sensitivity. Extraluminal appendicolith, abscess, appendiceal wall enhancement defect, extraluminal air, ileus, periappendiceal fluid collection, ascites, intraluminal air, and intraluminal appendicolith showed a pooled specificity greater than 70% (range, 74-100%), but sensitivity was limited (range, 14-59%). Periappendiceal fat stranding was the only feature that showed high sensitivity (94%) but low specificity (40%).[35]

CT scanning techniques

In evaluating patients with suspected appendicitis, a number of techniques have been advocated, including abdominal and pelvic CT scanning without oral or IV contrast, focused CT scanning of the cecum after administration of rectal contrast, or use of oral or IV contrast, with a delay of 45 minutes to 1 hour following oral contrast administration to permit passage of the contrast into the colon.

Patient acceptance of retrograde instillation of barium per rectum may limit the use of this focused technique. A limited scanning volume may be more appropriate in young, particularly female, patients in consideration of reducing radiation exposure to the internal and reproductive organs.

Visualization of the appendix is optimized in CT scanning with use of bowel opacification, IV contrast, fine collimation (< 5 mm), and/or thin-section reconstruction. The postacquisition reformatting capabilities of multislice technology should limit the necessity for rescanning a nonvisualized or poorly visualized appendix, although at a somewhat higher cost in the initial radiation dose.

CT scan findings in the normal appendix

The appendix can be visualized in approximately 50% of patients without appendicitis, although a higher visualization rate is expected in patients who are scanned with thin-section, multislice CT scanning (see the image below).

The appendix is demonstrated as a tubular structure when it lies in the plane of the CT scan section or as a ringlike structure when it is scanned in a cross-sectional plane. The appendix occasionally contains air or barium, but it may be collapsed. The wall thickness should be very thin, and the normal external diameter is usually less than 6 mm, although this may be slightly greater in patients with an elongated, retrocecal appendix (see the image below).

CT scan findings in appendicitis

With high-resolution CT scanning, an abnormal appendix can be observed, and variable CT scan findings can be evaluated in patients with acute appendicitis.

Specific CT scan findings of appendicitis include enlargement of the appendix (>10 mm in the outer diameter), hyperenhancement of the appendiceal wall, appendiceal wall thickening (>3 mm), lack of opacification in an enlarged appendix, increased intraluminal fluid, fat stranding in the periappendiceal region, and the presence of an appendicolith within the appendix. The appendicolith may be observed outside the appendix lumen, within an inflammatory mass, or in a fluid collection. Appendiceal diameters of 6-10 mm are usually equivocal, and the other associated findings listed above are needed to confirm the diagnosis of appendicitis.

A few secondary findings often accompany acute appendicitis. Focal thickening of the cecum may occur. The inflammatory process can be observed to separate the cecal lumen from the base of the appendix or an appendicolith (cecal bar). Contrast medium is observed within the cecum channeling to the point of the appendiceal obstruction, called the arrowhead sign (see the image below).[36]

Gangrenous appendicitis is recognized as enlargement of the appendix, with associated fluid and loculated air within the lumen (see the image below).

Distal appendicitis (seen in the image below) is diagnosed when CT scanning reveals appendicitis involving the distal appendix, with a normal-appearing proximal appendix.

Detection of phlegmons, abscesses, and RLQ inflammation adjacent to the cecum is strongly suggestive of—but not pathognomonic for—acute appendicitis. Perforated appendicitis is usually accompanied by pericecal phlegmon or abscess formation. Free intra-abdominal air can be observed either under the right diaphragm or in a retroperitoneal location in patients with a perforated appendix. Associated CT scan findings include extraluminal air, marked ileocecal thickening, localized lymphadenopathy, peritonitis, and small-bowel obstruction (see the image below).

Degree of confidence

Conventional and helical CT scanning techniques have documented high accuracy (96-98%), sensitivity (96-100%), specificity (95-97%), positive predictive value (97-99%), and negative predictive value (88-100%).[37, 38, 39, 40]

Initial studies evaluated sequential (nonhelical) CT scanning in the diagnosis of appendicitis. Malone evaluated nonenhanced, sequential CT in 211 patients and reported a sensitivity of 87% and a specificity of 97%.[41] The addition of intravenous and oral contrast agent increased sensitivity to 96-98%.

Thus, sequential CT scanning with oral and intravenous contrast enhancement is highly accurate. However, this examination is also time consuming and expensive; it is best used for equivocal presentations when helical CT scanning is not available.

Lane et al evaluated helical CT scanning without contrast enhancement and found a sensitivity of 90% and a specificity of 97%.[42] Other studies of noncontrast helical CT in adults with suspected appendicitis have shown a sensitivity of 91-96% and a specificity of 92-100% for this examination.[43, 44, 45, 46]

Kaiser et al found that nonenhanced CT was 66% sensitive.[47] Sensitivity increased to 90% with the use of intravenous contrast material.

In a study of 112 pediatric patients, Hoecker and Bilman found that unenhanced CT achieved a sensitivity of 87.5%, a specificity of 98.7%, a positive predictive value of 91.3%, and a negative predictive value of 90.8%.[13]

In a retrospective study of 210 pediatric patients who underwent abdominopelvic CT for suspected acute appendicitis, limited-range CT was found to be as accurate as full-range CT, with the radiation dose reduced by 46%.[12]

A meta-analysis of 9 studies (2957 patients) found that for acute appendicitis, low-dose CT had a sensitivity of 96.25% and a specificity of 93.22%, whereas standard-dose CT had a sensitivitiy of 96.4% and a specificity of 92.17%.[25]

Rao et al found that focused (lower abdominal and upper pelvic) helical CT with 3% Gastrografin instilled into the colon (without intravenous contrast agent) had a superior sensitivity of 98% and a specificity of 98%.[39]

Focused helical scanning without intravenous contrast agent eliminates the risk of anaphylaxis and reduces the cost. Acquisition time is less than 15 minutes. Radiation exposure is less than that of a standard obstruction series. Alternative diagnoses are revealed in up to 62% of patients and include diverticulitis, nephrolithiasis, adnexal pathology, RLQ tumor, small-bowel hernias, and ischemia.

The literature suggests that limited helical CT scanning with rectal contrast enhancement is a highly accurate, time-efficient, cost-effective way to evaluate adults with equivocal presentations for appendicitis.

Two studies of focused helical CT scanning with rectal contrast in children suggest a sensitivity of 95-97%. This is an excellent diagnostic approach in patients with equivocal presentations who are poor candidates for intravenous contrast.

A retrospective study of 173 adults found that helical CT scanning with intravenous contrast, but no rectal or oral contrast, had a sensitivity of 100%, a specificity of 97%, a positive predictive value of 95%, and negative predictive value of 100%.[48]

Another study that used only intravenous contrast in the CT scan diagnosis of appendicitis, this time on 78 patients, found the examination to have a sensitivity of 91.9%, a specificity of 87.5%, and an accuracy of 91%.[49]

In a retrospective review of 23 published reports, Anderson et al found that CT scanning without oral contrast was at least as accurate as CT scanning with oral contrast, achieving a sensitivity of 95%, a specificity of 97%, a positive predictive value of 97%, and a negative predictive value of 96%.[50] Elimination of oral contrast reduces emergency department length of stay and delay to operative intervention.

In a meta-analysis  of 7 prospective studies, the pooled sensitivity and specificity of noncontrast CT for detecting acute appendicitis were 90% and 94%, respectively.[51]

CT scanning is readily available, operator independent, and relatively easy to perform, and it provides results that are easy to interpret. Furthermore, CT scanning helps clinicians to diagnose other intra-abdominal conditions in 50-80% of patients who do not have acute appendicitis (see the images below).

CT scanning has high diagnostic accuracy in detecting complications, particularly in patients in whom appendiceal perforation is suspected. This modality can help clinicians to accurately differentiate mild inflammation from phlegmons and abscesses, as well to identify the extent of the disease. CT scanning can be used to guide surgical or percutaneous abscess drainage.

False positives/negatives

Some CT scan findings that are suggestive of or often accompany acute appendicitis are nonspecific and can be observed with other RLQ conditions, such as Crohn disease, ulcerative colitis, pancreatitis, perforation of a duodenal ulcer, and cholecystitis. Misinterpretation of these signs as indicative of appendicitis can lead to a false-positive result.

Conversely, specific signs can be misapplied. Without optimal cecal opacification, a distended, inflamed appendix can be mistaken for a small-bowel loop. Rarely, an appendiceal lumen filled with appendicoliths can mimic an opacified lumen on CT scans.

Distal appendicitis can potentially be the cause of a false-negative interpretation.

Two studies regarding the use of MRI in evaluating acute appendicitis support different techniques. A study by Incesu et al described significant enhancement of the inflamed appendix and surrounding fat on gadolinium-enhanced, fat-suppressed, T1-weighted, spin-echo images (see the first image below).[52] Mild enhancement was observed in the normal appendix and gut (see the second image below). Using fat-saturation techniques, contrast differences were observed between the inflamed appendix and the surrounding fat. Fat-suppressed, T2-weighted, axial and coronal images also helped in the detection and evaluation of appendicitis and its complications.

In another study, Hormann et al suggested that unenhanced, axial, T2-weighted, spin-echo imaging was the most sensitive sequence for the diagnosis of acute appendicitis.[53] The authors obtained axial, T1-weighted, turbo, spin-echo sequences; axial and coronal, T2-weighted, turbo, spin-echo sequences; and axial, fat-suppressed, short inversion-time, inversion-recovery, turbo, spin-echo sequences. In the study, on T2-weighted, ultraturbo, spin-echo images, acute appendicitis demonstrated a markedly central hyperintensity and markedly hyperintense periappendiceal tissue, as well as a slightly hyperintense thickened wall.

In a meta-analysis of 30 studies and 2665 patients, the sensitivity and specificity of MRI for the diagnosis of acute appendicitis were 96% and 96%, respectively. In pregnant patients, sensitivity was 94% and specificity was 97%; in children, sensitivity was 96% and specificity was 96%.[54]

In a study of 97 pediatric patients (age 4-18 yr) suspected of appendicitis who underwent MRI examination without contrast or sedation, sensitivity was 94%, specificity 95%, positive predictive value 91%, and negative predictive value 97%.[11]

In a study by Badr et al of 85 pregnant women with abdominal pain who underwent MRIs, the appendix was identified in all patients, and acute appendicitis was suspected in 7. The diagnosis was confirmed in 6 patients. Sensitivity was 100%, specificity 98.7%, positive predictive 85.7%, and negative predictive value 100%.[22]

Komanchuk et al studied 98 children aged 5-17 years with suspected appendicitis who underwent ultrasonography and found that 53 (54.1%) of the ultrasound studies were equivocal for appendicitis. MRI was performed and provided further diagnostic information in 41 of 53 (77.4%; 10 positive, 31 negative; 12 remained equivocal). Secondary findings of appendicitis on MRI included abdominal free fluid (24, 45.3%), periappendiceal fluid (12, 22.6%), intraluminal appendiceal fluid (9, 17.0%), fat stranding (8, 15.1%), appendicolith (2, 3.8%), and periappendiceal abscess (1, 1.9%).[23]

Gadolinium-based contrast agents have been linked to the development of nephrogenic systemic fibrosis (NSF) or nephrogenic fibrosing dermopathy (NFD). The disease has occurred in patients with moderate to end-stage renal disease after being given a gadolinium-based contrast agent to enhance MRI or MRA scans.

Degree of confidence

Fat-suppressed, gadolinium-enhanced MRI scans are sensitive (97%) and accurate (95%) in the detection of acute appendicitis; however, this technique is not routinely used in the diagnosis of acute appendicitis. Unenhanced MRI scanning is also used for the evaluation of appendicitis with an accuracy of 100%. When the results of the studies by Hormann et al and Incesu et al are compared, unenhanced MRI is found to be the more appropriate imaging modality.[52, 53]

A single retrospective study assessed the accuracy of MRI in 51 pregnant patients with suspected appendicitis in whom ultrasonography was nondiagnostic. Sensitivity, specificity, positive and negative predictive values, and accuracy for MRI was 100%, 93.6%, 91.4%, 100%, and 94.0%, respectively.[14]  In another multicenter restrospective study of pregnant women with confirmed acute appendicitis, sensitivity, specificity, accuracy, positive predictive value, and negative predictive values were 96.8%, 99.2%, 99.0%, 92.4%, and 99.7%, respectively.[15]

False positives/negatives

False-negative results in MRI usually result from technique-related limitations, such as inefficient fat saturation that causes the appendiceal wall enhancement to be obscured by mesenteric fat.[55]

Inflammatory diseases of the gut, such as ileal diverticular abscess and inflamed ileal segments of Crohn disease that may mimic appendicitis, have been reported as false-positive results on enhanced MRI.

Ultrasonography is a widely available and inexpensive modality that has the potential for highly accurate imaging in patients with suspected acute appendicitis.[56, 57, 16, 58, 59]  With the introduction of graded-compression US by Puylaert in 1986, the diagnostic use of this modality in acute appendicitis has increased dramatically.[60, 61]

The advantages of US include its noninvasiveness, short acquisition time, lack of radiation exposure, and potential for the diagnosis of other causes of abdominal pain, particularly in the subset of women of childbearing age. Many authorities believe that US should be the initial imaging test in pregnant women and in pediatric patients, because radiation exposure is particularly undesirable in these groups.

A study of point-of-care ultrasound to diagnose appendicitis found ultrasonography to have a sensitivity of 91%, a specificity of 97%, a positive predictive value of 91%, and a negative predictive value of 94%.[24]

A study by Davis et al found that point-of-care ultrasonography and MRI for pediatric appendicitis can be highly accurate. In a retrospective study of 209 patients, sensitivity and specificity of ultrasonography were 100% and 95.2%, respectively, in addition to 86 equivocal examinations, 6 of which had appendicitis. The sensitivity and specificity of MRI were 94.4% and 96.5%, respectively, in addition to 27 equivocal examinations, 2 of which had appendicitis.[59]

A retrospective study, carried out on 1,228 children with suspected appendicitis, evaluated the use of a staged protocol using ultrasound, and, subsequently, CT only if the findings were equivocal.[17] Results showed that in children with suspected acute appendicitis, the protocol of US first and then CT was highly accurate and may reduce the need for CT and thus exposure to radiation (sensitivity, 98.6%; specificity; 90.6%). The negative appendectomy rate was 8.1% (19 of 235 patients). The missed appendicitis rate was less than 0.5% (1 of 631 patients). CT was avoided in more than half (333 of the 631 patients) in whom the protocol was followed, and the US findings were definitive, representing a substantial reduction in radiation exposure to this population.

One study, by Poortman et al, suggests that ultrasonography should be incorporated as a first-line imaging modality for the diagnosis of acute appendicitis in adults. In this study, 151 patients with suspected appendicitis underwent the designed protocol. Graded-compression US was performed first. Patients with positive results on graded-compression US underwent surgery. Patients in this study who had inconclusive or negative results underwent contrast-enhanced, multidetector CT scanning. Patients with positive findings on CT scanning also underwent surgery. Patients with negative CT scan findings were admitted for observation. Positive ultrasonographic results were confirmed at surgery in 71 of 79 patients, and positive CT scan results were confirmed in 21 patients. Thirty-nine patients with normal CT scan results recovered and did not require surgery. The sensitivity and specificity of this protocol was 100% and 86%, respectively.[62]

Poortman et al concluded that this diagnostic pathway, using primary graded-compression US and complementary multidetector CT scanning, yields a high diagnostic accuracy for acute appendicitis, without adverse events from delay in treatment. Although US is less accurate than CT scanning, it can be used as a primary imaging modality and avoids the disadvantages of CT scanning. Observation is safe for patients with negative findings on US or CT scanning.[62]

Bedside ultrasound may be an appropriate initial test to evaluate patients with suspected acute appendicitis in the emergency department. One study found bedside ultrasound had a sensitivity of 67.65% and a specificity of 98.41%.[63]

Ultrasonography has been found not to be reliable in differentiating between simple and complex appendicitis in children. In one study, ultrasonography had a sensitivity of 46%, a specificity of 90%, a positive predictive value of 84%, and a negative predictive value of 60% in differentiating simple from complex appendicitis.[58]

The principal disadvantage is that US is operator dependent. Because nonvisualization is interpreted as a noninflamed appendix, technical expertise and commitment to a thorough examination are essential in obtaining maximum sensitivity.[57, 16, 60, 63, 64, 65]

Ultrasonographic technique

Ultrasonographic examination should begin with a thorough evaluation of the abdominal and pelvic organs.

In women with low pelvic tenderness in whom the appendix is not visualized on transabdominal US or in whom the diagnosis is not evident after abdominal and pelvic examinations have been performed, add an endovaginal ultrasonographic examination. Pelvic appendicitis is best demonstrated in women with endovaginal US; a posterolateral approach is suggested to evaluate the retrocecal area for retrocecal appendicitis.

Puylaert described a graded-compression technique for evaluating the appendix with transabdominal US.[60] A 5-MHz transducer is used. Gentle, but firm, pressure is applied on the RLQ to displace intervening bowel gas and to decrease the distance between the transducer and the appendix, improving image quality. An outer diameter of greater than 6 mm, noncompressibility, lack of peristalsis, or periappendiceal fluid collection characterizes an inflamed appendix. The normal appendix is not visualized in most cases. A posterolateral approach is suggested to evaluate the retrocecal area. Scattered case reports endorse transvaginal US in women with low pelvic tenderness, if the appendix is not visualized on transabdominal scans.

Graded-compression US allows successful examination of a patient who may have peritoneal irritation and sensitivity. During the compression, maximal tenderness of an abdominal point can provide an important diagnostic clue, which is often useful in focusing the ultrasonographic examination on the correct area in a patient with suspected appendicitis.

Ultrasonographic findings

Major ultrasonographic findings in acute appendicitis in the RLQ include the following:

• An aperistaltic, noncompressible, blind-ended, sausage-shaped structure that arises from the base of the cecum

• Distinct appendiceal wall layers

• An outer diameter greater than 6 mm

• A target appearance

• Appendicolith(s) (See the images below.)

• Periappendiceal fluid collection

• Echogenic, prominent pericecal fat

  Phlegmonous appendicitis; oblique-axial ultrasonogram. A pericecal fluid collection, which is walled off by small-bowel loops (arrowheads) is shown, and an appendicolith with an acoustic shadow (arrow) is observed.

  Suppurative appendicitis in a 15-year-old boy; longitudinal view, pathologic specimen. An inflamed appendix with appendicoliths in the lumen (arrow) is demonstrated.

Normal appendix

A normal appendix is infrequently observed using gray-scale US, but this structure can be visualized as a blind-ended, tubular, compressible intestinal loop that is continuous with the cecum and has a diameter of less than 6 mm, particularly in thin patients (see the image below).

Suppurative appendicitis

The longitudinal ultrasonographic view demonstrates a nonperforated, inflamed appendix that is characterized by an aperistaltic, noncompressible, blind-ended, tubular structure with a laminated wall that arises from the base of the cecum. When the inflammation is mild and visualization is optimal, 5 distinct appendiceal wall layers can be identified (see the image below).

In the transverse view, the abnormal appendix often has a target appearance, and its outer diameter is in excess of 6 mm in the RLQ. Researchers have studied the thickness of the appendix wall; walls surpassing 3 mm in thickness have been evaluated as pathologic.

An appendicolith, which is identified as an intraluminal echogenic focus with posterior shadowing, can be detected by US in approximately 30% of patients with acute appendicitis.

Gangrenous appendicitis

Generalized or focal loss of the echogenic submucosal layer of the appendiceal wall, as well as the prominent, surrounding, echogenic fat, is consistent with gangrenous appendicitis (see the images below).

Perforated appendicitis

In the diagnosis of a perforated appendicitis, gray-scale US is also a valuable diagnostic tool, despite the fact that the perforated appendix may not be visualized in the RLQ. Irregularity of and damage to the contour of the appendix by the presence of periappendiceal fluid and hyperechoic, prominent pericecal fat are diagnostic of perforation (see the image below). Gas bubbles occur within a fluid collection in cases of perforation or as a result of gas-forming organisms. A localized perforation of the appendiceal tip may also demonstrate gas pockets in the perforation site.

Periappendiceal phlegmon and abscess

A phlegmon appears as a localized fluid collection, which is walled off by the adjacent greater omentum and small-bowel loops. An appendiceal abscess appears as a complex, hypoechoic mass adjacent to the cecum or appendix. In these patients, the inflamed appendix may not be visualized (see the image below).

Alternative disease

Ultrasonographic examination is also useful for diagnosing alternative pathology, such as a tubo-ovarian abscess, ovarian torsion, ovarian cyst, or mesenteric adenitis, especially in women of childbearing age. Establishment of an alternative diagnosis does not exclude appendicitis. However, it is a double benefit when appendicitis can be excluded by US and an alternative diagnosis is made.

Color Doppler ultrasonographic findings

Color Doppler US is beneficial in the evaluation of inflammatory conditions of the intestinal tract, and according to most authors, this modality is a useful adjunct to conventional US in the assessment of acute appendicitis.[66, 67]

A normal appendix seldom shows findings of mild hyperemia on Doppler ultrasonographic examination. However, an inflamed appendix uniformly shows greater flow than a normal appendix, and circumferential color in the wall of the inflamed appendix (seen in the image below) as observed on color Doppler ultrasonographic images is a strong indicator of acute appendicitis.

A flowing appearance with Doppler US has been reported to be suggestive of a pathologic appendix, but absence of flow cannot differentiate a normal from an abnormal appendix.

Peripheral vascularity of the inflamed mesentery and omentum can be demonstrated; however, detection of hyperemia in the appendix wall is not possible in patients with gangrenous appendicitis because of the vascular necrosis adjacent to the appendix wall that has resulted from inflammation.

Degree of confidence

Numerous studies have documented that, in experienced hands, US has a sensitivity of 75-90%, a specificity of 86-100%, an accuracy of 87-96%, a positive predictive value of 91-94%, and a negative predictive value of 89-97% for the diagnosis of acute appendicitis. However, US is extremely operator dependent.

Five studies of graded-compression US in children showed overall sensitivities of 85-95% and specificities of 47-96%. One study found a sensitivity of 35% and a specificity of 98% in pediatric patients with perforated appendicitis.

One study indicated that in cases of appendicitis, US has a sensitivity and specificity of 77% and 86%, respectively (although other studies have reported higher figures for these), as compared to 100% sensitivity and specificity for CT scanning.

Similarly, a literature review from the Netherlands found CT scanning to be superior to graded-compression ultrasonography for the diagnosis of acute appendicitis.

One author reported the ability to identify a normal appendix in as many as 82% of patients without appendicitis. When a normal appendix cannot be demonstrated and when no pericecal fluid or echogenic mass is observed, the examination is usually considered not to be diagnostic for appendicitis.[68]

In a study by Rettenbacher et al, in which several hundred patients in whom appendicitis was suspected, as well as control subjects, were evaluated, the range of appendiceal diameters as measured by US was found to vary from 2 to 13 mm, with a diameter of greater than 6 mm in 23% of control subjects.[68]

Ultrasonographic examination is especially limited in obese patients or those who are in significant pain. Another important limitation of US is in the diagnosis of appendiceal perforation; in this scenario, the diagnosis depends on secondary findings, such as free pericecal fluid, echogenic pericecal fat, and gas bubbles in the perforation site.

Color Doppler US increases accuracy in the diagnosis of acute appendicitis, although potential limitations exist in using this modality for this purpose.

If a graded-compression ultrasonographic examination of the RLQ is positive for appendicitis, appendectomy should be performed. However, if the examination results are negative, this finding is not sufficiently sensitive to rule out the possibility of appendicitis. Consideration should be given to further observation and focused helical CT scanning with rectal contrast enhancement.

Tzanakis et al proposed a clinical scoring system that assigns 6 points if the results from an appendiceal ultrasonographic examination are positive, 4 points for RLQ tenderness, 3 points for rebound tenderness, and 2 points for a white blood cell (WBC) count of greater than 12,000.[69]  In their prospective study of 303 adults, using a total score cut-off of 8 points or greater, the authors reported that the scoring system had a sensitivity, a specificity, and an accuracy of 95.4%, 97.4%, and 96.5%, respectively. These findings should be confirmed by additional studies before routine clinical use.

The main limitation of US is that its reliability is completely user dependent.

False positives/negatives

Several major pitfalls are involved in the overdiagnosis of appendicitis with use of US, including misinterpretation of the terminal ileum as the appendix and misinterpretation of a normal appendix as an inflamed appendix.

The appendix may be inflamed secondarily as a result of a variety of inflammatory processes. In cases of intrinsic neoplasms, the appendix may be enlarged and noncompressible.

Some false-positive US findings have resulted from spontaneously resolving appendicitis.

Fluid-filled bowel loops and fecal and gaseous distention of the colon interfere with adequate visualization of the appendiceal area. Approximately 30% of appendicitis cases are missed by US as the result of a retrocecal appendix. This problem may be partially overcome by scanning in the coronal plane (with the transducer parallel to the iliac wing) to visualize posterior to the cecum.

A gas-filled appendix may cause diagnostic difficulty and may be misinterpreted as either a small-bowel loop or gas in a periappendiceal abscess.

False-positive results may occur in patients with Crohn disease.

Although several investigators have explored the potential of radioactive isotope imaging in patients with acute appendicitis, no practical role currently exists for this imaging modality in this patient population.

Indium-111 (111In)–labeled leukocytes are very sensitive and specific in the detection of appendicitis, with an overall accuracy of 91-95%.[70] However, indium is expensive, the optimal time for imaging is long (17-24 hr after injection), and the technique is not always readily available.

Several techniques exist for labeling leukocytes with technetium-99m (99mTc), an inexpensive and readily available isotope.[71] In this technique, whole blood is withdrawn, and neutrophils and macrophages are labeled with 99mTc, which is administered intravenously. Then, nuclear images of the abdomen and pelvis are obtained serially over 4 hours. Localized uptake of tracer material in the RLQ suggests appendiceal inflammation.

One study used 99mTc albumin colloid–labeled leukocytes for scanning and reported the sensitivity for this technique to be 89%.[72]

Another study used Tc-99m-hexamethylpropyleneamine oxime–labeled leukocytes to scan for acute appendicitis and reported the sensitivity to be 81% and the overall accuracy to be 89%.

Technetium-99m human immunoglobulin uptake has also been suggested as an accurate and safe method in the detection of acute appendicitis (see the image below).[73]

Scintigraphic techniques are usually completed within 5 hours, a time period that can delay operative treatment longer than the delay occasioned by the use of US and CT scanning.