Applied Radiology

Dr. Kurtz received his MD from Jefferson Medical College, Philadelphia, PA, in 1998. In 1999, he completed an internship in Internal Medicine and is now a third-year Diagnostic Radiology Resident at Yale­New Haven Hospital, New Haven, CT. In 2003 he plans to begin an MRI fellowship at Jefferson University Hospital, Philadelphia, PA.

Computed tomography (CT) has become a critical tool in the diagnostic algorithm for acute appendicitis. For the clinician, history and physical examination alone are frequently not accurate enough to evaluate diseases such as appendicitis, where emergent decisions must be made about patient treatment. Compared with clinical assessment alone, CT increases the sensitivity, specificity, and accuracy of diagnosing acute appendicitis. Radiologists are constantly seeking ways to further optimize CT protocols and improve diagnostic accuracy. CT has dramatically decreased the morbidity, mortality, and healthcare costs associated with acute appendicitis, and will likely maintain its role as an important tool for diagnosing acute appendicitis in the forseeable future.

Appendicitis extracts a heavy burden in the United States, in terms of patient morbidity and mortality, as well as in hospital-based resources, contributing to as many as 250,000 cases and 1 million hospital inpatient days a year. ¹ These cases may be difficult to diagnose on clinical exam, and they place a tremendous burden on U.S. hospital-based resources. Computed tomography (CT) is an important diagnostic tool for appendicitis that can decrease patient morbidity and healthcare costs.

The diagnostic algorithm for appendicitis

When should the clinician suspect appendicitis? In a meta-analysis of 10 studies involving 4000 patients, the three most prominent clinical findings that indicate a high probability of appendicitis are right lower quadrant pain, rigidity of the lower abdominal wall, and migration of pain from a generalized location in the abdomen to the right lower quadrant. Other signs and symptoms are pain before vomiting, psoas muscle irritation, fever, rebound tenderness, guarding, rectal tenderness, anorexia, nausea, and vomiting. ²

Traditionally, history and physical examination have been the most commonly used tools in developing a differential diagnosis of appendicitis. Unfortunately, these methods are often not sensitive or specific enough. ³ In fact, missed appendicitis based on only history and physical examination has become one of the most successful claims of malpractice against emergency physicians. ⁴ Thus, the clinician must choose the best method to further evaluate patients with suspected appendicitis. The choices at the physician's disposal at this juncture are observation, imaging, or surgery.

In the current U.S. healthcare system, observing a patient with suspected appendicitis as an inpatient has become cost prohibitive and is done only on a highly selective basis. In addition, the delay can be deadly for a patient with acute appendicitis. It has been well established that the longer the delay to diagnosis, the higher the risk of appendiceal perforation, which increases the risk of postoperative complications (figure 1). ⁵

Open (surgical) appendectomy and laparoscopy have been shown to have high negative appendectomy rates when appendicitis is diagnosed on clinical grounds alone. In a literature review from 1978 to 1988, negative appendectomy rates ranged from 15% to 22% in studies designed to compare open appendectomy with laparoscopy. ⁶

Despite its associated morbidity, in the past, exploratory laparotomy could be justified because it provided prompt therapy, mitigating against perforation and its associated risks. Today, few cases of clinically suspected appendicitis go directly to surgery without being evaluated by diagnostic imaging. After the history and physical examination have been performed, diagnostic imaging is frequently the next step in the diagnosis of clinically suspected appendicitis.

Two imaging modalities predominate in the diagnosis of appendicitis. Ultrasound, with graded compression of the right lower quadrant, is often utilized for diagnosing appendicitis in the pediatric and pregnant population. CT is used commonly in adults. One of the benefits of ultrasound is its greater availability, and possible decreased time to patient imaging. Another is its reasonably high accuracy.

A study in the pediatric surgery literature examined the results of 2056 ultrasound examinations performed by radiologists on children with suspected appendicitis. Statistical analysis revealed a sensitivity of 89%, a specificity of 95%, a positive predictive value of 86%, and a negative predictive value of 96%. An alternate ultrasound diagnosis was made in 157 children. ⁷

Some of the benefits of ultrasound over CT have been called into question. One study indicated that when compared with children who underwent ultrasound, children who underwent CT had a significantly lower negative appendectomy rate while maintaining a statistically equivalent perforation rate. ⁸ Those who favor CT over ultrasound claim it has higher diagnostic accuracy. One study compared 76 children in whom a noncontrast, focused CT scan of the abdomen was performed with 86 children who underwent graded compression ultrasound for suspected appendicitis. Sensitivity, specificity, and accuracy for CT were 97%, 100%, 99%, respectively, and for ultrasound were 100%, 88%, and 91%. ⁹ However, the lack of radiation and fairly good sensitivity and specificity of ultrasound have made it the more common initial imaging modality for children and pregnant women. Radiation concerns with CT in the pediatric age group have been much publicized in current literature. ¹⁰ Also, ultrasound requires no intravenous (IV) contrast, and is an efficient and relatively inexpensive technique. At our institution, ultrasound is the initial imaging modality of choice for the pediatric and pregnant population.

Technical advancements have made CT the preferred study in cases of clinically suspected appendicitis in adults. High-resolution images and shorter scanning times provided by helical CT equipment are important contributors to the modality's effectiveness in the diagnosis of appendicitis. Modern multidetector helical CT systems can perform a standard CT of the abdomen and pelvis in <10 minutes of table time. ¹¹

Techniques

A variety of helical CT protocols are currently utilized for optimal evaluation of appendicitis. Some protocols include the use of oral or rectal contrast agents. Some image the entire abdomen and pelvis, others image only the inferior abdomen and pelvis, and still others image only a focused area covering the right lower quadrant. Some include IV contrast, others do not. Many protocols require a collimation of ¾ 5 mm or less, particularly through the appendiceal region. A 5-mm slice thickness has proven to significantly decrease false negative evaluations when compared with 10-mm images. ¹²

The ability of modern helical CT to acquire volumetric data has greatly aided radiological diagnosis of many diseases, including appendicitis. If the radiologist cannot adequately visualize the appendix with 5-mm images, the stored volumetric data may serve as a convenient diagnostic aid. The radiologist can request image reconstruction in the area of the appendix at less than 5-mm intervals using this stored data. ¹³ With multidetector row helical CT, overlapping images that are thinner than those reviewed initially may be reconstructed (figure 2). These methods may reveal an appendix previously "hidden" in the computer's data banks.

The protocol used at our institution includes imaging the entire abdomen and pelvis using a multidetector helical CT scanner. Using a detector group thickness of 2.5 mm, 5-mm thick sections are reconstructed every 5 mm from data obtained with a beam pitch of 1.5 [detector pitch of 6, table feed per rotation of 15 mm]. Nonionic IV contrast and oral contrast are given to the patient. Nonionic contrast material (300 mgI/mL) administered at 3.0 mL/sec and scanning is begun 60 sec after initiation of the contrast bolus. Our oral contrast solutions consist of 900 mL of 2% diatrizoate sodium (Hypaque, Amersham Health, Princeton, NJ) or 30 mL of diatrizoate meglumine (Gastrografin, Bracco Diagnostics, Inc., Princeton, NJ) in 900 mL of water. If contrast is administered rectally, it is important to elicit any contraindications to this method, such as toxic megacolon, neutropenic colitis, or ischemic colitis.

After drinking the oral contrast, the patient waits approximately 60 to 90 minutes for the contrast to arrive at the cecum. The patient is then put on the scanner and a scout film is obtained. The radiologist can then examine the scout images and localize the oral contrast. If the distal portion of the oral contrast is still in the small bowel, then the study may be delayed for a short while until the cecum is contrast-filled. Oral contrast can be an important component of CT protocols, because nonfilling of the appendix with contrast is one important sign of appendicitis (figure 3). A recent Australian study examined 100 patients with clinically equivocal cases of appendicitis. Using focused appendiceal CT and no IV contrast, they obtained a 93% sensitivity, a 97% specificity, and a 96% accuracy. ¹⁴

Other studies have stressed the importance of IV contrast for an accurate diagnostic evaluation of appendicitis. A recent article compared focused, right lower quadrant helical CT technique with oral contrast only to a nonfocused CT of the abdomen and pelvis with both oral and IV contrast material. In this study, the accuracy for diagnosing appendicitis increased significantly with use of both IV and oral contrast agents. The three radiologists who read the nonfocused, enhanced CTs also reported greater confidence in diagnosing alternative conditions. Of the greatest concern was the fact that when using the focused CTs the diagnosis of appendicitis was missed entirely in two patients whose inflamed appendices were not included in the images. ¹⁵

The clinician's perspective

Although the optimal protocol remains debatable, the use of helical CT in the clinician's diagnostic algorithm for acute appendicitis has become commonplace. Recent articles in the emergency and surgical literature have supported the use of CT for this purpose.

A study based in Taiwan compared the abilities of both surgically trained and non-surgically trained residents to diagnose appendicitis in the Emergency Department. ¹⁶ Under the supervision of board-certified emergency physicians, there was no significant difference between surgically based and non-surgically based residents in their ability to make the diagnosis of appendicitis. However, non-surgically based residents ordered more CT studies (12% versus 5.1%). Significantly, CT was found to lower the negative appendectomy rate from 23% to12%. ¹⁶

Another study in the emergency literature examined the effectiveness of diagnosing appendicitis clinically. This study compared the accuracy of clinical assessment alone with clinical assessment combined with CT imaging in a patient with clinically suspected appendicitis. Clinical assessment alone, based on specific criteria, had a sensitivity of 91.6% and a specificity of 84.7%. With the assistance of CT scanning, sensitivity and specificity increased to 98.3% and 95.8%, respectively. ³

The surgical literature also has significant data supporting the use of CT for appendicitis. In one study, surgically trained physicians gave 99 patients with clinically suspected appendicitis a physical examination before and after CT scan. ¹⁷ Diagnostic accuracy and management of acute appendicitis was significantly improved when the results of the first physical exam were compared with second physical exam after CT. Patients were advanced from observation status to either operative appendectomy or discharge more efficiently. In this study, the most convincing evidence for the utility of CT in the diagnosis of appendicitis comes from the 18 patients who were diagnosed correctly on CT findings alone, and 6 patients who were spared from surgery following the CT scan and second examination. ¹⁷

Benefits for the healthcare system

The increased efficiency of helical CT and the significant decreases in healthcare spending associated with its use are two important factors in its acceptance in diagnosing clinically suspected appendicitis. One study explored the utility of focused appendiceal CT with rectal contrast in 100 consecutive emergency department patients. ¹⁸ Prior to CT imaging, history, physical exam, and laboratory results were used to determine if patients were to be observed for suspected appendicitis or sent to the operating room for surgical appendectomy. Following CT studies, the preliminary treatment plans were compared with the patient's eventual real treatment. The appendiceal CT was 98% accurate, with treatment plans changing in 59 patients based on the CT results.

The financial savings related to the use of CT in this study were significant. A total of 13 patients were spared needless appendectomy, for a savings of $47,281. In addition, 50 hospital inpatients days were avoided, for further savings of $20,250. After subtracting the $22,800 expenditure for the appendiceal CT scans, total savings for the 100 patients was $44,731. If 250,000 cases of appendicitis occur per year, ¹ the use of appendiceal CT in cases of clinically suspected appendicitis could save the U.S. economy $111,827,500 annually in hospital-based resources. ¹⁸

Image interpretation

One limitation to the above-mentioned study is that an academic emergency department radiologist interpreted all the appendiceal CT scans. Can emergency department physicians, surgeons, and other clinicians expect the same accuracy if helical CT studies are read by community radiologists? The following study addressed this concern. Radiologists working in a community hospital in Hawaii read 100 CT studies. ¹⁹ Significantly, none of the radiologists had prior experience reading appendiceal CT scans. Focused appendiceal CT scans were obtained with enteric contrast for appendicitis. A total of 33 CT scans were evaluated as positive for appendicitis and 67 were considered negative. Statistical analysis revealed a sensitivity of 97%, a specificity of 94%, and an accuracy of 95%. ¹⁹ Thus, excellent diagnostic performance can be expected in the university or community hospital setting when CT is performed for the diagnosis of appendicitis.

Significantly, in this community hospital study, CT led to a different diagnosis in 36 patients. The high-resolution images that modern CT equipment provides permit excellent anatomic and pathologic evaluation of the abdomen and pelvis. Many alternative diagnoses have been made at our institution in the month prior to submission of this article as well. These diagnoses have included sigmoid diverticulitis, tubo-ovarian abscess, and right ureterovesicular junction calculus with associated forniceal rupture. Mesenteric adenitis and terminal ileitis are two of the many other common conditions that can be indistinguishable clinically from appendicitis, but are readily diagnosable by CT.

Appendicitis and its mimics

To adequately understand how to characterize an inflamed appendix by its appearance on CT, it is first important to understand the features of a normal appendix. The vermiform appendix (L. vermis , worm, and forma , form) averages 8 cm in length and no more than 6 mm in width. Superior to the appendix, the ileum partly invaginates into the cecum, forming the ileocecal valve. Two to three centimeters inferior to the ileocecal valve, the three teniae coli of the cecum merge at the base of the appendix and form its outer longitudinal muscle coat. Two thirds of the time, the body of the appendix is in a retrocecal position (figure 4). When retrocecal, the appendix may be intraperitoneal or extraperitoneal. ²⁰ When extraperitoneal, the appendix can be seen at or near the convergence of the perinephric and lateral conal fascia. The appendiceal mesentery, or mesoappendix, suspends the appendix from the mesentery of the terminal ileum. The lymphatic vessels of the cecum and appendix drain into mesoappendiceal, ileocolic, and superior mesenteric lymph nodes. Cases of appendicitis are occasionally associated with small lymph nodes in these regions (figure 5). ²¹

Appendicitis is most often caused by fecal obstruction of the appendix. Normally, appendiceal secretions build up and decompress into the cecum. In cases of obstruction, these secretions cannot escape and the appendix becomes swollen, edematous, and inflamed (figure 6). This inflammation and swelling causes the pain of acute appendicitis. ²²

What are the signs of acute appendicitis by helical CT? One study examined nonenhanced CT scans in 120 consecutive patients with acute appendicitis in the differential diagnosis. In this population, the most significant signs of appendicitis were fat stranding (100%), enlarged appendix (>6 mm) (97%), adenopathy (63%), appendicoliths (43%), abscess (10%), and phlegmon (5%) (figures 3, 5, and 7). ²³ Enhancement of a thickened appendiceal wall is another important sign of acute appendicitis when IV contrast is administered (figures 2B, 4A, 6B, and 7). ⁹ Nonfilling of the inflamed appendix with oral contrast is also a useful sign. Cecal wall thickening adjacent to the inflamed appendix may be seen (figure 7). Other important CT signs are present when appendicitis is complicated by perforation. These signs are periappendiceal abscess, phlegmon, free intraperitoneal fluid, free air, and inflammatory changes of adjacent bowel (figures 1B and 8).

Optimization of technique is essential to identify the CT signs of acute appendicitis. If oral contrast does not opacify the terminal ileum or cecum adequately, a false-positive result may ensue. A nonopacified, thickened appendix often indicates appendicitis and is difficult to differentiate from nonopacified loops of distal and terminal ileum (figure 3). Without IV contrast, mesenteric vessels in the lower quadrant may be confused with mesenteric adenopathy, especially if these vessels cannot be adequately followed to their origin. ¹⁵ The radiologist must also determine if the inflamed appendix has been included in the images when using the focused right lower quadrant technique. A false-negative result may be reported without this determination.

Appendicitis should be diagnosed definitively only if the radiologist sees an abnormal appendix, or a calcified appendicolith associated with pericecal fat stranding or fluid collections. ¹³ Without either of these two signs present, the reader can include appendicitis as only one of many differential possibilities. Certain pathological processes may mimic appendicitis by causing thickening of the appendix itself. Some of these processes include appendiceal or cecal tumors, mucoceles, and endometriosis. Appendiceal tumors, such as carcinoid, and cecal tumors can directly obstruct the appendix, causing a secondary appendicitis. Mucin secreting tumors of the appendix can also cause obstruction and bloating of the mucin-filled appendix, or mucocele. Mucoceles are not usually associated with an appendicolith or pericecal inflammation. Endometriosis may cause luminal fibrosis and obstruction, leading to a picture that may mimic a mucocele. ^13,24

Inflammatory changes in the pericecal fat can be found with common mimickers of appendicitis such as cecal diverticulitis, epiploic appendagitis, mesenteric adenitis, pelvic inflammatory disease, transmural ileitis, and perforation of cecal carcinoma. These processes are often indistinguishable from acute appendicitis if a normal appendix is not visualized. Cecal diverticulitis may be diagnosed if discrete cecal diverticula or an intramural abscess with adjacent inflammation is present. ²⁵ Epiploic appendagitis results from torsion of an epiploic appendage, resulting in inflammation and abdominal pain. In a series of five patients with epiploic appendagitis, all had CT findings of a pericolic fatty mass with increased attenuation compared with normal mesenteric fat. In each case, the fatty mass had a high attenuation rim with focal fatty stranding. ²⁶

Mesenteric adenitis is another important clinical mimicker of appendicitis. In one study, a series of 18 CT scans among patients with a discharge diagnosis of mesenteric adenitis were reviewed. All 18 had a cluster of nodes in the right lower quadrant measuring a minimum of 5 mm in short axis, associated with a normal appendix. Eight had associated ileal or ileocecal wall thickening. ²⁷ An infectious, inflammatory, or ischemic process in adjacent bowel may also secondarily inflame the pericecal fat and the appendix itself.

Conclusion

Helical CT has become a critical diagnostic tool in the evaluation of the adult patient with clinically suspected appendicitis. Many current techniques and protocols have an impressive diagnostic performance record in identifying appendicitis. CT has also proven to be a cost-effective method of diagnosing appendicitis and its clinical mimics. The excellent sensitivity and specificity of CT for appendicitis have many direct benefits to the patient. When a CT study is interpreted as negative, it spares patients the unnecessary morbidity of a laparoscopic or open surgical procedure. When a study is positive for appendicitis, the patient most often undergoes appropriate surgery.

Acknowledgement

The author wishes to thank James A. Brink, MD, from Yale­New Haven Hospital, for his thoughtful input, insight, and assistance with this manuscript.