To describe the MR appearance of the normal appendix and the MR imaging characteristics of acute appendicitis with correlation to pathological severity.
To describe the MR appearance of the normal appendix and the MR imaging characteristics of acute appendicitis with correlation to pathological severity.
A total of 20 volunteers participated in this study to demonstrate normal appendices by MR imaging. A total of 37 consecutive patients with clinically diagnosed acute appendicitis were also scanned. T1-weighted (T1WI) spin-echo images, T2-weighted (T2WI) fast spin-echo, and fat-suppressed spectral presaturation inversion recovery T2-weighted (T2SPIR) fast spin-echo images were obtained. The MR criteria for considering acute appendicitis were as follows: 1) thickening of the appendiceal wall with high intensity on T2WI or T2SPIR; 2) dilated lumen filled with high intensity material on T2WI or T2SPIR; and 3) increased intensity of periappendiceal tissue on T2WI or T2SPIR.
The visibility of a normal appendix on MR imaging was 90% (18/20). It appeared as a cord-like structure of medium intensity without fluid collection in the lumen. A total of 30 cases with clinically diagnosed acute appendicitis had positive MR findings and all except one were pathologically proven. The one had cecal diverticulitis. These cases demonstrated filled lumen, with a hypointense wall on T1WI and slightly hyperintense on T2WI or T2SPIR. MR findings correlated well with pathological severity, especially a thicker wall, periappendiceal high intensity, and ascites were useful for suspecting severe appendicitis.
Correct diagnosis of acute appendicitis was obtained with MRI, and correlated well with its pathological severity. MRI is a powerful alternative for diagnosing acute appendicitis especially for the patients in whom the radiation is major concern. J. Magn. Reson. Imaging 2005;21:156–165. © 2005 Wiley-Liss, Inc.
ACUTE APPENDICITIS is one of the most common abdominal diseases and presents as an acute abdomen. The diagnosis is usually based on clinical and laboratory findings, however, approximately one-third of patients with acute appendicitis present with atypical clinical symptoms. This confusion leads to a high prevalence of unnecessary laparotomy, reported at 6% to 16% (1–5). The other cases demonstrate atypical and nonspecific symptoms shared with other abdominal diseases including cholecystitis, gastritis, enterocolitis, diverticulitis, mesenteric adenitis, gynecological disease (e.g., pelvic inflammatory disease [PID], tubo-ovarian abscess, ovarian cyst or torsion, endometriosis, degenerating uterine leiomyoma), and urological disorders (6–12). In such clinically questionable cases, it is not uncommon that radiological evaluations provide useful information in making a differential diagnosis. An appropriate imaging procedure can prevent surgery delays and minimize the rate of negative laparotomies. Currently, the imaging modalities chosen to demonstrate an inflamed appendix, as well as its complications, are ultrasound (US) and computed tomography (CT). However, MR has not been applied to acute appendicitis.
The first objective of this study was to describe the MR appearances of the normal appendix using asymptomatic volunteers. The second objective was to describe the appearances of unenhanced MR imaging in cases with acute appendicitis. A correlative study between the MR imaging and pathological severity was also undertaken.
A total of 20 normal volunteers (13 male, 7 female, 25–75 years old, mean age 45 years) participated in this study for the evaluation of normal appendices by MRI. All the participants were free of any abdominal symptoms at their MR examinations. All the participants were informed of the purpose of this study and details of the methods prior to giving their written consent.
MR imaging was performed using a 0.5-T superconductive magnet MR imaging unit (Gyroscan, T5/NT; Philips, Best, the Netherlands) with a phased array coil. Cases were acquired with T1-weighted spin-echo imaging (T1WI) (TR/TE = 595/15 msec; NEX = 2), T2-weighted fast spin-echo imaging (T2WI) (TR/TE = 3000/110 msec, NEX = 4) and fat-suppressed spectral presaturation inversion recovery T2-weighted fast spin-echo imaging (T2SPIR) (TR/TE = 3100/100 msec, NEX = 6) in the axial and coronal directions, with 5-mm slice thickness and 0.5-mm intersection gap. The imaging matrix size was 230 × 512, and the field of view (FOV) was 28 × 28 cm to 36 × 36 cm. The axial scans covered the area of the lower abdomen and pelvis. Total imaging time was approximately 20 minutes.
All scans were reviewed by three experienced radiologists (N.N., M.M., and M.T.) who reached a consensus on visualization of normal appendices, as well as their imaging characteristics. The position of the appendix was evaluated and classified as: 1) retrocecal; 2) pointed over brim of pelvis; 3) anterior to ileum; and 4) posterior to ileum (13).
A total of 37 patients (18 male, 19 female, 16–69 years old, mean age 37.1 years) with clinically diagnosed acute appendicitis underwent MR imaging. These clinical diagnoses were established on the basis of a combination of subjective symptoms, findings on physical examination, and the results of laboratory tests. Other imaging procedures, including US or CT, were not intentionally undertaken in this study, except for the cases whose MR findings were negative for acute appendicitis. They underwent CT or US to confirm the negative diagnosis of acute appendicitis. Informed consent was obtained from all the patients before the MR examination. Participants were informed of the purpose of this study and allowed a MR study to be performed instead of sonography or CT. The imaging parameters were the same as in the volunteers study.
The MR criteria for considering acute appendicitis were as follows: 1) thickening of the appendiceal wall with high intensity on T2WI or T2SPIR; 2) dilated lumen filled with high intensity material on T2WI or T2SPIR; and 3) increased intensity of periappendiceal tissue on T2WI or T2SPIR. A fluid collection or abscess formation may also exist. Partial obliteration of the appendiceal structure was also considered to be a MR finding indicating a severe case of acute appendicitis, such as the gangrenous type. Intensity of the luminal fluid on T1WI, T2WI, and T2SPIR was also evaluated. The position of the appendix was classified as in the normal volunteer study. All scans were retrospectively reviewed by three experienced radiologists (N.N., M.M., and M.T.). Readers were informed that all 37 patients had symptoms suspicious for acute appendicitis. They were unaware of the final diagnosis of these patients; however, they were informed that some patients had other diseases other than appendicitis. They reached a consensus on the imaging characteristics of appendices and periappendiceal structures and finally on MR interpretation regarding whether acute appendicitis was suspected or not. These MR diagnoses were compared with the diagnosis confirmed by surgery, pathology and clinical follow-up.
Normal appendices were demonstrated in 18 of 20 volunteers (90%). The two missed cases had little intraperitoneal fatty tissue. The normal appendix appeared as a cord-like structure with intensity similar to muscle on all sequences and did not demonstrate a lumen. The positions of appendices were classified into: 1) retrocecal: six cases (33%); 2) pointed over brim of pelvis: two cases (11%); 3) downward to right: four cases (22%); 4) anterior to ileum: three cases (17%); and 5) posterior to ileum: three cases (17%) (Fig. 1). The diameter of the normal appendices ranged from 1 to 6 mm, with a mean value of 2.94 mm. The length ranged from 5 to 8 cm, with a mean value of 7.81 cm.
In 30 of the 37 cases that were clinically suspected to have acute appendicitis, MR demonstrated the abnormal findings consistent with the MR criteria for diagnosing acute appendicitis, and the patients underwent surgery. The final surgical and histopathological diagnosis was 29 acute appendicitis and one cecal diverticulitis. The 29 cases of appendicitis were histopathologically classified into catarrhal appendicitis in seven (Fig. 2), phlegmonous appendicitis in 15 (Fig. 3), and gangrenous appendicitis in seven (Fig. 4). The catarrhal type is an early form of acute appendicitis showing scant neutrophilic infiltrates and vascular congestion. The phlegmonous type is characterized by prominent neutrophilic infiltrates, fibrinopurulent exudate, ulceration, and necrosis. The gangrenous type shows extensive ulceration, necrosis, and gangrene. Figure 5 shows the clinical course of the patients. Table 1 summarizes the MR findings of surgically proven cases with regard to the histopathological classifications. The thickness of the wall was less than 2 mm in six of seven cases (85.7%) in catarrhal appendicitis, whereas it was thicker than 2 mm in 11 of 15 cases (73.3%) in phlegmonous, and six of seven cases (85.7%) in gangrenous appendicitis. The high intensity change of the wall either on T2WI or T2SPIR was observed in 23 of 29 cases (79.3%) of acute appendicitis; two of seven cases (28.3%) in catarrhal appendicitis, 14 of 15 cases (92.3%) of phlegmonous appendicitis, and all cases (100%) of gangrenous appendicitis. The fluid-filled appendiceal lumen on T2WI or T2SPIR was observed in all cases of acute appendicitis. In some cases, the fluid appeared as slight high intensity on T1WI. Table 2 shows the pattern of the luminal fluid intensity with correlation to the severity of appendicitis. If compared with the intensity of the trunk muscle, 93.1% (27/29) of the fluid showed slightly high or isointensity on T1WI. A low intensity pattern was observed only in 6.9% (2/29) of cases. Periappendiceal high intensity on T2WI or T2SPIR was observed in 24 of 29 cases; in two of seven cases (28.6%) in catarrhal appendicitis; and in all of the phlegmonous and gangrenous cases. In catarrhal appendicitis, periappendiceal high intensity was observed only with T2SPIR images. In the phlegmonous type, all cases showed periappendiceal high intensity on T2SPIR, except for one case in which high intensity could be observed both with T2SPIR and T2WI. In all the gangrenous cases, periappendiceal high intensity was observed both with T2SPIR and T2WI. Ascites was observed in two of seven (28.6%) cases of catarrhal, and all cases of phlegmonous and gangrenous type. In one of 37 patients, cecal diverticulitis with a normal appendix was correctly diagnosed on MR imaging (Fig. 6). This patient had an operation to drain the pus. In this case, the MR appearance of the appendix was normal on all sequences. Pericecal high intensity both on T2SPIR and T2WI was observed. Surgery revealed a severe diverticulitis of the cecum. In one of the 30 operated patients, cecal diverticulitis was incorrectly diagnosed as appendicitis. In this case, the structure of the appendix was not demonstrated on any sequence of MR imaging due to little intraperitoneal fatty tissue. Pericecal high intensity both on T2SPIR and T2WI was observed. Surgery showed severe diverticulitis of the cecum. Six of 37 patients showed negative MR findings for appendicitis and were observed without surgery. All six cases subsequently underwent CT or US to confirm the negative diagnosis of appendicitis. The imaging findings and clinical follow up revealed that the six cases were consistent with acute enteritis. Follow-up had no relapses of symptoms suggesting acute appendicitis.
|Histopathological classifications||MR Findings (%)|
|Thickeness of wall||High intensity of the wall fluid filled lumen||Periappendiceal high intensity||Ascites|
|2 mm>||2 mm<||T2WI||T2SPIR||T2WI||T2SPIR|
|Catarrhal (7)||85.7% (6/7)||14.3% (1/7)||28.3% (2/7)||100% (7/7)||0% (0/7)||28.6% (2/7)||28.6% (2/7)|
|Phlegmonous (15)||26.7% (4/15)||73.3% (11/15)||100% (15/15)||100% (15/15)||60.0% (9/15)||100% (15/15)||80.0% (12/15)|
|Gangrenous (7)||14.3% (1/7)||85.7% (6/7)||57.1% (4/7)||100% (7/7)||100% (7/7)||100% (7/7)||100% (7/7)|
|Total (29)||37.9% (11/29)||62.1% (18/29)||72.4% (21/29)||100% (29/29)||55.2% (16/29)||82.8% (24/29)||72.4% (21/29)|
|Catarrhal appendicitis: 7 cases|
|Plegmonous appendicitis: 15 cases|
|Gangrenous appendicitis: 7 cases|
In the cases that underwent surgery, there was 100% compatibility between the MR findings and actual position of the appendix—ascending retrocecal appendix: 12 cases (37.5%); pointed over brim of pelvis: 14 cases (43.7%); anterior to ileum: three cases (9.4%); and posterior to ileum: three cases (9.4%).
Acute appendicitis is one of the most commonly encountered emergency diseases that need urgent surgical intervention (6–12). Overall mortality is reported as 0.01%; however, it rises to 5.5% for perforated cases, especially in the elderly (14–18). The mortality risk also depends on the pathological severity (6–12). A wound infection is one of the most common postoperative complications and relates to the postoperative mortality. Wound infection is markedly increased in cases of perforation or gangrenous appendicitis. Delayed appendectomies of greater than 72 hours from presentation evoke high incidentally serious effects, including perforation (90%), as well as other major complications (60%) (19). It is clear that a delay in diagnosis is directly related to increased mortality (6–12, 14–19)
The imaging modalities generally considered to be useful in diagnosing acute appendicitis are US and CT. High-resolution US performed with the graded compression technique introduced by Puylaert (20) in 1986 has proved to be of clinical value in diagnosing questionable cases of acute appendicitis. The inflamed appendix is a noncompressible, tubular structure that shows no peristalsis in the longitudinal view. A transverse diameter of more than 6 mm is a reproducible index suggesting appendicitis (21, 22). The reported sensitivity of US in diagnosing acute appendicitis ranges from 75% to 90%, the specificity and accuracy are greater than 90% (20, 22–25). Abu-Yousef et al (23) reported a specificity of 95%, a sensitivity of 80%, and an accuracy of 90% in 68 consecutive cases. Jeffrey et al (22) reported 89.9% sensitivity, 96.2% specificity, and accuracy of 93.9% in 245 consecutive cases. There are several disadvantages of using US to diagnose acute appendicitis. Its diagnostic accuracy depends on the examiner's skill and experience. The graded compression technique requires a high level of technical expertise and experience. The retrocecal appendix is hard to visualize by US. Approximately 30% of cases missed on US examination are attributed to the appendix being in the retrocecal position (22–25). The appendix in the obese patient is difficult to demonstrate by US. In addition, gaseous distension of the bowel loop in paralytic or obstructive conditions hinders the visualization of the appendix. Widespread inflammation is also not accurately demonstrated by US.
CT is also believed to be an accurate diagnostic modality for diagnosing acute appendicitis. According to previous reports, its sensitivity and specificity are over 80% even with patients complaining of atypical symptoms (21, 26–29). Reported accuracy is 93% to 94%, sensitivity is 87% to 98%, and specificity is 83% to 97% (21, 26–29). Some investigators have reported CT to be even more accurate than US (21). Balthazar et al (21) prospectively studied, with CT and US, 100 consecutive patients suspected of having appendicitis. Analysis of the data for CT and US revealed sensitivity of 96% vs. 76%, specificity of 89% vs. 91%, and accuracy of 94% vs. 83%, respectively. The advantages of CT over US in diagnosing appendicitis are as follows: the periappendiceal spread of inflammation is correctly identified by detecting increased fatty density, linear strands, or fluid collection. Complicated appendicitis such as abscess formation or severe phlegmonous change around the cecal region is easily demonstrated, as is extrapelvic extension of the inflammation (21, 27). CT can also easily assess the remainder of the abdomen and pelvis, allowing nonappendiceal diseases with the same symptoms including cholecystitis, pancreatitis, gynecological disease, and urological disease to be diagnosed (21, 27). Widespread availability both geographically and temporally is an important advantage of CT over MRI.
A significant disadvantage of CT is radiation exposure, especially since the peak incidence of acute appendicitis is between the late teenage years and early twenties, when pregnancy can be an issue (21, 27).
MRI is not listed as a commonly used diagnostic tool for acute appendicitis. This is attributed to the following facts: MR was a time-consuming procedure not suitable for emergent clinical situations; motion artifact is another drawback since the severe pain of acute appendicitis makes it difficult for patients to remain immobile; and the spatial resolution of MR is not comparable to CT or US. During the last decade, however, there has been marked development of MR imaging both in the hardware and software. Image quality has improved and scan time has shortened dramatically. Additionally, MR inherently has an excellent contrast resolution, which enables recognition of the inflammatory process more accurately than the other imaging modalities. Another advantage of MRI is the avoidance of ionizing radiation.
The aim of this study was to describe the MR appearances of acute appendicitis with correlation to postoperative histological results. Prior to these studies, MRI was used to evaluate the appendix in normal volunteers. Detection of the normal appendix was possible in 18 of 20 patients (90%) and was consistently demonstrated to the coronal section, both on the T1WI and T2WI images. Hormann et al (30) reported that the normal appendix was described by T2WI and T1WI, in 86% and 73% of children, respectively. On T2WI, the normal appendix had a hyperintense center and a hypointense wall, and was mostly hypointense on T1WI. Their result differs from this study's, in which the normal appendix was shown as a hypointense tubular structure both in T1WI and T2WI. The appendix has extremely rich lymphoid tissue in the mucosa and submucosa, which forms an entire layer of germinal follicles and lymphoid pulp in the young. With aging, this lymphoid tissue underlying the mucoepithelium and glands undergo progressive atrophy. The distal portion of the appendix sometimes shows fibrous obliteration in the elderly. The difference between the two studies is probably due to these morphological and pathological differences between children and adults. Incesu et al (31) reported the clinical usefulness of Gd-enhanced MRI in demonstrating acute appendicitis, and found that the normal appendix was not demonstrated on MRI even with Gd-contrast. This study showed that normal appendices were clearly demonstrated by MRI even without contrast. The ability to demonstrate suspected appendicitis as normal has great clinical significance in patient management, because it directly contributes to the acceptable negative predictive value. The patient avoids laparotomy for appendectomy when the imaging clearly demonstrates a normal appendix. In fact, this study showed that the negative predictive value of MR imaging in diagnosing acute appendicitis was 100%. In our study, the normal appendix was demonstrated to range from 1 to 6 mm in diameter. This value was consistent with the normal value derived from US studies. It has not been previously reported, and it should be stressed, that the normal appendix shows low intensity in any sequence and fluid collection was not demonstrated within the lumen.
In cases in which MRI failed to demonstrate normal appendices, the volume of the intraperitoneal fat, as well as periappendiceal fat was small. Accordingly, it is considered that the volume of fatty tissue surrounding the appendix may influence the visibility of normal appendix in MR imaging.
To our knowledge, there have been no reports about MR appearances of acute appendicitis regarding to its pathological severity.
Regarding correlation to pathological severity, the catarrhal type showed a mild thickening of the appendiceal wall and about 28% of the cases showed high intensity of the wall under T2WI or T2SPIR. The fluid accumulated in the appendiceal lumen appeared as slightly high intensity in 65.5% (12/29) and isointensity in 27.6% (8/29) on T1WI, if compared with the trunk muscle. Only 6.9% (2/29) showed low intensity consistent with the serous fluid. It was speculated that these intensity patterns on T1WI probably reflect the mucous nature of the luminal fluid in the cases with acute appendicitis. It is considered that this diagnostic information is specific for MR imaging, and can not be obtained from US or CT images. The fatty tissues surrounding the appendix showed slightly increased intensity; however, only on T2SPIR. A small amount of fluid collection around the appendix was also observed.
The phlegmonous type was appreciated as moderate to severe thickening of the wall. High intensity of the wall was observed in most cases under T2WI or T2SPIR. These findings were demonstrated along the entire length of the appendix and could be easily observed on the coronal images. Increased intensity of the fatty tissue around the appendix was observed on T2SPIR. One case, however, showed periappendiceal high intensity even without fat suppression. The rate of ascites increased to 80%.
In the gangrenous appendix, visualization of the entire length was difficult on coronal images, and partial obliteration of the wall was noted. A wide area of the surrounding fatty tissue showed marked high intensity both on T2WI and T2SPIR. This associated inflammatory change was also demonstrated on T1WI as an area of low intensity. The fluid collection was more prominent than with phlegmonous appendicitis (100%). Abscess formation was also observed in 5 of 29 cases, and were all classified as the gangrenous type. When the volume of fatty tissue is small, associated fatty changes were not well demonstrated and a distinction between phlegmonous and gangrenous appendicitis was difficult.
The MR findings were relatively well correlated with pathological severity of appendicitis. Especially, thicker appendiceal wall, periappendiceal high intensity on T2WI or T2SPIR and existence of ascites were thought to be good indicators of a more severe case of appendicitis. Among them, it was interesting to note that periappendiceal high intensity could be observed by T2SPIR even in the early stage of appendicitis. Although a correlative study with CT was not undertaken, T2WI with fat-suppressed technique may have an advantage over CT in demonstrating an early inflammatory process in the periappendiceal fatty tissue. With advanced disease, it was possible to detect periappendiceal abnormality not only by T2SPIR but also by T2WI. In advanced gangrenous stage, both imaging methods equally demonstrated the periappendiceal abnormality.
The patient diagnosed with cecal diverticulitis on MR was surgically proven. In this case, the appendix was positioned anterior to the ileum and the inflammation from diverticulitis did not extended to the appendix. MRI demonstrated a normal appearing appendix. Yet a case diagnosed as acute appendicitis on MR was surgically proven to be diverticulitis. On surgery, the appendix was positioned retrocecum and the inflammation from diverticulitis seemed to extend to the appendix. In this case, the structure of the appendix was not demonstrated on any MR images, probably due to little intraperitoneal fat. The amount of fatty tissue in the peritoneal space seems to be a key factor in diagnosing appendicitis using MR, and probably CT as well. Retrospective review of this case showed the main foci of inflammation localized within the colonic wall and both findings would have changed the diagnosis if they had been recognized.
In all cases diagnosed as surgically-revealed acute appendicitis, the positions of the appendices were correctly identified on MR imaging. This allows for surgeons to know the exact position of the appendix preoperatively. However, as mentioned above, visualization of the entire length of the appendix was difficult in cases of gangrenous appendicitis, and accurate positions were identified based upon other MR findings, including areas of high signal and fluid collection on fat-suppressed T2WI.
MRI correctly diagnosed the acute appendicitis, and correlated well with its pathological severity. MRI is a powerful alternative for diagnosing acute appendicitis, especially for the patients in whom the radiation is major concern.