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Magnetic Resonance of Acute Appendicitis: Pearls and Pitfalls
Magnetic Resonance of Acute Appendicitis: Pearls and Pitfalls Michael Lam, MD,a Ajay Singh, MD,a,b Rathachai Kaewlai, MD,b and Robert A. Novelline, MDb
Acute appendicitis is the most common surgical abdominal emergency. Although the clinical diagnosis can be made accurately in typical cases, imaging plays an important role in improving diagnostic accuracy of this condition, especially when the clinical diagnosis is uncertain. Magnetic resonance imaging is an emerging promising technique for the diagnosis of acute appendicitis, especially in patients with nondiagnostic ultrasound and in patients where radiation is a clinical concern. In the following review, the role of magnetic resonance in the diagnosis of appendicitis will be discussed.
Acute appendicitis is the most common indication for emergency abdominal surgery in the United States.1 There are approximately 250,000 cases of appendicitis annually in United States, accounting for 1 million days per year spent in a hospital.2 The clinical accuracy for diagnosing appendicitis is between 65 and 82%, which is less than optimal considering the increased morbidity and mortality in complicated cases.1,3 Imaging has played an important role in improving the diagnostic accuracy and reducing unnecessary surgeries. Currently in most institutions, computed tomography (CT) is the standard investigation in adults, while ultrasound (US) is the modality of choice in children and pregnant women. With advancements and wider availability, magnetic resonance (MR) imaging has emerged as a promising alternative modality for the diagnosis of this condition.
From the aDepartment of Radiology, University of Massachusetts Memorial Medical Center, Worcester, MA; and bDepartment of Radiology, Massachusetts General Hospital, Boston, MA. Reprint requests: Ajay Singh, MD, 10 Museum Way, #524, Boston, MA 02141. E-mail: [email protected]. Curr Probl Diagn Radiol 2008;37:57-66. © 2008 Mosby, Inc. All rights reserved. 0363-0188/2008/$34.00 ⫹ 0 doi:10.1067/j.cpradiol.2007.10.002
Curr Probl Diagn Radiol, March/April 2008
Pathogenesis The inflammation of appendix is commonly caused by obstruction of the appendiceal lumen by a fecalith or lymphoid hyperplasia. Less common causes of acute appendicitis include appendiceal obstruction from a foreign body, parasites, or a neoplasm. The natural history of untreated appendicitis is usually appendiceal perforation or abscess formation. Spontaneous resolution occurs in a minority of cases, of which 38% have recurrent appendicitis, often within the first year.4 The histological changes associated with appendicitis depend on the duration and severity of the disease. They range from minimal inflammation to marked necrosis with complete mural destruction and rupture.5
Clinical Presentation Classically, patients with acute appendicitis present with an initial vague periumbilical pain progressing to the right lower quadrant, fever, nausea, and leukocytosis. Other signs and symptoms such as vomiting, the Psoas sign, the Obturator sign, and the Rovsing sign may be detected. Given the classic presentation in young, previously healthy individuals, additional imaging testing may be unnecessary. Uncertainty in diagnosis occurs when the signs and symptoms are not typical for appendicitis. Variations in the location of appendix result in wide variations in the sites of pain, when the appendix is inflamed. Retrocecal appendicitis produces vague and poorly localized pain, while pelvic appendicitis causes midline or left-sided abdominal pain. Appendicitis extending to the urinary bladder or ureter may cause voiding symptoms, hematuria, or pyuria. Misdiagnosis can lead to a delay in treatment, resulting in increased complications such as gangrene, perforation, and ab-
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TABLE 1. Review of literature on acute appendicitis Reference
No. of positive or negative appendicitis
No. of patients
Sensitivity (%)
Specificity (%)
NPV (%)
PPV (%)
Accuracy (%)
Pedrosa, 2006 Nitta, 2005 Incesu, 1997
4 29 34
Pregnant Adults Adults
100 100 97
94 88 92
100 100 96
57 97 94
94 97 92
scess formation. Postoperative complications can be seen in 39% cases of appendiceal perforation.6,7 Low diagnostic accuracy for acute appendicitis is especially prevalent mainly at extremes of age and in females. The removal of a normal appendix in suspected appendicitis is associated with a complication rate of 4 to 15% and unnecessary hospitalization. Negative laparotomy rates at 10 to 15% in men and up to 45% in women suspicious of having acute appendicitis have been reported.3
Role of MR Imaging in Acute Appendicitis The addition of imaging to the clinical algorithm has improved diagnostic accuracy of acute appendicitis and reduced the number of unnecessary operations.8 When the appendix is normal by imaging, acute appendicitis can be excluded and alternative diagnoses can be made with the same imaging examination. Alternative diagnoses include diverticulitis, colon cancer, typhlitis, terminal ileitis, Crohn’s disease, and various gynecologic conditions. According to the American College of Radiology (ACR) Appropriateness Criteria, CT is considered the best imaging method for evaluation of acute appendicitis in the adult population.9 US with graded compression is the most suitable initial method for the diagnosis of acute appendicitis in pregnant women. When US is nondiagnostic or inconclusive, MR is the next most appropriate imaging modality to diagnose acute appendicitis in pregnant patients. MR imaging for the evaluation of acute appendicitis has many advantages over CT. MR imaging employs no radiation hazard, which is appealing for imaging of pregnant women and children, as these patients are more sensitive to the effects of radiation. The use of oral contrast is optional in MR examinations. When compared with ultrasound, MR has the advantage of being operator-independent. Using the standardized protocols, the sensitivity, specificity, and diagnos-
58
TABLE 2. MR protocol for suspected acute appendicitis 1. SS-FSE T2 (triplane) of the lower abdomen for localization of appendix (rest of the exam tailored to the appendix) 2. FSE T2 with fat suppression (axial and coronal) 3. SE T1 with fat suppression (axial) 4. STIR (axial) 5. SPGR T1 with fat suppression (axial), pre- and postgadolinium injection Total exam time is typically 30 minutes from the start to the end of scanning. For pregnant patient, follow steps 1 to 4 (Gadolinium is not used).
tic accuracy of MR are reproducible and do not rely on the performers. MR is more useful than US in certain patients who are overweight or obese, in pregnant women, in patients with gaseous abdomen and patients with retrocecal appendix or unusual appendix location. The appendix may be more consistently visualized along its entire length from the cecal base to the tip.10 Incesu and coworkers showed statistically significant improvement with MR over US in sensitivity (97% versus 76%), accuracy (95% versus 82%), and negative-predictive value (96% versus 74%).11 Hormann and coworkers found that MR detected more cases of appendicitis than US in the same population of patients.12 Several studies have shown the usefulness of MR imaging for the diagnosis of acute appendicitis, mostly in pregnant women and children. The diagnostic sensitivity, specificity, and accuracy are documented in Table 1. The limitations of MR imaging include the length of the examination and motion artifacts. These limitations have been partially resolved with the newer gradient echo MR sequences, which take seconds rather than minutes to acquire. At present, restriction in the use of MR imaging is because of limited availability in the acute setting, high cost of the examination, limited review of literature, and possibly, limited comfort of emergency radiologist in reading cases.
Curr Probl Diagn Radiol, March/April 2008
TABLE 3. Key imaging features of acute appendicitis on MR MR criteria Appendiceal changes
Dilated appendix, greater than 6 mm in total diameter Lumen of appendix filled with high intensity on T2-WI or STIR Thickening of the appendix wall with high intensity on T2-WI or STIR Marked enhancement of the appendix wall on fat-suppressed T1-WI Periappendiceal changes Increased intensity of periappendiceal tissue on T2-WI or STIR Suggestion of perforation Air outside the appendiceal lumen (signal void on all sequences) Fluid collection or abscess
FIG 1. Normal appendix. Axial T1-weighted (A), axial T2-weighted fat-saturated (B), and axial STIR (C) sequences demonstrate the normal appendix (curved arrows), located posteromedial to the appendix (arrowheads). The appendix is 6 mm in diameter and does not show any wall edema or periappendiceal inflammation.
Imaging Protocol In general, the protocol includes T2-weighted images without and with fat suppression, noncontrast T1weighted and postcontrast T1-weighted sequences, often obtained in three planes. Initially, a single-shot
Curr Probl Diagn Radiol, March/April 2008
FIG 2. Acute appendicitis. Axial T2-weighted fat-saturated (A) and SSFSE (B) sequence shows an 11-mm-thick appendix (curved arrows) medial to the cecum and demonstrating uniformly elevated signal intensity.
fast spin-echo (SSFSE) sequence of the lower abdomen is used to localize the cecum and appendix, followed by which we obtain short-tau inversion recovery (STIR) and T2-weighted sequence. The plane of imaging is planned by the radiologist who is monitoring the study. A tailored examination of the cecum and appendix reduces the scan time and allows
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FIG 3. Acute appendicitis. Coronal scout image (A) and sagittal contrast-enhanced T1-weighted fat-saturated sequence (B) shows a dilated appendix (curved arrows) measuring 12 mm in diameter. Sagittal postcontrast T1-weighted fat-saturated sequence shows enhancement of the appendiceal wall.
the best possible imaging of the area of interest. The detailed MR protocol is provided in Table 2. The average examination time at our institution using this protocol is approximately 30 minutes. Intravenous gadolinium administration is not used in pregnant women due to concern for unknown effects to the fetus. However, if used in the nonpregnant population, gadolinium enhancement is routinely used and may be helpful in increasing the diagnostic confidence and accuracy. We do not use oral or rectal contrast during MR assessment of the appendix.
MR of Normal Appendix The appendix is considered normal when it is not distended and measures less than 6 mm in total diameter and less than 2 mm in wall thickness. It is hypointense on both T1-weighted and T2-weighted images compared with skeletal muscle (Fig 1). No significant wall enhancement is demonstrated after
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intravenous contrast injection. It can be located in the pelvis, anterior to the ileum, posterior to the ileum, or posterior to the cecum. The periappendiceal fat has a uniform isointensity compared with fat seen in other regions of the body and should have low signal on fat-suppressed sequences if the fat saturation technique is adequate. A few groups of investigators had reported visualization rates for the normal appendix in each MR sequence. Nikolaidis and coworkers13 found that in 78% (55 of 71 patients) the normal appendix was seen in T1 spin-echo sequence and, if the appendix was not visualized in this sequence, it was not visible in others. HASTE (SSFSE) sequence was the second best sequence for visualization of the normal appendix in their study with a visualization rate of 60%. The results were comparable to another study of Hormann and coworkers12 that in 73% of patients the normal appendix was visible in T1 spin-echo sequence. Using a combination of multiple sequences, Nitta and coworkers14 found that in 90% of patients the normal
Curr Probl Diagn Radiol, March/April 2008
was identified in 83% of 47 cases. Overall, the ability to visualize a normal appendix by MR is reported to be 72 to 92%.11,14-16
Abnormal Appendix
FIG 4. Perforated appendicitis with pelvic abscess. Axial (A and B) postcontrast T1-weighted fat-saturated sequence shows pelvic abscess (arrows) with surrounding inflammation (arrowheads) secondary to perforated appendicitis. Axial contrast-enhanced CT (C) of the same patient shows enlarged appendix (arrow) in the right lower quadrant.
appendix was visible in their study of normal 20 volunteers. In children, T2-weighted sequences are superior to T1-weighted sequences for evaluation of the appendix.10,12 In the study of pregnant patients by Pedrosa and coworkers15 a normal appendix on MR
Curr Probl Diagn Radiol, March/April 2008
The findings of appendicitis are best depicted on T2-weighted images. Several imaging features are included in Table 3. T2-weighted fast spin-echo and inversion recovery sequences (such as STIR) are useful for detection of fluid in the lumen of the appendix, appendiceal wall inflammation, and periappendiceal inflammation. However, it should be noted that inversion recovery sequences have a lower yield for the detection of the normal appendix but are useful for the detection of inflammatory changes and edema. The abnormal appendix is distended with a caliber greater than 6 mm and increased wall thickness of more than 2 mm. The appendiceal wall is T1 hypointense and slightly T2 hyperintense due to edema. The fluid-filled lumen is hyperintense on T2-weighted sequences. This results in a “target sign,” resulting from central fluid in a distended appendix with a thick appendiceal wall. If intravenous gadolinium is used, the thickened wall has marked enhancement on fatsuppressed T1-weighted sequence (Figs 2-4). In patients with acute appendicitis, the periappendiceal fat is either replaced by a fluid or infiltrated with edema that is shown as T2 hyperintensity, best demonstrated on fat-suppressed T2-weighted images or STIR. The findings of periappendiceal inflammation can be particularly useful when the appendix itself cannot be definitively identified. Appendiceal perforation may be suggested by the presence of a T2 hyperintense fluid collection, abscess, or extraluminal gas adjacent to the appendix. Air within an abscess or free air is seen as signal void in all sequences. MR imaging may have certain limitations for evaluation of the entire length of the appendix and for the presence of an appendicolith. The latter is indistinguishable from air in the lumen of the appendix on MR.
Pregnant Women with Acute Appendicitis Appendicitis is the most common cause of acute abdominal pain during pregnancy.17 The incidence of appendicitis in pregnant women experiencing acute abdominal pain is similar to the rest of the population.17,18 The clinical presentation including
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FIG 5. Pelvic pain secondary to endometriotic cyst. Axial T2-weighted sequence (A) shows high signal intensity cystic lesion with dependent low signal from shading (arrow). Axial T1-weighted fat-saturated sequence (B) shows the high signal intensity in the endometriotic cyst (curved arrow), produced by subacute blood products. Ultrasound (C) of the endometriotic cyst shows dependent hemorrhagic products forming a fluid–fluid level (straight arrow).
history, physical examination, and laboratory values may be unreliable in pregnancy. A pregnant patient with acute abdominal pain can be a diagnostic challenge. The clinical accuracy for diagnosing acute appendicitis is lower in the second and third trimesters, and a higher rate of false-negative appendectomies on the basis of clinical evaluation alone has been reported.19,20 If further investigation is necessary, evaluation without ionizing radiation has been advocated by the International Commission on Radiologic Protection.21
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US is considered the primary imaging modality for pregnant patients with right lower quadrant pain, by the American College of Radiology. However, displacement of the appendix by the uterus can make its visualization and evaluation with US and graded compression more difficult.18 This is particularly true in the third trimester of pregnancy. In one study, an appendix could not be identified in 11 of 12 patients by US, while MR saw an appendix in 10 of 12 patients. This supports the use of MR when ultrasound is nondiagnostic or as an alternative to avoid ionizing
Curr Probl Diagn Radiol, March/April 2008
FIG 6. Endometriosis. T1-weighted fat-saturated sequence shows multiple endometriotic cysts (curved arrow) containing subacute blood.
radiation. The success in diagnosis of appendicitis using MR during pregnancy has been demonstrated in a number of studies.15,16,22-24 Pregnant patients should be informed that MR during pregnancy is considered generally safe but its safety has not been definitely proven.23 Moreover, there have been no documented teratogenic effects from the use of MR on a fetus. Although one should be rigorous in safeguarding a fetus from possible damage from radiation, the well-being of the mother must be given greater consideration. Gadolinium is considered a category C drug and has shown retarded fetal development and skeletal malformation in animals at higher doses. However, in humans at clinical doses, there is no known carcinogenic effect. When the benefits outweigh the risks and during the second and third trimester, gadolinium can be considered but should generally be avoided.25
Strategies for Successful Interpretation While many findings are associated with acute appendicitis, commonly helpful features are a distended appendix, thickening of the appendix wall, and periappendiceal inflammation. The presence of inflammation in the right lower quadrant suggesting the possibility of appendicitis may be informative when the appendix cannot be identified. Acute appendicitis can be effectively excluded by the appearance of a normal appendix on MR given its high negative-predictive value.11,15 Proper MR technique to acquire images with minimal motion artifact from physiologic bowel peristalsis
Curr Probl Diagn Radiol, March/April 2008
FIG 7. Mucinous cystadenoma in pregnant patient with pelvic pain. Sagittal (A) and axial (B) T2-weighted sequence shows a presacral multiloculated cystic mass (curved arrows) with mural nodules (arrowheads).The gravid uterus with a gestational sac (straight arrows) is displaced anteriorly by the pelvic mass.
and respiratory motion increases the likelihood of identifying the anatomy of the appendix, cecum, and terminal ileum. Searching for the cecum and the blindend structure of the appendix facilitates differentiation between the ileum and appendix. Adequate fat suppression on fat-saturated T1- and T2-weighted sequences improves image contrast for detection of enhancement and periappendiceal inflammation, respectively. Despite optimization of the image acquisition, anatomical detail can be confusing. The terminal ileum and appendix may be mistaken for each other. Several conditions have been recognized that can lead to a false-positive MR diagnosis of appendicitis including a fluid-filled terminal ileum, terminal ileum inflammation such as in Crohn’s disease, cecal diverticulitis, small bowel diverticulitis, fluid in the right lower
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FIG 9. Acute sigmoid colitis. T2-weighted fat-saturated sequence shows thickening of the sigmoid colonic wall (arrowheads) with surrounding pelvic free fluid (arrows).
FIG 8. Pelvic dermoid in pregnancy causing pelvic pain. Non-fatsaturated T1-weighted (A) and fat-saturated (B) sequence demonstrates a heterogeneous presacral mass containing fat signal intensity (curved arrows), which is hyperintense on non-fat-saturated and hypointense on fat-saturated sequence. The mass also contains non-fat-containing soft tissue (arrowheads), which does not decrease in signal intensity on fat-saturated sequence. The gestational sac (arrows) is located anterior to the presacral mass.
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FIG 10. Small bowel infarction. Contrast-enhanced T1-weighted fat-saturated sequence (A) shows small bowel wall thickening and serosal enhancement (arrowheads). The lack of enhancement of rest of the small bowel wall layers corresponded to transmural bowel wall infarction at surgery. Axial noncontrast CT (B) showed nonspecific thickening of the small bowel wall (arrow). The patient could not receive intravenous contrast because of renal failure.
Curr Probl Diagn Radiol, March/April 2008
patients. Patients with a paucity of intraperitoneal fat may be better assessed with US.
Conclusion MR is a useful alternative imaging technique for the diagnosis of acute appendicitis. It is an attractive alternative to US and CT in pediatric and pregnant patients. Although higher cost and restricted availability limit the utilization of MR in the emergency setting, the advantage of nonionizing radiation and better contrast resolution makes MR use a promising imaging modality in a limited subset of patients (Figs 5-11). Attention to anatomy and proper MR protocol is essential for maximizing diagnostic accuracy and optimizing the MR examination. The use of MR in the cases of suspected acute appendicitis could lead to rapid and accurate diagnosis.
REFERENCES
FIG 11. Acute cholecystitis. Coronal scout image (A) and axial T2-weighted fat-saturated (B) sequence shows distended gallbladder with wall thickening (curved arrows).
quadrant from a ruptured right ovarian cyst, and intraabdominal ascites. A false-negative result will occur if a distended fluid-filled appendix is mistaken for the terminal ileum. If ascites is present due to a comorbid condition, periappendiceal inflammation from appendicitis may be obscured. While nonvisualization of the appendix and a normal right lower quadrant on CT when observed by experienced radiologists suggests a normal study,26,27 the value of MR when the appendix cannot be reliably identified has not been specifically addressed. Interpretation of MR may be difficult if the appendix cannot be visualized due to lack of intraperitoneal fat that is common in the pediatric population and thin
Curr Probl Diagn Radiol, March/April 2008
1. Gupta H, Dupuy DE. Advances in the imaging of the acute abdomen. Surg Clin North Am 1997;77:1245-63. 2. Addiss DF, Shafer N, Fowler BS, et al. The epidemiology of appendicitis and appendectomy in the United States. Am J Epidemiol 1990;132:910-25. 3. Berry J, Malt RA. Appendicitis near its centenary. Ann Surg 1984;200:567-75. 4. Cobben LP, de Van Otterloo AM, Puylaert JB. Spontaneously resolving appendicitis: Frequency and natural history in 60 patients. Radiology 2000;215:349-52. 5. Fenoglio-Preiser CM, Noffsinger AE, Stemmermann GN, et al. In: Fenoglio-Preiser CM, editor. Gastrointestinal Pathology: An Atlas and Text. 2nd ed. Philadelphia, PA: Lippincott Williams & Wilkins, 1998. p. 519-42. 6. Jess P, Bjerregaard B, Brynitz S, et al. Acute appendicitis: Prospective trial concerning diagnostic accuracy and complications. Am J Surg 1981;141:232-4. 7. Andersson R, Hugander A, Thulin A. Diagnostic accuracy and perforation rate in appendicitis: Association with the age and sex of the patient and with appendectomy rate. Eur J Surg 1992;158:37-41. 8. Bendeck SE, Nino-Murcia M, Berry GJ, et al. Imaging for suspected appendicitis: Negative appendectomy and perforation rates. Radiology 2002;225:131-6. 9. American College of Radiology Appropriateness Criteria. Available at: http://www.acr.org/s_acr/bin.asp?CID⫽1207& DID⫽11763&DOC⫽FILE.PDF. Accessed March 15, 2007. 10. Hormann M, Paya K, Eibenberger K, et al. MR imaging in children with nonperforated acute appendicitis: Value of unenhanced MR imaging in sonographically selected cases. AJR 1998;171:467-70. 11. Incesu L, Coskun A, Selcuk MB, et al. Acute appendicitis: MR imaging and sonographic correlation. AJR 1997;168:669-74. 12. Hormann M, Puig S, Prokesch SR, et al. MR imaging of the normal appendix in children. Eur Radiol 2002;12:2313-6.
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13. Nikolaidis P, Hammond N, Marko J, et al. Incidence of visualization of the normal appendix on different MRI sequences. Emerg Radiol 2006;12:223-6. 14. Nitta N, Takahashi M, Furukawa A, et al. MR imaging of the normal appendix and acute appendicitis. J Magn Reson Imaging 2005;21:156-65. 15. Pedrosa I, Levine R, Eyvazzadeh AD, et al. MR imaging evaluation of acute appendicitis in pregnancy. Radiology 2006;238:891-9. 16. Oto A, Ernst RD, Shah R, et al. Right-lower-quadrant pain and suspected appendicitis in pregnant women: Evaluation with MR imaging—initial experience. Radiology 2005;234:445-51. 17. Babaknia A, Parsa H, Woodruff J. Appendicitis during pregnancy. Obstet Gynecol 1977;50:40-4. 18. Andersen B, Nielsen T. Appendicitis in pregnancy: Diagnosis, management and complications. Acta Obstet Gynecol Scand 1999;78:758-62. 19. Firstenberg M, Malangoni M. Gastrointestinal surgery during pregnancy. Gastroenterol Clin North Am 1998;27:73-88.
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20. Balthazar EJ, Megibow AJ, Siegel SE, et al. Appendicitis: Prospective evaluation with high-resolution CT. Radiology 1991;180:21-4. 21. International Commission on Radiological Protection. “Pregnancy and irradiation.” Ann IRCP 2000;30:1-43. 22. Cobben LP, Groot I, Hans L, et al. MRI for clinically suspected appendicitis during pregnancy. AJR 2004;183:671-5. 23. Shellock F, Kanal E. Policies, guidelines, and recommendations for MR imaging safety and patient treatment: SMRI safety committee. J Magn Reson Imaging 1991;1:97-101. 24. Birchard KR, Brown MA, Hyslop WB, et al. MRI of acute abdominal and pelvic pain in pregnant patients. AJR 2005; 184:452-8. 25. Product package insert for Omniscan. Amersham Health. Nycomed Amersham: Princeton, NJ. 26. Nikolaidis P, Hwang C, Miller F, et al. The non-visualized appendix: Incidence of acute appendicitis when inflammatory changes are absent. AJR 2004;183(4):889-92. 27. Ganguli S, Raptopoulos V, Komlos F, et al. Right lower quadrant pain: Value of the nonvisualized appendix at multidetector CT. Radiology 2006;241:175-80.
Curr Probl Diagn Radiol, March/April 2008
Acute appendicitis is the most common surgical abdominal emergency. Although the clinical diagnosis can be made accurately in typical cases, imaging plays an important role in improving diagnostic accuracy of this condition, especially when the clinical diagnosis is uncertain. Magnetic resonance imaging is an emerging promising technique for the diagnosis of acute appendicitis, especially in patients with nondiagnostic ultrasound and in patients where radiation is a clinical concern. In the following review, the role of magnetic resonance in the diagnosis of appendicitis will be discussed.
Acute appendicitis is the most common indication for emergency abdominal surgery in the United States.1 There are approximately 250,000 cases of appendicitis annually in United States, accounting for 1 million days per year spent in a hospital.2 The clinical accuracy for diagnosing appendicitis is between 65 and 82%, which is less than optimal considering the increased morbidity and mortality in complicated cases.1,3 Imaging has played an important role in improving the diagnostic accuracy and reducing unnecessary surgeries. Currently in most institutions, computed tomography (CT) is the standard investigation in adults, while ultrasound (US) is the modality of choice in children and pregnant women. With advancements and wider availability, magnetic resonance (MR) imaging has emerged as a promising alternative modality for the diagnosis of this condition.
From the aDepartment of Radiology, University of Massachusetts Memorial Medical Center, Worcester, MA; and bDepartment of Radiology, Massachusetts General Hospital, Boston, MA. Reprint requests: Ajay Singh, MD, 10 Museum Way, #524, Boston, MA 02141. E-mail: [email protected]. Curr Probl Diagn Radiol 2008;37:57-66. © 2008 Mosby, Inc. All rights reserved. 0363-0188/2008/$34.00 ⫹ 0 doi:10.1067/j.cpradiol.2007.10.002
Curr Probl Diagn Radiol, March/April 2008
Pathogenesis The inflammation of appendix is commonly caused by obstruction of the appendiceal lumen by a fecalith or lymphoid hyperplasia. Less common causes of acute appendicitis include appendiceal obstruction from a foreign body, parasites, or a neoplasm. The natural history of untreated appendicitis is usually appendiceal perforation or abscess formation. Spontaneous resolution occurs in a minority of cases, of which 38% have recurrent appendicitis, often within the first year.4 The histological changes associated with appendicitis depend on the duration and severity of the disease. They range from minimal inflammation to marked necrosis with complete mural destruction and rupture.5
Clinical Presentation Classically, patients with acute appendicitis present with an initial vague periumbilical pain progressing to the right lower quadrant, fever, nausea, and leukocytosis. Other signs and symptoms such as vomiting, the Psoas sign, the Obturator sign, and the Rovsing sign may be detected. Given the classic presentation in young, previously healthy individuals, additional imaging testing may be unnecessary. Uncertainty in diagnosis occurs when the signs and symptoms are not typical for appendicitis. Variations in the location of appendix result in wide variations in the sites of pain, when the appendix is inflamed. Retrocecal appendicitis produces vague and poorly localized pain, while pelvic appendicitis causes midline or left-sided abdominal pain. Appendicitis extending to the urinary bladder or ureter may cause voiding symptoms, hematuria, or pyuria. Misdiagnosis can lead to a delay in treatment, resulting in increased complications such as gangrene, perforation, and ab-
57
TABLE 1. Review of literature on acute appendicitis Reference
No. of positive or negative appendicitis
No. of patients
Sensitivity (%)
Specificity (%)
NPV (%)
PPV (%)
Accuracy (%)
Pedrosa, 2006 Nitta, 2005 Incesu, 1997
4 29 34
Pregnant Adults Adults
100 100 97
94 88 92
100 100 96
57 97 94
94 97 92
scess formation. Postoperative complications can be seen in 39% cases of appendiceal perforation.6,7 Low diagnostic accuracy for acute appendicitis is especially prevalent mainly at extremes of age and in females. The removal of a normal appendix in suspected appendicitis is associated with a complication rate of 4 to 15% and unnecessary hospitalization. Negative laparotomy rates at 10 to 15% in men and up to 45% in women suspicious of having acute appendicitis have been reported.3
Role of MR Imaging in Acute Appendicitis The addition of imaging to the clinical algorithm has improved diagnostic accuracy of acute appendicitis and reduced the number of unnecessary operations.8 When the appendix is normal by imaging, acute appendicitis can be excluded and alternative diagnoses can be made with the same imaging examination. Alternative diagnoses include diverticulitis, colon cancer, typhlitis, terminal ileitis, Crohn’s disease, and various gynecologic conditions. According to the American College of Radiology (ACR) Appropriateness Criteria, CT is considered the best imaging method for evaluation of acute appendicitis in the adult population.9 US with graded compression is the most suitable initial method for the diagnosis of acute appendicitis in pregnant women. When US is nondiagnostic or inconclusive, MR is the next most appropriate imaging modality to diagnose acute appendicitis in pregnant patients. MR imaging for the evaluation of acute appendicitis has many advantages over CT. MR imaging employs no radiation hazard, which is appealing for imaging of pregnant women and children, as these patients are more sensitive to the effects of radiation. The use of oral contrast is optional in MR examinations. When compared with ultrasound, MR has the advantage of being operator-independent. Using the standardized protocols, the sensitivity, specificity, and diagnos-
58
TABLE 2. MR protocol for suspected acute appendicitis 1. SS-FSE T2 (triplane) of the lower abdomen for localization of appendix (rest of the exam tailored to the appendix) 2. FSE T2 with fat suppression (axial and coronal) 3. SE T1 with fat suppression (axial) 4. STIR (axial) 5. SPGR T1 with fat suppression (axial), pre- and postgadolinium injection Total exam time is typically 30 minutes from the start to the end of scanning. For pregnant patient, follow steps 1 to 4 (Gadolinium is not used).
tic accuracy of MR are reproducible and do not rely on the performers. MR is more useful than US in certain patients who are overweight or obese, in pregnant women, in patients with gaseous abdomen and patients with retrocecal appendix or unusual appendix location. The appendix may be more consistently visualized along its entire length from the cecal base to the tip.10 Incesu and coworkers showed statistically significant improvement with MR over US in sensitivity (97% versus 76%), accuracy (95% versus 82%), and negative-predictive value (96% versus 74%).11 Hormann and coworkers found that MR detected more cases of appendicitis than US in the same population of patients.12 Several studies have shown the usefulness of MR imaging for the diagnosis of acute appendicitis, mostly in pregnant women and children. The diagnostic sensitivity, specificity, and accuracy are documented in Table 1. The limitations of MR imaging include the length of the examination and motion artifacts. These limitations have been partially resolved with the newer gradient echo MR sequences, which take seconds rather than minutes to acquire. At present, restriction in the use of MR imaging is because of limited availability in the acute setting, high cost of the examination, limited review of literature, and possibly, limited comfort of emergency radiologist in reading cases.
Curr Probl Diagn Radiol, March/April 2008
TABLE 3. Key imaging features of acute appendicitis on MR MR criteria Appendiceal changes
Dilated appendix, greater than 6 mm in total diameter Lumen of appendix filled with high intensity on T2-WI or STIR Thickening of the appendix wall with high intensity on T2-WI or STIR Marked enhancement of the appendix wall on fat-suppressed T1-WI Periappendiceal changes Increased intensity of periappendiceal tissue on T2-WI or STIR Suggestion of perforation Air outside the appendiceal lumen (signal void on all sequences) Fluid collection or abscess
FIG 1. Normal appendix. Axial T1-weighted (A), axial T2-weighted fat-saturated (B), and axial STIR (C) sequences demonstrate the normal appendix (curved arrows), located posteromedial to the appendix (arrowheads). The appendix is 6 mm in diameter and does not show any wall edema or periappendiceal inflammation.
Imaging Protocol In general, the protocol includes T2-weighted images without and with fat suppression, noncontrast T1weighted and postcontrast T1-weighted sequences, often obtained in three planes. Initially, a single-shot
Curr Probl Diagn Radiol, March/April 2008
FIG 2. Acute appendicitis. Axial T2-weighted fat-saturated (A) and SSFSE (B) sequence shows an 11-mm-thick appendix (curved arrows) medial to the cecum and demonstrating uniformly elevated signal intensity.
fast spin-echo (SSFSE) sequence of the lower abdomen is used to localize the cecum and appendix, followed by which we obtain short-tau inversion recovery (STIR) and T2-weighted sequence. The plane of imaging is planned by the radiologist who is monitoring the study. A tailored examination of the cecum and appendix reduces the scan time and allows
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FIG 3. Acute appendicitis. Coronal scout image (A) and sagittal contrast-enhanced T1-weighted fat-saturated sequence (B) shows a dilated appendix (curved arrows) measuring 12 mm in diameter. Sagittal postcontrast T1-weighted fat-saturated sequence shows enhancement of the appendiceal wall.
the best possible imaging of the area of interest. The detailed MR protocol is provided in Table 2. The average examination time at our institution using this protocol is approximately 30 minutes. Intravenous gadolinium administration is not used in pregnant women due to concern for unknown effects to the fetus. However, if used in the nonpregnant population, gadolinium enhancement is routinely used and may be helpful in increasing the diagnostic confidence and accuracy. We do not use oral or rectal contrast during MR assessment of the appendix.
MR of Normal Appendix The appendix is considered normal when it is not distended and measures less than 6 mm in total diameter and less than 2 mm in wall thickness. It is hypointense on both T1-weighted and T2-weighted images compared with skeletal muscle (Fig 1). No significant wall enhancement is demonstrated after
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intravenous contrast injection. It can be located in the pelvis, anterior to the ileum, posterior to the ileum, or posterior to the cecum. The periappendiceal fat has a uniform isointensity compared with fat seen in other regions of the body and should have low signal on fat-suppressed sequences if the fat saturation technique is adequate. A few groups of investigators had reported visualization rates for the normal appendix in each MR sequence. Nikolaidis and coworkers13 found that in 78% (55 of 71 patients) the normal appendix was seen in T1 spin-echo sequence and, if the appendix was not visualized in this sequence, it was not visible in others. HASTE (SSFSE) sequence was the second best sequence for visualization of the normal appendix in their study with a visualization rate of 60%. The results were comparable to another study of Hormann and coworkers12 that in 73% of patients the normal appendix was visible in T1 spin-echo sequence. Using a combination of multiple sequences, Nitta and coworkers14 found that in 90% of patients the normal
Curr Probl Diagn Radiol, March/April 2008
was identified in 83% of 47 cases. Overall, the ability to visualize a normal appendix by MR is reported to be 72 to 92%.11,14-16
Abnormal Appendix
FIG 4. Perforated appendicitis with pelvic abscess. Axial (A and B) postcontrast T1-weighted fat-saturated sequence shows pelvic abscess (arrows) with surrounding inflammation (arrowheads) secondary to perforated appendicitis. Axial contrast-enhanced CT (C) of the same patient shows enlarged appendix (arrow) in the right lower quadrant.
appendix was visible in their study of normal 20 volunteers. In children, T2-weighted sequences are superior to T1-weighted sequences for evaluation of the appendix.10,12 In the study of pregnant patients by Pedrosa and coworkers15 a normal appendix on MR
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The findings of appendicitis are best depicted on T2-weighted images. Several imaging features are included in Table 3. T2-weighted fast spin-echo and inversion recovery sequences (such as STIR) are useful for detection of fluid in the lumen of the appendix, appendiceal wall inflammation, and periappendiceal inflammation. However, it should be noted that inversion recovery sequences have a lower yield for the detection of the normal appendix but are useful for the detection of inflammatory changes and edema. The abnormal appendix is distended with a caliber greater than 6 mm and increased wall thickness of more than 2 mm. The appendiceal wall is T1 hypointense and slightly T2 hyperintense due to edema. The fluid-filled lumen is hyperintense on T2-weighted sequences. This results in a “target sign,” resulting from central fluid in a distended appendix with a thick appendiceal wall. If intravenous gadolinium is used, the thickened wall has marked enhancement on fatsuppressed T1-weighted sequence (Figs 2-4). In patients with acute appendicitis, the periappendiceal fat is either replaced by a fluid or infiltrated with edema that is shown as T2 hyperintensity, best demonstrated on fat-suppressed T2-weighted images or STIR. The findings of periappendiceal inflammation can be particularly useful when the appendix itself cannot be definitively identified. Appendiceal perforation may be suggested by the presence of a T2 hyperintense fluid collection, abscess, or extraluminal gas adjacent to the appendix. Air within an abscess or free air is seen as signal void in all sequences. MR imaging may have certain limitations for evaluation of the entire length of the appendix and for the presence of an appendicolith. The latter is indistinguishable from air in the lumen of the appendix on MR.
Pregnant Women with Acute Appendicitis Appendicitis is the most common cause of acute abdominal pain during pregnancy.17 The incidence of appendicitis in pregnant women experiencing acute abdominal pain is similar to the rest of the population.17,18 The clinical presentation including
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FIG 5. Pelvic pain secondary to endometriotic cyst. Axial T2-weighted sequence (A) shows high signal intensity cystic lesion with dependent low signal from shading (arrow). Axial T1-weighted fat-saturated sequence (B) shows the high signal intensity in the endometriotic cyst (curved arrow), produced by subacute blood products. Ultrasound (C) of the endometriotic cyst shows dependent hemorrhagic products forming a fluid–fluid level (straight arrow).
history, physical examination, and laboratory values may be unreliable in pregnancy. A pregnant patient with acute abdominal pain can be a diagnostic challenge. The clinical accuracy for diagnosing acute appendicitis is lower in the second and third trimesters, and a higher rate of false-negative appendectomies on the basis of clinical evaluation alone has been reported.19,20 If further investigation is necessary, evaluation without ionizing radiation has been advocated by the International Commission on Radiologic Protection.21
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US is considered the primary imaging modality for pregnant patients with right lower quadrant pain, by the American College of Radiology. However, displacement of the appendix by the uterus can make its visualization and evaluation with US and graded compression more difficult.18 This is particularly true in the third trimester of pregnancy. In one study, an appendix could not be identified in 11 of 12 patients by US, while MR saw an appendix in 10 of 12 patients. This supports the use of MR when ultrasound is nondiagnostic or as an alternative to avoid ionizing
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FIG 6. Endometriosis. T1-weighted fat-saturated sequence shows multiple endometriotic cysts (curved arrow) containing subacute blood.
radiation. The success in diagnosis of appendicitis using MR during pregnancy has been demonstrated in a number of studies.15,16,22-24 Pregnant patients should be informed that MR during pregnancy is considered generally safe but its safety has not been definitely proven.23 Moreover, there have been no documented teratogenic effects from the use of MR on a fetus. Although one should be rigorous in safeguarding a fetus from possible damage from radiation, the well-being of the mother must be given greater consideration. Gadolinium is considered a category C drug and has shown retarded fetal development and skeletal malformation in animals at higher doses. However, in humans at clinical doses, there is no known carcinogenic effect. When the benefits outweigh the risks and during the second and third trimester, gadolinium can be considered but should generally be avoided.25
Strategies for Successful Interpretation While many findings are associated with acute appendicitis, commonly helpful features are a distended appendix, thickening of the appendix wall, and periappendiceal inflammation. The presence of inflammation in the right lower quadrant suggesting the possibility of appendicitis may be informative when the appendix cannot be identified. Acute appendicitis can be effectively excluded by the appearance of a normal appendix on MR given its high negative-predictive value.11,15 Proper MR technique to acquire images with minimal motion artifact from physiologic bowel peristalsis
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FIG 7. Mucinous cystadenoma in pregnant patient with pelvic pain. Sagittal (A) and axial (B) T2-weighted sequence shows a presacral multiloculated cystic mass (curved arrows) with mural nodules (arrowheads).The gravid uterus with a gestational sac (straight arrows) is displaced anteriorly by the pelvic mass.
and respiratory motion increases the likelihood of identifying the anatomy of the appendix, cecum, and terminal ileum. Searching for the cecum and the blindend structure of the appendix facilitates differentiation between the ileum and appendix. Adequate fat suppression on fat-saturated T1- and T2-weighted sequences improves image contrast for detection of enhancement and periappendiceal inflammation, respectively. Despite optimization of the image acquisition, anatomical detail can be confusing. The terminal ileum and appendix may be mistaken for each other. Several conditions have been recognized that can lead to a false-positive MR diagnosis of appendicitis including a fluid-filled terminal ileum, terminal ileum inflammation such as in Crohn’s disease, cecal diverticulitis, small bowel diverticulitis, fluid in the right lower
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FIG 9. Acute sigmoid colitis. T2-weighted fat-saturated sequence shows thickening of the sigmoid colonic wall (arrowheads) with surrounding pelvic free fluid (arrows).
FIG 8. Pelvic dermoid in pregnancy causing pelvic pain. Non-fatsaturated T1-weighted (A) and fat-saturated (B) sequence demonstrates a heterogeneous presacral mass containing fat signal intensity (curved arrows), which is hyperintense on non-fat-saturated and hypointense on fat-saturated sequence. The mass also contains non-fat-containing soft tissue (arrowheads), which does not decrease in signal intensity on fat-saturated sequence. The gestational sac (arrows) is located anterior to the presacral mass.
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FIG 10. Small bowel infarction. Contrast-enhanced T1-weighted fat-saturated sequence (A) shows small bowel wall thickening and serosal enhancement (arrowheads). The lack of enhancement of rest of the small bowel wall layers corresponded to transmural bowel wall infarction at surgery. Axial noncontrast CT (B) showed nonspecific thickening of the small bowel wall (arrow). The patient could not receive intravenous contrast because of renal failure.
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patients. Patients with a paucity of intraperitoneal fat may be better assessed with US.
Conclusion MR is a useful alternative imaging technique for the diagnosis of acute appendicitis. It is an attractive alternative to US and CT in pediatric and pregnant patients. Although higher cost and restricted availability limit the utilization of MR in the emergency setting, the advantage of nonionizing radiation and better contrast resolution makes MR use a promising imaging modality in a limited subset of patients (Figs 5-11). Attention to anatomy and proper MR protocol is essential for maximizing diagnostic accuracy and optimizing the MR examination. The use of MR in the cases of suspected acute appendicitis could lead to rapid and accurate diagnosis.
REFERENCES
FIG 11. Acute cholecystitis. Coronal scout image (A) and axial T2-weighted fat-saturated (B) sequence shows distended gallbladder with wall thickening (curved arrows).
quadrant from a ruptured right ovarian cyst, and intraabdominal ascites. A false-negative result will occur if a distended fluid-filled appendix is mistaken for the terminal ileum. If ascites is present due to a comorbid condition, periappendiceal inflammation from appendicitis may be obscured. While nonvisualization of the appendix and a normal right lower quadrant on CT when observed by experienced radiologists suggests a normal study,26,27 the value of MR when the appendix cannot be reliably identified has not been specifically addressed. Interpretation of MR may be difficult if the appendix cannot be visualized due to lack of intraperitoneal fat that is common in the pediatric population and thin
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