acute appendicitis is the most common diagnosis suspected in patients presenting in emergency rooms with acute abdominal pain , and is the most common indication for an urgent abdominal intervention . yet , it is difficult to diagnose based solely on the patient 's medical history , physical examination , and laboratory findings . both delayed and unnecessary interventions may lead to adverse fetus outcomes.[13 ] a mean negative appendectomy rate of 26% ( 1647% ) have been reported when the diagnosis is based only on clinical and laboratory findings , dropping to 610% when imaging is performed . therefore , the use of imaging modalities is critical to confirm the diagnosis , when facing a clinical suspicion of appendicitis . currently , there is no uniform protocol regarding the use of imaging tests , with some centers starting with ultrasound ( us ) and others with computed tomography ( ct ) , according to their availability and local experience . hence , there is a need for guidelines regarding the optimal patient - tailored imaging modality to begin with , and an algorithm for determining further imaging procedures . meta - analyses of ct and us in the diagnosis of appendicitis were published by researchers from the university of amsterdam , based on articles published between january 1994 and february 2006 , and from the university of toronto , based on publications from 1986 to 2004 . in the present review , data from relevant articles on appendicitis imaging published from february 2006 to march 2011 was retrieved from a computerized database ( medline ) . the aim of this review is to analyze the diagnostic capabilities of us , ct , and mri in patients suspected of suffering from acute appendicitis , based on articles published in the last five years . high - resolution linear array transducers with harmonics and compound capabilities in modern us systems allow optimal definition of the structures at the right lower quadrant . in obese patients , a convex array lower frequency transducer may be needed to achieve better penetration . bowel wall and peristalsis , iliopsoas muscle , iliac vessels , mesenteric fat , and mesenteric lymph nodes are clearly defined . the normal appendix can be visualized from the base of the cecum as a blind - ended , gut - pattern aperistaltic tubular structure , with a wall thickness of 2 mm or less and a 67 mm or less diameter [ figure 1 ] . a 31-year - old - female with right lower quadrant abdominal pain for two days . normal appendix on us . ( b ) short axis view . a blind - ended , gut - pattern tubular structure ( arrows ) , with 2 mm wall thickness and 4.5 mm lumen width ( cursors ) is seen anterior to the right common iliac vessels on gray - scale us graded compression . the inflamed appendix is visualized as an incompressible , blind - ended , gut - pattern fluid - filled tubular structure , with a thickened wall , and a diameter greater than 67 mm . applying graded compression with the transducer , differentiation between normal displaceable loops and color and power doppler may aid in the diagnosis by showing a hyperemic wall [ figure 2 ] . manual compression of the lumbar fossa improves visualization of appendicitis , in particular when it is retrocecal . visualization of the whole appendix is needed to detect if inflammation is localized just at the end of the appendix , known as  tip  appendicitis . when there is clinical suspicion of appendicitis , the presence of a fecalith in the appendix confirms the diagnosis [ figure 3 ] . additional findings are pelvic or inter loop peritoneal fluid and hyperechoic inflamed mesenteric fat surrounding the appendix [ figure 4 ] . perforated appendicitis may be seen as an abscess in the right lower quadrant [ figure 5 ] . appendicitis - mimicking conditions of gastrointestinal and urogenital origin may be revealed on us images of the right lower quadrant [ figure 7 ] . an incompressible , blind - ended , gut - pattern fluid - filled tubular structure , with thickened wall and diameter greater than 7 mm is seen in the right lower quadrant ( arrows ) . following diagnosis of acute appendicitis on us examination , patient proceeded to surgery , without further imaging tests . a 28-year - old - female with pelvic pains and fever . a normal proximal appendix , 4.8 mm width followed by enlarged , 12.7 mm width fluid - filled distal appendix is seen on compression us . a hyperechogenic focus with posterior acoustic shadow compatible with a fecalith ( b ) short axis view of the appendix , showing the normal proximal appendix and the inflamed distal part ( measurements ) , the last containing an appendicolith . a 21-year - old - male with right pelvic pain and fever . hyperechoic thickened mesenteric fat ( arrows ) surrounding 9.5 mm inflamed appendix ( cursors ) is seen on compression us . a 10- year - old- female with abdominal pain for three days and rebound in right lower quadrant . dual gray scale compression us image ( right plot before compression ; left plot during compression ) showing an uncompressible distended appendix ( cursors ) surrounded by a fluid collection ( arrows ) . a 45- year - old - female with fever and abdominal pains , 10 days after laparoscopic appendectomy . a ) contrast enhanced mdct showing a retrocecal collection displacing the ascending colon ( arrows ) . ( b ) us of the right lower quadrant demonstrated fluid collection ( long arrows ) and in the bottom , an echogenic structure with posterior acoustic shadowing ( short arrows ) , compatible with a dropped appendicolith . ( c ) a 7f pig tail catheter ( arrow at the tip ) is introduced in the collection under us guidance . ( d ) the catheter is shown in the partially drained collection ( arrow ) . a six - year - old - female with pain in right lower abdomen and pelvis . ( a ) longitudinal us scan of the pelvis showing a hyperechoic linear structure with a  twinkle  artifact on color doppler compatible with a stone , in the right ureter at the uretero vesical junction . they were distinguished by factors including the use of oral or rectal contrast administration of either positive , neutral , or water contrast agents and whether non - contrast or contrast exams after intravenous injection were performed . they also varied in the anatomical extension of the scan , some including the entire abdomen and pelvis and others focusing on the anatomical area , from the level of the xyphoid to the ramus pubis . the normal appendix is visualized as a blind - ended tubular structure exiting from the cecum , filled with contrast media or gas [ figure 8 ] . the normal diameter ranges between 6 and 10 mm , although more than 7 mm is generally considered as pathological . a 10-year - old - male with persistent right lower quadrant abdominal pain and fever , after two negative us exams for appendicitis . a retrocecal subhepatic blind - ended gas filled tubular structure of normal wall thickness and normal diameter is seen ( large arrow ) . the inflamed appendix is seen as a blind - ended tubular structure exiting from the cecum , without contrast or air filling , more than 7 mm in diameter [ figure 9 ] . the presence of contrast or air from gas - forming organisms in the proximal part of the lumen does not exclude appendicitis . additional findings such as the presence of an appendicolith , interloop peritoneal fluid , cecal wall thickening , and periappendicular fat stranding are especially useful in an indeterminate appendix [ figure 10 ] . administration of intravenous contrast media , as a bolus injection of 80 - 100 cc non - ionic contrast media , allows evaluation of the appendix wall enhancement , which can be useful in borderline cases . a three - year - old - male with abdominal pains and rebound in right lower abdomen . ( a ) gray scale us performed by the resident on - duty showed a structure interpreted as mimicking intussusception in the subhepatic area ( arrows ) . due to the patient 's clinical condition and ( b ) contrast enhanced coronal oblique reformatted mdct showing a widened fluid filled tubular structure , without intraluminal air , of oral contrast media ( arrows ) , surrounded by infiltrated mesenteric fat , compatible with acute appendicitis . a 47-year - old - male with a history of inflammatory bowel disease , presenting with abdominal pain and fever . ( a ) a blind - ended tubular structure exiting from the cecum , without intraluminal oral contrast media or air , 12 mm in diameter , is seen ( large arrow ) . mesenteric fat stranding ( short arrow ) is seen in the proximity of the inflamed appendix . ( b ) mesenteric fat edema and reactive lymph nodes are seen in a scan proximal to ( a ) . ct followed a us exam positive for appendicitis , to rule - out signs of active crohn 's disease . alternative diagnosis of right lower quadrant pain may be achieved using ct , to determine if it is of gastrointestinal origin , such as mesenteric adenitis , intussusception , terminal ileitis , diverticulitis , epiploic appendagitis , and typhlitis , or of urogenital origin , such as ureteral stone and urinary tract infection , tubo ovarian abscess , ovarian torsion , ectopic pregnancy , hemorrhagic ovarian cyst or corpus luteum remnants [ figures 11 and 12 ] . cecum and terminal ileum with thickened walls ( arrows ) and adjacent peritoneal fluid ( f ) confirm crohn 's disease . an appendicolith ( lateral right arrow ) is seen at the base of the inflamed appendix , ( shown in figure 9 ) . mri is an alternative method to ct for pregnant patients , offering high soft tissue contrast without ionizing radiation . the imaging protocol for mri evaluation of acute appendicitis in the pregnant population includes t1- and t2- weighted images , and has been extensively described in the literature . the normal appendix is seen as a tubular structure exiting from the cecum , greater than 7 mm in diameter , filled with air or contrast media [ figure 13 ] . a 37- year - old - pregnant patient with lower abdominal pain and fever . a normal sized appendix with air into the lumen is seen in the right lower quadrant in this axial t1 weighted with fat suppression image . a previously performed us was negative for appendicitis , but the normal appendix could not be demonstrated . acute appendicitis is seen as an enlarged appendix , greater than 7 mm in diameter , and void of air or contrast media . signs of peri - appendicular inflammation , seen as band - like areas of high signal intensity on t2-weighted images , single - shot fast spin - echo images or fat saturation images , or the presence of an appendicolith , visualized as an intraluminal low signal intensity focus , confirm the diagnosis , especially in the borderline widened appendix [ figure 14].[1620 ] according to the american college of radiology guidelines for safe mri practices , contrast agents should not be routinely administered to pregnant patients . routine use of gadolinium - based contrast agents in pregnancy is not approved . a 28-year - old - female of 26 weeks pregnancy , presenting with lower abdominal pain and fever . ( a ) coronal t2 weighted image showing an enlarged ( 10 mm in diameter ) fluid filled appendix surrounded by a band - like area of high signal intensity ( arrow ) compatible with appendicitis with periappendicular inflammation . the inflamed appendix is seen as a tubular fluid filled structure ( arrow ) in the right lower quadrant , behind the pregnant uterus . on surgery , mri is also useful in identifying alternative sources of right lower quadrant pain in patients suspected of acute appendicitis [ figure 15 ] . a 13- year - old - male with a history of crohn 's disease , presenting with right lower abdominal pain . axial t2 weighted image after diluted oral contrast media administration , showing the thickened - wall terminal ileum ( long arrows ) compatible with crohn 's disease . a normal retrocecal appendix is seen ( short arrow ) . in two previous review studies , us and ct performance for the diagnosis of acute appendicitis the respective mean specificities for ct and graded compression us were 90% and 83% . in a head - to - head comparison , ct offered a better test performance than graded compression us in diagnosing appendicitis . therefore , ct is recommended in patients suspected of acute appendicitis . however , taking into consideration the drawbacks of radiation exposure in young , female and slender patients , graded compression us is recommended as the primary diagnostic test in these patients . based on 26 studies in children and 31 studies in adults from 1988 to 2004 , doria et al found the pooled sensitivity and specificity for us in the diagnosis of appendicitis in children to be 88% and 94% respectively , and for ct studies , 94% and 95% respectively . the pooled sensitivity and specificity for us studies in the diagnosis of appendicitis in adults were 83% and 93% respectively , and for ct studies , 94% and 94% respectively . however , the drawbacks of radiation exposure associated with ct should be considered , especially in children . methodological shortcomings in both of these studies , however , could influence reported diagnostic test accuracy . in our present review , we selected studies performed from february 2006 to march 2011 comparing graded compression us and ct performance , using surgery or clinical follow - up as the standard reference . the results are presented in table 1 . us , ct , and mri performance for the diagnosis of acute appendicitis poortman et al developed an imaging diagnostic pathway using us as the primary test . of a total population of 151 adult patients suspected of appendicitis , 79 patients diagnosed by us as positive for appendicitis were directed to appendectomy and 72 negative or inconclusive patients diagnosed by us proceeded to mdct . an alternative diagnosis was made in 12 patients and no source of abdominal pain was found in 39 patients . in this study , the sensitivity and specificity for ct were 100% , and for us , 77% and 86% respectively . although less accurate than ct , us can be used as a primary diagnostic modality avoiding the disadvantages of ct . gaitini et al conducted a retrospective study to evaluate the diagnostic accuracy of color doppler us and contrast - enhanced mdct in 420 adult patients referred from the emergency room with clinical suspicion of appendicitis . all the patients underwent graded compression us and color doppler of the right lower quadrant . ct was performed in 132 patients due to inconclusive sonographic findings or a discrepancy between clinical and sonographic diagnoses . sonography and ct correctly diagnosed acute appendicitis in 66 of 75 patients and in 38 of 39 patients , respectively , and correctly ruled out acute appendicitis in 312 of 326 and in 92 of 92 patients . sonography was inconclusive in 17 of 418 cases and ct , in one of 132 cases . the sensitivity and specificity of us were 74% and 97% respectively , with a 93% negative predictive value , while ct had a sensitivity and specificity of 100 % and 99% respectively , with a 100% negative predictive value . the positive predictive value and accuracy for sonography were 88% and 92% and for ct , 97% and 99% . due to its high negative predictive value ( 93% ) , us should be used as the first imaging test in adult patients for the diagnosis of appendicitis and triage of acute abdominal pain , reserving ct as a complementary study for selected cases . cuschieri et al , of the surgical care and outcomes assessment program ( scoap ) collaborative group evaluated the relationship between negative appendectomies ( na ) and negative ct / us over a two year period ( 20062007 ) . there was a significant increase in the use of ct / us and decrease in na over the study period ( p < .0001 ) . variation between hospitals was linked closely to ct / us accuracy , suggesting that ct / us accuracy should be considered a measure of quality in the care of patients with presumed appendicitis . us is widely used for the diagnosis of appendicitis in pediatric patients due to concerns regarding risks from exposure to ionizing radiation . in a recent paper , wan et al analyzed cost - effectiveness of us versus ct in young children based on a decision analytic model using costs , utilities and probabilities . the most cost - effective method was to start with a us study and follow negative us studies with a ct examination . this strategy is in concordance with the previously published study of garcia pena et al . pregnant patients present a special population , and clinical and laboratory findings lack specificity for the diagnosis of appendicitis in pregnancy . an unnecessary laparotomy increases the risk of pre - term contractions , while delay in the treatment of appendicitis leading to perforation increases fetal mortality to 637% . pedrosa et al investigated the role of mri in 148 pregnant patients suspected of appendicitis in lowering the negative laparotomy rate ( nlr ) and the perforation rate ( pr ) . an oral contrast media was administrated to facilitate identification of the appendix in mri examinations . us had a low sensitivity ( 36% ) but an excellent specificity ( 99% ) . the sensitivity and specificity of mri were 100% and 99% respectively . among the patients with a negative diagnosis for appendicitis , the normal appendix could be visualized on us in less than 2% ( 2 of 126 ) of cases and on mr in 87% ( 116 of 134 ) of cases ( p < .0001 ) . the use of mr imaging yields favorable combinations of nlr and pr compared with values previously reported in the literature . comparison of methods for imaging appendicitis the main advantage of us over ct is the lack of ionizing radiation , which is most important in the pediatric and young adult populations , among which appendicitis occurs more frequently , and who are most vulnerable to radiation 's detrimental effects.[2731 ] in the pediatric population , us also obviates the need for sedation or general anesthesia . us may be considered as an extension of the physical examination : patients can point to the region of tenderness and graded compression us may elicit a rebound with an increased sensation of pain . contrast media ingestion , with a consequent delay in surgery , or contrast injection , carrying risks of allergic reaction and nephrotoxicity , are not required for us examinations . us may not only diagnose an abscess or phlegmon in perforated appendicitis but can also guide percutaneous drainage . several alternative conditions , especially in the female pelvis , may be diagnosed on us examination.[24 ] the main disadvantage of us is its operator dependence . us has a lower sensitivity compared to ct for the diagnosis of appendicitis.[67222433 ] however , with a high prevalence of the disease and a strict protocol , a sensitivity and specificity of 98.5% and 98.2% respectively was achieved for the us diagnosis of appendicitis in children . the inability of us to properly scan a gas distended bowel or an obese patient , and lack of demonstration of a normal appendix , especially when retrocecal or deeply situated in the pelvis , may lead to indeterminate or false negative exams . in pregnant patients , the main advantage of ct for the diagnosis of appendicitis is its high sensitivity and specificity [ table 1].[3234 ] marked advancements in ct technology over the last decade have led to excellent image quality . the main disadvantage of ct is the radiation exposure , especially important in the radiosensitive pediatric and young adult appendicitis - suspected population . allergic reaction and nephrotoxicity risks of iodinated contrast media injection , delay in the diagnosis and therapy due to time invested in ingestion of contrast media , higher cost and lower availability , particularly in small or peripheral centers , are further disadvantages of ct . operator dependence related to ct protocols and radiologist skills for correct performance and interpretation are also limitations of ct exams . the main advantage of mri in pregnant patients is in avoiding radiation exposure to the fetus . mri is less operator - dependent than us , lacks exposure to intravenous iodinated contrast media , and may afford alternative diagnoses , such as ovarian torsion or renal obstruction.[203739 ] mri disadvantages reside in a longer examination time , limitations related to claustrophobia and metal devices , low availability , and higher cost . guidelines for the diagnosis of appendicitis are needed , related to choosing which is the optimal patient - tailored imaging modality to begin with , and an algorithm for further imaging procedures , if required . according to the present review , imaging of a patient of any age suspected of appendicitis should start with a graded compression us examination . a positive , good - quality us when there is a strong clinical suspicion is enough to proceed to surgery . a negative us exam for appendicitis , where the normal appendix is clearly visualized , or alternatively , when the appendix is not seen but the clinical suspicion for appendicitis is low , may end the investigation , leading to patient discharge or follow - up . furthermore , if an alternative diagnosis for the clinical presentation is reached , treatment of the alternative condition will be followed . a negative or indeterminate us exam in a non - pregnant adult with a high clinical suspicion of appendicitis a negative us with a high suspicion for appendicitis should lead to an mri examination.[3840 ] [ figure 16 ] . in the pediatric patient , a second us examination , especially when the first was performed by a less experienced professional , may render definitive results , while avoiding ct radiation risks . a positive us is enough to proceed to surgery or percutaneous drainage of a peri - appendicular abscess a negative us showing the normal appendix may end the investigation and lead to patient discharge . an indeterminate us , whether neither the normal nor the inflamed appendix is seen may lead to follow up or a repeated us if the clinical suspicion is low . this approach seems to offer excellent accuracy , with reported sensitivities of 94% to 99% and specificities of 94% to 95% . high - resolution linear array transducers with harmonics and compound capabilities in modern us systems allow optimal definition of the structures at the right lower quadrant . in obese patients , bowel wall and peristalsis , iliopsoas muscle , iliac vessels , mesenteric fat , and mesenteric lymph nodes are clearly defined . the normal appendix can be visualized from the base of the cecum as a blind - ended , gut - pattern aperistaltic tubular structure , with a wall thickness of 2 mm or less and a 67 mm or less diameter [ figure 1 ] . a 31-year - old - female with right lower quadrant abdominal pain for two days . a blind - ended , gut - pattern tubular structure ( arrows ) , with 2 mm wall thickness and 4.5 mm lumen width ( cursors ) is seen anterior to the right common iliac vessels on gray - scale us graded compression . the inflamed appendix is visualized as an incompressible , blind - ended , gut - pattern fluid - filled tubular structure , with a thickened wall , and a diameter greater than 67 mm . applying graded compression with the transducer , differentiation between normal displaceable loops and color and power doppler may aid in the diagnosis by showing a hyperemic wall [ figure 2 ] . manual compression of the lumbar fossa improves visualization of appendicitis , in particular when it is retrocecal . visualization of the whole appendix is needed to detect if inflammation is localized just at the end of the appendix , known as  tip  appendicitis . when there is clinical suspicion of appendicitis , the presence of a fecalith in the appendix confirms the diagnosis [ figure 3 ] . additional findings are pelvic or inter loop peritoneal fluid and hyperechoic inflamed mesenteric fat surrounding the appendix [ figure 4 ] . perforated appendicitis may be seen as an abscess in the right lower quadrant [ figure 5 ] . appendicitis - mimicking conditions of gastrointestinal and urogenital origin may be revealed on us images of the right lower quadrant [ figure 7 ] . an incompressible , blind - ended , gut - pattern fluid - filled tubular structure , with thickened wall and diameter greater than 7 mm is seen in the right lower quadrant ( arrows ) . following diagnosis of acute appendicitis on us examination , patient proceeded to surgery , without further imaging tests . a normal proximal appendix , 4.8 mm width followed by enlarged , 12.7 mm width fluid - filled distal appendix is seen on compression us . a hyperechogenic focus with posterior acoustic shadow compatible with a fecalith ( b ) short axis view of the appendix , showing the normal proximal appendix and the inflamed distal part ( measurements ) , the last containing an appendicolith . a 21-year - old - male with right pelvic pain and fever . hyperechoic thickened mesenteric fat ( arrows ) surrounding 9.5 mm inflamed appendix ( cursors ) is seen on compression us . a 10- year - old- female with abdominal pain for three days and rebound in right lower quadrant . dual gray scale compression us image ( right plot before compression ; left plot during compression ) showing an uncompressible distended appendix ( cursors ) surrounded by a fluid collection ( arrows ) . a 45- year - old - female with fever and abdominal pains , 10 days after laparoscopic appendectomy . ( a ) contrast enhanced mdct showing a retrocecal collection displacing the ascending colon ( arrows ) . lower quadrant demonstrated fluid collection ( long arrows ) and in the bottom , an echogenic structure with posterior acoustic shadowing ( short arrows ) , compatible with a dropped appendicolith . ( c ) a 7f pig tail catheter ( arrow at the tip ) is introduced in the collection under us guidance . ( d ) the catheter is shown in the partially drained collection ( arrow ) . a six - year - old - female with pain in right lower abdomen and pelvis . ( a ) longitudinal us scan of the pelvis showing a hyperechoic linear structure with a  twinkle artifact on color doppler compatible with a stone , in the right ureter at the uretero vesical junction . they were distinguished by factors including the use of oral or rectal contrast administration of either positive , neutral , or water contrast agents and whether non - contrast or contrast exams after intravenous injection were performed . they also varied in the anatomical extension of the scan , some including the entire abdomen and pelvis and others focusing on the anatomical area , from the level of the xyphoid to the ramus pubis . the normal appendix is visualized as a blind - ended tubular structure exiting from the cecum , filled with contrast media or gas [ figure 8 ] . the normal diameter ranges between 6 and 10 mm , although more than 7 mm is generally considered as pathological . a 10-year - old - male with persistent right lower quadrant abdominal pain and fever , after two negative us exams for appendicitis . a retrocecal subhepatic blind - ended gas filled tubular structure of normal wall thickness and normal diameter is seen ( large arrow ) . the inflamed appendix is seen as a blind - ended tubular structure exiting from the cecum , without contrast or air filling , more than 7 mm in diameter [ figure 9 ] . the presence of contrast or air from gas - forming organisms in the proximal part of the lumen does not exclude appendicitis . additional findings such as the presence of an appendicolith , interloop peritoneal fluid , cecal wall thickening , and periappendicular fat stranding are especially useful in an indeterminate appendix [ figure 10 ] . administration of intravenous contrast media , as a bolus injection of 80 - 100 cc non - ionic contrast media , allows evaluation of the appendix wall enhancement , which can be useful in borderline cases . a three - year - old - male with abdominal pains and rebound in right lower abdomen . ( a ) gray scale us performed by the resident on - duty showed a structure interpreted as mimicking intussusception in the subhepatic area ( arrows ) . due to the patient 's clinical condition and indeterminate us diagnosis , a ct was performed . ( b ) contrast enhanced coronal oblique reformatted mdct showing a widened fluid filled tubular structure , without intraluminal air , of oral contrast media ( arrows ) , surrounded by infiltrated mesenteric fat , compatible with acute appendicitis . a 47-year - old - male with a history of inflammatory bowel disease , presenting with abdominal pain and fever . acute appendicitis with mesenteric stranding on ct . ( a ) a blind - ended tubular structure exiting from the cecum , without intraluminal oral contrast media or air , 12 mm in diameter , is seen ( large arrow ) . mesenteric fat stranding ( short arrow ) is seen in the proximity of the inflamed appendix . ( b ) mesenteric fat edema and reactive lymph nodes are seen in a scan proximal to ( a ) . ct followed a us exam positive for appendicitis , to rule - out signs of active crohn 's disease . alternative diagnosis of right lower quadrant pain may be achieved using ct , to determine if it is of gastrointestinal origin , such as mesenteric adenitis , intussusception , terminal ileitis , diverticulitis , epiploic appendagitis , and typhlitis , or of urogenital origin , such as ureteral stone and urinary tract infection , tubo ovarian abscess , ovarian torsion , ectopic pregnancy , hemorrhagic ovarian cyst or corpus luteum remnants [ figures 11 and 12 ] . cecum and terminal ileum with thickened walls ( arrows ) and adjacent peritoneal fluid ( f ) confirm crohn 's disease . an appendicolith ( lateral right arrow ) is seen at the base of the inflamed appendix , ( shown in figure 9 ) . mri is an alternative method to ct for pregnant patients , offering high soft tissue contrast without ionizing radiation . the imaging protocol for mri evaluation of acute appendicitis in the pregnant population includes t1- and t2- weighted images , and has been extensively described in the literature . the normal appendix is seen as a tubular structure exiting from the cecum , greater than 7 mm in diameter , filled with air or contrast media [ figure 13 ] . a 37- year - old - pregnant patient with lower abdominal pain and fever . a normal sized appendix with air into the lumen is seen in the right lower quadrant in this axial t1 weighted with fat suppression image . a previously performed us was negative for appendicitis , but the normal appendix could not be demonstrated . acute appendicitis is seen as an enlarged appendix , greater than 7 mm in diameter , and void of air or contrast media . signs of peri - appendicular inflammation , seen as band - like areas of high signal intensity on t2-weighted images , single - shot fast spin - echo images or fat saturation images , or the presence of an appendicolith , visualized as an intraluminal low signal intensity focus , confirm the diagnosis , especially in the borderline widened appendix [ figure 14].[1620 ] according to the american college of radiology guidelines for safe mri practices , contrast agents should not be routinely administered to pregnant patients . a 28-year - old - female of 26 weeks pregnancy , presenting with lower abdominal pain and fever . ( a ) coronal t2 weighted image showing an enlarged ( 10 mm in diameter ) fluid filled appendix surrounded by a band - like area of high signal intensity ( arrow ) compatible with appendicitis with periappendicular inflammation . the inflamed appendix is seen as a tubular fluid filled structure ( arrow ) in the right lower quadrant , behind the pregnant uterus . on surgery , mri is also useful in identifying alternative sources of right lower quadrant pain in patients suspected of acute appendicitis [ figure 15 ] . a 13- year - old - male with a history of crohn 's disease , presenting with right lower abdominal pain . axial t2 weighted image after diluted oral contrast media administration , showing the thickened - wall terminal ileum ( long arrows ) compatible with crohn 's disease . in two previous review studies , us and ct performance for the diagnosis of acute appendicitis were compared . the respective mean specificities for ct and graded compression us were 90% and 83% . in a head - to - head comparison , ct offered a better test performance than graded compression us in diagnosing appendicitis . however , taking into consideration the drawbacks of radiation exposure in young , female and slender patients , graded compression us is recommended as the primary diagnostic test in these patients . based on 26 studies in children and 31 studies in adults from 1988 to 2004 , doria et al found the pooled sensitivity and specificity for us in the diagnosis of appendicitis in children to be 88% and 94% respectively , and for ct studies , 94% and 95% respectively . the pooled sensitivity and specificity for us studies in the diagnosis of appendicitis in adults were 83% and 93% respectively , and for ct studies , 94% and 94% respectively . however , the drawbacks of radiation exposure associated with ct should be considered , especially in children . methodological shortcomings in both of these studies , however , could influence reported diagnostic test accuracy . in our present review , we selected studies performed from february 2006 to march 2011 comparing graded compression us and ct performance , using surgery or clinical follow - up as the standard reference . us , ct , and mri performance for the diagnosis of acute appendicitis poortman et al developed an imaging diagnostic pathway using us as the primary test . of a total population of 151 adult patients suspected of appendicitis , 79 patients diagnosed by us as positive for appendicitis were directed to appendectomy and 72 negative or inconclusive patients diagnosed by us proceeded to mdct . an alternative diagnosis was made in 12 patients and no source of abdominal pain was found in 39 patients . in this study , the sensitivity and specificity for ct were 100% , and for us , 77% and 86% respectively . although less accurate than ct , us can be used as a primary diagnostic modality avoiding the disadvantages of ct . gaitini et al conducted a retrospective study to evaluate the diagnostic accuracy of color doppler us and contrast - enhanced mdct in 420 adult patients referred from the emergency room with clinical suspicion of appendicitis . all the patients underwent graded compression us and color doppler of the right lower quadrant . ct was performed in 132 patients due to inconclusive sonographic findings or a discrepancy between clinical and sonographic diagnoses . sonography and ct correctly diagnosed acute appendicitis in 66 of 75 patients and in 38 of 39 patients , respectively , and correctly ruled out acute appendicitis in 312 of 326 and in 92 of 92 patients . sonography was inconclusive in 17 of 418 cases and ct , in one of 132 cases . the sensitivity and specificity of us were 74% and 97% respectively , with a 93% negative predictive value , while ct had a sensitivity and specificity of 100 % and 99% respectively , with a 100% negative predictive value . the positive predictive value and accuracy for sonography were 88% and 92% and for ct , 97% and 99% . due to its high negative predictive value ( 93% ) , us should be used as the first imaging test in adult patients for the diagnosis of appendicitis and triage of acute abdominal pain , reserving ct as a complementary study for selected cases . cuschieri et al , of the surgical care and outcomes assessment program ( scoap ) collaborative group evaluated the relationship between negative appendectomies ( na ) and negative ct / us over a two year period ( 20062007 ) . there was a significant increase in the use of ct / us and decrease in na over the study period ( p < .0001 ) . variation between hospitals was linked closely to ct / us accuracy , suggesting that ct / us accuracy should be considered a measure of quality in the care of patients with presumed appendicitis . us is widely used for the diagnosis of appendicitis in pediatric patients due to concerns regarding risks from exposure to ionizing radiation . in a recent paper , wan et al analyzed cost - effectiveness of us versus ct in young children based on a decision analytic model using costs , utilities and probabilities . the most cost - effective method was to start with a us study and follow negative us studies with a ct examination . this strategy is in concordance with the previously published study of garcia pena et al . pregnant patients present a special population , and clinical and laboratory findings lack specificity for the diagnosis of appendicitis in pregnancy . an unnecessary laparotomy increases the risk of pre - term contractions , while delay in the treatment of appendicitis leading to perforation increases fetal mortality to 637% . pedrosa et al investigated the role of mri in 148 pregnant patients suspected of appendicitis in lowering the negative laparotomy rate ( nlr ) and the perforation rate ( pr ) . an oral contrast media was administrated to facilitate identification of the appendix in mri examinations . us had a low sensitivity ( 36% ) but an excellent specificity ( 99% ) . the sensitivity and specificity of mri were 100% and 99% respectively . among the patients with a negative diagnosis for appendicitis , the normal appendix could be visualized on us in less than 2% ( 2 of 126 ) of cases and on mr in 87% ( 116 of 134 ) of cases ( p < .0001 ) . the use of mr imaging yields favorable combinations of nlr and pr compared with values previously reported in the literature . comparison of methods for imaging appendicitis the main advantage of us over ct is the lack of ionizing radiation , which is most important in the pediatric and young adult populations , among which appendicitis occurs more frequently , and who are most vulnerable to radiation 's detrimental effects.[2731 ] in the pediatric population , us also obviates the need for sedation or general anesthesia . us may be considered as an extension of the physical examination : patients can point to the region of tenderness and graded compression us may elicit a rebound with an increased sensation of pain . contrast media ingestion , with a consequent delay in surgery , or contrast injection , carrying risks of allergic reaction and nephrotoxicity , are not required for us examinations . us may not only diagnose an abscess or phlegmon in perforated appendicitis but can also guide percutaneous drainage . several alternative conditions , especially in the female pelvis , may be diagnosed on us examination.[24 ] the main disadvantage of us is its operator dependence . us has a lower sensitivity compared to ct for the diagnosis of appendicitis.[67222433 ] however , with a high prevalence of the disease and a strict protocol , a sensitivity and specificity of 98.5% and 98.2% respectively was achieved for the us diagnosis of appendicitis in children . the inability of us to properly scan a gas distended bowel or an obese patient , and lack of demonstration of a normal appendix , especially when retrocecal or deeply situated in the pelvis , may lead to indeterminate or false negative exams . in pregnant patients , the main advantage of ct for the diagnosis of appendicitis is its high sensitivity and specificity [ table 1].[3234 ] marked advancements in ct technology over the last decade have led to excellent image quality . the main disadvantage of ct is the radiation exposure , especially important in the radiosensitive pediatric and young adult appendicitis - suspected population . allergic reaction and nephrotoxicity risks of iodinated contrast media injection , delay in the diagnosis and therapy due to time invested in ingestion of contrast media , higher cost and lower availability , particularly in small or peripheral centers , are further disadvantages of ct . operator dependence related to ct protocols and radiologist skills for correct performance and interpretation are also limitations of ct exams . the main advantage of mri in pregnant patients is in avoiding radiation exposure to the fetus . mri is less operator - dependent than us , lacks exposure to intravenous iodinated contrast media , and may afford alternative diagnoses , such as ovarian torsion or renal obstruction.[203739 ] mri disadvantages reside in a longer examination time , limitations related to claustrophobia and metal devices , low availability , and higher cost . guidelines for the diagnosis of appendicitis are needed , related to choosing which is the optimal patient - tailored imaging modality to begin with , and an algorithm for further imaging procedures , if required . according to the present review , imaging of a patient of any age suspected of appendicitis should start with a graded compression us examination . a positive , good - quality us when there is a strong clinical suspicion is enough to proceed to surgery . a negative us exam for appendicitis , where the normal appendix is clearly visualized , or alternatively , when the appendix is not seen but the clinical suspicion for appendicitis is low , may end the investigation , leading to patient discharge or follow - up furthermore , if an alternative diagnosis for the clinical presentation is reached , treatment of the alternative condition will be followed . a negative or indeterminate us exam in a non - pregnant adult with a high clinical suspicion of appendicitis must be followed by ct . in pregnant patients , a negative us with a high suspicion for appendicitis should lead to an mri examination.[3840 ] [ figure 16 ] . in the pediatric patient , a second us examination , especially when the first was performed by a less experienced professional , may render definitive results , while avoiding ct radiation risks . a positive us is enough to proceed to surgery or percutaneous drainage of a peri - appendicular abscess a negative us showing the normal appendix may end the investigation and lead to patient discharge . an indeterminate us , whether neither the normal nor the inflamed appendix is seen may lead to follow up or a repeated us if the clinical suspicion is low . this approach seems to offer excellent accuracy , with reported sensitivities of 94% to 99% and specificities of 94% to 95% . imaging is increasingly important in the diagnosis and management of appendicitis , avoiding unnecessary interventions and delay in treatment that may lead to perforation . us findings must be considered in conjunction with the clinical evaluation . if these guidelines are followed , the number of ct examinations performed will be considerably reduced . clinical guidelines defining the use of each particular imaging modality allow uniformity , a targeted approach for different patient populations , and cost - effectiveness of medical services .
the goal of this review is to present the state of the art in imaging tests for the diagnosis of acute appendicitis . 
 relevant publications regarding performance and advantages / disadvantages of imaging modalities for the diagnosis of appendicitis in different clinical situations were reviewed . 
 articles were extracted from a computerized database ( medline ) with the following activated limits : humans , english , core clinical journals , and published in the last five years . 
 reference lists of relevant studies were checked manually to identify additional , related articles . 
 ultrasound ( us ) examination should be the first imaging test performed , particularly among the pediatric and young adult populations , who represent the main targets for appendicitis , as well as in pregnant patients . a positive us examination for appendicitis or an alternative diagnosis of possible gastrointestinal or urological origin , or a negative us , either showing a normal appendix or presenting low clinical suspicion of appendicitis , should lead to a final diagnosis . 
 a negative or indeterminate examination with a strong clinical suspicion of appendicitis should be followed by a computed tomography ( ct ) scan or alternatively , a magnetic resonanace imaging ( mri ) scan in a pregnant patient . 
 a second us examination in a patient with persistent symptoms , especially if the first one was performed by a less experienced imaging professional , is a valid alternative to a ct .