colonic diverticulitis ( cd ) is a common entity with well - known clinical and radiological findings , especially with regard for ct features . moreover , computed tomography ( ct ) is widely accepted as the standard of reference technique for diagnosis of cd [ 13 ] . however , the diagnosis may be misleading in the case of unusual presentation that may mimic several acute or even chronic abdominal pathological conditions . the objective of this work is to delineate the pitfalls of ct imaging in patients with suspected cd . we will discuss pitfalls related to technical factors , anatomical factors , cd complications and conditions that can mimic cd . ct is well suited to the evaluation of cd because it is able to demonstrate the wall of the colon as well as the surrounding pericolic fat . for imaging suspected cd and associated complications , careful attention to technique timing of ct : the most common reason for ct failure in diagnosing cd is an excessively long interval between antibiotic therapy initiation and ct . ct should be done within 48 h after onset of the symptoms suggesting cd ( fig .  1 ) . 1abdominal ct performed within 24 h of symptoms onset demonstrating typical features of sigmoid diverticulitis ( a , b ) , with colonic wall thickening ( black arrow ) and paracolic inflammation ( white arrow ) . ct performed three weeks later , does not show any suggestion of diverticulitisnon - contrast - enhanced ct : this is the first step in ct examination . performed before any opacification , it may demonstrate free - air perforation and/or severe intestinal occlusion , which are relative contra - indications of digestive opacification . sometimes it gives additional arguments in favour of the diagnosis , such as an intradiverticular stercolith that may be less visible on a contrast - enhanced phase ( fig . 2stercolith within a diverticulum of the right colon is more clearly identified on non - contrast - enhanced ct ( a ) ( white arrow ) than on contrast - enhanced ct ( b ) . the association with mild paracolic inflammation ( black arrows ) is diagnostic for diverticulitis of the right coloncolonic opacification : although colonic opacification is not mandatory for the diagnosis of cd , it helps prevent confusion between collapsed bowel and mural thickening due to inflammation by distending the rectum and colon , and provides better images of the colon wall and pericolic abnormalities . in a few cases , furthermore , colonic opacification could be diagnostic even without intravenous contrast - enhanced ct , which is of great interest in patients with renal failure or proven allergy to contrast material ( fig . some authors use oral contrast material but it should not be advised because of the increased patient preparation time and an often insufficient anterograde colonic opacification . 
 fig . 3diverticulitis of the right colon in a 67-year - old woman with a history of renal failure . only non - contrast - enhanced ct was performed in this patient and the assessment of ct features of diverticulitis ( white arrows ) was feasible thanks to colonic opacificationcontrast - enhanced ct : intravenous contrast material administration is crucial for ct examination . portal venous phase performed after a 70- to 80-s delay , usually sufficient for arterial visualisation and excellent for venous opacification , is critical for enabling detection of subtle bowel wall abnormalities and vascular complications that otherwise might not be visible ( fig .  4 ) . 4abdominal ct in a 63-year - old woman , demonstrating typical features of non - complicated sigmoid diverticulitis ( arrowheads ) . however , left uterine vein thrombosis with vein enlargement and filling defect ( white arrows ) was detected on portal phase contrast - enhanced ctinvestigation of the entire abdominal cavity : it is mandatory and far more frequently performed now than in the past , thanks to the development of multidetector ct . failure to comply with this rule carries a risk of missing other diagnoses , such as pylephlebitis , hepatic abscess or even cd somewhere other than in the sigmoid colon.multiplanar reformations ( mprs ) : mprs are not actually technical pitfalls but insufficiencies of the interpretation methodology . mprs improve advantageously the assessment of colic and pericolic abnormalities , especially in the case of unusual anatomical location or complications ( fig .  5 ) . they are widely used and recommended in the assessment of acute abdominal conditions [ 1315 ] . 5diverticulitis of the right colon , with an inflamed diverticulum of the posterior wall of the caecum with fat stranding ( arrow ) . mpr showing adjacent normal appendix ( arrowheads ) confirmed the diagnosis timing of ct : the most common reason for ct failure in diagnosing cd is an excessively long interval between antibiotic therapy initiation and ct . ct should be done within 48 h after onset of the symptoms suggesting cd ( fig .  1 ) . 1abdominal ct performed within 24 h of symptoms onset demonstrating typical features of sigmoid diverticulitis ( a , b ) , with colonic wall thickening ( black arrow ) and paracolic inflammation ( white arrow ) . ct performed three weeks later , does not show any suggestion of diverticulitis abdominal ct performed within 24 h of symptoms onset demonstrating typical features of sigmoid diverticulitis ( a , b ) , with colonic wall thickening ( black arrow ) and paracolic inflammation ( white arrow ) . ct performed three weeks later , does not show any suggestion of diverticulitis non - contrast - enhanced ct : this is the first step in ct examination . performed before any opacification , it may demonstrate free - air perforation and/or severe intestinal occlusion , which are relative contra - indications of digestive opacification . sometimes it gives additional arguments in favour of the diagnosis , such as an intradiverticular stercolith that may be less visible on a contrast - enhanced phase ( fig . 2stercolith within a diverticulum of the right colon is more clearly identified on non - contrast - enhanced ct ( a ) ( white arrow ) than on contrast - enhanced ct ( b ) . the association with mild paracolic inflammation ( black arrows ) is diagnostic for diverticulitis of the right colon stercolith within a diverticulum of the right colon is more clearly identified on non - contrast - enhanced ct ( a ) ( white arrow ) than on contrast - enhanced ct ( b ) . the association with mild paracolic inflammation ( black arrows ) is diagnostic for diverticulitis of the right colon colonic opacification : although colonic opacification is not mandatory for the diagnosis of cd , it helps prevent confusion between collapsed bowel and mural thickening due to inflammation by distending the rectum and colon , and provides better images of the colon wall and pericolic abnormalities . in a few cases , failure to opacify the colonic lumen may hinder the diagnosis of cd . furthermore , colonic opacification could be diagnostic even without intravenous contrast - enhanced ct , which is of great interest in patients with renal failure or proven allergy to contrast material ( fig . some authors use oral contrast material but it should not be advised because of the increased patient preparation time and an often insufficient anterograde colonic opacification . 
 fig . 3diverticulitis of the right colon in a 67-year - old woman with a history of renal failure . only non - contrast - enhanced ct was performed in this patient and the assessment of ct features of diverticulitis ( white arrows ) was feasible thanks to colonic opacification diverticulitis of the right colon in a 67-year - old woman with a history of renal failure . only non - contrast - enhanced ct was performed in this patient and the assessment of ct features of diverticulitis ( white arrows ) was feasible thanks to colonic opacification contrast - enhanced ct : intravenous contrast material administration is crucial for ct examination . portal venous phase performed after a 70- to 80-s delay , usually sufficient for arterial visualisation and excellent for venous opacification , is critical for enabling detection of subtle bowel wall abnormalities and vascular complications that otherwise might not be visible ( fig .  4 ) . 4abdominal ct in a 63-year - old woman , demonstrating typical features of non - complicated sigmoid diverticulitis ( arrowheads ) . however , left uterine vein thrombosis with vein enlargement and filling defect ( white arrows ) was detected on portal phase contrast - enhanced ct abdominal ct in a 63-year - old woman , demonstrating typical features of non - complicated sigmoid diverticulitis ( arrowheads ) . however , left uterine vein thrombosis with vein enlargement and filling defect ( white arrows ) was detected on portal phase contrast - enhanced ct investigation of the entire abdominal cavity : it is mandatory and far more frequently performed now than in the past , thanks to the development of multidetector ct . failure to comply with this rule carries a risk of missing other diagnoses , such as pylephlebitis , hepatic abscess or even cd somewhere other than in the sigmoid colon . multiplanar reformations ( mprs ) : mprs are not actually technical pitfalls but insufficiencies of the interpretation methodology . mprs improve advantageously the assessment of colic and pericolic abnormalities , especially in the case of unusual anatomical location or complications ( fig .  5 ) . they are widely used and recommended in the assessment of acute abdominal conditions [ 1315 ] . 5diverticulitis of the right colon , with an inflamed diverticulum of the posterior wall of the caecum with fat stranding ( arrow ) . mpr showing adjacent normal appendix ( arrowheads ) confirmed the diagnosis diverticulitis of the right colon , with an inflamed diverticulum of the posterior wall of the caecum with fat stranding ( arrow ) . colonic diverticulitis usually occurs in the left side of the colon ( 90% ) but may occur anywhere , except in the rectum . clinical and imaging features are highly dependent on the location of the inflamed diverticulum within the colon and its anatomical connections with intra- and retroperitoneal spaces . diverticulitis of the right colon : the incidence of this form is low in western countries ( 1.5% ) but is significantly higher ( 5570% ) in asian countries . it can present a significant diagnostic challenge , especially with acute appendicitis ( fig .  5 ) [ 17 , 18 ] . actually , the clinical preoperative diagnosis rate of right cd is as low as 535% [ 17 , 19 ] . performing ct improves significantly the diagnosis accuracy in right cd by showing typical features of an  inflamed diverticulum 6 ) , which is defined as a rounded , paracolic outpouching centred within paracolic inflammation , with a measured soft tissue attenuation . therefore , ct may prevent the patient from undergoing unnecessary surgery . 
 fig . 6diverticulitis of the right colon with an inflamed anterior diverticulum ( white arrow ) and thickening of the caecal wall , demonstrating the  arrowhead sign  ( black arrow)diverticulitis of the transverse colon : this represents a very unusual location of cd with only a few reports in the medical literature [ 21 , 22 ] . it can masquerade clinically as cholecystitis , appendicitis , hepatic abscess , gastroduodenal perforation , pancreatitis , splenic and renal infection or infarction ( fig .  7 ) . 7a 49-year - old patient , presented with acute pain of the upper right quadrant of the abdomen . ct showed a significant fat densification of the upper right quadrant ( white arrows ) mimicking segmental omental infarction . however , thickening of the right transverse colon associated with a typical  inflamed diverticulum  ( black arrow ) were diagnostic for diverticulitis of the transverse colondiverticulitis of the descending colon : much more common than the previous location , it is actually similar to sigmoid diverticulitis in its uncomplicated form ( fig .  8) . nevertheless , in its complicated forms and because of its close anatomical connection with the retroperitoneum , it becomes closer to  retroperitoneal  forms [ 23 , 24 ] . thus , the differential diagnosis with any kind of retroperitoneal disease may arise , even if it is less likely because left - sided symptoms are often suggestive . 8diverticulitis of the descending colon with an anterior  inflamed diverticulum  ( white arrows ) associated with intraperitoneal paracolic inflammationretroperitoneal  diverticulitis : diverticula in the posterior colon wall that are in close contact with the posterior peritoneum may produce retroperitoneal abnormalities [ 23 , 24 ] . 9diverticulitis of the descending colon with a posterior  inflamed diverticulum  ( arrowhead ) associated with retroperitoneal inflammation ( arrows)fig . 10contrast - enhanced abdominal ct performed for acute left - sided abdominal pain in a 56-year - old woman . b lower sections revealed  inflamed diverticulum  within the posterior wall of the descending colon ( black arrow ) confirming the diagnosis of left colonic diverticulitis with almost purely retroperitoneal involvementectopic  diverticulitis : this term encompasses cd in a long sigmoid loop lying in the right iliac fossa ( fig . 11sigmoid diverticulitis in a 29-year - old patient with a sigmoid loop lying in the right iliac fossa masquerading clinically as appendicitis . pericolic abscess ( black arrow)giant diverticulum : diverticula may vary in size but usually range from 2 to 3  mm up to 2  cm . giant diverticulum , defined as a colonic diverticulum measuring 4  cm in size or larger , is a rare complication of this common disease with fewer than 150 cases reported in the literature . preoperative diagnosis requires a high degree of suspicion and needs to be differentiated from sigmoid volvulus , caecal volvulus , intestinal duplication cyst , pneumatosis cystoides intestinalis , and similar conditions . awareness of this unusual condition and ct study are the keys to diagnosis.synchronous multifocal diverticulitis : multiplicity is a characteristic of diverticular disease . however , the occurrence of cd in distinct colonic sites at the same time is quite exceptional and may be misdiagnosed if the entire colon is not thoroughly analysed ( fig . visible on the same ct examination : two of them are located in the right ascending colon ( white and black arrows ) and one in the left descending colon ( arrowhead)recurrent multifocal diverticulitis : recurrence is characteristic of cd , usually occurring more or less at the same location as the previous episode , which is most often the sigmoid colon , justifying , in some reference centres , elective surgery after the second episode of acute cd . recurrence of cd at a different location in the colon ( e.g. right or transverse colon ) is quite a rare condition , raising controversy about the most appropriate management of patients following two episodes of acute cd ( fig . 13a a 61-year - old patient was admitted to hospital in december 2004 with acute abdominal pain . the patient was scheduled for surgery and right colonic resection was performed two months after recovery . d the patient was readmitted in july 2007 , with a fourth episode of diverticulitis involving the sigmoid colon diverticulitis of the right colon : the incidence of this form is low in western countries ( 1.5% ) but is significantly higher ( 5570% ) in asian countries . it can present a significant diagnostic challenge , especially with acute appendicitis ( fig . actually , the clinical preoperative diagnosis rate of right cd is as low as 535% [ 17 , 19 ] . performing ct improves significantly the diagnosis accuracy in right cd by showing typical features of an  inflamed diverticulum 6 ) , which is defined as a rounded , paracolic outpouching centred within paracolic inflammation , with a measured soft tissue attenuation . 6diverticulitis of the right colon with an inflamed anterior diverticulum ( white arrow ) and thickening of the caecal wall , demonstrating the  arrowhead sign  ( black arrow ) diverticulitis of the right colon with an inflamed anterior diverticulum ( white arrow ) and thickening of the caecal wall , demonstrating the  arrowhead sign  ( black arrow ) diverticulitis of the transverse colon : this represents a very unusual location of cd with only a few reports in the medical literature [ 21 , 22 ] . it can masquerade clinically as cholecystitis , appendicitis , hepatic abscess , gastroduodenal perforation , pancreatitis , splenic and renal infection or infarction ( fig .  7 ) . 7a 49-year - old patient , presented with acute pain of the upper right quadrant of the abdomen . ct showed a significant fat densification of the upper right quadrant ( white arrows ) mimicking segmental omental infarction . however , thickening of the right transverse colon associated with a typical  inflamed diverticulum  ( black arrow ) were diagnostic for diverticulitis of the transverse colon a 49-year - old patient , presented with acute pain of the upper right quadrant of the abdomen . ct showed a significant fat densification of the upper right quadrant ( white arrows ) mimicking segmental omental infarction . however , thickening of the right transverse colon associated with a typical  inflamed diverticulum  ( black arrow ) were diagnostic for diverticulitis of the transverse colon diverticulitis of the descending colon : much more common than the previous location , it is actually similar to sigmoid diverticulitis in its uncomplicated form ( fig .  8) . nevertheless , in its complicated forms and because of its close anatomical connection with the retroperitoneum , it becomes closer to  retroperitoneal  forms [ 23 , 24 ] . thus , the differential diagnosis with any kind of retroperitoneal disease may arise , even if it is less likely because left - sided symptoms are often suggestive . 8diverticulitis of the descending colon with an anterior  inflamed diverticulum  ( white arrows ) associated with intraperitoneal paracolic inflammation diverticulitis of the descending colon with an anterior  inflamed diverticulum  ( white arrows ) associated with intraperitoneal paracolic inflammation diverticulitis : diverticula in the posterior colon wall that are in close contact with the posterior peritoneum may produce retroperitoneal abnormalities [ 23 , 24 ] . 9diverticulitis of the descending colon with a posterior  inflamed diverticulum  ( arrowhead ) associated with retroperitoneal inflammation ( arrows)fig . 10contrast - enhanced abdominal ct performed for acute left - sided abdominal pain in a 56-year - old woman . b lower sections revealed  inflamed diverticulum  within the posterior wall of the descending colon ( black arrow ) confirming the diagnosis of left colonic diverticulitis with almost purely retroperitoneal involvement diverticulitis of the descending colon with a posterior  inflamed diverticulum  ( arrowhead ) associated with retroperitoneal inflammation ( arrows ) contrast - enhanced abdominal ct performed for acute left - sided abdominal pain in a 56-year - old woman . b lower sections revealed  inflamed diverticulum  within the posterior wall of the descending colon ( black arrow ) confirming the diagnosis of left colonic diverticulitis with almost purely retroperitoneal involvement  ectopic diverticulitis : this term encompasses cd in a long sigmoid loop lying in the right iliac fossa ( fig .  11 ) and the few cases of cd within inguinal or parietal herniation . 11sigmoid diverticulitis in a 29-year - old patient with a sigmoid loop lying in the right iliac fossa masquerading clinically as appendicitis . pericolic abscess ( black arrow ) sigmoid diverticulitis in a 29-year - old patient with a sigmoid loop lying in the right iliac fossa masquerading clinically as appendicitis . pericolic abscess ( black arrow ) giant diverticulum : diverticula may vary in size but usually range from 2 to 3  mm up to 2  cm . giant diverticulum , defined as a colonic diverticulum measuring 4  cm in size or larger , is a rare complication of this common disease with fewer than 150 cases reported in the literature . preoperative diagnosis requires a high degree of suspicion and needs to be differentiated from sigmoid volvulus , caecal volvulus , intestinal duplication cyst , pneumatosis cystoides intestinalis , and similar conditions . however , the occurrence of cd in distinct colonic sites at the same time is quite exceptional and may be misdiagnosed if the entire colon is not thoroughly analysed ( fig . visible on the same ct examination : two of them are located in the right ascending colon ( white and black arrows ) and one in the left descending colon ( arrowhead ) synchronous multifocal diverticulitis with three visible on the same ct examination : two of them are located in the right ascending colon ( white and black arrows ) and one in the left descending colon ( arrowhead ) recurrent multifocal diverticulitis : recurrence is characteristic of cd , usually occurring more or less at the same location as the previous episode , which is most often the sigmoid colon , justifying , in some reference centres , elective surgery after the second episode of acute cd . recurrence of cd at a different location in the colon ( e.g. right or transverse colon ) is quite a rare condition , raising controversy about the most appropriate management of patients following two episodes of acute cd ( fig . 13a a 61-year - old patient was admitted to hospital in december 2004 with acute abdominal pain . the patient was scheduled for surgery and right colonic resection was performed two months after recovery . d the patient was readmitted in july 2007 , with a fourth episode of diverticulitis involving the sigmoid colon a a 61-year - old patient was admitted to hospital in december 2004 with acute abdominal pain . abdominal ct was diagnostic for diverticulitis of the right transverse colon . the patient was scheduled for surgery and right colonic resection was performed two months after recovery . d the patient was readmitted in july 2007 , with a fourth episode of diverticulitis involving the sigmoid colon colonic diverticulitis is associated with regional inflammation affecting the pericolonic fat , the adjacent mesentery , the retroperitoneum or the pelvis resulting in several complications that could come to the forefront , misleading the primary diagnosis . perforation and abscess formation : ct evidence of a pericolic abscess or extraluminal air or contrast material is a well - established risk factor for failure of non - surgical treatment ( figs .  14 and 15 ) . even in the case of successful medical treatment , presence of extraluminal air or pericolic abscess indicates a need for prophylactic surgery [ 33 , 34 ] . ct demonstrated free gas within the mesosigmoid inflamed fat ( white arrows)small bowel obstruction : cd is an uncommon cause of small bowel obstruction and may be overlooked as a cause . small bowel obstruction occurs when diverticular inflammation , most often located at the antimesocolic side of a sigmoid loop , comes into contact with the mesenteric side of the small bowel ( fig . preoperative ct diagnosis is of first interest to rule out organic small bowel obstruction and prevent unnecessary surgery because inflammation can usually be healed with medical treatment , secondarily relieving the small bowel obstruction . 16sigmoid diverticulitis with abscess formation ( black arrows ) associated with thickening of adjacent small bowel wall ( white arrowhead ) responsible for intestinal obstruction ( white arrows)large bowel obstruction : cd is supposed to be the cause of 10% of large bowel obstructions . the large bowel can be obstructed in two ways : acute inflammation and oedema of the affected segment of bowel , with perhaps a pericolic abscess narrowing the lumen , or chronic inflammation , after recurrent attacks of diverticulitis , can result in fibrous bands across the bowel lumen causing obstruction ( fig . ct alone may not distinguish the benign from the malignant causes of luminal narrowing and colonoscopy or sometimes surgery is required where diagnostic uncertainty remains . 17chronic inflammatory narrowing of the sigmoid colon ( black arrows ) , due to recurrent diverticulitis and responsible for large bowel obstruction and retraction of small bowel loops around an inflammatory mass ( white arrows)fistulisation : fistulisation most frequently involves the bladder in men ( fig . 18 ) , which is less frequently involved in women when the uterus is present [ 11 , 37 ] . 19 ) . in other cases , fistulisation concerns the abdominal wall or the retroperitoneum and may have several presentations varying with the organ involved ( hydronephrosis , psoas abscess , spondylodiscitis , etc . ) [ 24 , 39 ] . the diagnostic difficulty remains , proving the relationship between fistula and cd in the case of mild or absent colonic abnormalities . 18contrast - enhanced abdominal ct demonstrating features of sigmoid  inflamed diverticulum  ( black arrows ) adjacent to a thickened bladder wall associated with free air within the bladder ( white arrows ) , establishing the diagnosis of sigmoid diverticulitis with bladder fistulisationfig . 19sigmoid diverticulitis with abscess formation ( black arrow ) and left adnexal extension of the inflammatory process ( white arrow)pylephlebitis : pylephlebitis is the result of the extension of the septic process into the venous drainage of the affected portion of the colon . spontaneous hyperdensity of the inferior mesenteric vein may be seen on non - contrast - enhanced ct . narrowing ct windows is required when looking for this hyperdensity which may be unseen with a large window which are more adapted to analysis of peritoneal abnormalities . luminal filling defect may be missed on contrast - enhanced ct if the injection delay is not optimal ( < 7080  s ) for portal vein assessment ( figs . abdominal contrast - enhanced ct demonstrated a superior mesenteric vein thrombosis ( white arrows ) , with no regional source identified on this first ct scan ( a , b ) . the septic source of this pylephlebitis was diverticulitis of the right colon ( black arrow ) detected by a second ct examination performed later with colonic opacification ( c)fig . 21contrast - enhanced ct demonstrating hepatic abscess ( black arrow ) and pylephlebitis with intravenous free air ( white arrowhead ) in the superior mesenteric vein . the lower ct sections revealed the septic source which was sigmoid diverticulitis with a small paracolic abscess ( white arrows)liver abscesses : these are rare complications of cd . a colonic septic source sometimes paucisymptomatic or hidden by an immunosuppressive treatment must be sought , especially when biliary tract disease is ruled out and no other septic origin is identified ( fig . perforation and abscess formation : ct evidence of a pericolic abscess or extraluminal air or contrast material is a well - established risk factor for failure of non - surgical treatment ( figs .  14 and 15 ) . even in the case of successful medical treatment , presence of extraluminal air or pericolic abscess indicates a need for prophylactic surgery [ 33 , 34 ] . ct demonstrated free gas within the mesosigmoid inflamed fat ( white arrows ) complicated sigmoid diverticulitis with two paracolic abscesses ( white arrows ) diverticular perforation complicating sigmoid diverticulitis in a 35-year - old man . ct demonstrated free gas within the mesosigmoid inflamed fat ( white arrows ) small bowel obstruction : cd is an uncommon cause of small bowel obstruction and may be overlooked as a cause . small bowel obstruction occurs when diverticular inflammation , most often located at the antimesocolic side of a sigmoid loop , comes into contact with the mesenteric side of the small bowel ( fig . preoperative ct diagnosis is of first interest to rule out organic small bowel obstruction and prevent unnecessary surgery because inflammation can usually be healed with medical treatment , secondarily relieving the small bowel obstruction . 16sigmoid diverticulitis with abscess formation ( black arrows ) associated with thickening of adjacent small bowel wall ( white arrowhead ) responsible for intestinal obstruction ( white arrows ) sigmoid diverticulitis with abscess formation ( black arrows ) associated with thickening of adjacent small bowel wall ( white arrowhead ) responsible for intestinal obstruction ( white arrows ) large bowel obstruction : cd is supposed to be the cause of 10% of large bowel obstructions . the large bowel can be obstructed in two ways : acute inflammation and oedema of the affected segment of bowel , with perhaps a pericolic abscess narrowing the lumen , or chronic inflammation , after recurrent attacks of diverticulitis , can result in fibrous bands across the bowel lumen causing obstruction ( fig . ct alone may not distinguish the benign from the malignant causes of luminal narrowing and colonoscopy or sometimes surgery is required where diagnostic uncertainty remains . 17chronic inflammatory narrowing of the sigmoid colon ( black arrows ) , due to recurrent diverticulitis and responsible for large bowel obstruction and retraction of small bowel loops around an inflammatory mass ( white arrows ) chronic inflammatory narrowing of the sigmoid colon ( black arrows ) , due to recurrent diverticulitis and responsible for large bowel obstruction and retraction of small bowel loops around an inflammatory mass ( white arrows ) fistulisation : fistulisation most frequently involves the bladder in men ( fig . 18 ) , which is less frequently involved in women when the uterus is present [ 11 , 37 ] . the retroperitoneum and may have several presentations varying with the organ involved ( hydronephrosis , psoas abscess , spondylodiscitis , etc . ) the diagnostic difficulty remains , proving the relationship between fistula and cd in the case of mild or absent colonic abnormalities . 18contrast - enhanced abdominal ct demonstrating features of sigmoid  inflamed diverticulum  ( black arrows ) adjacent to a thickened bladder wall associated with free air within the bladder ( white arrows ) , establishing the diagnosis of sigmoid diverticulitis with bladder fistulisationfig . 19sigmoid diverticulitis with abscess formation ( black arrow ) and left adnexal extension of the inflammatory process ( white arrow ) contrast - enhanced abdominal ct demonstrating features of sigmoid  inflamed diverticulum ( black arrows ) adjacent to a thickened bladder wall associated with free air within the bladder ( white arrows ) , establishing the diagnosis of sigmoid diverticulitis with bladder fistulisation sigmoid diverticulitis with abscess formation ( black arrow ) and left adnexal extension of the inflammatory process ( white arrow ) pylephlebitis : pylephlebitis is the result of the extension of the septic process into the venous drainage of the affected portion of the colon . spontaneous hyperdensity of the inferior mesenteric vein may be seen on non - contrast - enhanced ct . narrowing ct windows is required when looking for this hyperdensity which may be unseen with a large window which are more adapted to analysis of peritoneal abnormalities . luminal filling defect may be missed on contrast - enhanced ct if the injection delay is not optimal ( < 7080  s ) for portal vein assessment ( figs . abdominal contrast - enhanced ct demonstrated a superior mesenteric vein thrombosis ( white arrows ) , with no regional source identified on this first ct scan ( a , b ) . the septic source of this pylephlebitis was diverticulitis of the right colon ( black arrow ) detected by a second ct examination performed later with colonic opacification ( c)fig . 21contrast - enhanced ct demonstrating hepatic abscess ( black arrow ) and pylephlebitis with intravenous free air ( white arrowhead ) in the superior mesenteric vein . the lower ct sections revealed the septic source which was sigmoid diverticulitis with a small paracolic abscess ( white arrows ) a 68-year - old man with abdominal pain and fever . abdominal contrast - enhanced ct demonstrated a superior mesenteric vein thrombosis ( white arrows ) , with no regional source identified on this first ct scan ( a , b ) . the septic source of this pylephlebitis was diverticulitis of the right colon ( black arrow ) detected by a second ct examination performed later with colonic opacification ( c ) contrast - enhanced ct demonstrating hepatic abscess ( black arrow ) and pylephlebitis with intravenous free air ( white arrowhead ) in the superior mesenteric vein . the lower ct sections revealed the septic source which was sigmoid diverticulitis with a small paracolic abscess ( white arrows ) liver abscesses : these are rare complications of cd . a colonic septic source sometimes paucisymptomatic or hidden by an immunosuppressive treatment must be sought , especially when biliary tract disease is ruled out and no other septic origin is identified ( fig . the following conditions may be included in the differential diagnosis 
 acute appendicitis : as the most frequent abdominal emergency , appendicitis is important to consider systematically because of its extraordinary variability in clinical presentation , until or unless a normal appendix is visualised . appendicitis and diverticulitis of the right colon share a younger patient population than sigmoid diverticulitis and clinical presentation is usually about the right lower quadrant . however , therapeutic issues diverge with emergency surgery for appendicitis and medical treatment for cd if uncomplicated , which make the differential diagnosis crucial . on ct , differentiating appendicitis and right cd can be very challenging in the case of ( 1 ) pericaecal inflammatory stranding in the absence of a visualised appendix , ( 2 ) both thickened diverticular caecal wall and appendix with fat stranding or ( 3 ) appendicitis in a medial or pelvic location may simulate cd if the distal part of the appendix is in contact with a sigmoid loop ( fig .  22 ) [ 45 , 46 ] . factors that may contribute to a missed diagnosis include a misleading clinical history , paucity of intra - abdominal fat , incomplete contrast material filling of the caecum and small bowel ileus . 22paracolic abscess with thickening of the adjacent sigmoid wall was very suggestive of sigmoid diverticulitis ( white arrows ) . curved mpr revealed a mild thickening of the extremity of the appendix ( black arrows ) , which was pointing to the paracolic abscess . a final diagnosis of a perforated appendicitis was confirmed at surgeryappendagitis : epiploic appendagitis is a rare condition that consists of inflammatory and ischaemic changes related to torsion or spontaneous venous thrombosis of one the epiploic appendices . a misdiagnosis of acute appendagitis as cd may result in unnecessary hospital admission and antibiotic therapy . because the ct appearance ( well - defined oval or round area of fat with an enhancing rim located immediately adjacent to the colon ) usually suggests the correct diagnosis non - invasively , most cases can be managed conservatively ( fig . approximately 7.1% of patients investigated to exclude cd have imaging findings of primary epiploic appendagitis . although the differential diagnosis is often easy , appendagitis may simulate cd including mild thickening of the colonic wall which is a rare but classically reported finding . on the other hand , inflammation from cd may extend to involve secondarily epiploic appendages , which is a common finding in the case of cd , with the resultant increased difficulty of diagnosis on the basis of ct images [ 48 , 50 ] . actually , extraluminal air , a lengthy segment of thickened colonic wall , fistula , abscess formation and bowel obstruction are extremely rare in primary acute appendagitis . 23primary appendagitis of the descending colon with a fatty oval - shaped nodule ( white arrow ) located antero - laterally to the colon associated with mild thickening of the parietal peritoneum ( white arrowhead)fig . 24non - contrast - enhanced and contrast - enhanced ct revealing typical features of sigmoid diverticulitis associated with secondary appendagitis ( white arrows)colitis : most acute inflammatory diseases of the colon , including infectious , non - infectious , and ischaemic disorders , are centred in the colon wall . for these diseases , the degree of colonic wall thickening typically exceeds the degree of associated fat stranding , and not uncommonly ; fat stranding may be subtle despite marked mural abnormality [ 10 , 51 ] . in cd , however , fat stranding is described as ( i.e. stranding more severe than expected for the degree of bowel wall thickening present ) , which is considered to be a reliable sign for the diagnosis of cd in patients with acute abdominal pain ( fig abdominal ct demonstrates extensive wall thickening of a diverticular left colon ( arrowheads ) with  disproportionate  pericolic fat standing ( arrows ) and no identified  inflamed diverticulum  , suggesting the diagnosis of ischaemic colitis rather than diverticulitiscolon cancer : the most important entity in the differential diagnosis of cd to exclude is colon cancer . several studies have described the ct features differentiating cd from colon cancer , and have found statistically significant differences in the frequency of different ct findings in patients with colon cancer and those with cd [ 4 , 5254 ] . the ct signs that are most suggestive of cd are ( 1 ) a stenosis longer than 10  cm , ( 2 ) sloping transition zones , ( 3 ) colon wall thickness less than 1  cm , ( 4 ) fluid in the colonic mesentery , ( 5 ) engorgement of mesenteric vessels and ( 6 ) absence of enlarged pericolonic lymph nodes ( figs .  26 and 27 ) . in spite of this good correlation , many authors reported a considerable overlap in the ct diagnosis of these two conditions , reaching 50% of cases . by using strict criteria , however , one can make a correct unequivocal diagnosis of cd or cancer in approximately 50% of cases . in those cases , the patients need not undergo further diagnostic evaluation , and further evaluation may be carried out for surgical planning . besides , the diagnosis may be twice as misleading when ischaemic colitis occurs , which is a classical associated feature in both colonic cancer and cd with mechanical obstruction . nevertheless , making the diagnosis on the basis of only imaging findings can be very difficult in approximately half of the patients , and colonoscopy at a distance from the acute flare is strongly recommended . recently , a few studies have reported that functional ct perfusion measurements may facilitate differentiation and discrimination , in combination with morphological criteria , between cancer and cd . 26sigmoid diverticulitis mimicking a sigmoid cancer ( white arrows ) with left ureteral involvement resulting in hydronephrosis ( white arrowheads)fig . 27abdominal ct in an 80-year - old patient admitted for acute abdominal pain and fever , demonstrating sigmoid thickening suggesting colonic cancer ( white arrows ) associated with the ct finding of upstream diverticulitis ( black arrows ) . evidence of the association between these two pathological conditions was surgically proven acute appendicitis : as the most frequent abdominal emergency , appendicitis is important to consider systematically because of its extraordinary variability in clinical presentation , until or unless a normal appendix is visualised . appendicitis and diverticulitis of the right colon share a younger patient population than sigmoid diverticulitis and clinical presentation is usually about the right lower quadrant . however , therapeutic issues diverge with emergency surgery for appendicitis and medical treatment for cd if uncomplicated , which make the differential diagnosis crucial . on ct , differentiating appendicitis and right cd can be very challenging in the case of ( 1 ) pericaecal inflammatory stranding in the absence of a visualised appendix , ( 2 ) both thickened diverticular caecal wall and appendix with fat stranding or ( 3 ) appendicitis in a medial or pelvic location may simulate cd if the distal part of the appendix is in contact with a sigmoid loop ( fig . 22 ) [ 45 , 46 ] . factors that may contribute to a missed diagnosis include a misleading clinical history , paucity of intra - abdominal fat , incomplete contrast material filling of the caecum and small bowel ileus . 22paracolic abscess with thickening of the adjacent sigmoid wall was very suggestive of sigmoid diverticulitis ( white arrows ) . curved mpr revealed a mild thickening of the extremity of the appendix ( black arrows ) , which was pointing to the paracolic abscess . a final diagnosis of a perforated appendicitis was confirmed at surgery paracolic abscess with thickening of the adjacent sigmoid wall was very suggestive of sigmoid diverticulitis ( white arrows ) . curved mpr revealed a mild thickening of the extremity of the appendix ( black arrows ) , which was pointing to the paracolic abscess . a final diagnosis of a perforated appendicitis was confirmed at surgery appendagitis : epiploic appendagitis is a rare condition that consists of inflammatory and ischaemic changes related to torsion or spontaneous venous thrombosis of one the epiploic appendices . a misdiagnosis of acute appendagitis as cd may result in unnecessary hospital admission and antibiotic therapy . because the ct appearance ( well - defined oval or round area of fat with an enhancing rim located immediately adjacent to the colon ) usually suggests the correct diagnosis non - invasively , most cases can be managed conservatively ( fig .  23 ) . approximately 7.1% of patients investigated to exclude cd have imaging findings of primary epiploic appendagitis . although the differential diagnosis is often easy , appendagitis may simulate cd including mild thickening of the colonic wall which is a rare but classically reported finding . on the other hand , inflammation from cd may extend to involve secondarily epiploic appendages , which is a common finding in the case of cd , with the resultant increased difficulty of diagnosis on the basis of ct images [ 48 , 50 ] . actually , extraluminal air , a lengthy segment of thickened colonic wall , fistula , abscess formation and bowel obstruction are extremely rare in primary acute appendagitis . 
 fig .  23primary appendagitis of the descending colon with a fatty oval - shaped nodule ( white arrow ) located antero - laterally to the colon associated with mild thickening of the parietal peritoneum ( white arrowhead)fig . 24non - contrast - enhanced and contrast - enhanced ct revealing typical features of sigmoid diverticulitis associated with secondary appendagitis ( white arrows ) primary appendagitis of the descending colon with a fatty oval - shaped nodule ( white arrow ) located antero - laterally to the colon associated with mild thickening of the parietal peritoneum ( white arrowhead ) non - contrast - enhanced and contrast - enhanced ct revealing typical features of sigmoid diverticulitis associated with secondary appendagitis ( white arrows ) colitis : most acute inflammatory diseases of the colon , including infectious , non - infectious , and ischaemic disorders , are centred in the colon wall . for these diseases , the degree of colonic wall thickening typically exceeds the degree of associated fat stranding , and not uncommonly ; fat stranding may be subtle despite marked mural abnormality [ 10 , 51 ] . in cd , however , fat stranding is described as ( i.e. stranding more severe than expected for the degree of bowel wall thickening present ) , which is considered to be a reliable sign for the diagnosis of cd in patients with acute abdominal pain ( fig abdominal ct demonstrates extensive wall thickening of a diverticular left colon ( arrowheads ) with  disproportionate  pericolic fat standing ( arrows ) and no identified  inflamed diverticulum  , suggesting the diagnosis of ischaemic colitis rather than diverticulitis an 81-year - old man presenting with acute abdominal pain . abdominal ct demonstrates extensive wall thickening of a diverticular left colon ( arrowheads ) with  disproportionate  pericolic fat standing ( arrows ) and no identified inflamed diverticulum  , suggesting the diagnosis of ischaemic colitis rather than diverticulitis colon cancer : the most important entity in the differential diagnosis of cd to exclude is colon cancer . several studies have described the ct features differentiating cd from colon cancer , and have found statistically significant differences in the frequency of different ct findings in patients with colon cancer and those with cd [ 4 , 5254 ] . the ct signs that are most suggestive of cd are ( 1 ) a stenosis longer than 10  cm , ( 2 ) sloping transition zones , ( 3 ) colon wall thickness less than 1  cm , ( 4 ) fluid in the colonic mesentery , ( 5 ) engorgement of mesenteric vessels and ( 6 ) absence of enlarged pericolonic lymph nodes ( figs .  26 and 27 ) . in spite of this good correlation , many authors reported a considerable overlap in the ct diagnosis of these two conditions , reaching 50% of cases . by using strict criteria , however , one can make a correct unequivocal diagnosis of cd or cancer in approximately 50% of cases . in those cases , the patients need not undergo further diagnostic evaluation , and further evaluation may be carried out for surgical planning . besides , the diagnosis may be twice as misleading when ischaemic colitis occurs , which is a classical associated feature in both colonic cancer and cd with mechanical obstruction . nevertheless , making the diagnosis on the basis of only imaging findings can be very difficult in approximately half of the patients , and colonoscopy at a distance from the acute flare is strongly recommended . recently , a few studies have reported that functional ct perfusion measurements may facilitate differentiation and discrimination , in combination with morphological criteria , between cancer and cd . 26sigmoid diverticulitis mimicking a sigmoid cancer ( white arrows ) with left ureteral involvement resulting in hydronephrosis ( white arrowheads)fig . 27abdominal ct in an 80-year - old patient admitted for acute abdominal pain and fever , demonstrating sigmoid thickening suggesting colonic cancer ( white arrows ) associated with the ct finding of upstream diverticulitis ( black arrows ) . evidence of the association between these two pathological conditions was surgically proven sigmoid diverticulitis mimicking a sigmoid cancer ( white arrows ) with left ureteral involvement resulting in hydronephrosis ( white arrowheads ) abdominal ct in an 80-year - old patient admitted for acute abdominal pain and fever , demonstrating sigmoid thickening suggesting colonic cancer ( white arrows ) associated with the ct finding of upstream diverticulitis ( black arrows ) . unusual presentations and complications of cd must be kept in mind by the radiologist who must perform ct examination with an optimised technical procedure . when the diagnosis of cd is made , the radiologist has to look for all possible complications that might modify the therapeutic strategy .
colonic diverticulitis ( cd ) is a common entity whose diagnosis is particularly based on computed tomography ( ct ) examination , which is the imaging technique of choice . 
 however , unusual ct findings of cd may lead to several difficulties and potential pitfalls : due to technical errors in the management of the ct examination , due to the anatomical situation of the diseased colon , in diagnosing unusual complications that may concern the gastrointestinal tract , intra- and retroperitoneal viscera or the abdominal wall , and in differentiating cd from other abdominal inflammatory and infectious conditions or colonic cancer . 
 the aim of this work is to delineate the pitfalls of ct imaging and illustrate misleading ct features in patients with suspected cd .