imaging in crohn disease

1432543716768.123 Imaging in Crohn Disease

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Imaging in Crohn Disease Author: YungHsin Chen, MD; Chief Editor: Eugene C Lin, MD more...

Updated: Oct 24, 2013

OverviewCrohn disease is not a distinct histopathologic entity. Although described and named after its author in 1932, Crohndisease was not clinically, histologically, or radiographically distinguished from ulcerative colitis until 1959.

Currently, the diagnosis of Crohn disease entails an analysis of clinical, radiologic, endoscopic, pathologic, and stoolspecimen results. Contrastenhanced radiography is used to localize the extent, severity, and contiguity of disease;CT scanning provides crosssectional images for assessing mural and extramural involvement; endoscopy enablesdirect visualization of the mucosa and provides the ability to obtain a biopsy specimen for histopathologiccorrelation; and ultrasonography and MRI are adjuncts that provide alternative crosssectional images in populationsin whom radiation exposure is a concern.

For excellent patient education resources, visit eMedicineHealth's Digestive Disorders Center. Also, seeeMedicineHealth's patient education articles, Inflammatory Bowel Disease, Crohn Disease, and Crohn DiseaseFAQs.

Examples of Crohn disease are provided in the images below.

Crohn disease. Aphthous ulcers. Doublecontrast barium enema examination in Crohn colitis demonstrates numerous aphthousulcers.

Crohn disease of the terminal ileum with CT and sonographic correlation. Smallbowel followthrough study demonstrates the stringsign in the terminal ileum. Also note pseudodiverticula of the antimesenteric wall of the terminal ileum, secondary to greaterdistensibility of this lessinvolved segment of the wall.

Crohn disease of the terminal ileum with CT and sonographic correlation. Note terminal ilealwall thickening and adjacent mesentericinflammatory stranding.

Crohn disease. Mesenteric inflammation. CT scan demonstrates inflammatory mass in the right lower quadrant associated withthickening of the wall and narrowing of the lumen of the terminal ileum.

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Crohn disease. Crohn colitis. Doublecontrast barium enema study demonstrates marked ulceration, inflammatory changes, andnarrowing of the right colon.

Crohn disease. MRI with CT correlation. MRI demonstrates thickening of the wall of the right colon with intramural increased signalon a T1weighted image. This was believed to represent intramural fat deposition.

Pathophysiology

The etiology of Crohn disease is largely unknown. Genetic, infectious, immunologic, and psychological factors haveall been implicated in influencing the development of the disease. The disease is characterized by chronicinflammation extending through all layers of the intestinal wall and involving mesentery as well as regional lymphnodes.

Early mucosal involvement consists of longitudinal and transverse aphthous ulcerations, which are responsible for acobblestone appearance. As the disease progress, deep fissures, sinuses, and fistulae develop. Eventually,communication between diseased bowel segments, the abdominal wall, retroperitoneal structures, and the urinarytract occurs.

Because of the transmural nature of the disease, mesenteric and perianal manifestations are fairly common.Because of the inflammation, strictures resulting from edema, inflammation, and, ultimately fibrosis and scaring,are frequent. Crohn disease is pervasive. The basic pathologic process of disease can occur at any segment of thealimentary tract.

Crohn disease and ulcerative colitis share similar inflammatory changes. Cryptitis and subsequent crypt abscessesconsisting of polymorphonuclear cells are identical for both diseases. However, during the inflammatory flareups,Crohn disease involves increases in the number of cells containing immunoglobulin G2 (IgG2) and ulcerative colitisinvolves a predominant increase in immunoglobulin G1 (IgG1) and immunoglobulin G3 (IgG3) cell types.

The inflammatory infiltrate of the lamina propria in Crohn disease leads to loose aggregations of macrophages, andthey organize into noncaseating granulomas, which involve all layers of the bowel wall from mucosa to serosa.Occasionally, they can be seen on laparoscopy as miliary nodules, and they function as contiguous spread of thedisease from the intestine. With chronic inflammation, the bowel walls become thickened, fibrotic, and stenotic inCrohn disease, and an extension of inflammation and fistula formation often occurs as a result of a transmuralfissure.

In ulcerative colitis, hemorrhagic and ulcerative inflammation is mostly limited to the mucosa, with recurrenceleading to atrophic mucosa. Ulcers often have irregular borders, giving rise to a collarstud effect. In recurrentdisease, inflammatory polyps develop from exuberant epithelial regeneration. When inflammation infiltrate extendsinto the submucosa and muscularis propria, it does so in a diffuse pattern, in contrast to Crohn disease, in whichthey appear as lymphoid aggregates. Why Crohn disease has a skipdistribution as opposed to that seen inulcerative colitis is uncertain.

Epidemiology

Findings from studies in the United States and Western Europe indicate that the incidence of Crohn disease is 2cases per 100,000 population. The prevalence is estimated to be 2040 cases per 100,000 population. Recent datashow that at least in Europe, rates in Southern European countries are catching up to those of their northernneighbors.

Approximately 15% of the cases of Crohn disease appear in persons older than 50 years.

The relative risk for adenocarcinoma of the ileum is at least 100fold greater in Crohn disease patients comparedwith age and sexmatched controls. Smallbowel cancers typically arise at sites of macroscopic disease after meanage of 18 years.

Mortality and morbidity

Crohn disease is associated with higher rate of mortality, as compared with that of the general population,independent of GI tract involvement. The excess mortality is most pronounced in the first few years after diagnosis.This observation has been attributed to complications of Crohn disease, which include abscesses, fistulas, intestinalobstructions and perforations, and colorectal cancer.

Approximately 15% of the cases of Crohn disease appear in persons older than 50 years. In the older population,Crohn disease tends to involve the colon, and more obstructive and inflammatory complications tend to develop.



However, despite this fact, older patients have been shown to tolerate medical and surgical therapy as well asyoung patients.

Abscesses develop in approximately 1520% of patients with Crohn disease as a result of sinus track formation or asa complication of surgery. Abscesses can be found in the mesentery, peritoneal cavity, or retroperitoneum or in anextraperitoneal location. The most common sites of retroperitoneal abscesses are the ischiorectal fossa, thepresacral space, and the iliopsoas region. The terminal ileum is the most common site of origin of abscesses. It isone of leading causes of mortality in Crohn disease.

Obstruction occurs in 2030% of patients during the course of the disease. Early in the disease, it appears asreversible intermittent postprandial obstruction due to edema and bowel spasm. Over several years, this persistentinflammation gradually progresses to fibrostenotic narrowing and stricture, which may require regional resection.

Fistula formation is a frequent complication of Crohn disease of the colon. Fistulas can be categorized into 3groups: benign, nuisance, and intractable. Benign fistulas are simple and include ileoileal, ileocecal, and ileosigmoidfistulas, which might produce only mild or moderate diarrhea. They may even remain asymptomatic for yearswithout any treatment. Nuisance fistulas must be closed because of annoying symptoms and troublesomepathophysiologic consequences, but neither the complications nor the underlying bowel disease is severe enough torequire surgery. This intermediate group includes enterovesicular, enterocutaneous, cologastric, and coloduodenalfistulas.

Complicated fistulas with abscesses or severe underlying bowel disease (either ulcerating inflammation or distalobstruction) are the most difficult to manage. They occur in 50% of patients with Crohn disease. The role of medicaltherapy is simply to control the obstructing, inflammatory, or suppurative processes before definitive surgery isperformed. The goal of the operation is evacuation of the abscess and, if not contraindicated by associated sepsis,resection of the diseased bowel. This form of fistula leads to spontaneous intestinal perforation in 12% of patients.

GI cancer has been the leading cause of mortality in Crohn disease. Adenocarcinoma usually arises in areas ofchronic disease. The cancer risk is higher in both the small intestine and the colon, as compared with that of generalpopulation. The relative risk for adenocarcinoma of the ileum is at least 100fold greater in age and sexmatchedcontrols. Smallbowel cancers typically arise at sites of macroscopic disease after mean age of 18 years.

Unfortunately, most cancers related to Crohn disease are not detected until advanced stages, and the patients havepoor prognoses. Mounting evidence from studies indicates that Crohn disease is associated a cancer risk equal tothat of ulcerative colitis. Some extraintestinal cancers (eg, squamous cell cancer in patients with chronic perianal,vulvar, or rectal disease) and Hodgkin or nonHodgkin lymphomas have also been shown to be more common inpatients with Crohn disease.

Preferred Examination

The preferred examinations are plain radiography, doublecontrast barium enema examination, singlecontrast upperGI series with smallbowel followthough or enteroclysis with CT, and doublecontrast evaluation of the small bowel.Ultrasonography and MRI can be used as adjuncts if radiation exposure is an issue in monitoring disease activity.[1,2, 3]

In general, the clinician should select CT first in evaluation of Crohn disease. CT has is not as sensitive indelineating fissure or fistula as barium studies, but it is superior to barium studies in showing the extraluminalsequelae of Crohn disease. Residual contrast material from barium studies leads to severe streak artifact on CTscans due to hyperattenuating contrast suspension used in barium studies. On the other hand, CT contrast residuedoes not preclude a barium study.

Barium contrast studies are limited in the evaluation of transluminal inflammation in Crohn disease; distention ofsmall bowel with contrast material is required for proper evaluation. Slow passage of the contrast agent through thepylorus can result in nonvisualization of smallbowel lesions in small bowel series. Enteroclysis is one way tocircumvent the dilemma by passing a catheter to the duodenal jejunal junction.

Abdominal radiographic findings are not specific for Crohn disease. Radiography is useful in evaluation of bowelloop distention and pneumoperitoneum. Sonographic findings have high variability because of operator dependencein detection of the bowelwall changes seen in Crohn disease. Transmission of ultrasound waves through fattytissues is limited, and detection may be severely limited by the patient's body habitus.

Traditionally, MRI was limited in the evaluation of the abdomen and pelvis because of motion artifact. With strongergradients, breathhold imaging, and faster sequences, MRI of the abdomen and pelvis can be readily performed inmost patients. It is currently actively used in routine assessment of pelvis fistulae and sinus tracks. In many medicalcenters, MRI enterography and enteroclysis are actively used in surveillance of small bowel disease and extraluminalmesenteric disease. MRI is an attractive alternative to traditional fluoroscopy and CT scanning, especially inpediatric population in which longterm radiation exposure is a concern.

CTguided therapy

CT has become the procedure of choice not only in diagnosing Crohn disease but also in managing abscesses. Agrowing body of literature shows that CTguided percutaneous abscess drainage may obviate surgery. In studies, CTpercutaneous abscess drainage has shown great success either as a temporizing measure or as definitive therapywith a decreased rate of recurrence, as compared with that of surgery. Because about 7090% of patients withregional enteritis eventually require surgery, avoiding an operation to treat an abscess is a tangible benefit of CT.[4]

Complications and contraindications

The oral administration of contrast material is to be avoided when moderate or highgrade colonic obstruction ispresent. Doublecontrast (air contrast) barium enema examination is contraindicated in patients with severe colitis,because injection of air with contrast agent may precipitate toxic megacolon or colonic perforation. Barium studiesare contraindicated when there are signs and symptoms of peritonitis or when there are radiographic signs of gas inthe bowel wall or pneumoperitoneum.[5]

The intravenous injection of contrast material for CT studies should be avoided when chronic renal insufficiency ispresent, when there is continued use of Glucophage, or when there are signs and symptoms of acute renal failure.CT and barium studies use ionizing radiation, which may result in considerable radiation burden. This exposure is a



relative contraindication in pregnancy and childhood. Sonography and MRI may prove to be useful alternativeimaging modalities.

RadiographyThe role of plain radiography with barium studies in Crohn disease is fairly limited. The 2 major purposes that itserves are (1) to assess the presence of intestinal obstruction and (2) to evaluate pneumoperitoneum prior to furtherradiological workup. Additional extraintestinal findings of sacroiliitis or oxalate kidney stones may be present. Thesefurther support the diagnosis of Crohn disease.

Crohn disease versus ulcerative colitis

Doublecontrast barium enema study is useful for diagnosing inflammatory bowel disease and for differentiatingCrohn disease from ulcerative colitis, especially in the early phase of the disease. On doublecontrast studies, earlyCrohn disease is characterized by discrete aphthoid ulcers, which are seen as punctate or slitlike collections ofbarium surrounded by radiolucent mounds of edema. The aphthoid ulcers are often separated by normal bowel andpresent as skip lesions. On the contrary, ulcerative colitis extends proximally at various degrees from the rectum asa continuous area of disease that eventually leads to pancolitis. Early ulcerative colitis is characterized by a granularappearance on doublecontrast examination as a result of edema and hyperemia of the mucosa. Thus, the 2diseases can be differentiated on basis of radiographic findings.[6]

In a comparison of 23 patients with ulcerative colitis with 27 patients with Crohn disease, Laufer et al establishedthe differentiating features the conditions using barium study. They found that ulcerative colitis involves granularmucosa, diffuse rectal involvement, and continuous inflammatory changes in the bowel with sparing of the terminalileum. Crohn disease involves patchy rectal involvement with punchedout ulcers, ulcers on normal mucosa, anddiscontinuous bowel inflammation typically with involvement of the terminal ileum. These criteria are distinguishingin 95% of patients. The difficulty in differentiating the 2 illnesses occurs at later chronic stages, during whichnumerous remissions and exacerbations can result in discontinuous ulcerative colitis. Despite this difficulty, carefulexamination of the mucosal surface with barium study enables the distinction.[6]

Aphthoid ulcers

Aphthoid ulcers, as shown in the image below, are detected on barium studies in 2550% of patients with Crohndisease. These are identified in as many as 75% of surgical specimens with Crohn disease. Endoscopy is slightlysuperior to barium studies in the demonstration of isolated or a few aphthoid ulcers.

Crohn disease. Aphthous ulcers. Doublecontrast barium enema examination in Crohn colitis demonstrates numerous aphthousulcers.

Severe Crohn disease

As more severe Crohn disease develops, the small ulcers become enlarged and deeper, and they connect to oneanother, forming stellate, serpiginous, and linear ulcers. These ulcers are found most frequently in terminal ileumalong the mesenteric border. These are pathognomonic of Crohn disease. On smallbowel series or enteroclysis, amesenteric border ulcer appears as a long 1 to 2mm barium collection that parallels a short, straight mesentericborder. A radiolucent collar usually parallels the linear barium collection at the margin of the ulcer. Theantimesenteric border of the bowel is usually uninvolved and pulled into the ulcer collar, creating radiating folds.

Cobblestoning

As inflammation penetrates the submucosa and muscularis layers, deep knifelike linear clefts form the basis of"cobblestoning" and fissure or fistula formation. They appear as a bariumfilled reticular network of grooves thatsurround round or ovoid radiolucent islands of mucosa. Eventually, transmural inflammation leads to decreasedluminal diameter and limited distensibility. This leads to a radiographic string sign that represents long areas ofcircumferential inflammation and fibrosis resulting in long segments of luminal narrowing. See the images below.

Crohn disease. Cobblestoning. Spot view of the terminal ileum from a smallbowel followthrough study demonstrates linearlongitudinal and transverse ulcerations that create a cobblestone appearance. Also note the relatively greater involvement of themesenteric side of the terminal ileum and the displacement of the involved loop away from the normal small bowel secondary tomesenteric inflammation and fibrofatty proliferation.

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Crohn disease. Spot view of the terminal ileum from a smallbowel followthrough study demonstrates several narrowing andstricturing, consistent with the string sign. Also note a sinus tract originating from the medial wall of the terminal ileum and theinvolvement of the medial wall of the cecum.

Enlarged villi

Chronic inflammation in the lamina propria of the small intestine results in enlarged villi radiographically manifestedas 0.5 to 2mm, round or polygonal nodules. This fine mucosal nodularity occurs in the small intestine and shouldnot be confused with the mucosal granularity seen in the colon of patients with ulcerative colitis.

Limitations

Mucosal nodularity or granularity in a smallbowel series is a nonspecific finding that can be seen in diseases thatinfiltrate or inflame the lamina propria, such as amyloidosis or radiation enteritis.

Smallbowel followthough examination is limited by the speed of barium passage through the pylorus. If too slow,incomplete distention in the lumen of the bowel can cause short skip lesions, masses, or obstructing lesions in thesmall bowel to be missed.

Additional barium studies images of Crohn disease are shown below.

Crohn disease. Crohn colitis. Doublecontrast barium enema study demonstrates marked ulceration, inflammatory changes, andnarrowing of the right colon.

Crohn disease. Singlecontrast barium enema study demonstrates stricturing of the caput cecum, the socalled coned cecum.

Crohn disease. Enterocolic fistula. Doublecontrast barium enema study demonstrates multiple fistulous tracts between the terminalileum and the right colon adjacent to the ileocecal valve, the socalled doubletracking of the ileocecal valve.



Crohn disease. Smallbowel followthrough study demonstrates narrowing of the lumen and multiple enteroenteric fistulae, but it failsto show the enterovesical fistula.

Crohn disease. Cystogram demonstrates a filling defect and inflammatory changes of the dome of the bladder, but it fails todemonstrate the enterovesical fistula.

In general, 1820% of findings are falsenegative on barium study, as compared with endoscopic detection.However, barium enema has a 95% accuracy rate in distinguishing Crohn disease from ulcerative colitis.

Computed TomographyThe role of CT in the evaluation of Crohn disease is well accepted. The ability of CT to depict bowel involvementand extraluminal pathology (eg, abscess, obstruction, fistula) makes it an essential imaging tool for patient care.The earliest CT finding of Crohn disease is bowel wall thickening, which usually involves the distal small bowel andcolon, although any segment of the GI tract can be affected. Typically, the luminal thickening is 515 mm.[7, 8, 9, 10]

CT should be the first radiologic procedure performed in patients with acute symptoms and suspected or knownCrohn disease. The ability to directly demonstrate the bowel wall, adjacent abdominal organs, mesentery, andretroperitoneum makes CT superior to barium studies in diagnosing the complications of Crohn disease. CT directlydemonstrates bowel wall thickening, mesenteric edema, and lymphadenopathy, as well as phlegmon and abscess.See the images below.

Crohn disease of the terminal ileum with CT and sonographic correlation. Smallbowel followthrough study demonstrates the stringsign in the terminal ileum. Also note pseudodiverticula of the antimesenteric wall of the terminal ileum, secondary to greaterdistensibility of this lessinvolved segment of the wall.


Crohn disease. Active smallbowel inflammation. CT scan demonstrates smallbowel wall thickening, mesenteric inflammatorystranding, and mesenteric adenopathy.



Crohn disease. Mesenteric inflammation. CT scan demonstrates inflammatory mass in the right lower quadrant associated withthickening of the wall and narrowing of the lumen of the terminal ileum.

Although barium is more sensitive in demonstrating the presence of fissures and fistulas, CT is superior indemonstrating the sequelae of these tracks (eg, air in the urinary bladder in enterovesical fistula).

The sensitivity of CT for Crohn disease is estimated to be 71%, with lower detection of early mucosal disease ascompared with barium studies. A recent study by Philpotts et al has shown that the CT findings of Crohn diseaseconsiderably overlap with those of infectious, radiation, ulcerative, and ischemic colitides.[11]

Certain distinguishing features have been cited in delineating Crohn disease from other forms of enterocolitis,including differences in wall thickness and attenuation; the distribution of colonic wall involvement; and the presenceor absence of abscesses, fistulas, smallbowel disease, and mesenteric fibrofatty proliferation. In using thementioned features, CT can attain positive predictive value above 90% and a diagnostic accuracy as high as 93%.[11] See the image below.

Crohn disease. Fibrofatty proliferation. CT scan in a patient with Crohn colitis in the chronic phase demonstrates wall thickening ofthe right colon, an absence of adjacent mesenteric inflammatory stranding, and a large amount of fatty proliferation around the rightcolon separating the colon from the remainder of the gut, socalled creeping fat.

The introduction of multidetectorrow CT scanners with thinner collimation and faster intravenous injections ofcontrast material have allowed more detailed evaluation of the bowel. The enhancement of the bowel wall afterintravenous contrast enhancement is correlated with the enlargement of the feeding vessel and hyperemia duringactive disease. In an article by Del Campo et al, patients with active disease had a bowel wall attenuation of 95 HU,as compared with 65 HU in patients with disease in remission.[12] The ability to measure bowel wall enhancementmay prove valuable in treating patients with Crohn disease.

One limitation of CT has been in the area of delineating active versus inactive disease. The presence of mesentericstranding does not reliably signify active disease because residual mesenteric thickening can remain duringremission.

Ulcerations

Ulcerations in the mucosa can be detected on thinsection CT, although smallbowel series or enteroclysis is moresensitive to the early mucosal changes of Crohn disease. In addition, mesenteric stranding, increase in mesentericfat, local adenopathy, fistula, and abscess are readily and commonly identified on CT scans.

Hazy fat

Edema or mild inflammation of the mesenteric fat results in fat of increased attenuation, the socalled hazy fat onCT. Greater inflammation or fibrosis of fat results on CT in attenuating linear bands of soft tissue coursing throughthe mesentery. On CT, an illdefined inflamed mass of mixed attenuation may represent a phlegmon or earlyabscess formation. Enlarged lymph nodes are usually seen in proximity to the bowel wall along the mesentericcourse of the vascular bundle. See the image below.

Crohn disease. Mesenteric inflammation. CT scan demonstrates an inflammatory mass in the right lower quadrant associated withthickening of the wall and narrowing of the lumen of the terminal ileum.

Abscesses

On CT scans, abscesses appear as welldefined, round or oval masses of fluid attenuation, and they are oftenmultilocular. Pockets or bubbles of gas usually result from fistulous communication with bowel or, less likely, frominfection by gasproducing organisms.

Crohn disease versus ulcerative colitis

There is considerable overlap between CT findings of ulcerative colitis and Crohn disease. Despite this fact, certaindefining features of each disease have been characterized. Ulcerative colitis is predominantly a mucosal disease.However, with progression of illness, there is hypertrophy of the muscularis often by 40fold, increase submucosafatty deposition, and thickening of the lamina propria from round cell infiltration, which all leads to bowel wallthickening. On average, thickening of the luminal wall is 7.8 mm in ulcerative colitis, which is less than the amount



of wall thickening typically seen with Crohn disease.

Submucosal fat is a prominent finding in chronic ulcerative colitis and is one of the defining features of the muralstratification seen in ulcerative colitis. In comparison, Crohn disease has transmural involvement that over timeleads to replacement of submucosal fat with fibrosis and loss of mural stratification. In distinction to ulcerativecolitis, Crohn disease also has several extraluminal CT findings, including mesenteric fibrofatty proliferation andabscess.

Additional images of Crohn disease on CT scanning are provided below.

Crohn disease. CT with MRI correlation. CT scan in a patient with chronic inactive Crohn disease demonstrates thickening of the wallof the right colon with intramural lucency. This was believed to represent intramural fat deposition.

Crohn disease. MRI with CT correlation. MRI demonstrates thickening of the wall of the right colon with intramural increased signalon a T1weighted image. This was believed to represent intramural fat deposition.

Crohn disease. Perianal abscesses. CT scan demonstrates multiple fluid, contrast material, and air collections around theanorectum. Note the presence of a rectal tube.

Crohn disease. Perianal abscesses. CT scan demonstrates multiple fluid, contrast agent, and air collections around the anorectum.Note the presence of a rectal tube.

Crohn disease. Smallbowel obstruction in a patient with recurrence proximal to an anastomosis. CT scan in a patient with a priorileocolectomy demonstrates smallbowel dilatation and wall thickening of the small bowel proximal to the anastomosis.

Crohn disease. Enteroenteric fistula. CT scan demonstrates the tract of an enteroenteric fistula.



Crohn disease. Enterocutaneous fistula. CT scan demonstrates enterocutaneous and colocutaneous fistula formation.

Magnetic Resonance ImagingTraditionally, MRI has had a welldefined role in evaluation of anorectal complications of Crohn disease. With aregular fast spinecho technique, the pathologic entities of a fistula, a sinus tract, and an abscess can be detected inthe static anorectal region by using MRI.[13] More recently, MRI enterography and enteroclysis has allowed detectionof small bowel Crohn disease similar to the use of small bowel series, with the additional benefits for assessment ofextraluminal disease.[14] Highresolution imaging with the current MRI technology has increased the accuracy ofassessing the grade and severity of patient with Crohn disease.[14, 15, 16, 17, 18] Its increased use and availability willlikely rival traditional imaging in the assessment of Crohn disease.

Perianal sinus tracts and fistulas

Sinus tracts and fistulas often appear hyperintense on T1weighted images and hyperintense on T2weightedimages because of their fluid content. With fat suppression, the fluid signal is further intensified and easily seen asbeing hyperintense on T2weighted images. An abscess often appears as an isolated collection of highsignalintensity areas on the T2weighted image, especially in ischioanal fossa. Defining whether an abscess, fistula, orsinus tract is above or below the levator ani muscle is important for drainage, because any part of the abscessabove the levator ani muscle will not drain adequately in the inferior direction, and vice versa.

MRI sequences

The development of faster pulse sequences (eg, singleshot fast spinecho, steadystate free precession, andgradientecho sequences) and highergradient systems has made T1 and T2weighted breathhold imagingpossible. This breathhold imaging has been a major breakthrough in overcoming physiologic motion artifacts inabdominal imaging. It has made routine abdominal MRI feasible.

The singleshot fast spinecho sequence, in which T2weighted images are acquired by using halfFouriertransformation and a long echo train. Each image section is acquired independently in less than 1 second, and themethod eliminates physiologic motion from the bowel and the need for breath holds. Fat suppression can be addedto increase specificity for bowel and mesenteric edema.

The steadystate free precession imaging is based on a low flip angle gradient echo series with short repetition time.It is another series of sequences that is insensitive to motion artifacts and can provide T2type imaging. It can haveblack boundary artifacts along the bowel wall that mask small lesions, but fat suppression can reduce the artifact.The major feature of the sequence is the ability to acquire an entire series within a single breath hold. In fact, MRfluoroscopy is performed with cine of steadystate free precession imaging and a frame rate of 0.52 sections perseconds along the long axis of the affected segments.

Contrast evaluation is often imaged with 3dimensional spoiled gradient echo T1 fatsuppressed sequences. Imagesare acquired with breath hold. For bowel imaging, series are taken after intravenous glucagon at 30 and 70 secondspost contrast.

Because of a decrease in cumulative radiation exposure and because of the capability of attaining highqualitycoronal images correlating with barium studies, MRI is currently an alternative for monitoring disease activity inCrohn disease.

MRI enterography and enteroclysis

With the development of faster imaging sequence and with the use of intravenous 0.2 mg glucagon to decreasebowel motility, the small bowel is distended in both MRI enterography or enteroclysis, often with 1.52 L of solutioncontaining biphasic intraluminal contrast agents (low T1 and high T2). Some of these agents are Volumen (EZEM;Westbury, NY), mannitol (2.5%), methylcellulose, sorbitol (2%), and polyethylene glycol. Image is performed for 4060 minutes subsequent to luminal distension for MRI enterography and enteroclysis.

Oral ingestion of the intraluminal contrast is performed in enterography, while nasojejunal intubation and infusion ofintraluminal contrast is performed in enteroclysis. Nasojejunal intubation under fluoroscopy is required for MRIenteroclysis; it provides excellent bowel distention and provides detailed luminal information.[19] However, there isadded procedure time for the fluoroscopy nasojejunal intubation, along with increased invasiveness and patientdiscomfort.[20, 21] These factors can be major drawback, especially in pediatric population.

MRI enterography has less patient discomfort, but the bowel preparation may not produce the uniform distentionachieved with enteroclysis. Nevertheless, several studies have shown better patient tolerance of enterography overenteroclysis, and some studies have shown similar sensitivity for both techniques.[20]

Initial imaging with serial coronal steadystate free precession for the small bowel can demonstrate decreasedmotility in areas of small bowel disease, along with luminal thickening (wall thickness >4 mm) while monitoringadequate distention of the ileocecal lumen. After adequate luminal distention, intravenous 0.2 mg of glucagon or 1mg of intramuscular glucagon is administered to reduce motion artifacts, followed by T2weighted singleshot fastspinecho series. The coronal and axial T2weighted singleshot fast spinecho images can show edema in the smallbowel mesentery and small bowel wall deep ulcers, while fatsaturated images can determine chronic mural fatchanges. Lastly, after administration of a gadoliniumbased intravenous contrast agent, coronal volume gradientecho sequences are acquired to assess vascular engorgement, mucosal hyperemia, mural enhancement,inflammatory hyperenhancing lymph nodes, abscess, and fistula.



There are currently ongoing investigations into the use of MRI enterography and assessment of active disease in thecolon.

Active Crohn disease

Assessment of inflammatory activity is fundamental to the managing Crohn disease. There is no single referencestandard for defining active disease. Clinical scores such as the Crohn disease activity index and biochemicalmarkers such as Creactive protein are widely used, but they lack utility for assessment of the entire bowel.[17]Endoscopy is the current reference standard for evaluation of disease of the colon and terminal ileum. It has the keyadvantages of direct visualization and sampling of disease, but it is limited in its assessment of the entire smallbowel. MRI currently serves as a viable method for global assessment.

Many of MRI criteria of disease activity are based on luminal and extraluminal disease. In terms of luminal disease,active disease includes ulcer, wall thickening, mural and perimural T2 intensity, and bowel wall enhancement withgadoliniumbased contrast agents.[17] Extraluminal active disease includes enlarged inflammatory lymph nodes,fibrofatty proliferation, and mesenteric edema.

During active inflammation, gadolinium enhancement of the bowel wall can be seen on T1weighted images, and itis easily differentiated from normal adjacent bowel. There are 3 main patterns of enhancement in determining thelevel of disease. The layered or stratified enhancement is seen with enhancement of the mucosa, and relativelypoor submucosal enhancement and submucosal edema are seen in active disease.[22, 23, 24] Diffuse intensehomogenous enhancement of the entire bowel wall thickness occurs from transmural inflammation.[25] Lowlevelheterogeneous enhancement occurs with fibrosis.[26] See the image below.

MRI enterography 3dimensional gradient postcontrast series demonstrates stratified hyperenhancement of the mucosa of the distalileum in a patient with active Crohn disease.

Wall thickening is variable in active disease, as described in many reports. The general consensus is that concentricbowel wall thickening greater than 4 mm is suggestive of active disease. In study by Maccioni et al, active disease ischaracterized by a thickened bowel wall with gadolinium enhancement, but inactive disease is not.[27] With theadvent of MRI enteroclysis, and to a lesser degree optimal enterography, alteration of folds in early active Crohndisease can be seen as diffuse thickened folds as in a picketfence pattern, reduction and distortion of foldssecondary to ulceration, and cobblestoning on singleshot spinecho series.[26] See the image below.

MRI enterography with coronal fatsaturated T2weighted singleshot fast spin echo imaging demonstrates mesenteric edema andmural wall edema and thickening in a patient with active Crohn disease in the distal ileum.

Fatsuppressed T2weighted images can also be used for differentiation because they show highsignal intensity inactive disease and low or absent signal intensity in nonactive disease. Mural T2 increased signal intensity is a wellvalidated marker of disease activity.[23, 27, 28, 29, 30, 31] In fact, the fat suppression in T2weighted images helpsdifferentiate fibrofatty changes of mural disease from mural wall edema.

Ulceration in active Crohn disease is highly dependent on the quality of luminal distention and is best depicted byMRI enterography and enteroclysis. Deep ulcers appear as thin lines of high signal within a thickened bowel wall onsingleshot fast spinecho series and can be seen more readily on MRI enterography, whereas aphthous ulcer, witha nidus of high signal with surrounding intermediate signal, can be seen on highresolution MRI enteroclysis. Seethe image below.

MRI enterography coronal singleshot fast spinecho images in a patient with active Crohn disease demonstrates wall thickening andwall deep fissuring along an ascending limb of the distal ileum small bowel loop in the right lower quadrant of the abdomen.

Fibrofatty proliferation is hyperintense on T2weighted images and is related to regional mesenteritis or edema and



dilatation of local vessels. The dilatation of the local vessels is seen supplying a local inflamed bowel segment, akinto the “comb sign” seen on CT examination; this finding is well depicted in postcontrast gradientecho images andsteadystate freeprocession images. Inclusive in the term fibrofatty proliferation is “fat wrapping,” whereby there ischronic enlarged mesenteric fat leading to increased separation of the mesenteric bowel loops. Fat proliferation is adistinguishing feature of Crohn disease and is indicative of the diagnosis. See the image below.

MRI enterography, steadystate freeprocession image, demonstrates vascular engorgement of the vasa recta or "comb" sign in athickened ileum bowel loop during active disease.

Mesenteric edema in active disease is seen accompanying bowel wall edema and hyperenhancement and is seenoften with the comb sign of the mesentery.

Active lymph nodes are enlarged, hyperenhancing, and edematous, typically along the vascular supply of affectedbowel segment. The nodal enhancement is usually homogenous and is greater than or equal to one of the adjacentlymph node for active disease.[23]

Low et al[32] and Marcos and Semelka[33] have found gradientecho imaging to be more sensitive than othermethods in determining the severity of Crohn disease, and they favor the use of gadoliniumenhanced gradientechoMRI.

Gadoliniumenhanced spoiled gradientecho MRI has a reported sensitivity of 8589%, a specificity of 9694%, andan accuracy of 9491% for active disease, as compared with singleshot fast spinecho MRI, which has a sensitivityof 5152%, specificity of 9896%, and accuracy of 8384%.[32]

Chronic Crohn disease and complications of Crohn disease

There are 3 major forms of chronic Crohn disease: fistula and perforating disease, fibrostenotic disease, reparativeor regenerative disease. Fistulae are sequelae of deep transmural ulcers that extend through the musculature,leading to the formation of small abscesses and sinus tracts. The sinus tract can involve and communicate withadjacent hollow organ and form fistulae. MRI depiction of fistulae is often seen as fibrotic starshaped reactions ofthe mesentery, with tethering of the adjacent communicating structures. It has avid enhancement post contrast and,on occasion, shows a linear T2hyperintense tract. The chronic mesenteric inflammation can eventually form fibrousbands with enhancement similar to sinuses and fistulas. However, they often lead to tenting and segmentalobstruction with MR fluoroscopy, showing kinking and stretching of the adjacent bowel loops.

Fibrostenotic disease is often depicted as bowel obstruction without bowel wall thickening. The bowel stricture haslow T1 and T2 mural signal and has mild nonhomogenous enhancement. With asymmetric bowel wall involvement,pseudosacculation can occur.

Regenerative disease is depicted on MRI as luminal narrowing without inflammation or obstruction. On steadystatefreeprocession imaging, filiform polyposis may be suggested without enhancement or obstruction. Neoplasia is alsoa concern in chronic disease, since Crohn disease patients are at increased risk of developing adenocarcinoma ofthe affected bowel segment.[34, 35] Eccentric bowel wall thickening or shouldering, mesenteric infiltration, andlymphadenopathy on MRI enterography or enteroclysis are often early signs of malignancy.

Warning about gadoliniumbased contrast agents

Gadoliniumbased contrast agents (gadopentetate dimeglumine [Magnevist], gadobenate dimeglumine[MultiHance], gadodiamide [Omniscan], gadoversetamide [OptiMARK], gadoteridol [ProHance]) have been linked tothe development of nephrogenic systemic fibrosis (NSF) or nephrogenic fibrosing dermopathy (NFD). The diseasehas occurred in patients with moderate to endstage renal disease after being given a gadoliniumbased contrastagent to enhance MRI or MR angiography scans.[36] NSF/NFD is a debilitating and sometimes fatal disease.Characteristics include red or dark patches on the skin; burning, itching, swelling, hardening, and tightening of theskin; yellow spots on the whites of the eyes; joint stiffness with trouble moving or straightening the arms, hands,legs, or feet; pain deep in the hip bones or ribs; and muscle weakness.

Degree of confidence and impact of MRI

Gadoliniumenhanced spoiled gradientecho MRI has a reported sensitivity of 8589%, a specificity of 9694%, andan accuracy of 9491% for active disease, as compared with singleshot fast spinecho MRI, which has a sensitivityof 5152%, a specificity of 9896%, and an accuracy of 8384%.[32] MRI enterography and enteroclysis comparedwith conventional imaging has sensitivity of 8898% and specificity of 78100%.[14] In a recent retrospective study,MRI enterography has been confirmed to impact the management of patient care with additional information fromendoscopy and clinical assessment. In a retrospective study of 120 MRI enterography patients, 53% underwentadditional medical treatment of active disease and 16% underwent surgery with intraoperative findings concordant toMR enterography.[37]

UltrasonographyUltrasonography can be an alternative to CT in the evaluation of the intraluminal and extraluminal manifestations ofCrohn disease. The normal GI wall appears as 5 concentric, alternating echogenic and hypoechoic layers; thisappearance is known as the gut signature. The GI wall has an average thickness of less than 5 mm.[38, 39]



In the case of active Crohn disease, the wall thickness can range from 5 mm to 2 cm with either partial or total lossof layering, which reflects transmural edema, inflammation, or fibrosis. With severe inflammation, the wall appearsdiffusely hypoechoic with a central hyperechoic line that corresponds to the narrowed lumen. Peristalsis is reducedor absent, and the diseased segment is noncompressible and rigid with a loss of haustra.

Ultrasonography can depict ballooning of the less involved segments, which is seen as focal sacculation oroutpouching. These findings reflect the skip lesions found in Crohn disease. The accuracy of ultrasonography isfurther improved with the use of color Doppler imaging. The use of Doppler imaging is helpful in the detection ofhyperemia of an inflamed bowel wall and adjacent fat during active disease.

With transmural inflammation, edema and fibrosis of the adjacent mesentery occurs, leading to fingerlike projectionsof mesenteric fat that creeps over the serosal surface of the bowel. This creeping fat eventually envelops thediseased bowel segment. On sonograms, this appears as a uniform hyperechoic mass, which is classically seen atthe cephalic margin of the terminal ileum. With longstanding disease, this becomes more heterogeneous or evenhypoechoic.

In active Crohn disease, reactive mesenteric nodes are enlarged and may coalesce to form a conglomerate mass.On sonograms, enlarged nodes can be seen as oval hypoechoic masses in the mesentery. With confluence, theybecome lobulated masses of various sizes.

Many complications of Crohn disease can also be seen in their ultrasonographic forms. Phlegmon appears as ahypoechoic mass with irregular borders and no identifiable wall or fluid. Abscess appears as a fluid collection with athickened wall containing air or echogenic debris. Obstruction appears as dilated hyperperistaltic fluid filledsegments. Perforation appears as bright echoes with distal acoustic shadows outside the boundaries of bowel loops.

A fistula, on the other hand, appears as a hypoechoic tract. If gas is present in the fistulous tract, it containshyperechoic foci with acoustic shadowing. Palpation of diseased loops during sonography enables tractidentification. In addition, sonography should be able to identify gas bubbles in abnormal locations, such as air inthe bladder or vaginal vault, the retroperitoneum, the subcutaneous tissue, and the urachal remnant.

The detection of bowel wall thickening varies widely. Detection rates range from 2289%. The large variationpresumably reflects differences in technique, operator experience, and ultrasound equipment. Determination of theextent of the disease is not always possible, and correlation between wall thickening and the clinical activity ofdisease is poor.

The loss of gut signature and bowel wall thickening is a nonspecific finding. It is found in infectious, ischemic,neoplastic, and radiationinduced conditions. In addition, the detection of bowel wall changes in Crohn diseasevaries significantly because of operator dependence.

See the images below.


Crohn disease of the terminal ileum with CT and sonographic correlation. Note hypoechoic wall thickening, loss of the gut signature,and the hyperechoic line representing the narrowed lumen.

Crohn disease of the terminal ileum with CT and sonographic correlation. Note hypoechoic wall thickening, loss of the gut signature,and the hyperechoic line representing the narrowed lumen.



Crohn disease. Sonogram of a thickened bowel wall demonstrates the socalled pseudokidney appearance.

The literature states that the differentiation between hypoechoic foci from creeping fat and that from phlegmon oredema may be difficult or nearly impossible. Proponents of CT have also stated that the specificity of color Dopplerimaging is still unknown. In general, the confidence level of the radiologist in interpreting the results is operatordependent, and it is often lower than that of CT. For these reasons, ultrasonography has not been the favoredmodality for imaging Crohn disease.

Nuclear ImagingLeukocytes labeled with either technetium99mHMPAO (hexamethylpropylamine oxime) or indium111 can be usedto assess for active bowel inflammation in inflammatory bowel disease. Compared to the111 In label, the99m TcHMPAO label has better imaging characteristics and can be imaged much sooner after injection. However, imagingmust typically be done within an hour after injection of99m TcHMPAOlabeled leukocytes, as there is normalexcretion into the bowel after this time, unlike with111 labeled leukocytes, which have no normal bowel excretion.

Molnar et al found that a99m TcHMPAO leukocyte scan in active Crohn disease had a sensitivity of 76.1% and aspecificity of 91.0%, as compared to CT sensitivity of 71.8% and specificity of 83.5%. While leukocyte scans may bebetter in the detection of segmental inflammatory activity, CT is superior for the detection of complications.[40]

Falsepositive bowel activity can be seen with gastrointestinal bleeding, swallowed leukocytes (eg, from uptakerelated to sinusitis or nasogastric tubes), or activity related to indwelling enteric tubes. In addition, leukocyte uptakeis not specific for Crohn disease and will be seen in most infectious or inflammatory bowel processes. As mentionedabove, there is often normal bowel excretion of99m TcHMPAO leukocytes if imaging occurs within the first hourafter injection.

Contributor Information and DisclosuresAuthorYungHsin Chen, MD Director of MRI, Section Chief of Musculoskeletal Radiology, Alliance MRI Norton;Assistant Clinical Professor, Tufts University School of Medicine

YungHsin Chen, MD is a member of the following medical societies: American Roentgen Ray Society,Radiological Society of North America, and Society of Skeletal Radiology

Disclosure: Nothing to disclose.

Coauthor(s)Dahua Zhou, MD Staff Physician, Department of Radiology, Nassau University Medical Center

Dahua Zhou, MD is a member of the following medical societies: Radiological Society of North America


David I Weltman, MD Consulting Staff, S & D Medical, LLP; Director, Department of Radiology, SouthsideHospital

David I Weltman, MD is a member of the following medical societies: American College of Radiology, AmericanRoentgen Ray Society, Association of Program Directors in Radiology, New York County Medical Society, andRadiological Society of North America


Specialty Editor BoardBernard D Coombs, MB, ChB, PhD Consulting Staff, Department of Specialist Rehabilitation Services, HuttValley District Health Board, New Zealand


Spencer B Gay, MD Professor of Radiology, Department of Radiology and Medical Imaging, University ofVirginia School of Medicine


Robert M Krasny, MD Resolution Imaging Medical Corporation

Robert M Krasny, MD is a member of the following medical societies: American Roentgen Ray Society andRadiological Society of North America


Chief EditorEugene C Lin, MD Attending Radiologist, Teaching Coordinator for Cardiac Imaging, Radiology ResidencyProgram, Virginia Mason Medical Center; Clinical Assistant Professor of Radiology, University of WashingtonSchool of Medicine

Eugene C Lin, MD is a member of the following medical societies: American College of Nuclear Medicine,American College of Radiology, Radiological Society of North America, and Society of Nuclear Medicine


Additional ContributorsJohn L Haddad, MD Clinical Associate Professor, Department of Radiology, Weill Medical College of CornellUniversity; Director of Body MRI, Department of Radiology, Methodist Hospital in Houston

John L Haddad, MD is a member of the following medical societies: American College of Radiology, AmericanMedical Association, and Radiological Society of North America

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