The gastrointestinal tract

Large bowel

 

Imaging techniques

Plain radiography is useful mainly in acute disease such as obstruction, ischaemic colitis or acute inflammatory colitis (see below). In chronic disease it is of limited use; the extent of faecal residue may be approximately assessed in patients with constipation. The double contrast barium enema (DCBE) is now the contrast examination of choice in most patients with suspected large bowel pathology. The single contrast enema is undertaken if the DCBE cannot be performed or in an unprepared colon if there is suspected large bowel obstruction or leak, in which latter case a water-soluble contrast should be used. The DCBE has been shown to be superior to the single contrast technique in the detection of small polyps and subtle mucosal disease. The single contrast barium enema may be used in the very old, disabled or ill patient where moblity is limited and to exc1ude obstruction. Some authors advocate a biphasic technique, a limited single contrast study of the sigmoid being performed after the double-contrast examination. This is effective where there is diverticular disease which may obscure polyps on the double-contrast study.

For routine studies, single or double contrast, colon cleansing is essential. A variety of regimens is available, some of which require a preparatory period of low-residue diet and then clear fluids. Laxatives are then given which are of ten a combination of magnesium citrate and Bisacodyl tablets, followed by suppositories. Colonic cleansing enemas may be given, but an interval of at least 45-60 minutes is required before the examination can be performed. Several regimes, for example sodium picosulphate or oral colonic lavage solution, dispense with the need for strict low-residue diets and cleansing enemas.

Barium is introduced via a rectal tube. Balloon retention catheters should be avoided, as most complications of barium enema - perforation of the rectum or more proximally - have occurred in association with their use. If a balloon catheter is used, as when there is laxity of the anal sphincter, this should be inflated under fluoroscopy using a low-capacity bulb with a valve. The bariums used for DCBE are of high density. Low density, dilute barium should be used for the single contrast barium studies. lonic water soluble contrast is used when there is suspected perforation or in obstruction when the patient is expected to be operated upon soon after the study. Contra-indications to DCBE inc1ude obstruction (use single contrast), potential technical difficulties (such as immobility, etc), severe inflammatory bowel disease, acute diverticulitis (use water-soluble contrast or, preferably, CT scanning) and deep biopsy within the last 6-7 days (superficial mucosal biopsy is not a contraindication). Contraindications to single contrast barium enema include acute severe colitis, suspected perforation (use water-soluble contrast) and recent deep biopsy.

Many different techniques have been described for the performance of the DCBE. Some are complicated and depend upon the configuration of the sigmoid loops seen during the examination. Other are simple standardised methods which work in the great majority of patients. There is an increasing tendency to use carbon dioxide instead of room air for insufflation since, because of its more rapid absorption from the bowel, it causes less post-procedure discomfort. A pressure limiting system is

Figure 61. Detail of double contrast examination of sigmoid colon showing typical innominate groove pattern in proximal loop (open arrows) and fine granularity of active ulcerative colitis (solid arrows).

required to give carbon dioxide; it must not be administered directly from high pressure cylinders. Views of the large bowel taken during the DCBE are designed to show all parts of the bowel in double contrast. Optimal positioning for spot films is determined by fluoroscopy.

Thin grooves may be seen in profile on DCBE in the normal colon, known as innominate lines; these are a feature of the normal mucosal pattern but are not seen as frequently as the areae gastricae in the stomach. It is important to distinguish them from the granular pattern of fine mucosal ulceration (Fig. 61). Most errors in DCBE are perceptive; multiple readings by more than one radiologist significantly increase lesion detection rate.

As in small bowel disease, transabdominal ultrasound may be useful in the delineation of extramural disease such as paracolic abscess but its application is limited by bowel gas. Some authors have used transabdominal colonic sonography for the detection of neoplasms, using colonic water enemas, but the role of this technique is yet to be defined. Endoluminal ultrasound is of limited availability but appears to be accurate in the staging of colorectal neoplasms in a similar way to upper gastrointestinal cancer staging with endoscopic ultrasound.

CT scanning is still the method of choice in most centres for the imaging of extramural colorectal disease. Major applications include the staging of neoplasms and the assessment of paracolic inflammation and abscesses in inflammatory bowel disease and diverticulitis. There are limitations to CT; bowel wall thickening is largely non-specific and in neoplastic disease the depth of intramural invasion cannot be determined; the demonstration of enlarged lymph nodes is also non-specific and metastatic disease may occur in normal-sized nodes. The technique for CT scanning in large bowel disease must include adequate oral contrast administration to allow good luminal distension and opacification. Bowel cleansing preparation may be required for imaging primary colonic disease. A dose of dilute oral contrast should be given, if possible, several hours before the examination - the evening before, if the study is scheduled for the morning - or else a hurrying agent such as Sorbitol or Maxolon may be added to the oral contrast. In selected patients 200-300 ml of dilute contrast may be administered by rectal catheter to opacify the distal colon and rectum. An alternative is to insufflate the rectum with air which provides good luminal distension and contrast. In most cases intravenous contrast should be given to outline vascular structures and the urinary tract. Dynamic intravenous bolus contrast techniques are used to image gastrointestinal lesions, if possible, during the arterial enhancement phase, since the presence, degree and pattern of enhancement are important parameters used in diagnosis. Dynamic techniques are also used for detection of liver metastases in colorectal cancer staging. It may be desirable to acquire extra scans in decubitus or prone positions, depending on the site of the lesion, to image subtle abnormality, and to obtain extra thin sections through an area of interest for better resolution.

The use of fast scanning techniques in MR imaging and the development of intraluminal contrast agents are likely to lead to wider application of this modality in large bowel disease. There is evidence of MR's utility in detecting fistulae and abscesses in inflammatory bowel disease. MR suffers from the same limitations as CT in relation to bowel wall disease. There is little advantage of MR over CT for staging colonic carcinoma, but it does appear to be superior in invasive rectal carcinoma staging and in the diagnosis of presacral recurrent tumour following abdomino-perineal resection.

Pathology

Diverticular disease
Colonic diverticula are of the acquired pulsion type. In Western societies they are present in 30-50% of the population over the age of 50 years. Diverticula are seen predominantly in the sigmoid and distal descending colon and occur laterally between the mesenteric and antimesenteric taenia or sometimes in the antimesenteric inter-taenial area, in which position they are of ten small or intramural. In 10% they are seen in the right side of the colon only, and in 17% they are scattered throughout the colon.

Figure 62. Views of a sigmoid colon diverticulum on double contrast barium enema. Note in (a) the diverticulum is seen en face; it is empty of barium, the resultant ring shadow having an outer well defined rim and an inner rim that ''fades away". The mouth of the diverticulum is seen in the centre of the ring giving a "Mexican hat" sign. In (b) the diverticulum is seen obliquely and protrudes beyond the lumen proving the nature of the lesion.

The term "diverticulosis" is often used for multiple diverticula in asymptomatic individuals; "diverticular disease" is used when there are symptoms and "diverticulitis" when there is associated inflammation. "Pre-diverticular disease" is sometimes employed to denote the appearance of thickened circular folds in the sigmoid colon and a spikey irregular outline along the antimesenteric ridge, changes assumed to represent the early phase of diverticular disease. When a large number of diverticula are present in the sigmoid colon, often with spasm and overlapping loops, it is easy to miss co-existing polypoid lesions. Indeed, this area is one of the weak points of the barium enema examination. Some authors advocate a biphasic examination, following the double-contrast study with a limited single contrast one with compression of the sigmoid colon, to demonstrate small polyps as filling defects within the dilute barium. The appearance of diverticula on barium enema depends on the angle from which they are viewed, the degree of filling with barium and/or air, and whether there are retained faceoliths within them. En face they are seen as rounded collections of barium of variable size, or to contain an air/barium level on decubitus or erect views, or as a ring shadow if they are empty of barium. When viewed in profile or obliquely, diverticula appear as barium-coated or barium-filled outpouchings (Figs. 62, 63).

Figure 63. Double contrast barium enema showing left sided diverticula, some appearing as ring shadows, others as barium filled outpouchings. In addition there is a pericolic inflammatory mass causing impression on the medial aspect of the sigmoid (arrowed).

Diverticulitis
Acute diverticulitis typically presents with left iliac fossa pain, with or without a palpable mass, fever and leucocytosis. There is an increasing tendency to investigate these patients radiologically with CT scanning, though some authors maintain that there is a continued place for water-soluble contrast enema, particularly if the CT is equivocal. CT has many advantages: as well as being more comfortable for the patient, it is able to confirm the presence of diverticula and the site of disease, demonstrate peri-colic inflammation - assess abscess formation and help plan management, whether it be medical or, in the case of abscess, surgical or percutaneous drainage. As well as showing even small paracolic fluid collections, other CT signs include wall thickening and peridiverticular inflammatory infiltrate into the surrounding fat (Fig. 64). If CT is unavailable, ultrasound is often able to show pericolic abscess, but suffers from the usual limitations of DS in the presence of bowel gas. If conservative management is undertaken, it is reasonable to perform a delayed double-contrast barium enema when the patient has recovered, to assess the extent of diverticular disease and any co-existing pathology, since the CT appearance of wall thickening is non-specific and CT

a	Figure 64. Acute diverticulitis. (a) CT scan showing sigmoid loop with marked mural thickening and pericolic inflammation. A row of diverticula are interconnected by linear inflammatory stranding (arrowed). (b) Another case. Double contrast enema showing impression from pericolic abscess (arrowed). Gas is present in the adjacent pericolic gutter (curved arrow). (c) Same patient as (b). Small pericolic abscess with inflammatory infiltrate in to pericolic fat and thickening of anterior Para renal and lateroconal fascia.
b
c

cannot exclude colonic neoplasm.

If a contrast enema is performed during the acute attack, water-soluble contrast should be used and introduced with care. If a significant leak is present this will be seen as extravasation, contrast usually tracking parallel to the bowel wall and sometimes forming interconnections with a number of diverticula (see Fig. 64 a). Deformed distended diverticular sacs may suggest microperforation. Mucosal changes that are seen include the "drape sign" - the bending of adjacent diverticula around a presumed pericolic abscess and multiple crowded transverse pleat-like folds which may be deformed into a crumpled pattern. Other signs of pericolic inflammation inc1ude a soft tissue mass which may cause impression, compression or displacement of the adjacent lumen (Figs. 63,64), and may have visible gas within it. Occasionally, contrast extravasates into a definite cavity. A pattern of small bowel obstruction may be apparent on plain radiographs or CT, due to loops of bowel adherent to an inflammatory mass.

Other complications of diverticular disease
Diverticulitis may cause large bowel obstruction due to a pericolic inflammatory mass and associated narrowing and spasm. It is often very difficult on contrast enema to distinguish this from neoplastic obstruction. The strictures caused by carcinomas tend to be shouldered and associated with destruction of mucosal folds. Acute haemorrhage from diverticulitis may be a massive affair and is not uncommon. Contrast studies are not indicated. The patient will usually undergo endoscopy. If bleeding continues and the source of bleeding remains in doubt or there is an indication for non-operative intervention, then angiography is undertaken. This may be preceded by a nuclear medicine blood pool scan. Diverticular disease is not regarded as a cause of anaemia by occult bleeding.
Other complications of diverticulitis include spread of inflammation, resulting in fistula formation to the urinary or genital tract or retroperitoneal spread to the perirenal space or inferiorly to the buttock or groin through the sacrosciatic foramen. Free perforation into the peritoneal cavity is rare. Distant spread of infection can occur leading to portal pyaemia and/or hepatic abscess.

Polyps versus diverticula
A frequent problem on double contrast barium enema is to distinguish the ring shadows of polyps and diverticula (Fig. 62). A polyp will appear intraluminal on all views; within the barium pool it will be seen as a filling defect; a stalk may be present which may be seen in long or oblique axis, or present a "Mexican-hat sign" when the stalk is seen en face through the head of the polyp (Fig. 65). Ring shadows due to polyps typically have an inner rim which is well-defined and an outer rim which tends to fade away, in contradistinction to the ring shadows formed by diverticula, which demonstrate the reverse pattern. Diverticula are usually seen on at least one view as extraluminal when image d obliquely or tangentially, or to contain barium and/or air. The "bowler-hat" sign is a feature of both polyps and diverticula and is due to the base of the lesion

Figure 65. View of sigmoid/descending colon junction showing constricting carcinoma (open arrow) and "sentinel" polyp in barium pool (solid arrows). Note the "Mexican hat" sign related to the latter.

being seen tangentially. However, if the "hat" always points towards the centre of the long axis of the bowel the les ion should be intraluminal, i.e. a polyp. This rule is useful unless the lesion lies in the midline of the bowel or is parallel to its long axis.

lnflammatory diseases
Of the non-infective causes of inflammatory bowel disease (IBD) to affect the large intestine, ulcerative colitis (UC) and Crohn's disease (CD) are by far the commonest. In the patient with acute symptoms, the plain radiograph may be useful. In VC, the extent of disease can sometimes be ascertained by the distribution of faecal residue; residue is not seen in that part of the bowel where there is active inflammation. However, this sign is not helpful if the colon happens to be empty of faeces nor in CD where there are of ten "skip" lesions. The extent and severity of the disease may be apparent from the visualised mucosal fold pattern. Mucosal pseudopolyposis, submucosal oedema ("thumbprinting") and wall thickening may be seen where luminal air provides adequate radiographic contrast. In severe colitis, actual or impending toxic megacolon may be evident.

Most patients with IBD can be examined by DCBE, but the bowel preparation should be modified in patients with acute symptoms or severe diarrhoea. Contraindications to the double contrast examination include severe acute colitis, and contraindications to any contrast enema study include toxic dilatation and perforation. Some authors advocate an "instant-enema" in patients with acute colitis to assess the extent and severity of the disease and to monitor progress with treatment. It is helpful in DC where inflammation starts in the rectum and extends proximally in a contiguous fashion, but is of less use in CD. However, with the increased availability of flexible endoscopy, the "instant enema" is not commonly required. It may be of use if endoscopy is unable to reach the proximal extent of the inflammation. The technique is performed on an unprepared colon. Barium is run in to the splenic flexure or until faecal residue is encountered (if this is more distal) - in DC there is unlikely to be active disease where there is faceal residue. The rectum is then drained and air (or preferably carbon dioxide) is carefully introduced to produce gentle distension. A short-acting hypotonic agent may be given.

Other modalities that are employed in the investigation of IBD include radionuclide-labelled white cell scanning to assess activity and severity of disease, and DS or CT imaging for extramural complications, such as abscesses and fistulae. DS is hindered by the presence of bowel gas. CT should be regarded as complementary to barium studies. It has limitations in milder degrees of DC where the disease affects the mucosa only, but in more severe DC and other colitides, CT will demonstrate thickened bowel wall, in a distribution corresponding to the type of colitis, and may show transmural ulcers and pericolic inflammation or abscesses.
Regarding MR scanning, see previous comments under "Inflammatory bowel disease" in the section small intestine.

Ulcerative colitis
This disease is characterised by episodes of exacerbation and remission. The rectum is virtually always involved, with a variable but contiguous extent of the colon being affected proximally. The main categories of involvement are: proctitis, distal (or left-sided) colitis and so-called "extensive" colitis. The latter term is used to denote disease radiographically extending as far proximally as the hepatic flexure. In practice this almost always means that the whole colon is involved. The earliest sign of DC on DCBE is a finely granular mucosal pattern which is uniform and confluent. Progression is manifest as superficial erosions which give a stippled appearance to the mucosa (Fig. 61). As these heal a coarsely granular

Figure 66. Double contrast enema showing left sided ulcerative colitis. Deep "collar stud" ulcers are present on a background of abnormal mucosa.

appearance is seen. More severe disease leads to deeper ulceration, with "collar-stud" ulcers which are due to undermining of the mucosa and appear as linear and parallel to the long axis of the bowel (Fig. 66). If these are long they produce a double-track appearance to the wall. These changes are seen on a background of diffuse granularity (cf. Crohn's disease ). Extensive acute ulceration results in islands of relatively intact mucosa between ulcers. This intact and usually hyperplasic, oedematous mucosa appears polyp-like on contrast studies (and often on plain films) - so called pseudopolyposis. Inflammatory polyps are se en due to masses of granulation tissue. The haustrations of the colon are blunted and thickened, but become effaced when ulceration is present. (It should be noted that haustrations are often normally absent from the left hemicolon.) In the rectum, the mucosal changes described above are seen and there is thickening of the rectal folds. Apparent relative sparing of the rectum may be seen in patients receiving topical steroid enema treatment.

In longstanding chronically active VC there is loss of haustral folds and the whole affected colon becomes narrowed, shortened and featureless ("pipe-stem" colon; Fig. 67). The rectaI foIds are effaced and there

Figure 67. Chronic ulcerative colitis. The left hemicolon is diffusely mildly narrowed and has lost haustral markings. The filling defect in the splenic flexure region was artefactual.

is widening of the post-rectal space. The rectum is narrowed and consequently of low capacity. In total colitis "backwash ileitis" may be present manifest as dilatation, hypomotility and granularity of the distal ileum and incompetence of the ileocaecal valve (Fig. 45).

lntestinal complications of ulcerative colitis
In the acute attack, plain abdominal films should be obtained. The musocal edge pattern and the distribution of faceal residue should be observed. Sometimes an exacerbation of symptoms is associated with impaction of faeces proximal to active colitis. A "gasless" abdomen often points to an extensive active colitis. Dilatation of the colon, most commonly involving the transverse colon, but affecting any segment of the large bowel, may signify toxic megacolon. This is a diagnosis made on a combination of clinical and plain radiographic criteria. The latter include the presence of dilatation to a calibre of 8 cm, mucosal islands and thumbprinting, usually associated with evidence of a small bowel ileus. If this progresses, intramural gas followed by local or free perforation may be apparent. When dilatation extends to 5 cm, then the patient should be regarded at high risk for developing toxic dilatation, and frequent monitoring with plain abdominal radiographs is called for.

Figure 68. Chronic total ulcerative colitis complicated by carcinoma. There is an annular carcinoma of the ascending colon (open arrow), superimposed on changes of chronic colitis - shortened, narrowed colon lacking haustrae. Plaque-like areas of dysplasia were also evident (solid arrows in sigmoid colon; these reproduce poorly in the illustration).

The complications of long-standing VC include benign strictures and colonic malignancies. Benign strictures occur mostly in the sigmoid colon and are symmetrically tapered with a uniform mucosal texture. However, endoscopic biopsies are usually required to exclude malignancy and dysplastic changes. There is a significantly increased risk of colonic neoplasm in UC; this is particularly so in patients with total colitis of greater than 10 years duration. The presence of dysplastic mucosal changes is associated with a high risk. Neoplasms are evenly distributed in the large bowel and tend to be annular (Fig. 68), although scirrhous growths are also seen. Synchronous neoplasms are relatively common. Dysplasia can occasionally be recognised radiographically when it gives rise to elevated plaques which appear as a nodular polygonal pattern on double contrast views. However, radiology is insensitive in detecting dysplasia, and monitoring of at-risk patients should be by serial colonoscoples.

An interesting and characteristic appearance may be seen on DCBE, following healing of a severe colitis, which is usually due to VC but may be seen with other inflammatory colitides. As ulceration heals, the hyperplastic

Figure 69. Post inflammatory filiform polyps in a patient with a history of previous acute colitis.

mucosal islands between the ulcers undergo further hyperplasia and may give rise to branching mucosal tags of various configurations often Y or V or inverted V shapes -leading to post-inflammatory filiform ("worm-shaped") polyposis (Fig. 69).

Crohn's (granulomatous) colitis
Crohn's disease is a chronic disorder with acute exacerbations. The transmural nature of the abnormality explains the propensity for sinus, fistula and stricture formation. Approximately 15 % of patients with this disease have colonic involvement only. The small bowel only is affected in 30%, and ileocolic involvement is present in 55%. Early changes of Crohn's disease in the colon are similar to those previously described in the small bowel. Discrete aphthoid ulcers may be demonstrated on DCBE surrounded by normal mucosa (Fig. 70). The differential diagnosis of this appearance includes Yersinia, Amoebic, CMV and other infective colitides, Behcet's disease and ischaemic colitis. The accompanying submucosal oedema is less prominent than seen in small bowel Crohn's disease and the haustral pattern and mucosal folds may therefore be normal. Occasionally, a granular mucosa may be seen as in VC. A subgroup

Figure 70. Crohn 's colitis, overview of colon. There are discontinuous changes with severe involvement of transverse colon (cobblestoning and ulceration) and mild sigmoid disease (aphthoid ulcers on a background of normal mucosa). Note also the terminal ileal stricture.

of colitis occurs in which it is impossible to distinguish, on radiographic criteria, VC and Crohn's disease. In some of these the distinction may also be impossible on endoscopic and histological grounds.

More advanced disease is characterised by discontinuous, asymmetrical changes (Fig. 70). This asymmetry is seen as involvement of one wall with ulceration during the active phase, and, later, by sacculation as fibrosis causes shortening of one wall and redundancy of the opposite wall. Large ulcers which may be of "collar-stud" type may be seen, as in UC, but unlike UC the intervening mucosa tends to be normal and not granular. Serpiginous longitudinal and transverse u1cers, with or without cobblestoning, strictures, fistulae and sinuses are seen as in small bowel Crohn's disease. Abscesses may be imaged by ultrasound, CT or MR scanning. MR appears useful in delineation of perianal fistulae and abscesses. Rectal involvement occurs in 50% of patients with colonic disease. Inflammatory polyps and pseudopolyposis are seen as in UC. Regression of acute changes often leads to scarring in colonic Crohn's disease, unlike UC which may heal to a virtually normal appearance. Comparative features of large bowel Crohn's disease and ulcerative colitis are summarised in Table 8.

Table 8.Features of colonic Crohn 's disease & ulcerative colitis

---

                                                 UC                                               CROHN'S

---

Rectum involved	always	approx.50 %
Extent	variable, proctitis to total	variable
Ileum	occ backwash ileitis with total UC	commonly involved
Contiguity	contiguous disease	"skip" lesions
Background	granular	normal mucosal pattern
Ulceration	collar-stud on granular background	tend to deeper ulcers on normal background
Sinuses/fistulae	rare	common
Strictures	uncommon	common
Malignancy	increased	slightly increased

 

lnfectious diseases

Entamoeba Histolytica
This is dealt with in the chapter on "Tropical disease".

Other infective colitides
In most patients with acute colonic infections a plain abdominal radiograph is the only radiological examination performed. This may give an indication of the extent and severity of disease and will show the development of toxic dilatation, which can occur with most of the acute infections.

Shigella dysentery produces discrete aphthoid ulcers predominantly in the left side of the colon which may progress to exensive deep ulceration like DC. Salmonella colitis and Pathogenic E. coli infection are rarely investigated radiologically, but can produce a similar picture to Shigella. Campylobacter infection is like DC or Crohn's disease. Toxic dilatation can occur in any of these. Yersinia enterocolitica affects the distal small bowel (q. v.) but occasionally involves the colon where it produces aphthoid ulceration; the ulcers tend to be smaller than those seen in Crohn's disease.

Chlamydia infection causes lymphogranuloma venereum, a disease of the tropics but also seen in immunocompromised hosts. Involvement of the rectum occurs mainly in women but also in men practising anal receptive intercourse. A chronic proctitis occurs which results in stricturing which may extend for a variable length up to the sigmoid colon or even more proximally. The sigmoid loop may be elevated by the fibrotic reaction. Pararectal, paravaginal and paracolic sinuses can be demonstrated often associated with abscesses. Rectal gonorrhoea results in small rectal ulcers usually without radiolucent halos on a background of normal mucosa. Rarely strictures and fistulae may occur. Actinomycosis affects the right side of the colon and distal ileum where it has a propensity for causing complex sinuses and fistulae and fibrous masses which may mimic neoplasms. Mycobacterium tuberculosis infrequently causes pure colonic disease. Ileocolic involvement has been described in the small bowel section. Scarring of the ileocaecal region and ascending colon, annular strictures and/or deformity of the ascending colon occur and the picture may be indistinguishable f