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 from Crohn's disease or from neoplasm. Prevalence of the respective diseases in local practice will obviously help. Tuberculosis is suggested by circumferential or stellate ulcers, "hourglass" type strictures and a constricted caecum with a patulous ileocaecal valve. Occasionally, diffuse colonic disease will mimic UC.

Schistosomiasis affecting the bowel is usually due to S. mansoni or S. japonicum. The appearances are described in the chapter on "Tropical disease".
AIDS-related colitis may be due to the co-existence of multiple organisms ("gay-bowel"). CMV infection may be relatively mild, associated with diffuse mucosal granularity and aphthoid ulceration, or fulminant, causing multiple large discrete ulceration, submucosal haemorrhage, toxic dilatation, pneumatosis and gangrenous necrosis. There is a predilection for caecal involvement with deep ulcers; this may also be seen in renal transplant patients. The haemorrhagic colitis and necrotic features are related to the vasculitis seen in infection with this organism. Cryposporidium and atypical mycopbacterial organisms do not produce a specific radiological picture in the large intestine. CT may be useful in AIDS-related proctocolitis. Inflammatory infiltrate into the perirectal fat is seen, but in addition, CT may help distinguish neoplastic from inflammatory disease. Diffuse mural thickening with a "target-sign" favours inflammatory disease.

Pseudomembranous colitis is due to endotoxin-producing Clostridium difficile and is most commonly related to the administration of broad-spectrum antibiotics. Sigmoidoscopy is usually diagnostic, showing the typical pseudomembranes, together with the isolation of the toxin from the stools. The clinical course is variable. Fulminant disease may occur with toxic dilatation demonstrable on plain radiographs. Other plain radiographic findings in less severe disease include an adynamic ileus (which may presage toxic dilatation), "thumbprinting", irregularity of the mucosal edge due to ulceration and pseudomembrane, and haustral thickening. If contrast studies are indicated, these show an irregular shaggy mucosa due to confluent shallow necrotic ulceration and pseudomembrane, elevated plaques where the ulcers are covered by pseudomembrane, and "thumbprinting" due to submucosal oedema. The whole colon may be affected and there may be an associated small bowel enteritis. Occasionally there is rectal sparing.

Acute ischaemic colitis
Acute ischaemic colitis typically presents with abdominal pain and rectal bleeding. In the majority of patients the splenic flexure and proximal descending colon are involved, this being the "water-shed" region between superior and inferior mesenteric artery circulations. Three categories of disease are described. The benign "transient" form responds to conservative therapy and the colon returns to a radiologically normal appearance. The "gangrenous" form requires surgical management. The third "stricturing" form is more controversial and is associated with the later development of an ischaemic stricture.

The "transient" form leads to submucosal haemorrhage or oedema, manifest as "thumbprinting" seen on plain films and contrast studies, mural thickening, spiculation and spasm causing narrowing of the lumen. The "instant enema" (see under ulcerative colitis) is an appropriate method of performing a contrast examination on these patients. Overdistension on barium examination may cause effacement of the thumbprinting. The mucosa may be radiologically intact, or, in more severe cases, there may be ulceration and, occasionally, pseudopolyps. Transverse ridging can be seen, the presence of which may be more common when the patient proceeds to the stricturing form. The colon in most patients will return to normal at a variable rate, of ten up to several weeks or months from the acute episode. Narrowing of the lumen during the healing phase is often eccentric, tapered and associated with sacculation (Fig. 71). There is some evidence that the latter finding may be an indicator of a delayed return to normal or to the formation of a stricture.

In the gangrenous form of ischaemic colitis, plain films may show intramural gas or, later, free gas or gas in the portal venous system. CT

Figure 71. Ischaemic colitis involving "water shed" region around the splenic flexure. The proximal descending colon is shown. There are eccentric changes with mild narrowing, sacculation and fine nodularity.

scanning may be useful in ischaemic colitis, although its role is as yet controversial. In the diagnosis of the less severe forms the findings are non-specific and inc1ude relatively mild (0.5-1 cm) symmetrical bowel wall thickening with a "double-halo", "target" sign or homogeneous density on intravenous bolus contrast enhancement. In more severe disease, CT is sensitive in detecting intramural and portal venous gas. In addition, CT can sometimes determine the cause of ischaemia by showing arterial occlusion or thrombus in SMA or portal venous system. Angiography is seldom indicated, since the information provided is infrequently decisive; even if an occlusion is detected, this usually affects a peripheral branch.

Figure 72. Pneumatosis cystoides coli affecting the sigmoid colon. Note radiolucent gas blebs which in profile are seen to parallel the bowel wall. More proximally the sigmoid is normal.

Pneumatosis coli

Gas in the bowel wall may occur for reasons other than ischaemic necrosis. Pneumatosis (cystoides) coli is a benign idiopathic condition associated with gas collections, usually in blebs or "cysts" within the wall and paralleling the lumen. It is most frequently distributed in the sigmoid and descending colon, often with rectal sparing. There is luminal narrowing and scalloping of the contour by the submucosal impressions of the gas cysts but obstruction is not seen (Fig. 72). Patients are either asymptomatic or present with intermittent abdominal pain and diarrhoea. Occasionally, a bleb may perforate and cause pneumoperitoneum without peritonitis. The small bowel may be involved in addition to the colon. A secondary form may be seen where there is a cause of raised intraluminal pressure in association with mucosal ulceration, such as may be present proximal to an obstructing stricture of the colon.

Colorectal neoplasms

Detection
Colorectal cancer (CRC) is the commonest internal malignancy in many Western countries. It occurs at all adult ages, but the incidence rises sharply after 40 years of age. Although some individuals fall into high or above-average risk groups (for example, prior history of colorectal neoplasia, family history of polyposis syndromes, hereditary non-polyposis colorectal cancer families, first degree relative(s) with CRC, chronic ulcerative colitis) most CRCs occur in patients with no known increased risk. As the cause of CRC is unknown, primary prevention is unlikely to be possible in the near future for most individuals, despite the imminent development of molecular genetic techniques to identify those at inherited risk. It is generally accepted that the great majority of CRCs develop in pre-existing benign adenomas, the detection and removal of which should therefore effect secondary prevention of CRC. For these reasons screening of asymptomatic persons has been advocated using faecal occult blood testing, flexible sigmoidoscopy, colonoscopy, radiology or a combination of techniques, to detect early-stage CRC and adenomas. The role of radiography in the screening of asymptomatic individuals has yet to be resolved. Some authors have advocated screening with periodic DCBE every 3-5 years in combination with annual faecal occult blood testing commencing at the age 40-50; there is a push by the American College of Radiology to adopt this programme but at present this is controversial.

Colonoscopy is undoubtedly superior to DCBE in demonstrating small polyps and has the potential for polypectomy, and this is the preferred examination (with a combination of DCBE and flexible sigmoidoscopy an acceptable alternative to colonoscopy where this is not available) in persons with positive occult blood tests or, some would suggest, as a screening technique in its own right. It is the screening procedure of choice in high risk groups.

Leaving aside the question of screening asymptomatic individuals, it behoves radiologists performing contrast enemas to optimise their techniques to enable them to pick up small cancers and polyps in patients undergoing studies for any indication. By diagnosing asymptomatic early neoplasms and polyps they will be contributing to the reduction of CRC mortality. Double reporting of DCBEs increases the yield of these examinations.

Colonic epithelial polyps
Neoplastic epithelial polyps are very common, increasing with age. Most are in the rectosigmoid region, but there is a relative shift to the right side of the colon with increasing age. They are multiple in about 50% of patients. In those with polyposis syndromes they may be innumerable. The importance of benign polyps lies in their malignant potential. Neoplastic polyps are tubular adenomas (much the most common), villous adenomas

Figure 73. Polypoid carcinoma of the caecum (large arrows) with synchronous sessile (small arrow) and pedunculated (arrowheads) sigmoid polyps.

and tubulovillous adenomas. Those with villous features have a higher malignant potential. Lesions may be pedunculated or sessile (Figs. 65, 73). Villous adenomas are more likely to be sessile polyps or plaque-like when they may give rise to a characteristic "carpet-like" growth, especially in the rectum or sigmoid colon, extending over several centimetres. The difficulties of radiological detection of small or diminutive polyps has already been alluded to, particularly in the sigmoid colon when there is co-existing diverticular disease and overlapping loops. Although the double-contrast method is superior to single contrast and, in the best of hands may detect up to 90% of polyps greater than 10 mm in diameter, there is only a 60-70% sensitivity for smaller lesions. Some authors advocate a biphasic technique. Combining DCBE with flexible sigmoidoscoy increases sensitivity. How important is it to find diminutive polyps? The majority of distal lesions smaller than 4 mm in size are hyperplastic and thus have no malignant potential. However, of lesions of 4-5 mm approximately 50% will be adenomas and will have this potential.

In determining whether a radiologically demonstrated polyp is currently benign or malignant, size is the only reliable indicator. For lesions less than 5 mm in diameter, the presence of malignancy is so unlikely as to be virtually discounted. One to 2 % of lesions of 5-10 mm in diameter are malignant. About 10% of 1-2 cm polyps are malignant, while the incidence rises to approximately 50% for polyps greater than 2 cm. Pedunculated polyps are less likely to be malignant than sessile ones. Indentation of the polyp base suggests malignancy. The surface pattern of the polyp is an unreliable indicator, but ulceration suggests malignancy.

The radiographic features of polyps and their distinction from diverticula have been discussed previously.

Polyposis syndromes
Familial adenomatous polyposis and Gardner's syndrome are associated with multiple, usually innumerable, colonic adenomas. Carcinoma is inevitable without colectomy. Polyps may be se en throughout the bowel and have been described elsewhere in this text. In Gardner's syndrome, soft tissue desmoid tumours and osteomas are also seen. There is a tendency to develop periampullary duodenal malignant neoplasms. Juvenile polyps are a form of hyperplastic benign lesion, usually solitary and pedunculated in the rectosigmoid area; they have a propensity for autoamputation. Peutz-Jegher's syndrome is associated with hamartomas in the bowel, bur rarely malignant degeneration can occur.

Colorectal carcinoma
Diagnosis
The rectum and sigmoid regions are the commonest sites for carcinoma, but there has been a relative shift of distribution to the right side of the colon in recent years. Multiple cancers are seen in about 5% or there are benign polyps present within the same colon, frequently so-called "sentinel" polyps that occur near the malignant tumour. The double contrast barium enema, in the best hands, detects approximately 90-95% of colonic cancers. Most missed tumours are in the sigmoid, often when there is co-existing diverticular disease, and in the caecum. Plaque-like lesions are more likely to be overlooked than polypoid or annular ones. The vast majority of tumours are adenocarcinomas. The radiographic appearances on barium studies are:

i) annular constricting lesions: the most common, seen as typically short "apple-core" segments of narrowing with destruction of the mucosal pattern (Fig. 65); ulceration may be present.
ii) Polypoid lesions which are fungating and intraluminal producing a large mass with irregular surfaces and an indrawn base; these are frequently caecal (Fig. 73).
iii) Infiltrating, plaque-like, lesions with submucosal spread, similar to linitis plastica. Subtle examples may be missed on barium enema, the only clues to the presence of a lesion may be its elevated edges with a deformity of the mucosal contour. This type of lesion is often the one associated with malignant change in ulcerative colitis when long and rather smooth contoured strictures may be seen.
iv) Ulcerating tumours with deep excavating craters and raised margins. These are the least common.
v) Mixed types.

Although most carcinomas conform to these patterns, early invasive cancers are occasionally seen. They have been classified by Japanese authors in much the same way as early gastric cancers, i.e. elevated or polypoid and flat or depressed lesions. Occasionally, colonic carcinoma may present with signs of localised or free perforation. Local perforation gives rise to an inflammatory mass, the nature of which is difficult to assess radiologically. Another uncommon pattern is that of ischaemic colitis proximal to a chronically obstructing annular carcinoma, presumably due to vascular compromise as a result of distension of the bowel wall.

Staging
The most frequent surgical/pathological method of staging colorectal carcinoma is by the modified Dukes classification. In recent years the TNM method has gained increasing acceptance. A single modality that allows accurate pre-operative staging is not available. Such staging would facilitate appropriate therapy to be planned and monitored. Accurate staging demands the determination of local tumour spread, lymph node metastasis and distant (liver) metatasis. Liver imaging is de alt with elsewhere in this publication. CT staging of the primary tumour (Fig. 74) and lymph node metastasis in colorectal carcinoma, while initially encouraging, has been shown to be only 48-74% accurate. This is due to the inability of CT to detect minor degrees of perirectal or pericolic

Figure 74. CT showing rectal carcinoma (T) infiltrating perirectal fat laterally and posteriorly (arrows) and seminal vesicles anteriorly.

tumour infiltration and due to the incidence of metastatic disease in non-enlarged lymph nodes. It is possible that recent refinements in technique have improved results, such as the use of colonic cleansing preparation, positional variation such as prone scans for the rectum, and air distention of the rectum. Lowering the size threshold for diagnosing lymph node metastasis increases sensitivity but lowers specificity.

Endorectal ultrasound using rigid probes for rectal carcinoma staging and flexible endoscopic ultrasonography for colonic cancers has proved promising. In rectal carcinoma accuracies of 80-90% have been achieved. Sensitivity is higher than specificity. As in the upper GI tract there are problems related to specificity for lymph node metastases and to non-transversable lesions. EUS is more accurate than CT in assessing local spread, but the advantage is less than is apparent in upper GI tract cancer staging. There is a tendency to overstage due to the presence of peritumourous inflammation. EUS is of particular clinical importance because of the expanding range of treatment options for rectal carcinoma, determined by the stage of disease. The clinical value of T -staging of colonic cancers is less certain, and data are relatively few. MR imaging is preferable to CT scanning for local staging of rectal cancer. Although suffering the same limitations in its inability to assess depth of intramural infiltration and nodal deposits, however, its multiplanar imaging potential offers special advantages. Invasion into pelvic muscles and bone by rectal tumours may be better seen by MR imaging. The use of endorectal coils has improved T staging, but problems remain particularly related to detection of nodal metastases. The advent of pelvic phased-array coils may lead to further improvements. Early results have been reported using lndium-labelled monoclonal antibodies for radionuclide imaging of colorectal cancers. There appears to be high sensitivity in the detection of tumour sites in the abdomen, but there is a significant false positive rate.

Recurrent tumours
Investigations have shown both CT and MR scanning to be able to detect asymptomatic tumour recurrence when the carcinoembryonic antigen levels are normal. Tumour recurrence after surgery for rectal carcinoma occurs in about one third of patients within the first two years and is most frequent in the area contiguous with the surgery. Sixty percent of these patients will have only local recurrence. It has be en shown that resection of recurrent tumour increases survival time. It has therefore been suggested that a baseline CT (or MR) be performed 2-3 months after initial surgery, followed by imaging every 6-9 months for 2-3 years. The protocol for imaging should be directed to the detection of both local recurrence and hepatic metastasis, since there is evidence that resection of the latter, where possible, may also increase survival time. CT accuracy for local tumour recurrence suffers from the same limitations as in staging the primary lesion, that is the inability to detect microscopic invasion of perirectal or pericolonic fat and to assess metastatic deposits in nodes of normal size. A further problem occurs after abdominoperineal resection for rectal cancer since there is frequently a soft tissue presacral mass due to oedema, haemorrhage, granulation tissue or fibrosis, for many months after surgery. This is particularly so if radiotherapy has been performed. Hence, the rationale for undertaking a post-operative base line study at 2-3 months. Any enlargement of the mass should be cause for concern and lead to percutaneous biopsy. MR scanning is reported to help distinguish recurrent rectal tumour from scar tissue, recurrent tumour having a high signal intensity on T2-weighted images. Position emission tomography with fluorine-labelled D-glucose has recently been shown to be useful in this function.

Less common benign tumours
Lipomas are the second most common benign colonic tumours. They are usually asymptomatic but can bleed or intussuscept. Most appear as submucosal pedunculated lesions, often on the right side of the colon, which tend to change shape during the barium examination with posture and compression. CT scanning is definitive in demonstrating the fatty attenuation of the lesion. Large bowel carcinoids mostly occur in the rectum and comprise 1-2 % of polyps less than 5 mm in size. They are sessile polyps and should be considered potentially malignant. Nearly all lesions greater than 2 cm in diameter are malignant. Endometriosis occurs at the rectosigmoid junction in 85 % of cases, due to deposits of endometrial tissue in the pouch of Douglas. On barium enemas they are seen as smooth, often scalloped, submucosal impressions on the bowel lumen on the anterior wall of the rectum causing mild narrowing. Symptoms are related to menstrual periods.
Prominent colonic lymphoid follicles may be seen as a normal variant on DCBE in young patients. Follicles less than 3 mm in diameter may be seen in 50% of patients less than 30 years of age. However, when follicles of 4 mm or more are seen, usually in the rectosigmoid area, there is an increased incidence of inflammatory bowel disease or lymphoma. Some authors have found a 70% incidence of colorectal neoplasia in patients over 40 years of age with prominent follicles 1-3 mm in diameter, especially when diffuse or left-sided. This finding should precipitate a very careful search for neoplasia.

Metastases to the colon
Metastasis to the colon can occur by direct invasion from contiguous organs, by spread along the mesentery or its lymphatics, by intraperitoneal seeding or less commonly by haematogenous embolisation. Typically serosal involvement leads to a mass effect on the bowel, fixation, angulation, traction changes with tethering and spiculation and narrowing (Fig. 75). En face, pleating of transverse folds which do not completely traverse the lumen, gives rise to a "stripe sign" due to tethering. Commoner causes of direct spread of neoplasm to the bowel include primaries in the genital tract, kidney and pancreas. Peritoneal spread especially involves the pouch of Douglas and thence the anterior rectal wall (Fig. 76), but also the sigmoid, ileocoecal and paracolic regions. Omental involvement produces an "omental cake" which typically affects the superior aspect of the transverse colon. US and CT will show coexisting mesenteric and peritoneal deposits and ascites, as well as soft tissue masses involving the serosal surfaces.

 

	Figure 75. Serosal metastases (squamous carcinoma, primary unknown) involving proximal descending colon. Note narrowing, fixation, tethering and spiculation.
	Figure 76. Transperitoneal spread to pouch of Douglas from carcinoma of caecum. There is narrowing of the rectosigmoid region with anterior spiculation.

Haematogenous metastasis, especially from breast, lung and melanoma, produce submucosal nodules which may become circumferential and may ulcerate.

 

a	Figure 77. MR images, T2 weighted (a) axial and (b) coronal showing fistula-in-ano track (arrowed) running from left to right posterior to anorectum (r) but inferior to levator muscles (lm).
b

Anorectal evacuation disorders

Radiology has a role in the investigation of chronic constipation, faecal incontinence and in various other functional anorectal disorders. In refractory constipation in adults a barium enema should be performed to exclude stricturing lesions. Bowel transit studies may be undertaken by nuclear medicine or radio-opaque marker methods. Techniques for imaging the anorectal region include evacuation proctography (defaecography) and anal endosonography. Magnetic resonance imaging is useful in staging anorectal neoplasms and assessing inflammatory conditions such as fistulae due to its multiplanar imaging capacity (Fig. 77).

Figure 78. Evacuation proctogram showing no significant abnormality. (a) at rest (b) during "squeeze" or lift (c) during straining (d) during evacuation. See text for illustrated features. Solid line depicts level of ischial tuberosities. V=vaginal contrast; dotted line=anorectal angle.

Evacuation proctography
Barium paste, the approximate consistency of soft stool is introduced into the rectum. A vaginal marker is inserted and some workers opacify the pelvic small bowel with oral barium. The patient is then seated laterally on a special radiolucent commode in front of the screening unit. Hard copy images are obtained with a 100 mm camera, and videofluoroscopic recording is made at rest, during "squeezing", during straining without evacuation, and then during attempted evacuation (Fig. 78). The investigation gives morphological and functional information. Parameters that are measured include the anorectal angle, the level of the anorectal junction relative to a bony landmark (such as the ischial tuberosities) and the anal canal width. Changes in these parameters in the various phases of the examination are more important than the absolute values. Features that are observed with each manoeuvre include the state of opening of the anal canal, the configuration and position of the anorectal junction, the degree of rectal mucosal prolapse and any rectocele or enterocele formation. A subjective assessment is made of the efficiency and degree of rectal evacuation.

Normally, at rest the anal canal is closed, the anorectal angle is about 90° and there is a well formed posterior impression at the anorectal junction from the puborectalis muscle. On "squeezing" there is contraction of the puborectalis sling and the external anal sphincter resulting in a decrease in the anorectal angle, elevation of the anorectal junction and further closure of the anal canal. On straining there is depression of the anorectal junction which should be no more than 3.5 cm from its resting position. Evacuation should proceed rapidly and efficiently and be accompanied by relaxation of puborectalis with loss of the posterior impression from this muscle, effacement of the anorectal angle, depression of the anorectal junction which should take on a funnel-like configuration, and opening of the anal canal. A "zone of evacuation" in the rectum develops as the rectum empties. Even in normal individuals, particularly women, there is some anterior bulging of the rectal wall (rectocele) and minor rectal mucosal prolapse. Abnormal features that may be observed are as follows:
- Weakness of the internal anal sphincter may be seen as an open anal canal at rest with incontinence.
- Puborectalis and external anal sphincter weakness is demonstrated as a weak "squeeze" with poor anorectal elevation and lack of anorectal angle change.
- Failure of relaxation or paradoxical contraction of puborectalis on evacuation seen as lack of effacement of the anorectal angle, causing straining against an unrelaxed pelvic floor - so called anismus or dyskinetic puborectalis muscle resulting in obstructed defaecation.
- Anterior rectocele formation during straining and defaecation. This may be seen to a minor degree in normals. Larger rectoceles may retain barium and be a cause of incomplete evacuation. They are often associated with other abnormalities such as rectal prolapse and perineal descent.
- Intussusception, or prolapse, of rectal mucosa. A minor degree is se en in many normals. There is invagination of the rectal wall which may be anterior, posterior or circumferential. The degree of descent is variable, reaching the internal anal os or, in more severe degrees, becoming intra-anal or prolapsing externally. Intussusception is often associated with sequestration of barium in a rectocele. When the prolapse spontaneously reduces after evacuation, the content of the rectocele may then leak out and produce incontinence.
- An enterocele, which may be suspected by separation of the posterior vaginal wall and the anterior rectal wall during evacuation. This may be confirmed by repeating the test after opacification of pelvic small bowel loops with oral barium. Enteroceles may be associated with incomplete evacuation.
- "Perineal descent" seen as an abnormal degree of descent of the anorectal junction with straining and evacuation. This is related to a decrease in tone of the pelvic floor musculature and gives rise to a sensation of incomplete evacuation. This leads to persistent straining, stretching of the pudendal nerve and pudendal neuropathy and, later, external sphincter weakness and incontinence. There is evidence that patients with a history of chronic constipation and straining at stool may progress through this course of events since cases of outlet obstruction constipation and incontinence demonstrate a similar pelvic floor neuropathy. A "vicious spiral" of deterioration is thus likely to ensue.
- Features of the "solitary rectal ulcer" syndrome. This is a misnomer since there is of ten focal inflammation but no ulceration and sometimes there are multiple ulcers. The mechanism of damage is probably due to trauma to intussuscepted mucosa during straining against an unrelaxed pelvic floor.

Constipation
Debilitating, chronic constipation in adults may be due to a number of systemic disorders (e.g. hypothyroidism). Idiopathic constipation may be related to slow bowel transit, anorectal outlet obstruction or a combination of these. Imaging investigations after exclusion of systemic conditions will therefore inc1ude barium enema (for stenosing lesions and megacolon), transit studies and evacuation proctography. Transit studies may be performed by the use of radio-opaque ingested markers - serial radiographs are taken to assess the number of markers remaining in the bowel - or by a nuclear medicine method. In the latter study, 111Indium-labelled resin microspheres or other material are given by mouth and images acquired serially. The radiograhic method has the virtue of cheapness and simplicity and is a reasonable screening test. The scintigraphic technique is more specific and allows measurement of segmental transit which may be important in management. The features seen on evacuation proctography have been discussed. A subjective assessment of rate and efficiency of evacuation may be made as well as the demonstration of puborectalis dysfunction and morphological abnormalities

Figure 79. Endo-anal ultrasound image showing relatively hypoechoic internal sphincter. There is a focal posterior defect (long arrows) with a longer neighbouring segment of thinning (short arrows). More laterally the sphincter is normal.

such as rectocele, prolapse and enterocele which may give rise to a sensation of incomplete emptying. Recently, radio-isotope proctography has been used to permit a quantitative measure of evacuation efficiency; this also provides some morphological information.

lncontinence
Incontinence may be associated with other evacuation disorders, such as the descending perineum syndrome or prolapse, and is accompanied by a history of multiparity and/or chronic straining leading to pudendal neuropathy and loss of anorectal sensation and deficient sphincters. Obstetric injury or anorectal surgery may result in sphincter defects. While evacuation proctography may help characterise some of the associated features of incontinence, the role of this examination is less dear cut than in constipation. Endo-anal sonography has become the most useful method of imaging the internal and external anal sphincters. Detailed images may be obtained which can localise focal defects and influence management (Fig. 79). Endosonography may replace needle electromyographic mapping of the external sphincter. It is also useful in imaging anorectal fistulae and abscesses, in local staging of anorectal carcinoma and in imaging local recurrence after rectal surgery for neoplasms.

 

Richard M. Mendelson