COLORECTAL PATHOLOGIES

The article reviews the role of computed tomography (CT) and magnetic resonance (MR) in the evaluation of colorectal pathology with specific reference to malignancy of the large bowel. CT and MR though limited in the identification and evaluation of early stages of colonic pathology are extremely useful as minimally invasive modalities to evaluate and stage bowel neoplasms, as well as extensive malignancy along with other large bowel pathology.

Plain X-ray abdomen is generally useful to evaluate for free air in the peritoneum and in patients suspected with intestinal obstruction. Double-Contrast Barium enema performed under image-intensifier television monitoring is still performed to evaluate the luminal surface of the large bowel as well as the strictures and diverticulosis. CT and MR provide for cross-sectional imaging where the bowel wall and the perienteral region can be evaluated as well. Sometimes endoluminal sonography, PET and immunoscintigraphy supplement these modalities.

TECHNIQUES

It is important to tailor the imaging technique based on the colorectal pathology. Most colorectal pathology is best evaluated using oral and/or rectal contrast to opacify the bowel loops. A plain CT study is followed by a dynamic intravenous (IV) contrast enhanced CT study of the abdomen and pelvis. The Helical/Spiral CT is the advanced form of CT which offers a unique advantage of rapid volumetric scanning.[1],[2] This enables scanning the abdomen and pelvis in less than a minute with minimum respiratory or motion artefacts. The intravenous contrast is also optimized and a smaller dose is required in comparison to a conventional scanner. Importantly, the volumetric data so acquired can be utilized to obtain thinner sections without subjecting the patient to additional radiation. Sagittal and coronal reconstruction with significantly reduced artefacts can be generated. More recently, further advancement in computer technology has resulted in the development of virtual colonoscopy.[3] Three-dimensional images are reconstructed using post-processing techniques simulating colonoscopy examination.

MR though limited in its evaluation of colorectal pathology in comparison to the CT, has an advantage of superior soft tissue contrast resolution and multiplanar capability.[5],[6] It is useful in the evaluation of liver, pancreas, biliary tree and pancreatic duct. MR is limited by the fact that as of today there is no reliable way to label bowel loops and the easy availability and extensive collective experience of the radiologists world over in the diagnosis of colorectal abnormalities on CT. MR with Gadolinium (IV contrast agent), sometimes with endorectal coil compliments CT in the evaluation of colorectal pathology. More recently MR colonography using dilute Gadolinium and intravenous Gadolinium with MR imaging guidance has been tried to study polyps. The diagnostic efficacy of both CT and MR is considerably enhanced with proper bowel preparation especially in the detection of subtle mucosal abnormalities.

Inflammatory Diseases

Apart from the ability of CT in differentiating inflammatory and neoplastic diseases, the cross sectional imaging also helps in evaluating the complications (Table 1). The normal wall thickness is 2-3 mm when optimally distended. A measurement of 3-6 mm is considered inderterminant while a dimension exceeding 6 mm is definitely abnormal.

The hallmark of inflammatory bowel disease on CT examination is mild (< 1 cm) symmetric and circumferential bowel wall thickening in association with pericolonic inflammatory change. The presence of a "double halo" or "target sign" pattern because of presence of submucosal oedema, inflammation and/or fat virtually assures the inflammatory aetiology, though not always an active disease.[7],[8],[10]

The other infective inflammatory disease of colon rarely require cross-sectional imaging. Nonspecific features of diffuse or more often segmental distribution of wall thickening contrast enhancement, and pericolonic inflammation are usually seen.

Traumatic lesions of the large bowel

The transverse colon, sigmoid colon and the caecum are the most common portions to be injured in a compressive or blunt injury to abdomen. The CT scan may reveal pneumoperitoneum, free intraperitoneal fluid and bowel wall findings such as circumferential or eccenteric thickening.[7],[8] The latter when associated with intense contrast enhancement of the wall and free fluid in abdomen is suggestive of perforation peritonitis. Injuries to the mesentery and vascular supplements of the colon may also be demonstrated, along with other rarer sequelae of trauma such as traumatic ventral herniation of transverse colon.

Benign Colonic Neoplasms

A benign polyp, on CT examinations, is a sessile or pedunculated soft tissue mass that protrudes into the lumen of the bowel without thickening the wall beyond its location in the colon. Villous adenoma may have characteristic appearance based on their high content of mucus (homogeneous water density of less than 10 HU) located eccentrically on the luminal side of mass and occupying more than half of the lesion with absence of air-fluid level. Other rare benign tumours such as the lipoma, leiomyoma, hamartomas (Peutz-Jegher syndrome) and retention polyps (Cronkhite-Canada syndrome) can also be demonstrated on imaging.

Malignant Colorectal Tumours

Although both endoscopic and double-contrast radiographic studies afford high accuracy, neither permits assessment of the depth of tumour infiltration or the local/distant spread of tumour. CT of the abdomen and pelvis provides precise measurements of the thickness of the colonic wall, determination of the mass with adjacent visceral structures, as well as detection of the presence of metastatic lymph nodes or metastasis to bone and viscera.

Despite the initial enthusiasm about the role of CT and MR in evaluation of colorectal carcinoma, there are several limitations of the two modalities in providing the surgeon a reliable road map to perform surgery. The following features are important when evaluating a patient of colorectal malignancies with CT and/or MR.[1-3],[6],[8],[10]

1.Early and subtle changes of the mucosal surfaces and lesions less than 5 mm in diameter usually are not detected by CT.

2.CT and MR may image a carcinoma as a discrete mass or a focal wall thickening that may involve entire circumference with or without pericolonic spread. The recent reports.[11-13] show that CT is more accurate in assessing T4 (disease including adjacent structures) rather than differentiating T2 from T3 (i.e. disease localised to the bowel wall versus spread into the pericolonic fat). Both CT and MR cannot detect microscopic pericolonic spread and at the same time, fat streaking may represent tumour infiltration, surrounding desmoplasia or oedema. Despite these limitation, patients with pericolorectal soft tissue infiltration have been found to have increased chances of local recurrence. The accuracy of MR (49%) is less than CT scan (76%) in staging the disease. Hence CT and MR must be reserved to image patients with advanced local disease.[14],[15]

TABLE 1
Inflammatory/infective diseases of bowel-imaging features
Disease	Features	Complications
Crohn'sDisease Ulcerative Colitis(Fig. 1) Pseudomem-branous colitis Ischaemic Colitis NeutropenicColitis (Typhlitis) Diverticulitis(Fig. 2) Tuberculosis(Fig.3)	-Normal in early course -Thickened (11-30 mm), homogenously enhancing wall. -Skip/segmental distributionFibro fatty proliferation (creeping fat sign) of mesentery . -Pericolonic fat streaking -Normal in early course of disease -Thickened wall (> 8 mm but less than in Crohn's) with inhomogenous contrast enhancement -"Target" or "Double Halo" sign caused by deposition of fat in the submucosal region. -Widened presacral space -Pericolonic fat stranding -Pancolitis:Marked bowel distension with mural thickening that may be irregular or polypoidal with thickened inflamed haustral folds (accordion sign) caused by cytotoxic enterotoxin produced by Clostridium Difficile -Absence of adjacent fat stranding. -Nonspecific, circumferential bowel wall thickening in segmental pattern corresponding to vascular distribution. Bowel wall thickening is secondary to mucosal/ submucosal oedema or haemorrhage. -Target or Double halo sign on IV contrast study. -Mesenteric fluid/oedema/haemorrhage -Vascular thrombosis/portal vein gas -Reappearance of normal findings in 7-14 days. -Nonspecific concentric thickening of bowel wall especially caecum and ascending colon. Pericolonic inflammation/fluid. -Colonic out pouching -Pericolonic inflammation -Thickening of bowel peritoneal reflection -Mural thickening -Ileocaecal region most commonly involved -In early or ulcerated form may be normal. -In hypertrophic form-diffuse/focal mural Thickening may be seen most commonly Involving the ileocaecal region. -Pericolonic inflammation	-Bowel stricture -Mesenteric inflammation,abscess, lymphadenopathy -Sinus/fistula tract formation --Superimposed malignancy -Renal/gall bladder calculus -Hepatitis and pericholangitis -Sacriliitis -Bowel strictures --Superimposed malignancy -Sacroiliitis . -Hepatitis and pericholongitis -Sinus and fistula are uncommon. -Toxic Megacolon. -Toxic Megacolon -Perforation peritonitis -Perforation peritonitis -Stricture -Perforation peritonitis -15-30% of patients with diverticulosis subsequently develop diverticulitis. -intramural sinus tracts. -Bowel obstruction. -Fistulas -Paracolic phlegmon, abscess. -Distant abscess can occur in liver or subphrenic space. -Colonic stricture -Mesenteric lymph nodes with low attenuation suggestive of caseation. -Peritoneal fluid/omental thickening.

TABLE 2
CT staging of primary or recurrent colorectal tumour and correlation to TNM classification
CT Scan Stage	TNM	CT scan features
I. II. IIIa. IIIb. IV .	T1 T2 T3 T4 T4 MI	Intraluminal mass without wall thickening (Fig. 4). Focal/diffusely thickened wall (> 0.6 cm) or pelvic mass no invasion/extension to side walls. Thickened wall/pelvic mass with invasion of adjacent fat but not into adjacent structures or to pelvic side walls or abdominal wall. Thickened wall or pelvic mass with extension into adjacent structures side walls or abdominal wall without distant metastasis. Distant metastasis with or without local abnormality.
*With ot without lymphadenopathy.

Fig : 1 Coronal reformation of contrast enhanced Helical CT scan of the abdomen demonstrating a stricture (arrow) in the descending colon in a known case of ulcerative colitis. Note also the diffuse bowel wall thickening in the proximal descending colon as well.

Fig : 2 Contrast enhanced CT scan demonstrating diverticulosis (open arrow) involving the sigmoid colon with vesico-colic fistula. Note air in the superior portion of the urinary bladder.

Fig : 3 Axial contrast enhanced CT image demonstrating mural thickening of the terminal ileum and caecum (open arrow) in a case of ileocaecal tuberculosis.

3.CT and MR can demonstrate the invasion of the adjoining organs like seminal vesicle, prostate, bladder, uterus, cervix, ovaries, small or large bowel and sciatic nerve and ureters. However, invasion can be simulated by absence of tissue planes between the viscera and mass. Obliteration of fat planes can also be caused by vascular or lymphatic congestion, inflammation or actual absence of fat. Therefore invasion should be considered definite only if a major portion of the mass envelopes the neighbouring structure or if obvious mass involves an adjacent organ (Fig. 5).

Fig 4 : An emergency CT in a case of large bowel obstruction demonstrating an exophytic growth in descending colon (arrow) with no lymph nodal/pericolonic extension (surgically proven adenocarcinoma).

Fig 5 : A contrast enhanced CT scan demonstrating ascending colon malignancy with ascites and omental seedlings (open arrows).

Fig 6a : A Contrast enhanced CT scan demonstrating a case of rectal malignancy (large arrow) with perirectal extension and perirectal lymphadenopathy (small arrow) (T3N1). Such perienteral streakiness can be also caused by oedema or desmoplasma besides tumour.

Fig 6b : Another case of ascending colon malignancy (large arrow) with paracolonic (open arrow) and para-aortic (small arrow) lymphadenopathy (T3N3).

4.The presence of nodal metastases significantly alters prognosis. Both CT and MR does not allow distinguishing normal sized nodes without tumour from those with tumour, especially since average size of lymph nodes affected with metastatic disease is less than 5 mm (Fig. 6a, 6b). The best criteria proposed for detection of lymph node involvement are:

- any node that is 1 cm or larger
- a cluster of 3 or more nodes less than 1 cm.
- demonstration of any size nodes within the mesentery.

Despite these, the overall accuracy of CT and MR in detecting lymph node infiltration is only about 60%. More recently experimental MR lymphography trials have been conducted utilizing intravenous ultrafine super particle iron oxide (USPIO), contrast agent. Gradient echo (T2* weighted) images obtained after the IV injection shows hypointense signal in normal or hyperplastic lymphnodes as the USPIO are taken up by these nodes as opposed to the metastasis nodes which do not turn hypointense.[16] The liver metastasis are most accurately detected with CT portography or MR. Complications such as perforation, abscess formation, intussception and fistula formation may also be appreciated.

5.Post operative or postradiation changes produce streaky densities, presacral masses, diffuse wall thickening at re-anastomosis, which are indistinguishable from tumour recurrence. Any increase in the size of the mass on follow-up, invasion of adjacent structures, presence of large lymph nodes, enhancement of mass (Fig. 7) with contrast material or development of a pericolonic soft tissue density on the follow up CT or MR, should be considered to be suggestive of recurrence and percutaneous guided biopsy is usually useful in confirming the aetiology. Recent reports suggest higher sensitivity of MR (95%) than CT (68%) in detection of tumour recurrence. CT and MR have an important role in evaluating for post-operative complication.

Fig : 7 Contrast enhanced CT scan reveals post-operative recurrence of ascending colon malignancy. Sometimes a guided FNAC/Biopsy is the only means of distinguishing tumour recurrence from post-operative fibrosis.

ROLE OF 3-D VIRTUAL COLONOSCOPY

The virtual colonoscopy (Fig. 8) provides a unique non-invasive method for evaluation of colonic pathology, even in cases where flexible endoscopy is contraindicated or unsuccessful in demonstrating the extent of the stricture.[17],[18] The axial CT images obtained for virtual colonoscopy provide excellent information regarding pericolonic spread, lymphatic and distant abdominal metastasis. In the current scenario, though recent reports suggest a reasonably good efficacy in detection of early and subtle pathology, its role in India for surveillance of colorectal malignancy would be limited due to the inherent limitations of virtual colonoscopy and relatively low incidence of colorectal malignancy in the Indian subcontinent compared to the West.

CT and MR have an important role in evaluating colorectal pathology, most importantly in colorectal malignancy.[19],[20] Despite the limitations of CT and MR, advances like virtual colonoscopy in 3-D imaging of colon and increased resolution are likely to provide better information and further complement colonoscopy in the evaluation of colo-rectal pathology.

Fig 8 : CT Virtual Colonoscopic rendering provides a comprehensive endoscopic view of the concentric intraluminal growth. (note the axial image in the inset at the same level showing the malignant growth).

REFERENCES