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Pitfalls of Using Three-Dimensional CT Colonography with Two-Dimensional Imaging Correlation

¹ Both authors: Department of Radiology, Abdominal Imaging, NYU Medical Center, Tisch Hospital, 560 First Ave., Ste. HW 206, New York, NY 10016.

Received June 2, 2000; accepted after revision June 28, 2000.

 
Address correspondence to M. Macari.

Introduction

Top Introduction Materials and Methods Normal Colon Intrinsic Abnormalities Extrinsic Defects Conclusion References

 
CT colonography is an imaging technique that enables both two-dimensional (2D) and three-dimensional (3D) evaluation of the colon [1,2,3]. Although debate continues regarding which rendering technique should be used for primary evaluation, the techniques (2D and 3D) are complementary, each increasing the value of CT for colonic lumen evaluation. Two-dimensional CT colonography, when used as the primary imaging technique, may be advantageous in that data may be evaluated in a more time-efficient fashion [2, 3]. When a potential filling defect is identified on 2D CT colonography, 3D imaging is used to determine whether its morphology is linear (interhaustral fold) or polypoid (stool or polyp) [2, 3].

Three-dimensional CT colonography has the potential to visualize smaller lesions than 2D CT colonography [4]. Although some additional small lesions may be identified, evaluating CT colonography data entirely with 3D imaging is time-consuming. Moreover, often lesions are detected using 3D CT colonography that show morphology consistent with a polyp or neoplasm. When these same areas are investigated with 2D CT colonography, a variety of normal structures may be found to represent these morphologic abnormalities.

Whether one uses 2D or 3D as the primary rendering method, the pitfalls on 3D imaging need to be recognized. Three-dimensional morphologic abnormalities can be divided into processes that are intrinsic or extrinsic to the colon. Our purpose is to describe the normal colon with 3D CT colonography and to show how 2D imaging can be used to establish the cause of an abnormality depicted with 3D rendering.

Materials and Methods

Top Introduction Materials and Methods Normal Colon Intrinsic Abnormalities Extrinsic Defects Conclusion References

 
Patients discussed in this essay underwent bowel preparation with either polyethylene glycol solution (GoLytely; Braintree Laboratories, Braintree, MA) or 45 mL of sodium phosphate (24-hr Fleet Prep Kit 1; Fleet, Lynchburg, VA). Immediately before CT colonography, patients evacuated any residual fluid or fecal material from the rectum. One milligram of glucagon was administered IV. A catheter was inserted into the rectum, and the colon was insufflated with room air (minimum of 40 puffs). A scout CT image was obtained to verify adequate bowel distention. If adequate bowel distention was present, the CT examination was performed. If bowel distention was not adequate, additional air (10 puffs) was insufflated into the rectum. Two acquisitions, the first with the patient in the supine position and the second, in the prone position, were performed. All CT examinations were performed with helical HiSpeed Advantage or CTI units (General Electric Medical Systems, Milwaukee, WI) or a Volume Zoom unit (Siemens, Erlangen, Germany). All CT images were transferred to a remote Advantage workstation (General Electric Medical Systems) equipped with commercial Navigator software (General Electric Medical Systems) capable of performing the 2D and 3D data rendering.

Normal Colon

Top Introduction Materials and Methods Normal Colon Intrinsic Abnormalities Extrinsic Defects Conclusion References

 
Adequate insufflation with gas (either room air or carbon dioxide gas) results in a well-distended colon. Data acquisition with the patient in both supine and prone positions ensures that complementary segments of the colon are adequately distended [5]. Depending on the degree of distention, the appearance of the normal colonic lumen will vary with 3D rendering. The mucosa appears featureless if the interhaustral folds are completely effaced by the pressure of the gas (Fig. 1). This featureless appearance is more often detected in the descending colon and rectum, where the haustra are relatively sparse [6]. In the cecum and ascending, transverse, and sigmoid colons thin curvilinear interhaustral folds will be visualized either randomly oriented or evenly spaced along the colonic surface (Fig. 2A,2B). The colon typically has a circular contour to its wall when well distended. In the transverse colon, the appearance of the endoluminal view may be more triangular in configuration [6].

View larger version (113K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1. —50-year-old man with normal colon. Three-dimensional thresholdrendered endoluminal CT colonograph of descending colon shows good distention (arrow) without interhaustral fold delineation.

View larger version (132K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 2A. —60-year-old man with normal colon. Three-dimensional threshold-rendered endoluminal CT colonograph of splenic flexure shows multiple thin linear filling defects (arrow) haphazardly arranged, consistent with interhaustral folds.

View larger version (124K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 2B. —60-year-old man with normal colon. Three-dimensional threshold-rendered endoluminal CT colonograph of transverse colon reveals multiple thin linear filling defects (arrow) arranged in continuous pattern, consistent with interhaustral folds.

If the colon is not properly distended, 3D endoluminal visualization will be limited, and adequate rendering may not be possible (Fig. 3A,3B). When the colon is evaluated with the 3D endoluminal technique, a circumferential constricting neoplasm may be difficult to distinguish from a collapsed segment. In addition, polyps are more difficult to perceive in collapsed segments. In general, adequate distention is recognized by obtaining a scout topogram after colonic insufflation with gas. If distention appears adequate, the CT colonography data set is obtained. Once the data set is obtained, the degree of distention is more easily appreciated on 2D than on 3D images.

View larger version (127K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 3A. —64-year-old man with incompletely distended colon. Three-dimensional threshold-rendered endoluminal CT colonograph of sigmoid colon shows poor distention limiting endoluminal perspectives. An 8-mm polyp (arrow) is difficult to perceive because of incomplete distention (same arrow).

View larger version (123K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 3B. —64-year-old man with incompletely distended colon. Two-dimensional axial CT scan of same region again reveals poor distention. Polyp (arrow) is easier to appreciate on 2D imaging in this case.

Intrinsic Abnormalities

Top Introduction Materials and Methods Normal Colon Intrinsic Abnormalities Extrinsic Defects Conclusion References

 
Residual Fecal Material
The major pitfall of 3D endoluminal evaluation is mistaking residual fecal material for polyp or neoplasm (Figs. 4A,4B and 5A,5B,5C). The colon needs to be rigorously cleansed before CT colonography [7]. Even in compliant patients, small amounts of residual fecal material may persist. The following observations aid in distinguishing fecal residue from true polyp. Most fecal debris will remain on the dependent surface of the bowel when the patient is moved from the supine to the prone position. Often, gas will be identified in the fecal material, confirming stool (Figs. 4A,4B and 5A,5B,5C). Small amounts of gas are easily detected using 2D techniques and adjustment of window and level settings. Both 3D rendering techniques (surface- and volume-rendering) currently in use for endoluminal display are not sensitive to the presence of air; however, the air is readily apparent on the 2D images, reinforcing the need to constantly correlate 2D and 3D information. Finally, residual barium in the fecal material may help differentiate stool from neoplasm (Fig. 6A,6B). There is currently interest in developing orally ingested bowel preparations that would "label" residual fecal material with barium, potentially aiding in the differentiation of stool and polyp [7].

View larger version (96K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 4A. —51-year-old man with residual fecal material. Three-dimensional threshold-rendered endoluminal CT colonograph of cecum shows large filling defect (arrows). Differential diagnosis includes polyp, neoplasm, stool, and extrinsic compression.

View larger version (122K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 4B. —51-year-old man with residual fecal material. Coronal reformatted CT scan at same level as A reveals filling defect that contains bubbles of gas (arrow), consistent with residual fecal material.

View larger version (137K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 5A. —75-year-old woman with known rectal neoplasm who has two filling defects seen on CT colonography. IV contrast material was administered in this case for staging purposes. Three-dimensional threshold-rendered endoluminal CT colonograph of rectum shows two indistinguishable filling defects (arrows). Differential diagnosis for each lesion includes neoplasm and stool.

View larger version (113K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 5B. —75-year-old woman with known rectal neoplasm who has two filling defects seen on CT colonography. IV contrast material was administered in this case for staging purposes. Axial CT scan of rectum at level of upper vagina reveals homogeneously enhancing mass (arrow) consistent with neoplasm. Mass corresponds with small arrow in A.

View larger version (122K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 5C. —75-year-old woman with known rectal neoplasm who has two filling defects seen on CT colonography. IV contrast material was administered in this case for staging purposes. Axial CT scan 1 cm cephalad to B reveals residual fecal material (arrow). Note small bubbles of gas in filling defect, confirming stool. This finding corresponds with large arrow in A.

View larger version (107K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 6A. —70-year-old woman with impacted diverticulum. Three-dimensional threshold-rendered endoluminal CT colonograph of sigmoid colon shows raised filling defect (arrow), suggesting polyp.

View larger version (101K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 6B. —70-year-old woman with impacted diverticulum. Coronal reformatted CT scan at same level as filling defect in A reveals impacted diverticulum (arrow) filled with high-density stool and residual barium from previous examination.

Diverticula
Diverticula may simulate polyps on 3D endoluminal displays (Fig. 7A,7B). A diverticulum will be noted to have a dense ring around it identifying the orifice [8]. In contrast, a polyp does not have a complete ring shadow surrounding it because it is a raised structure (Fig. 8A,8B). When a diverticulum is impacted with fecal material, it may appear as a raised lesion and mimic a polyp (Fig. 6A,6B). In these cases, 2D imaging is necessary to show both the higher density in the impacted diverticulum and a portion of the diverticulum extending outside the colonic wall.

View larger version (102K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 7A. —66-year-old man with diverticulum. Three-dimensional threshold-rendered endoluminal CT colonograph of transverse colon shows 1-cm defect surrounded by complete ring shadow (arrow), suggesting diverticulum.

View larger version (92K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 7B. —66-year-old man with diverticulum. Coronal reformatted CT scan at level of defect seen in A reveals defect represents a diverticulum (arrow).

View larger version (105K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 8A. —56-year-old woman with small polyp. Three-dimensional threshold-rendered endoluminal CT colonograph of cecum shows 5-mm filling defect with incomplete ring (arrow). Differential diagnosis includes polyp, residual fecal material, and atypical diverticulum.

View larger version (95K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 8B. —56-year-old woman with small polyp. Axial CT scan at same level as defect in A reveals pedunculated polyp (arrow).

Ileocecal Valve
A prominent ileocecal valve is a common pitfall at 3D CT colonography. Three appearances of the normal ileocecal valve have been characterized by colonoscopists: a papillary type (Fig. 9A,9B), with a domelike protrusion having its mouth at the apex; a labial type (Fig. 10A,10B), appearing as a slightly raised fold with the mouth separating the fold margins; and an intermediate type [6]. Occasionally the opening of the valve may be visualized during 3D evaluation (Fig. 10A,10B). Differentiating the valve from neoplasm is usually not difficult because the valve is in a characteristic location. However, the morphologic appearance of the ileocecal valve on 3D CT colonography may be similar to that of a polyp or neoplasm (Fig. 9A,9B). Two-dimensional imaging allows the terminal ileum to be evaluated, and it can then be followed directly to the valve (Figs. 9A,9B and 10A,10B).

View larger version (146K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 9A. —70-year-old man with papillary type ileocecal valve. Three-dimensional threshold-rendered endoluminal CT colonograph of ascending colon shows 1.5-cm raised filling defect (arrow). Note depression in center (arrowhead) of defect. Differential diagnosis includes ulcerated neoplasm and ileocecal valve.

View larger version (124K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 9B. —70-year-old man with papillary type ileocecal valve. Coronal reformatted CT scan at same level as A reveals filling defect is actually prominent ileocecal valve (arrow). Note cephalad and caudal leaflets of valve and terminal ileum.

View larger version (131K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 10A. —76-year-old man with labial type ileocecal valve. Three-dimensional threshold-rendered endoluminal CT colonograph of ascending colon shows small opening in expected region of ileocecal valve (arrow). Differential diagnosis includes incompetent valve or diverticulum.

View larger version (96K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 10B. —76-year-old man with labial type ileocecal valve. Axial CT scan at same level as A reveals ileocecal valve. Note opening of leaflets of valve (arrow).

Extrinsic Defects

Top Introduction Materials and Methods Normal Colon Intrinsic Abnormalities Extrinsic Defects Conclusion References

 
Any organ or structure that is outside the colon can cause external compression. Because extrinsic structures usually compress the colon along a focal area of the distended colon, they do not appear as occlusive lesions. Rather, when evaluated from an endoluminal 3D perspective, these external structures compressing the wall may appear to be focal neoplasms. We have noted external compression from the liver (Fig. 11A,11B), other loops of bowel (Fig. 12A,12B), the psoas muscle (Fig. 13A,13B), and the aorta (Fig. 14A,14B). Similar external compression effects may be detected related to the spleen and kidneys. These external compressions may be more common in thin patients and stress the need for 2D imaging correlation whenever an abnormality on 3D imaging is detected.

View larger version (145K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 11A. —50-year-old man with external compression from posteroinferior segment (segment VI) of liver. Three-dimensional threshold-rendered endoluminal CT colonograph of ascending colon viewing toward hepatic flexure shows large filling defect (arrows). Differential diagnosis includes neoplasm, stool, and extrinsic compression.

View larger version (119K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 11B. —50-year-old man with external compression from posteroinferior segment (segment VI) of liver. Coronal reformatted CT scan at same level as A reveals impression on hepatic flexure (arrow) by inferior surface of liver segment VI (6).

View larger version (132K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 12A. —52-year-old woman with external compression from adjacent loop of distended bowel. Three-dimensional threshold-rendered endoluminal CT colonograph of ascending colon shows large filling defect (arrow). Differential diagnosis includes neoplasm, stool, and extrinsic compression.

View larger version (103K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 12B. —52-year-old woman with external compression from adjacent loop of distended bowel. Axial CT scan reveals impression on ascending colon from adjacent loop of distended bowel (arrow).

View larger version (117K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 13A. —82-year-old man with external compression from psoas muscle. Three-dimensional threshold-rendered endoluminal CT colonograph of ascending colon shows long filling defect (arrows). Appearance is atypical for polyp or neoplasm.

View larger version (96K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 13B. —82-year-old man with external compression from psoas muscle. Axial CT scan reveals impression on ascending colon from psoas muscle (arrow) in this thin patient.

View larger version (119K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 14A. —74-year-old man with external compression from aorta. Three-dimensional threshold-rendered endoluminal CT colonograph of sigmoid colon shows filling defect (arrow). Differential diagnosis includes neoplasm, stool, and extrinsic compression.

View larger version (137K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 14B. —74-year-old man with external compression from aorta. Axial CT scan reveals impression on sigmoid colon is from aorta (arrow).

Conclusion

Top Introduction Materials and Methods Normal Colon Intrinsic Abnormalities Extrinsic Defects Conclusion References

 
A combination of 2D and 3D rendering techniques optimizes evaluation of the lumen of the colon for polyps and neoplasms [1, 2, 3, 7]. The techniques are complementary, and both need to be available to the radiologists when evaluating CT colonography data sets. We have described the normal 3D endoluminal view of the colon and some of the more common pitfalls that may be encountered with 3D rendering. We have not described every pitfall that may be encountered during CT colonography. Other pitfalls using both 2D and 3D CT colonography have been previously described [9].

When an abnormality is detected on 3D CT colonography, the area should be closely investigated with a combination of axial and multiplanar reformatted images. Such investigation will often help to establish the cause of the 3D abnormality.

References

Top Introduction Materials and Methods Normal Colon Intrinsic Abnormalities Extrinsic Defects Conclusion References

 

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