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Electronic Cleansing and Stool Tagging in CT Colonography: Advantages and Pitfalls with Primary Three-Dimensional Evaluation

¹ Department of Radiology, National Naval Medical Center, 8901 Wisconsin Ave., Bethesda, MD 20889-5600.
² Department of Radiology, F. Edward Hébert School of Medicine, Uniformed Services University of the Health Sciences, Bethesda, MD 20814.
³ Department of Radiology, Walter Reed Army Medical Center, Washington, DC 20307.

Received December 13, 2002; accepted after revision February 10, 2003.

 
The opinions and assertions contained herein are the private views of the authors and are not to be construed as official or as reflecting the views of the Department of the Navy, the Department of the Army, or the Department of Defense.

Address correspondence to P. J. Pickhardt.

Introduction

Top Introduction CT Colonography Technique Electronic Cleansing of Luminal... Barium Tagging of Particulate... Conclusion References

 
CT colonography is a rapidly evolving technique for detection of colorectal neoplasia [1]. Although the performance characteristics of CT colonography have been encouraging in populations with above-average risk for polyps, its utility for screening an average-risk population remains unproven [1-3]. Most studies to date have relied on the two-dimensional (2D) CT images for primary polyp detection, reserving the three-dimensional (3D) rendering for confirmation or problem solving [1]. A recognized advantage of 3D CT colonography over 2D imaging is its improved depiction of surface morphology, particularly in distinguishing polypoid lesions from haustral folds [4]. The main barrier to primary 3D endoluminal imaging has been its time-consuming nature [5]. However, continued improvements in the endoluminal display of some systems now allow time-efficient 3D fly-through of the colon for primary polyp detection. The 2D images remain important for correlation of lesions seen on 3D rendering.

For our ongoing multicenter screening trial in average-risk patients, we have found that polyp conspicuity is greatly increased using the 3D endoluminal perspective for primary detection. Although less sensitive for initial detection, the complementary 2D images improve specificity and are therefore indispensable for distinguishing true polyps from a variety of pseudopolyps seen on 3D imaging. Several well-known causes of 3D pseudopolyps, such as residual fecal debris, impacted diverticula, extrinsic compression, and the ileocecal valve have been previously described [4]. To further increase the diagnostic ability of CT colonography, we use tagging of residual fluid and debris with oral contrast material, as well as digital removal of the opacified fluid ("electronic cleansing"). Although both stool tagging and electronic cleansing offer certain diagnostic advantages, unique pitfalls exist beyond the usual 3D pseudopolyps listed previously. Our purpose is to show the various advantages and pitfalls of these techniques on CT colonography.

CT Colonography Technique

Top Introduction CT Colonography Technique Electronic Cleansing of Luminal... Barium Tagging of Particulate... Conclusion References

 
Patients undergo a standard colonic preparation with phosphosoda (24-hr Fleet 1 preparation, Fleet Pharmaceuticals, Lynchburg, VA). As part of their clear liquid diet, patients consume a total of 500 mL of 2.1% barium contrast material (Scan C, Lafayette Pharmaceuticals, Lafayette, IN) and 120 mL of diatrizoate meglumine and diatrizoate sodium (Gastrografin, Bracco Diagnostics, Princeton, NJ) in divided doses.

Briefly, our CT protocol is as follows: After rectal tip insertion, pneumoncolon is obtained to patient tolerance by self-administered hand bulb insufflation of room air before each scan. After scout images have been acquired, single-breath-hold acquisitions with the patient in the supine and prone positions are obtained using a four- or eight-channel CT scanner (Light-Speed and LightSpeed Ultra, General Electric Medical Systems, Milwaukee, WI). The CT technique entails 1.25- to 2.5-mm collimation, 13.5-15 mm/sec table speed (high-speed mode), 1-mm reconstruction intervals, 100 mAs, and 120 kVp. Total CT room time is generally 10 min or less. In our study patients, optical colonoscopy immediately follows CT.

Image processing and interpretation are performed using a commercially available CT colonography system (Viatronix V3D, version 1.3, Viatronix, Stony Brook, NY). This software package achieves electronic cleansing of the residual luminal fluid using a unique segmentation ray technique to address the partial volume effects that occur at air, fluid, and bowel wall interfaces. After partial volume is eliminated, the remaining tagged fluid is removed by simple thresholding. Unlike previously described techniques for digital subtraction bowel cleansing that require lengthy postprocessing [6], our studies are ready for interpretation within 10-15 min of being sent to the processor.

The V3D diagnostic interface allows virtual fly-through of the volume-rendered 3D images along an automated center-line path, interspersed with manual mouse-driven detours as needed. Seamless navigation between the 3D and 2D image displays allows rapid 2D correlation of any suspected 3D abnormality. The entire interpretation time (including extracolonic evaluation) is generally completed in 10-15 min or less for uncomplicated cases.

Electronic Cleansing of Luminal Fluid

Top Introduction CT Colonography Technique Electronic Cleansing of Luminal... Barium Tagging of Particulate... Conclusion References

 
Although originally intended to reduce or eliminate the need for colon purgation [6], electronic cleansing is also of benefit in the prepared colon. Digital removal of opacified residual fluid allows 3D evaluation of colonic mucosa that would have otherwise been obscured (Figs. 1A, and 1B). With effective electronic cleansing, the entire anterior and posterior walls can be evaluated on 3D images obtained with patients in both the prone and supine positions. In our experience, water-soluble iodinated contrast material works best for opacification of fluid, whereas diluted barium is more effective for tagging particulate debris. Although electronic cleansing raises hope for radiation-dose reduction by eliminating the scan obtained in either the prone or the supine position, we believe that obtaining scans in both positions is needed to ensure adequate distention and evaluation of all colonic segments.

View larger version (120K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1A. —Electronic cleansing of contrast-opacified luminal fluid on three-dimensional CT colonography in asymptomatic 65-year-old man undergoing screening for colorectal polyps. Uncleansed (A) and cleansed (B) images from same endoluminal perspective show increase in visualized mucosal surface after digital subtraction of residual fluid. Characteristic linear artifact where air-fluid level interfaces with colon wall (arrowheads, B) has been likened to residue that remains in tub after bath.

View larger version (129K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1B. —Electronic cleansing of contrast-opacified luminal fluid on three-dimensional CT colonography in asymptomatic 65-year-old man undergoing screening for colorectal polyps. Uncleansed (A) and cleansed (B) images from same endoluminal perspective show increase in visualized mucosal surface after digital subtraction of residual fluid. Characteristic linear artifact where air-fluid level interfaces with colon wall (arrowheads, B) has been likened to residue that remains in tub after bath.

Detection of polyps submerged in fluid represents the major advantage of electronic fluid cleansing (Figs. 2A, 2B, 2C, and 2D), although other abnormalities are occasionally uncovered (Figs. 3A, 3B, and 3C). Contrast opacification of the luminal fluid also allows improved detection of submerged lesions on uncleansed 2D images (Figs. 2A, 2B, 2C, 2D and 3A, 3B, 3C).

View larger version (111K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 2A. —CT colonography of submerged polyp in average-risk 58-year-old man undergoing screening for colorectal polyps. Three-dimensional endoluminal image obtained without electronic cleansing shows no abnormality. Note abrupt cutoff of haustral fold as it enters fluid pool.

View larger version (115K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 2B. —CT colonography of submerged polyp in average-risk 58-year-old man undergoing screening for colorectal polyps. Three-dimensional endoluminal image obtained with electronic cleansing uncovers pedunculated polyp (arrow) that was submerged under fluid. Lesion was less conspicuous on imaging obtained with patient in prone position (not shown). Note subtle linear artifact occurring at air-fluid-wall interface. Eight-millimeter adenomatous polyp was confirmed at optical colonoscopy and pathologic evaluation.

View larger version (118K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 2C. —CT colonography of submerged polyp in average-risk 58-year-old man undergoing screening for colorectal polyps. Uncleansed sagittal two-dimensional (2D) image obtained with patient in supine position and with wide window setting (2000/0 H) shows pedunculated polyp (arrow) in contrast-opacified fluid. Lesion could be mistaken for normal fold on this single image.

View larger version (128K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 2D. —CT colonography of submerged polyp in average-risk 58-year-old man undergoing screening for colorectal polyps. Cleansed sagittal 2D image shows complete removal of opacified luminal fluid surrounding polyp (arrow).

View larger version (135K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 3A. —CT colonography of extramucosal rectal mass (gastrointestinal stromal tumor) uncovered with electronic cleansing in 66-year-old asymptomatic woman undergoing screening. Uncleansed (A) and cleansed (B) three-dimensional endoluminal images, obtained with patient in supine position and simulating retroflexed endoscopic image of rectum, show broad-based lesion (asterisk, B) that was submerged in luminal fluid. Note tip of rectal catheter (arrow, B) and familiar linear artifact at air-fluid-wall interface (arrowheads, B). Lesion was less conspicuous on imaging with patient in prone position (not shown).

View larger version (128K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 3B. —CT colonography of extramucosal rectal mass (gastrointestinal stromal tumor) uncovered with electronic cleansing in 66-year-old asymptomatic woman undergoing screening. Uncleansed (A) and cleansed (B) three-dimensional endoluminal images, obtained with patient in supine position and simulating retroflexed endoscopic image of rectum, show broad-based lesion (asterisk, B) that was submerged in luminal fluid. Note tip of rectal catheter (arrow, B) and familiar linear artifact at air-fluid-wall interface (arrowheads, B). Lesion was less conspicuous on imaging with patient in prone position (not shown).

View larger version (131K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 3C. —CT colonography of extramucosal rectal mass (gastrointestinal stromal tumor) uncovered with electronic cleansing in 66-year-old asymptomatic woman undergoing screening. Uncleansed axial two-dimensional image obtained with patient in supine position and with soft-tissue window setting shows broad-based mass (arrowhead) forming obtuse angles with adjacent rectal mucosa. Mass extends into perirectal fat posteriorly.

The major drawback to electronic cleansing is the creation of 3D artifacts that simulate true polyps. Incomplete cleansing due to suboptimal opacification of luminal fluid can result in bizarre artifacts that may have the appearance of polyposis (Figs. 4A, 4B, 4C, and 4D). The geographic planar distribution of these abnormalities generally prevents them from being confused with true abnormalities. Although these artifacts can be distracting initially, they are easily ignored with experience.

View larger version (125K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 4A. —Electronic cleansing artifact seen on three-dimensional (3D) CT colonography resulting from suboptimal fluid opacification in 62-year-old man undergoing screening. Cleansed 3D endoluminal image shows multiple polypoid lesions restricted to one side of colon wall. With experience, radiologists can easily recognize distribution of findings as cleansing artifacts but could miss superimposed polyps. This result would necessitate more careful evaluation of two-dimensional (2D) images and use of alternate position (prone or supine).

View larger version (112K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 4B. —Electronic cleansing artifact seen on three-dimensional (3D) CT colonography resulting from suboptimal fluid opacification in 62-year-old man undergoing screening. Uncleansed 3D endoluminal image shows complete replacement of artifact by smooth fluid level.

View larger version (106K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 4C. —Electronic cleansing artifact seen on three-dimensional (3D) CT colonography resulting from suboptimal fluid opacification in 62-year-old man undergoing screening. Cleansed (C) and uncleansed (D) axial images show 2D correlate of cleansing artifact, with jagged appearance at air-fluid level on subtracted image (arrows, C).

View larger version (105K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 4D. —Electronic cleansing artifact seen on three-dimensional (3D) CT colonography resulting from suboptimal fluid opacification in 62-year-old man undergoing screening. Cleansed (C) and uncleansed (D) axial images show 2D correlate of cleansing artifact, with jagged appearance at air-fluid level on subtracted image (arrows, C).

Untagged or poorly tagged fecal debris lying dependently in a luminal fluid pool is another cause of pseudopolyps on 3D images related to electronic cleansing (Figs. 5A, 5B, and 5C). Unless adherent, these lesions move with the change in patient to prone or supine position. The dependent location of these objects in the fluid hints at their mobile nature but is not confirmatory by itself. The additional time needed to evaluate and exclude these lesions during primary 3D reviewing represents a trade-off with the increased detection of submerged true polyps.

View larger version (133K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 5A. —Pseudopolyp due to untagged fecal material in 59-year-old woman undergoing screening with CT colonography. Three-dimensional endoluminal image obtained with electronic cleansing shows 10-mm sessile polypoid lesion.

View larger version (105K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 5B. —Pseudopolyp due to untagged fecal material in 59-year-old woman undergoing screening with CT colonography. Cleansed (B) and uncleansed (C) axial two-dimensional images obtained with patient in supine position show rounded soft-tissue lesion (arrow) in dependent aspect of ascending colon. Lesion was found to be mobile on images obtained with patient in prone position (not shown). Mobility is also suggested by undermining of contrast material.

View larger version (93K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 5C. —Pseudopolyp due to untagged fecal material in 59-year-old woman undergoing screening with CT colonography. Cleansed (B) and uncleansed (C) axial two-dimensional images obtained with patient in supine position show rounded soft-tissue lesion (arrow) in dependent aspect of ascending colon. Lesion was found to be mobile on images obtained with patient in prone position (not shown). Mobility is also suggested by undermining of contrast material.

Polypoid artifacts related to partial volume effects represent a significant pitfall in 3D colonography with electronic cleansing. Such lesions typically occur where air-fluid levels interface with the colon wall, particularly when a meniscus effect is present (Figs. 6A, 6B, and 6C). Polypoid lesions also tend to occur at air-fluid-haustral fold interfaces (Figs. 7A, 7B, and 7C). For each of these artifacts associated with air-fluid levels, evaluation of the uncleansed images, particularly 2D images, is vital for excluding true abnormalities because a corresponding pseudopolyp is often present on the cleansed 2D images as well (Figs. 6A, 6B, 6C and 7A, 7B, 7C). For this reason, we always display the uncleansed images as our default 2D mode. Last of all, trapped air bubbles in an otherwise fluid-filled lumen can also give rise to convincing pseudopolyps on 3D images (Figs. 8A, 8B, and 8C). In such cases, the uncleansed 2D images yield a rapid explanation.

View larger version (128K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 6A. —Pseudopolyp due to partial volume effect at air-fluid-wall interface in average-risk 54-year-old man undergoing screening with CT colonography. Three-dimensional (3D) endoluminal image obtained with electronic cleansing shows 6-mm sessile polypoid lesion.

View larger version (116K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 6B. —Pseudopolyp due to partial volume effect at air-fluid-wall interface in average-risk 54-year-old man undergoing screening with CT colonography. Cleansed supine axial two-dimensional (2D) image shows polypoid soft-tissue lesion (arrow) that corresponds to 3D findings.

View larger version (132K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 6C. —Pseudopolyp due to partial volume effect at air-fluid-wall interface in average-risk 54-year-old man undergoing screening with CT colonography. Uncleansed axial 2D image obtained with patient in supine position shows air-fluid-wall interface (arrow) corresponding to location of "polyp" on cleansed images (A and B). No polyp or other correlate was seen on images obtained with patient in prone position or at endoscopy. Note utility of uncleansed images in avoiding this common pitfall.

View larger version (111K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 7A. —Pseudopolyp due to artifact at air-fluid-haustral fold interface in asymptomatic 66-year-old woman undergoing screening with CT colonography. Cleansed three-dimensional (3D) endoluminal image shows 10-mm oval polypoid lesion (arrows) extending from haustral fold.

View larger version (89K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 7B. —Pseudopolyp due to artifact at air-fluid-haustral fold interface in asymptomatic 66-year-old woman undergoing screening with CT colonography. Cleansed axial two-dimensional (2D) image obtained with patient in prone position shows large polypoid lesion (arrow) corresponding to 3D findings (A).

View larger version (93K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 7C. —Pseudopolyp due to artifact at air-fluid-haustral fold interface in asymptomatic 66-year-old woman undergoing screening with CT colonography. Uncleansed axial 2D image obtained with patient in prone position shows partial volume effect as air, opacified fluid, and haustral fold converge (arrow). No correlate was seen on image obtained with patient in supine position or at endoscopy. Again, note how findings on uncleansed image prevent this result from becoming false-positive.

View larger version (124K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 8A. —Three-dimensional (3D) pseudopolyp due to trapped air bubble in asymptomatic 50-year-old man undergoing screening for polyps with CT colonography. Cleansed 3D endoluminal image shows 10-mm polypoid lesion adjacent to haustral fold.

View larger version (102K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 8B. —Three-dimensional (3D) pseudopolyp due to trapped air bubble in asymptomatic 50-year-old man undergoing screening for polyps with CT colonography. Cleansed axial two-dimensional (2D) image obtained with patient in prone position shows apparent air cyst with thin wall (arrow) that corresponds to 3D findings (A).

View larger version (108K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 8C. —Three-dimensional (3D) pseudopolyp due to trapped air bubble in asymptomatic 50-year-old man undergoing screening for polyps with CT colonography. Uncleansed axial 2D image obtained with patient in prone position shows that "lesion" was created by small gas collection (arrow) in otherwise fluid-filled lumen. Findings on cleansed 2D image would not be confused with true polyp in this patient.

Barium Tagging of Particulate Fecal Material

Top Introduction CT Colonography Technique Electronic Cleansing of Luminal... Barium Tagging of Particulate... Conclusion References

 
Stool tagging for CT colonography has been studied primarily as a means of avoiding colon purgation [7, 8]. However, as with electronic cleansing, this technique is also beneficial in the prepared colon. In our experience, adherent stool represents the major cause of false-positive findings on CT colonography. Although internal heterogeneity of a polypoid lesion on 2D imaging is suggestive of stool, the presence of identifiable air in adherent fecal material is uncommon in the prepared colon. Barium tagging of adherent stool increases the specificity for true polyps on CT colonography. These lesions will often appear polypoid on the 3D endoluminal image, but the dense nature is apparent on the 2D images with soft-tissue window settings (Figs. 9A, and 9B). This increased attenuation could be overlooked on 2D images with the wider window setting that is typically used for polyp detection. We are currently investigating the use of 3D translucency rendering to provide internal density information more rapidly during the endoluminal fly-through, thereby saving time by avoiding the need for 2D correlation.

View larger version (134K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 9A. —Three-dimensional (3D) pseudopolyp from barium-tagged adherent stool in average-risk 58-year-old man undergoing screening on CT colonography. Three-dimensional endoluminal image shows polypoid lesion (arrow) between haustral folds in ascending colon.

View larger version (125K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 9B. —Three-dimensional (3D) pseudopolyp from barium-tagged adherent stool in average-risk 58-year-old man undergoing screening on CT colonography. Axial two-dimensional image obtained with patient in prone position and soft-tissue window settings shows uniformly dense lesion (arrow) in nondependent aspect of ascending colon, corresponding to 3D findings (A). It would be difficult to exclude true polyp in this patient without barium tagging of adherent fecal material.

Barium coating of a true colonic polyp is the main diagnostic pitfall in solid-stool tagging (Figs. 10A, 10B, and 10C). Dense lesions should be scrutinized for the presence of a homogeneous soft-tissue core that could represent a true polyp. Electronic cleansing may remove fluid adjacent to a polyp but might not remove the entire layer of barium coating (Figs 10A, 10B, and 10C.).

View larger version (129K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 10A. —Adenomatous polyp coated with barium in 58-year-old man undergoing screening with CT colonography. Uncleansed three-dimensional (3D) endoluminal image shows polypoid lesion (arrow) adjacent to or involving haustral fold.

View larger version (156K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 10B. —Adenomatous polyp coated with barium in 58-year-old man undergoing screening with CT colonography. Axial two-dimensional (2D) image obtained with patient in prone position shows correlate to 3D image (A) (arrow) which, at first glance, appears to be barium-tagged stool. However, note central homogeneous soft-tissue component, which raises suspicion of coated polyp.

View larger version (130K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 10C. —Adenomatous polyp coated with barium in 58-year-old man undergoing screening with CT colonography. Cleansed 3D endoluminal image shows subtraction of small amount of fluid, which fortuitously uncovers stalk of pedunculated polyp (arrowhead). Polyp itself retained coat of contrast material on cleansed 2D images (not shown).

Conclusion

Top Introduction CT Colonography Technique Electronic Cleansing of Luminal... Barium Tagging of Particulate... Conclusion References

 
Recent technical advances have paved the way for using 3D endoluminal rendering for primary polyp detection in CT colonography. Solid-stool tagging and electronic cleansing of opacified fluid are useful diagnostic adjuncts to colon purgation. In addition to the advantages, however, diagnostic pitfalls associated with these techniques must be recognized to maintain an acceptable level of specificity.

References

Top Introduction CT Colonography Technique Electronic Cleansing of Luminal... Barium Tagging of Particulate... Conclusion References

 

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				P. J. Pickhardt, P. A. Nugent, J. R. Choi, and W. R. Schindler Flat Colorectal Lesions in Asymptomatic Adults: Implications for Screening with CT Virtual Colonoscopy Am. J. Roentgenol., November 1, 2004; 183(5): 1343 - 1347. [Abstract] [Full Text] [PDF]

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				P. J. Pickhardt, P. A. Nugent, P. A. Mysliwiec, J. R. Choi, and W. R. Schindler Location of Adenomas Missed by Optical Colonoscopy Ann Intern Med, September 7, 2004; 141(5): 352 - 359. [Abstract] [Full Text] [PDF]

				P. J. Pickhardt, J. R. Choi, I. Hwang, and W. R. Schindler Nonadenomatous Polyps at CT Colonography: Prevalence, Size Distribution, and Detection Rates Radiology, September 1, 2004; 232(3): 784 - 790. [Abstract] [Full Text] [PDF]