Screening CT Colonography: How I Do It

doi:10.2214/AJR.07.2136

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Screening CT Colonography: How I Do It

Perry J. Pickhardt¹

¹ Department of Radiology, University of Wisconsin Medical School, E3/311 Clinical Science Center, 600 Highland Ave., Madison, WI 53792-3252.

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Fig. 1A —Contrast coating of polyp surface on screening CT colonography in 56-year-old man. Three-dimensional endoluminal colonographic image shows 1.5-cm sessile polyp within rectum.

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Fig. 1B —Contrast coating of polyp surface on screening CT colonography in 56-year-old man. Two-dimensional transverse CT colonographic images with polyp (B) and soft-tissue (C) window settings show lesion of uniform soft-tissue density with rim of adherent contrast material coating posterior surface (arrowhead), which should not be confused with fecal tagging. Contrast material appears to enlarge or "bloom" in C, decreasing conspicuity of polyp. Polyp windows are important for appreciating homogeneous nature of internal soft-tissue component of these lesions.

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Fig. 1C —Contrast coating of polyp surface on screening CT colonography in 56-year-old man. Two-dimensional transverse CT colonographic images with polyp (B) and soft-tissue (C) window settings show lesion of uniform soft-tissue density with rim of adherent contrast material coating posterior surface (arrowhead), which should not be confused with fecal tagging. Contrast material appears to enlarge or "bloom" in C, decreasing conspicuity of polyp. Polyp windows are important for appreciating homogeneous nature of internal soft-tissue component of these lesions.

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Fig. 1D —Contrast coating of polyp surface on screening CT colonography in 56-year-old man. Digital photograph from optical colonoscopy shows tenacious mucus clinging to portion of polyp surface, which may correspond to dense coating at CT colonography. Polyp proved to be tubulovillous adenoma. Lesions with villous component tend to have this coating effect.

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Fig. 2A —Addition of decubitus view because of inadequate sigmoid distention with supine and prone positioning in 53-year-old woman. Supine (A) and prone (B) 2D transverse CT colonographic images show focal segmental collapse (arrowheads) and fold thickening involving sigmoid colon. Distention was adequate throughout remainder of large intestine (not shown). In immediate review of 2D images, CT technologist recognized collapse at same position on both supine and prone images and obtained additional right lateral decubitus image.

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Fig. 2B —Addition of decubitus view because of inadequate sigmoid distention with supine and prone positioning in 53-year-old woman. Supine (A) and prone (B) 2D transverse CT colonographic images show focal segmental collapse (arrowheads) and fold thickening involving sigmoid colon. Distention was adequate throughout remainder of large intestine (not shown). In immediate review of 2D images, CT technologist recognized collapse at same position on both supine and prone images and obtained additional right lateral decubitus image.

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Fig. 2C —Addition of decubitus view because of inadequate sigmoid distention with supine and prone positioning in 53-year-old woman. Two-dimensional transverse CT colonographic image obtained with patient in right lateral decubitus position shows adequate distention of segment in question, allowing diagnostic examination and avoiding need for flexible sigmoidoscopy.

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Fig. 2D —Addition of decubitus view because of inadequate sigmoid distention with supine and prone positioning in 53-year-old woman. Prone scout radiograph does not show area of focal collapse, in part because presence of overlapping loops makes assessment difficult. Finding on scout image can lead to both overestimation and underestimation of distention and necessitates online review of 2D transverse images by CT technologist or radiologist for confident assessment of left colonic distention.

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Fig. 3A —Performance of 3D CT colonography in detection of adenomas 8 mm or larger in Department of Defense CT colonography screening trial [1]. Bar graphs show sensitivity (green) and specificity (blue) at 8-mm size threshold according to temporal quarter (Qtr) of trial (A) and study site (B). Uniform performance characteristics range from 91% to 95% in all cases. That most radiologists in study were not based at academic medical centers and had relatively little experience (25-50 of fewer CT colonography cases) suggests that learning curve with primary 3D polyp detection is much simpler than that for primary 2D approach. Site 1 = National Naval Medical Center, site 2 = Naval Medical Center San Diego, site 3 = Walter Reed Army Medical Center.

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Fig. 3B —Performance of 3D CT colonography in detection of adenomas 8 mm or larger in Department of Defense CT colonography screening trial [1]. Bar graphs show sensitivity (green) and specificity (blue) at 8-mm size threshold according to temporal quarter (Qtr) of trial (A) and study site (B). Uniform performance characteristics range from 91% to 95% in all cases. That most radiologists in study were not based at academic medical centers and had relatively little experience (25-50 of fewer CT colonography cases) suggests that learning curve with primary 3D polyp detection is much simpler than that for primary 2D approach. Site 1 = National Naval Medical Center, site 2 = Naval Medical Center San Diego, site 3 = Walter Reed Army Medical Center.

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Fig. 4 —Mucosal coverage on 3D endoluminal CT colonography in 59-year-old woman. Still image from 3D endoluminal fly-through shows visualized surface (green) after unidirectional navigation along automated centerline in opposite direction. Typically, more than 20% of endoluminal surface is not seen during unidirectional navigation with 90° field of view, necessitating bidirectional navigation. Previously unseen areas are relative blind spots at optical colonoscopy. Nonstandard 3D displays, such as virtual dissection view, avoid bidirectional evaluation and may become standard practice. Increasing field of view may also eliminate need for bidirectional endoluminal navigation.

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Fig. 5A —Three-dimensional translucency rendering for assessment of internal density characteristics in 58-year-old woman. Three-dimensional endoluminal CT colonographic image shows 7-mm polypoid lesion in rectum.

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Fig. 5B —Three-dimensional translucency rendering for assessment of internal density characteristics in 58-year-old woman. Same 3D image as A with translucency rendering shows lesion composed of soft tissue (red) surrounded by thick collar of adherent contrast material at base (white).

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Fig. 5C —Three-dimensional translucency rendering for assessment of internal density characteristics in 58-year-old woman. Two-dimensional coronal CT colonographic image confirms presence of rectal soft-tissue polyp with high-attenuation contrast material at base.

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Fig. 5D —Three-dimensional translucency rendering for assessment of internal density characteristics in 58-year-old woman. Digital photograph from same-day optical colonoscopy shows polyp depicted in A-C. Lesion proved to be tubular adenoma. Collar of adherent contrast material probably was washed away before photograph was obtained.

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Fig. 6A —Difficult polyp location for detection and polypectomy with optical colonoscopy in 68-year-old woman. Three-dimensional endoluminal CT colonographic image from perspective of cecal tip shows relatively subtle 1.5-cm sessile polyp (arrowheads) located behind fold and adjacent to ileocecal valve (arrow).

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Fig. 6B —Difficult polyp location for detection and polypectomy with optical colonoscopy in 68-year-old woman. Two-dimensional coronal CT colonographic image confirms presence of soft-tissue lesion (arrowhead) next to ileocecal valve (arrow).

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Fig. 6C —Difficult polyp location for detection and polypectomy with optical colonoscopy in 68-year-old woman. Three-dimensional colonic map shows anatomic location of cecal polyp (red dot), extensive sigmoid diverticulosis, and automated centerline (green). Blue arrow indicates 3D vantage point shown in A. Polyp was found and removed at optical colonoscopy and proved to be tubulovillous adenoma with high-grade dysplasia. Because of difficult location of polyp, gastroenterologist noted that he would have missed this lesion without detailed knowledge of its existence obtained with CT colonography.

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