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Influence of Tagged Fecal Material on Detectability of Colorectal Polyps at CT: Phantom Study

Ayso H. de Vries1, Henk W. Venema1,2, Jasper Florie1, Chung Y. Nio1 and Jaap Stoker1

1 Department of Radiology, Academic Medical Center, University of Amsterdam, Meibergdreef 9, 1105 AZ Amsterdam, The Netherlands.
2 Department of Medical Physics, Academic Medical Center, University of Amsterdam, Amsterdam, The Netherlands.


Figure 1
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  		Fig. 1A —Influence of tagged material on visibility. 75-year-old man with colonic polyp. CT scan (40 mAs; window width, 1,250 HU; level, –50 HU) shows colonic wall and polyp (white arrow). Gray arrow indicates haustral fold in transverse colon surrounded by air.

 

Figure 2
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  		Fig. 1B —Influence of tagged material on visibility. Simulated CT scans at tube charges lower than in A: 20 mAs (B), 10 mAs (C), and 5 mAs (D). White arrow indicates 6-mm polyp submerged in fecal material in descending colon proven at colonoscopy. Polyp is highly visible in A but hardly visible in D. In comparison, haustral fold in transverse colon surrounded by air (gray arrow) remains highly visible in D.

 

Figure 3
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  		Fig. 1C —Influence of tagged material on visibility. Simulated CT scans at tube charges lower than in A: 20 mAs (B), 10 mAs (C), and 5 mAs (D). White arrow indicates 6-mm polyp submerged in fecal material in descending colon proven at colonoscopy. Polyp is highly visible in A but hardly visible in D. In comparison, haustral fold in transverse colon surrounded by air (gray arrow) remains highly visible in D.

 

Figure 4
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  		Fig. 1D —Influence of tagged material on visibility. Simulated CT scans at tube charges lower than in A: 20 mAs (B), 10 mAs (C), and 5 mAs (D). White arrow indicates 6-mm polyp submerged in fecal material in descending colon proven at colonoscopy. Polyp is highly visible in A but hardly visible in D. In comparison, haustral fold in transverse colon surrounded by air (gray arrow) remains highly visible in D.

 

Figure 5
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  		Fig. 2 —Phantom (width, 1,250 HU; level, –50 HU). CT scan shows water-filled drum (1) with centrally placed polymethyl methacrylate cylinder (2), representing colon. Cylinder is filled with contrast material (3). Contrast between lumen and border is 480 HU. Filling defect represents 6-mm polyp.

 

Figure 6
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  		Fig. 3A —Colonic phantom in first phase of study. CT scans (300 mAs; 300 HU; width, 1,000 HU; level, 200 HU) show polyp in one of eight possible locations. Half past one-o'clock position.

 

Figure 7
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  		Fig. 3B —Colonic phantom in first phase of study. CT scans (300 mAs; 300 HU; width, 1,000 HU; level, 200 HU) show polyp in one of eight possible locations. Twelve-o'clock position.

 

Figure 8
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  		Fig. 3C —Colonic phantom in first phase of study. CT scans (300 mAs; 300 HU; width, 1,000 HU; level, 200 HU) show polyp in one of eight possible locations. Half past ten-o'clock position.

 

Figure 9
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  		Fig. 3D —Colonic phantom in first phase of study. CT scans (300 mAs; 300 HU; width, 1,000 HU; level, 200 HU) show polyp in one of eight possible locations. Nine-o'clock position.

 

Figure 10
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  		Fig. 3E —Colonic phantom in first phase of study. CT scans (300 mAs; 300 HU; width, 1,000 HU; level, 200 HU) show polyp in one of eight possible locations. Half past seven-o'clock position.

 

Figure 11
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  		Fig. 3F —Colonic phantom in first phase of study. CT scans (300 mAs; 300 HU; width, 1,000 HU; level, 200 HU) show polyp in one of eight possible locations. Six-o'clock position.

 

Figure 12
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  		Fig. 3G —Colonic phantom in first phase of study. CT scans (300 mAs; 300 HU; width, 1,000 HU; level, 200 HU) show polyp in one of eight possible locations. Half past four-o'clock position.

 

Figure 13
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  		Fig. 3H —Colonic phantom in first phase of study. CT scans (300 mAs; 300 HU; width, 1,000 HU; level, 200 HU) show polyp in one of eight possible locations. Three-o'clock position.

 

Figure 14
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  		Fig. 4 —Coronal section of part of virtual colon (24 of 120 rings) composed of scans at 80 mAs (width, 1,000 HU; level, 200 HU) for second phase of study. Contrast between colon and its contents is 300 HU. Arrow indicates 6-mm sessile polyp.

 

Figure 15
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  		Fig. 5 —Polymethyl methacrylate colon phantom in center of 34-cm-diameter water-filled drum. CT scan (10 mAs; width, 2,000 HU; level, 0 HU) shows 6-mm polyp (arrow) surrounded by air. Polyp was considered visible beyond doubt.

 

Figure 16
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  		Fig. 6 —Graph shows percentages of correct responses for each tube charge level for three lowest contrast levels. Data points are mean values for three observers. Ranges from lowest to highest scores are indicated. Curves are psychometric curves fitted to these data points. Dashed line indicates score of 90% correct. • = 300 HU, {blacksquare} = 400 HU, {blacktriangleup} = 790 HU.

 

Figure 17
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  		Fig. 7 —Graph shows sensitivity and number of false-positive findings at contrast levels of 300 HU (28–80 mAs) and 480 HU (20–40 mAs). Data points are average values for three observers. Sensitivity ranges from lowest to highest value are indicated. Horizontal dashed line represents 90% sensitivity. {blacksquare} = sensitivity at 480 HU, • = sensitivity at 300 HU, {square} = number of false-positive findings at 480 HU, {circ} = number of false-positive findings at 300 HU.

 

Figure 18
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  		Fig. 8A —Measuring contrast level and noise. Graphs show mean (A) and SD (B) of attenuation in regions of interest with shape of concentric rings within lumen of phantom and within polymethyl methacrylate border. Circles indicate measured values; lines, fitted values. Value of contrast and SD at site of polyp was taken as difference between fitted values of contrast and SD for ring with diameter of 47 mm (indicated by dashed vertical line), on which each polyp is centered. SDs for regions of interest at 48 and 52 mm are high because these regions of interest contain pixels of both lumen and border. SD at 50 mm is higher than 100 HU and therefore not shown.

 

Figure 19
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  		Fig. 8B —Measuring contrast level and noise. Graphs show mean (A) and SD (B) of attenuation in regions of interest with shape of concentric rings within lumen of phantom and within polymethyl methacrylate border. Circles indicate measured values; lines, fitted values. Value of contrast and SD at site of polyp was taken as difference between fitted values of contrast and SD for ring with diameter of 47 mm (indicated by dashed vertical line), on which each polyp is centered. SDs for regions of interest at 48 and 52 mm are high because these regions of interest contain pixels of both lumen and border. SD at 50 mm is higher than 100 HU and therefore not shown.

 

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