Adaptive correction of the pseudo-enhancement of CT attenuation for fecal-tagging CT colonography - PubMed (original) (raw)

Adaptive correction of the pseudo-enhancement of CT attenuation for fecal-tagging CT colonography

Janne Näppi et al. Med Image Anal. 2008 Aug.

Abstract

In fecal-tagging CT colonography (ftCTC), positive-contrast tagging agents are used for opacifying residual bowel materials to facilitate reliable detection of colorectal lesions. However, tagging agents that have high radiodensity tend to artificially elevate the observed CT attenuation of nearby materials toward that of tagged materials on Hounsfield unit (HU) scale. We developed an image-based adaptive density-correction (ADC) method for minimizing such pseudo-enhancement effect in ftCTC data. After the correction, we can confidently assume that soft-tissue materials and air are represented by their standard CT attenuations, whereas higher CT attenuations indicate tagged materials. The ADC method was optimized by use of an anthropomorphic phantom filled partially with three concentrations of a tagging agent. The effect of ADC on ftCTC was assessed visually and quantitatively by comparison of the accuracy of computer-aided detection (CAD) without and with the use of the ADC method in two different types of clinical ftCTC databases: 20 laxative ftCTC cases with 24 polyps, and 23 reduced-preparation ftCTC cases with 28 polyps. Visual evaluation indicated that ADC minimizes the observed pseudo-enhancement effect. With ADC, the free-response receiver operating characteristic curves indicating CAD performance in polyp detection yielded normalized partial area-under-curve values of 0.91 and 0.80 for the two databases, respectively, with statistically significant improvement over conventional thresholding-based approaches (p<0.05). The results indicate that ADC is a useful method for reducing the pseudo-enhancement effect and for improving CAD performance in CTC.

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Figures

Fig. 1

Fig. 1

Examples of the problems introduced by pseudo-enhancement. (a) A 6-mm polyp that is pseudo-enhanced by the surrounding high-density tagging. The polyp is visible in the LCT display on the left, but it is invisible in the STCT display on the right. (b) A 10-mm polyp covered by low-density tagging. The polyp is not pseudo-enhanced, and therefore it is clearly visible in both LCT and STCT displays. (c) The three images from left to right show a pseudo-enhanced fold (arrowhead) in LCT and STCT displays, and in endoscopic display, respectively. The endoscopic view demonstrates how the fold can generate a pseudo-polyp. (d) Poorly tagged feces seen in STCT display.

Fig. 2

Fig. 2

Overview of the study design. The pseudo-enhancement correction model was developed and optimized by use of a phantom study. The effect of the optimized pseudo-enhancement correction method was assessed both visually and quantitatively by use of CAD with two different CTC databases.

Fig. 3

Fig. 3

Illustration of the pseudo-enhancement effect in a colon phantom. The two images at top left indicate the location of the line samples that are depicted in the image to the right. In the plots, the line labeled ’Non-tagged’ indicates a line sample without tagging, whereas the lines labeled ’P1’, ’P2’, and ’P3’ indicate samples from phantoms filled partially with 300 HU, 600 HU, and 900 HU tagging concentrations, respectively. In these cases, the CT data were acquired with 1.25-mm slice thickness, 1.25-mm reconstruction interval, and a soft reconstruction kernel. The line ’P4’ indicates another line sample with 900 HU tagging concentration, where the CT data were acquired with 2.5 mm slice thickness, 2.5 mm reconstruction interval, and a standard reconstruction kernel. The lower figure shows a magnified view of the right part of the plot at the upper right corner.

Fig. 4

Fig. 4

Illustration of the effect of the pseudo-enhancement correction on a region of interest in the phantom. (a) LCT display of the original data. (b) STCT display of the original data. (c) Top figures show the estimated pseudo-enhancement energy at subsequent iterations, and the bottom figures show the corresponding density-corrected images.

Fig. 5

Fig. 5

Example of automated detection of a 6-mm polyp located next to the interface region between air and tagged fluid. The region of interest is visualized in cut-plane view by use of an LCT display. (a) Original region. (b) Output of ADC. (c) Result of density mapping (Näppi and Yoshida (2007)) to the output of ADC. (d) The automatically extracted region of the polyp is shown in white color.

Fig. 6

Fig. 6

Endoscopic and cut-plane example images illustrating differences between the two databases. (a) DB-1: laxative cleansing with fecal tagging. The detection of polyps is straightforward. (b) DB-2: reduced cleansing with fecal tagging. The detection of polyps is complicated by the presence of solid feces imitating polyps.

Fig. 7

Fig. 7

Plots of the optimized parameter functions of the ADC. (a) σ1. (b) σ2.

Fig. 8

Fig. 8

Cut-plane views of simulated 8-mm and 12-mm polyps submerged within 900-HU tagging concentration, seen before and after the application of the optimized ADC. (a) Original data in LCT display. (b) Original data in STCT display. (c) After ADC, in LCT display. (d) After ADC, in STCT display.

Fig. 9

Fig. 9

Profile plots of simulated 8-mm and 12-mm polyps and colonic wall submerged within a 900 HU tagging solution, before (dotted line) and after (solid line) the application of ADC. (a) The location of the lines along which the profiles were calculated. (b) Profile plot 1 (small polyp). (c) Profile plot 2 (large polyp). (d) Profile plot 3 (wall region). In each case, ADC has reduced the CT attenuation values within the simulated soft-tissue region to <100 HU.

Fig. 10

Fig. 10

Four examples of the application of ADC to clinical cases. In each example, the top three figures show a region of interest with LCT (left) and STCT displays (middle), and the result of the application of ADC in the STCT display (right). The plots show the CT attenuation before (dotted line) and after (solid line) the application of ADC along the line sample shown in the leftmost top image. The 100 HU CT attenuation level is shown by a thin horizontal dashed line in the plots. (a) 6-mm polyp covered by high-density tagging. (b) 12-mm polyp covered by high-density tagging. (c) 10-mm polyp covered by low-density tagging. The two line-sample plots overlap, because the effect of ADC is negligible in this case. (d) Solid stool with polypoid shape labeled with moderate-density tagging.

Fig. 11

Fig. 11

LOPO performance of CAD on database DB-1 for polyps ≥6 mm.

Fig. 12

Fig. 12

LOPO performance of CAD on database DB-2 for polyps ≥6 mm.

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