Structure-analysis method for electronic cleansing in cathartic and noncathartic CT colonography - PubMed (original) (raw)

Structure-analysis method for electronic cleansing in cathartic and noncathartic CT colonography

Wenli Cai et al. Med Phys. 2008 Jul.

Abstract

Electronic cleansing (EC) is an emerging method for segmentation of fecal material in CT colonography (CTC) that is used for reducing or eliminating the requirement for cathartic bowel preparation and hence for improving patients' adherence to recommendations for colon cancer screening. In EC, feces tagged by an x-ray-opaque oral contrast agent are removed from the CTC images, effectively cleansing the colon after image acquisition. Existing EC approaches tend to suffer from the following cleansing artifacts: degradation of soft-tissue structures because of pseudo-enhancement caused by the surrounding tagged fecal materials, and pseudo soft-tissue structures and false fistulas caused by partial volume effects at the boundary between the air lumen and the tagged regions, called the air-tagging boundary (AT boundary). In this study, we developed a novel EC method, called structure-analysis cleansing, which effectively avoids these cleansing artifacts. In our method, submerged soft-tissue structures are recognized by their local morphologic signatures that are characterized based on the eigenvalues of a three-dimensional Hessian matrix. A structure-enhancement function is formulated for enhancing of the soft-tissue structures. In addition, thin folds sandwiched between the air lumen and tagged regions are enhanced by analysis of the local roughness based on multi-scale volumetric curvedness. Both values of the structure-enhancement function and the local roughness are integrated into the speed function of a level set method for delineating the tagged fecal materials. Thus, submerged soft-tissue structures as well as soft-tissue structures adhering to the tagged regions are preserved, whereas the tagged regions are removed along with the associated AT boundaries from CTC images. Evaluation of the quality of the cleansing based on polyps and folds in a colon phantom, as well as on polyps in clinical cathartic and noncathartic CTC cases with fluid and stool tagging, showed that our structure-analysis cleansing method is significantly superior to that of our previous thresholding-based EC method. It provides a cleansed colon with substantially reduced subtraction artifacts.

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Figures

Figure 1

(a) Schematic illustration of a colonic polyp (a cap), a haustral hold (a ridge), and the colonic wall (a cylinder). (b) Planar map of the colon illustrates the morphologic shapes of the polyp and the fold. The polyp is depicted as a cap-like structure, whereas the fold is depicted as a ridge-like structure on the colonic planar surface. The three vectors **e**1, **e**2, and **e**3 represent the eigenvectors of the Hessian matrix for the polyp and the fold.

Figure 2

Profiles of a submerged fold and a submerged polyp on a colon phantom. (a) Two sampling lines on the cross-sectional image of a fold. CT values and eigenvalues of the Hessian matrix are sampled along the short and long axes of the fold. (b) Coronal view of the fold in (a). (c) A sampling line on the cross-sectional image of a polyp. (d) Coronal view of the polyp in (c). (e) Plot of the change in CT values and eigenvalues of the Hessian matrix along the short axis in (a). (f) Plot of the change in CT values and eigenvalues of the Hessian matrix along the long axis in (a). (g) Plot of the change in CT values and eigenvalues of the Hessian matrix along the sampling line in (c).

Figure 3

Effect of discrimination function F A on the differentiation of folds from other structures in the colon phantom. (a) Curves show the change in the values of F A as a function of R a at different values of α. (b) Coronal view of the CTC images of a colon phantom. All voxels above 200 HU are shaded. We observe that parts of the folds and polyps are pseudo-enhanced. (c) The response images of the phantom image in (b) resulting from the application of F A at α=0.1, 0.25, 0.5, and 1.0.

Figure 4

Effect of the discrimination function F B on the differentiation of folds from other structures in the colon phantom. (a) Curves show the change in the values of F B as a function of R b at a fixed value of γ=0.2 and at different values of β. (b) The response images of F B from the application of γ=0.2 and β=0.1, 0.3, and 0.5. (c) Curves show the change in the values of F a as a function of R b at a fixed value of γ=0.3 and at different values of β. (d) The response images of F B from the application of γ=0.3 and β=0.1, 0.3, and 0.5.

Figure 5

Effect of the discrimination function F C on the differentiation of polyps from other structures in the colon phantom. (a) Curves show the change in the values of F C as a function of R C at different values of η. (b) The response images resulting from the application of F C to the phantom image in Fig. 3b, when η was set to 0.1, 0.2, and 0.5.

Figure 6

Hessian response of submerged colonic structures: (a) and (c) show coronal and axial images of the colon phantom, respectively: all voxels above 200 HU are shaded. (b) and (d) are the Hessian response field of (a) and (c), respectively, in which the corresponding voxels above 200 HU are shaded. (e) Response images resulting from application of the rut-enhanced function _F_rut, the cup-enhancement function _F_cup, and structural enhancement function H A.

Figure 7

Demonstration of the effect of the structural enhancement functions on folds and polyps. (a) Portion of the colonic lumen filled with tagged materials. (b) Result of the Hessian response field to the lumen in (a). The folds submerged in the tagging materials are well enhanced. (c) Portion of the colonic lumen that is half filled with tagged materials. A submerged thin fold and a small polyp are indicated by black arrows. (d) Result of the Hessian response field to the lumen in (c). The submerged folds and polyps are well enhanced, as indicated by the white arrows.

Figure 8

Similarity between an AT boundary and the soft-tissue structure sandwiched between the air lumen and a tagged region. (a) The profile across the thin fold is shown by a gray arrow. (b) Two AT boundary profiles are shown by gray arrows. In both images (a) and (b), voxels of which the CT value is between −200 HU and 200 HU are shaded. Three profiles including the CT value and gradient value were sampled along the lines, from the filled circle to the arrow point. (c) Plot of the change in the gradient of CT values along the profiles in (a) and (b). Series 1 shows the change in the gradient along the profile in (a), whereas Series 2 and 3 show the change in the gradient along the profiles in (b). The three profiles show a very similar pattern of change in the CT values and gradient values.

Figure 9

(a) Image of a large block of tagged fecal material that has an AT boundary (top dots) and a thin fold sandwiched between the air lumen and the tagged region (bottom triangles). (b) Local roughness images generated from the image in (a). Here, a bright pixel represents a low roughness value. (c) Plots of the curvedness values across the scales. The dotted curves show the change in the curvedness values at the voxels along the AT boundary, as represented by circles in (a). The solid curves show the change in the curvedness values at the voxels, represented by triangles in (a), along the thin haustral fold sandwiched between the tagged region and the air lumen.

Figure 10

Local roughness of the iso-value voxels at [−200 HU,200 HU] distinguishes the AT boundaries and the thin folds within the ATT layer. (a) and (b) show CTC images in which arrows indicate either the AT boundaries or thin folds sandwiched between lumen air and tagged materials. (c) and (d) show the roughness response fields corresponding to (a) and (b). In (c), the thin fold [white arrow in (a)] is enhanced, whereas the AT boundary [black arrow in (a)], which has the same profile as the thin fold, is de-enhanced. The image in (d) demonstrates that the local roughness enhances the folds that are buried underneath the tagged objects [white arrows in (b)], whereas the AT boundary against gravity [black arrow in (b)] is de-enhanced.

Figure 11

Comparison of coronal images between the original phantom and the phantom cleansed by use of the SA- and T-cleansing methods. (a) Coronal image without tagged materials. (b) Coronal images after being filled with tagged materials. (c) and (d) show the coronal view of the cleansed images of (b) by use of the SA- and T-cleansing methods, respectively.

Figure 12

Evaluation results of EC on polyps in a colon phantom. (a)–(d) Examples of the cleansing results of a submerged phantom polyp by use of SA- and T-cleansing methods. (a) Detailed view of the original phantom polyp. (b) Detailed view of the polyp in (a) after SA cleansing. (c) Detailed view of the polyp in (a) after T cleansing. (d) 3D view of the original phantom polyp before the colon phantom was filled with tagged materials. (e) MIP image of the difference between (a) and (b). (f) MIP image of the difference between (a) and (c). (g), (h) Results for the 11 submerged polyps. (g) RMSE values of the difference VOIs for individual submerged polyps. (h) The difference in the number of soft-tissue voxels, defined as those with CT values higher than −600 HU, in the resulting VOIs from SA and T cleansing with respect to the reference standard. The _t_-test showed that both RMSE and the difference in the number of soft-tissue voxels were significantly lower for SA cleansing than for T cleansing (_p_=0.0003 and 0.0001, respectively).

Figure 13

Evaluation results of EC on folds in a colon phantom. (a)–(c) Examples of the cleansing results of a submerged phantom fold by use of SA- and T-cleansing methods. (a) Detailed view of the phantom fold before the colon phantom was filled with tagged materials. (b) Detailed view of the fold in (a) after SA cleansing. (c) Detailed view of the fold in (a) after T cleansing. (d),(e) Results for the seven submerged folds. (d) RMSE values of the difference VOIs for individual submerged folds. (e) The difference in the number of soft-tissue voxels, defined as those with CT values higher than −600 HU, in the resulting VOIs from SA and T cleansing with respect to the reference standard. The _t_-test showed that both RMSE and the difference in the number of soft-tissue voxels were significantly lower for SA cleansing than for T cleansing (_p_=0.0007 and 0.0003, respectively).

Figure 14

Evaluation results from the manual cleansing experiment. (a)–(c) Examples of the cleansing results of a 9 mm polyp submerged in tagged fluid in clinical CTC cases by use of the SA- and T-cleansing methods. (a) Detailed view of the polyp after manual cleansing. (b) Detailed view of the polyp in (a) after SA cleansing. (c) Detailed view of the polyp in (a) after T cleansing. We observe the fold and polyp degradation as well as a false fistula in (c). (d),(e) Results for the ten submerged polyps. (d) RMSE values of the difference VOIs for ten individual submerged polyps. (e) The difference in the number of soft-tissue voxels, defined as those with CT values higher than −600 HU, in the resulting VOIs from SA and T cleansing with respect to the reference standard. The _t_-test showed that both MSE and the difference in the number of soft-tissue voxels were significantly lower for SA cleansing than for T cleansing (_p_=0.0005 and 0.0012, respectively).

Figure 15

Illustration of the reduction in the artifact of the degradation of the soft-tissue structures in SA cleansing. (a) A thin haustral fold (white arrow) is submerged in the tagged semisolid stool. (b) The thin fold is preserved in the SA-cleansing method. (c) The thin fold in (a) was erroneously removed after application of the T-cleansing method. (d) A small polyp (white arrow) is submerged in the tagged materials. (e) The submerged polyp is preserved in the SA-cleansing method. (f) The polyp in (d) was erroneously removed by the T-cleansing method.

Figure 16

Illustration of the reduction of an artifact of pseudo soft-tissue structures and false fistulas in SA cleansing. (a) The boundary between the lumen air and tagging stool (AT boundaries) is indicated by the white and black arrows. (b) Application of SA cleansing to the image in (a) generates no pseudo-soft-tissue structures. (c) Application of T cleansing to the image in (a) generates pseudo-soft-tissue structures as indicated by the white and black arrows. (d) A thin colonic wall sandwiched between the tagged regions and the lumen air is indicated by the white arrow. (e) The colonic wall in (d) is preserved by use of the SA-cleansing method. (f) The colonic wall in (d) is erroneously removed by T cleansing as the boundaries between the lumen and tagged materials, creating a false fistula, as indicated by the white arrow.

Figure 17

Examples of the preservation of soft-tissue structures in SA cleansing. (a) and (c): Original CTC images with semisolid tagged stool. (b) and (d): Images after SA cleansing of the CTC images in (a) and (c), respectively. Both images demonstrate the soft-tissue preservation of submerged folds and thin folds within ATT layers (arrows) in the SA-cleansing method.

Figure 18

Comparison of scale parameter σ in Hessian response fields. (a) CTC phantom image. (b)–(g) Hessian response fields are calculated with different scales ranging from σ=0.5 to 8 in voxel units.

Figure 19

Illustration of cleansing of a fold (a–c) and a polyp (d–f) partially submerged in tagged semisolid stool in SA cleansing. (a) Fold indicated by the arrow is partially submerged in the tagged stool. (b) Response image of the structural enhancement function for both the submerged and nonsubmerged parts of the fold. (c) Fold cleansed by SA cleansing. (d) The polyp indicted by the arrow is partially submerged in the tagged stool. (e) Response image of the structural enhancement function for both the submerged and nonsubmerged parts of the polyp. (f) Polyp cleansed by SA cleansing.

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Structure-analysis method for electronic cleansing in cathartic and noncathartic CT colonography - PubMed (original) (raw)