Conspicuity of Colorectal Polyps at CT Colonography

Rationale and Objectives

The factors that influence the conspicuity of polyps on computed tomographic (CT) colonography (CTC) are poorly understood. The aim of this study is to compare radiologists’ visual assessment of polyp conspicuity to quantitative image features and show the relationship between visual conspicuity and the detection of colonic polyps by computer-aided detection (CAD) on CTC.

Methods

One polyp (size range 6—10 mm) was selected from the CTC examination of each of 29 patients from a larger cohort. All patients underwent oral contrast-enhanced CTC with same-day optical colonoscopy with segmental unblinding. The polyps were analyzed by a previously validated CAD system and placed into one of two groups (detected [ n = 12] or not detected [ n = 17] by CAD). The study population was intentionally enriched with polyps that were not detected by the CAD system. Four board-certified radiologists, blinded to the CAD results, reviewed two- and three-dimensional CTC images of the polyps and scored the conspicuity of the polyps using a 4-point scale (0 = least conspicuous, 3 = most conspicuous). Polyp height and width were measured by a trained observer. A t -test (two-tailed, unpaired equal variance) was done to determine statistical significance. Intra- and interobserver variabilities of the conspicuity scores were assessed using the weighted κ test. Regression analysis was used to investigate the relationship of conspicuity to polyp height and width.

Results

A statistically significant difference was found between the average conspicuity scores for polyps that were detected by CAD compared to those that were not (2.3 ± 0.6 vs. 1.4 ± 0.8) ( P = .004). There was moderate intraobserver agreement of the conspicuity scores (weighted κ 0.57 ± 0.09). Interobserver agreement was fair (average weighted κ for six pair-wise comparisons, 0.38 ± 0.15). Conspicuity was correlated with manual measurement of polyp height ( r 2 = 0.38–0.56, P < .001).

Conclusions

This CAD system tends to detect 6—10 mm polyps that are more visually conspicuous. Polyp height is a major determinant of visual conspicuity. The generalizability of these findings to other CAD systems is currently unknown. Nevertheless, CAD developers may need to specifically target flatter and less conspicuous polyps for CAD to better assist the radiologist to find polyps in this clinically important size category.

Accurate detection and diagnosis of colonic polyps using computed tomographic (CT) colonography (CTC) is dependent upon many factors. One of the most important factors is the ability of a human observer to visually locate and properly classify polyp candidates. Unfortunately, this ability is inconsistent as demonstrated by studies that reported high intra- and interobserver variabilities among radiologists ( ).

To ensure that patients are properly diagnosed, it is desirable to both reduce observer variability and increase average diagnostic performance. One way to achieve these goals is through the use of computer-aided detection (CAD) systems. For example, CAD systems are available to help detect breast cancers on mammography, pulmonary nodules on chest computed tomography and polyps on CTC.

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Materials and methods

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Patient Population

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Patient Preparation

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CT Scanning

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Polyp Identification

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Computer-aided Polyp Detection

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Data Preparation

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Data Collection—Observer

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Quantitative Information

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Qualitative Information

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Data Collection—Measurements

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Statistical Analysis

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Results

Polyp Size

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Reader Agreement

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Table 1

Intraobserver Agreement of Conspicuity Score

Reader Supine Prone Supine and Prone A 0.58 0.64 0.62 B 0.51 0.63 0.48 C 0.30 0.47 0.50 D 0.68 0.67 0.67 Mean ± SD 0.52 ± 0.16 0.60 ± 0.09 0.57 ± 0.09

SD, standard deviation.

Table values are weighted κ for each of the four radiologist’s conspicuity scores comparing the first and second reads. Weighted κ is shown separately for the supine, prone, and combined (supine and prone) reads.

Table 2

Interobserver Agreement of Conspicuity Score

Reader B C D A 0.53 0.38 0.26 B — 0.30 0.23 C — — 0.60

Table values are weighted κ for each pairwise comparison of radiologist conspicuity scores for the first of the two reads, when using the supine and prone images together (the “combined read”) to grade conspicuity.

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Conspicuity and CAD

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Conspicuity and Image Features

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Conspicuity and True Polyp Size

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Qualitative Features

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Table 3

Readers’ Description of Less Conspicuous Polyps

Physical Characteristics of Polyps Polyps Not Found by CAD Polyps Found by CAD Flat 21%, 12/56 9%, 2/23 Edges not prominent (ie, less well defined) 50%, 28/56 22%, 5/23 Difficult to distinguish from a normal fold 21%, 12/56 35%, 8/23 Looks like noise or a normal bump on the colonic surface 68%, 38/56 57%, 13/23 Looks like stool 16%, 9/56 9%, 2/23

Data are numbers of readings having each description for less conspicuous polyps (conspicuity scores <3). Assessments are for the first reading session on review of the supine and prone images together.

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Discussion

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Acknowledgments

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References

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Conspicuity of Colorectal Polyps at CT Colonography

Rationale and Objectives

Methods

Results

Conclusions

Materials and methods

Patient Population

Patient Preparation

CT Scanning

Polyp Identification

Computer-aided Polyp Detection

Data Preparation

Data Collection—Observer

Quantitative Information

Qualitative Information

Data Collection—Measurements

Statistical Analysis

Results

Polyp Size

Reader Agreement

Conspicuity and CAD

Conspicuity and Image Features

Conspicuity and True Polyp Size

Qualitative Features

Discussion

Acknowledgments

References

Further Reading

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Abstracts of Funded National Institutes of Health Grants

Assessment of Radiologist Performance in the Detection of Lung Nodules