Peripheral liver metastases are more frequently missed than central metastases in contrast-enhanced CT: insights from a 25-reader performance study

Abstract

Purpose: Subtle liver metastases may be missed in contrast enhanced CT imaging. We determined the impact of lesion location and conspicuity on metastasis detection using data from a prior reader study.

Methods: In the prior reader study, 25 radiologists examined 40 CT exams each and circumscribed all suspected hepatic metastases. CT exams were chosen to include a total of 91 visually challenging metastases. The detectability of a metastasis was defined as the fraction of radiologists that circumscribed it. A conspicuity index was calculated for each metastasis by multiplying metastasis diameter with its contrast, defined as the difference between the average of a circular region within the metastasis and the average of the surrounding circular region of liver parenchyma. The effects of distance from liver edge and of conspicuity index on metastasis detectability were measured using multivariable linear regression.

Results: The median metastasis was 1.4 cm from the edge (interquartile range [IQR], 0.9-2.1 cm). Its diameter was 1.2 cm (IQR, 0.9-1.8 cm), and its contrast was 38 HU (IQR, 23-68 HU). An increase of one standard deviation in conspicuity index was associated with a 6.9% increase in detectability (p = 0.008), whereas an increase of one standard deviation in distance from the liver edge was associated with a 5.5% increase in detectability (p = 0.03).

Conclusion: Peripheral liver metastases were missed more frequently than central liver metastases, with this effect depending on metastasis size and contrast.

Keywords: Eye tracking; Liver metastases; Search errors.

Fig. 1. Proposed model of visual search.

Each panel shows the same slice of a liver which contains two metastases (black arrows in Panel A), one obvious and one subtle. In Panel A, fixations (red arrows) are surrounded by small circles that represent the useful field of view for the detection of subtle metastases, and larger circles that represent the useful field of view for detection of typical metastases. Panels B, C, D present real eye tracker data for three human readers interpreting this case. Eye gaze is shown in cyan, and clusters of cyan correspond to eye fixations. Readers circumscribed metastases they detected, shown here as open boxes. All readers found the obvious metastasis, but only one reader (Panel D) found the subtle metastasis. In all cases, reader fixations were more centrally located; only the reader in Panel D also had fixations in the area near the subtle metastasis. Note that eye tracking measurement error may cause eye fixations associated with metastasis detection to be located adjacent to the metastasis rather than directly superimposed

Fig. 2

Metastases as a function of diameter, contrast, and conspicuity index. Each circle represents a metastasis. The conspicuity index is the product of metastasis diameter and contrast. Lines represent curves of equal conspicuity index that separate the three conspicuity tertiles: the white region on the bottom left is subtle (small and low contrast), the red region on the top right is obvious, and the intermediate colored region is moderate. Diameter values have been jittered (randomly offset by up to 1 pixel) to improve readability. The mean pixel size was 0.76 mm. The images for metastases A, B, C, and D are shown in Fig. 4

Fig. 3

Relationship between conspicuity index and detectability. As expected, more conspicuous metastases are more likely to be detected. The images for metastases A, B, C, and D are shown in Fig. 4

Fig. 4

Example metastases from the dataset. In the top row, (A) is in the obvious tertile, (B) is in the obvious tertile but bordering the moderate tertile, (C) is in the moderate tertile, and (D) is in the subtle tertile. In the bottom row, metastases at select percentiles of conspicuity index and their corresponding normalized conspicuity indices, denoted as “C” in panel titles

Fig. 5

Detectability of metastases grouped by tertiles of distance from liver capsule (peripheral [i.e., subcapsular], intermediate, central) and conspicuity (subtle, moderate, obvious)