Scanners and drillers: Characterizing expert visual search through volumetric images

Abstract

Modern imaging methods like computed tomography (CT) generate 3-D volumes of image data. How do radiologists search through such images? Are certain strategies more efficient? Although there is a large literature devoted to understanding search in 2-D, relatively little is known about search in volumetric space. In recent years, with the ever-increasing popularity of volumetric medical imaging, this question has taken on increased importance as we try to understand, and ultimately reduce, errors in diagnostic radiology. In the current study, we asked 24 radiologists to search chest CTs for lung nodules that could indicate lung cancer. To search, radiologists scrolled up and down through a "stack" of 2-D chest CT "slices." At each moment, we tracked eye movements in the 2-D image plane and coregistered eye position with the current slice. We used these data to create a 3-D representation of the eye movements through the image volume. Radiologists tended to follow one of two dominant search strategies: "drilling" and "scanning." Drillers restrict eye movements to a small region of the lung while quickly scrolling through depth. Scanners move more slowly through depth and search an entire level of the lung before moving on to the next level in depth. Driller performance was superior to the scanners on a variety of metrics, including lung nodule detection rate, percentage of the lung covered, and the percentage of search errors where a nodule was never fixated.

The Effect of Teaching Search Strategies on Perceptual Performance

“…This differs from the study of Drew et al, reporting both "scanners" and "drillers" among radiologists in a lung nodule detection task, although scanners were in the minority (11). Interestingly, in that study the scanners were less experienced readers than the drillers, which would raise the expectation of finding even more scanners among the junior residents in our study.…”

“…One interesting finding was that, when given the option to search freely, more experienced readers tend to select "drilling" as a search pattern (the more effective pattern), suggesting it might be a pattern that has, consciously or unconsciously, evolved through instruction or practice. The relationship between search patterns and experience has been noted in several other studies (4,(9)(10)(11), although it is unknown if experts unconsciously adopt these patterns or deliberately chose or had acquired one, as a strategy.…”

“…Scanners tend to visually search a single slice, before scrolling further through the stack, whereas drillers focus their eyes on one quadrant of the lung fields and quickly scroll through the stack in depth before moving to another quadrant. Drillers outperformed scanners with respect to higher true positive rates and a larger lung coverage (11). One interesting finding was that, when given the option to search freely, more experienced readers tend to select "drilling" as a search pattern (the more effective pattern), suggesting it might be a pattern that has, consciously or unconsciously, evolved through instruction or practice.…”

“…Some specific visual search patterns are found to be related to high performance (4,(9)(10)(11). Most patterns apply to visual search in X-rays, such as chest X-rays or mammography.…”

“…Most patterns apply to visual search in X-rays, such as chest X-rays or mammography. Two visual search types are distinguished for searching chest computed tomography (CT) images: "scanners" and "drillers" (11). Scanners tend to visually search a single slice, before scrolling further through the stack, whereas drillers focus their eyes on one quadrant of the lung fields and quickly scroll through the stack in depth before moving to another quadrant.…”