Monday, April 7, 2008

What connects 30" computer displays, eye-tracking, and pathologists' levels of experience?

An interesting threshold has been crossed by computer displays relative to modern light microscopes.
A well corrected light microscope uses so-called plano or "Plan" objectives (as in "PlanApo"). Such objectives are made to have a particularly large, flat field of view. The diameter of a 20X PlanApo objective's field of view is approximately 1 mm.
In a different context, 30" computer displays have become relatively common today. They are offered by Apple, Dell, and HP, for instance. A 30" computer display has a format of 2,560 pixels by 1,600 pixels.
A slide scanner that captures images at an equivalent magnification of 20X, creates images with pixels whose size on the specimen is typically 0.5 microns/pixel. The area of the specimen that can be displayed on a 30" computer display therefore measures 1.25 mm by 0.8 mm. A 30" display therefore shows a specimen area that is 27% larger than the area visible through eyepieces on a modern analog microscope (1 sq-mm vs. 0.78 sq. mm). Any smaller format display (e.g., a 24" display with 1,920 by 1,200 pixels) does not exceed the "PlanApo" threshold. Viewing images on a smaller computer display is equivalent to turning down the field stop in an analog microscope.

This is interesting news in its own right, but what is its real impact on digitizing pathology? A recent human-factors study by Krupinski, et al., compared eye movements by medical students, residents, and fully trained pathologists when reviewing breast core biopsy cases:

“…Our objective in this study was to take advantage of virtual slide technology to compare the eye movements of virtual slide readers with different levels of experience. …”
“…Unlike either the medical students or the residents, the [fully trained] pathologists frequently choose areas for viewing at higher magnification outside of areas of foveal (central) vision. …” [emphasis added by me]

(The Krupinski paper [Krupinski, et al., Human Pathology (2006) 37, 1543–1556] includes several amazing figures (especially Figures 7 and 8) which show the scan-paths of fully trained pathologists, residents, and medical students, respectively.)

The conclusion is that 30” computer displays are the minimum size that allow experienced pathologists to work naturally instead of being constrained to working as they had at earlier stages in their training, if paired with a smaller-format computer display. It's very hard to use one's peripheral vision when there is nothing to see with it!
This conclusion is further supported by anecdotal evidence. DMetrix has been demonstrating its scanners with 30" displays since 2005.

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