January 27, 2012
Visual Nudge Improves Accuracy Of Mammogram Readings
In 2011 – to the consternation of women everywhere – a systematic review of randomized clinical trials showed that routine mammography was of little value to younger women at average or low risk of breast cancer.
The review showed, for example, that for every 50-year-old woman whose life is prolonged by mammography, dozens are treated unnecessarily – some with harmful consequences – or treated without benefit. Hundreds are told they have breast cancer when they do not.Cindy M. Grimm, PhD, associate professor of computer science and engineering in the School of Engineering & Applied Science at Washington University in St. Louis, was not surprised by the review, a prestigious Cochrane review of the scientific evidence for a medical treatment.
“It´s not just the mammogram that´s the problem,” she says, “it´s accurately interpreting the mammogram.
“People aren´t good at it. Even expert radiologists aren´t good at it. Results vary widely from person to person, even when people have gone through the same training.”
But Grimm thought a perceptual trick she and colleagues had invented, called subtle gaze direction, might be used to improve training.
An experiment showed that a novice could be subtly guided to follow an expert´s scanpath across a mammogram and that this subtle nudging improved the novice´s accuracy.
The experimental results will be presented at the Eye Tracking Research & Application Symposium this March.
Grimm and her colleagues say the technique, should it prove durable, is widely applicable to visual search tasks. Not only might it improve the reading of mammograms and other types of medical images, such as MRIs and PET scans, but it might also be used to improve the accuracy of airport screening and learning in virtual environments.
Directing the gaze
Grimm invented subtle gaze direction together with colleagues Reynold Bailey, PhD, then her graduate student, and Ann McNamara, PhD, then of Saint Louis University, a conference acquaintance.
“I had double-majored in art and computer science as an undergraduate at the University of California, Berkeley,” Grimm says. “So I was aware that artists have all sorts of tricks for guiding viewers to look at particular areas in a painting, sometimes, in the case of narrative art, in a particular sequence.
“They might make an area brighter than the background, increase the contrast or have strong edges (borders) that attract the eye.
“Movie producers do the same thing in post processing,” Grimm says. “For example, when one actor is talking and others are listening, the audience tends to watch the talker. But the producer can direct attention to a listener´s reaction instead by changing the color or brightness of that part of the image.”
Subtle gaze direction is a high-tech version of this time-honored craft. It works, says Grimm, by exploiting the difference between peripheral and central (foveal) vision.
“We use a small area in the central part of our retina called the fovea to see detail,” she says. “But foveal vision doesn´t actually cover much of our field of view.
“If you hold out your thumb, your foveal vision – the part of your surroundings you´re actually seeing in detail – covers about the same area as your thumbnail.
“We use our foveal vision to read or drive or for other detail-oriented tasks. At the same time, we are monitoring the rest of our environment with our peripheral vision, which has lower resolution but responds faster than our foveal vision.
“When our peripheral vision picks up a stimulus, our eyes move to focus our foveal vision on it so that we can see it clearly.
“During those quick eye movements, called saccades, vision is suppressed, or masked, so that the motion of the eye, the motion blur of the image and the gap in visual perception are not noticeable to the viewer. We lose an astonishing 40 minutes of vision a day to saccadic masking.”
To direct the gaze, Grimm and her colleagues changed the brightness or “warmth” of an area in the peripheral field of view to draw the novice´s focus to this area.
The stimulus remained subtle, however, because the viewer´s gaze is monitored in real-time by an eye-tracking device and the modulations to the peripheral vision are terminated before the eye fixates on them.
“The idea,” says Grimm “is to get someone to look in a particular direction while altering their experience of viewing the image as little as possible.”
“In the case of mammograms,” for example, “you want to get a learner to look at the tumor region but you don´t want to do anything that makes the tumor region look different than it does on the mammogram itself.”
The mammography study
Reading mammograms is a good target for computer assistance because training is time-consuming and expensive, typically requiring a four-year residency and a two-year fellowship.
Despite advances in technology, novices are still trained by working as an apprentice to an expert.
The mammography study, led by Bailey, now an assistant professor of computer science at the Rochester Institute of Technology, brought together the same group of scientists as the subtle gaze direction experiment. McNamara is now assistant professor of visualization at Texas A&M University.
For the study, Grimm and her colleagues used a database of images provided by the Mammographic Image Analysis Society that includes both images and text files that contains coordinates of abnormalities and their size.
“Expert diagnostic radiologists have a particular search pattern that is not the same as that of a novice,” Grimm says. “We don´t know exactly what they´re doing, but they tend to do a fairly broad scan and then fixate on parts of the image that have a tumor-like texture. A novice might instead attend to brighter spots in the image or fail to scan all of it.”
Bailey hired an expert radiologist at the Rochester Institute of Technology to view and mark 65 images from the database. The expert´s scanpath was recorded during this process by an eye-tracking system.
During the experiment, subtle gaze direction was used to guide a group of novices along the expert scanpath. A control group viewed the mammograms without gaze manipulation.
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