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Interesting New Yorker article about Deep-Learning, Medicine and Radiology
[link=http://www.newyorker.com/magazine/2017/04/03/ai-versus-md]http://www.newyorker.com/…017/04/03/ai-versus-md[/link]
In mammography, too, computer-aided detection is becoming commonplace. Pattern-recognition software highlights suspicious areas, and radiologists review the results. But here again the recognition software typically uses a rule-based system to identify a suspicious lesion. Such programs have no built-in mechanism to learn: a machine that has seen three thousand X-rays is no wiser than one that has seen just four. These limitations became starkly evident in a 2007 study that compared the accuracy of mammography before and after the implementation of computer-aided diagnostic devices. One might have expected the accuracy of diagnosis to have increased dramatically after the devices had been implemented. As it happens, the devices had a complicated effect. The rate of biopsies shot up in the computer-assisted group. Yet the detection of small, invasive breast cancersthe kind that oncologists are most keen to detect[i]decreased[/i]. (Even later studies have shown problems with false positives.)
Thrun was convinced that he could outdo these first-generation diagnostic devices by moving away from rule-based algorithms to learning-based onesfrom rendering a diagnosis by knowing that to doing so by knowing how. Increasingly, learning algorithms of the kind that Thrun works with involve a computing strategy known as a neural network, because its inspired by a model of how the brain functions. In the brain, neural synapses are strengthened and weakened through repeated activation; these digital systems aim to achieve something similar through mathematical means, adjusting the weights of the connections to move toward the desired output. The more powerful ones have something akin to layers of neurons, each processing the input data and sending the results up to the next layer. Hence, deep learning.Thrun began with skin cancer; in particular, keratinocyte carcinoma (the most common class of cancer in the U.S.) and melanoma (the most dangerous kind of skin cancer). Could a machine be taught to distinguish skin cancer from a benign skin conditionacne, a rash, or a moleby scanning a photograph? If a dermatologist can do it, then a machine should be able to do it as well, Thrun reasoned. Perhaps a machine could do it even better.
Traditionally, dermatological teaching about melanoma begins with a rule-based system that, as medical students learn, comes with a convenient mnemonic: ABCD. Melanomas are often asymmetrical (A), their borders (B) are uneven, their color (C) can be patchy and variegated, and their diameter (D) is usually greater than six millimetres. But, when Thrun looked through specimens of melanomas in medical textbooks and on the Web, he found examples where none of these rules applied.
Thrun, who had maintained an adjunct position at Stanford, enlisted two students he worked with there, Andre Esteva and Brett Kuprel. Their first task was to create a so-called teaching set: a vast trove of images that would be used to teach the machine to recognize a malignancy. Searching online, Esteva and Kuprel found eighteen repositories of skin-lesion images that had been classified by dermatologists. This rogues gallery contained nearly a hundred and thirty thousand imagesof acne, rashes, insect bites, allergic reactions, and cancersthat dermatologists had categorized into nearly two thousand diseases. Notably, there was a set of two thousand lesions that had also been biopsied and examined by pathologists, and thereby diagnosed with near-certainty.
Esteva and Kuprel began to train the system. They didnt program it with rules; they didnt teach it about ABCD. Instead, they fed the images, and their diagnostic classifications, to the neural network. I asked Thrun to describe what such a network did.Imagine an old-fashioned program to identify a dog, he said. A software engineer would write a thousand if-then-else statements: [i]if[/i] it has ears, and a snout, [i]and[/i] has hair, and [i]is not[/i] a rat . . . and so forth, ad infinitum. But thats not how a child learns to identify a dog, of course. At first, she learns by seeing dogs and being told that they are dogs. She makes mistakes, and corrects herself. She thinks that a wolf is a dogbut is told that it belongs to an altogether different category. And so she shifts her understanding bit by bit: this is dog, that is wolf. The machine-learning algorithm, like the child, pulls information from a training set that has been classified. Heres a dog, and heres not a dog. It then extracts features from one set versus another. And, by testing itself against hundreds and thousands of classified images, it begins to create its own way to recognize a dogagain, the way a child does. It just knows [i]how[/i] to do it.
In June, 2015, Thruns team began to test what the machine had learned from the master set of images by presenting it with a validation set: some fourteen thousand images that had been diagnosed by dermatologists (although not necessarily by biopsy). Could the system correctly classify the images into three diagnostic categoriesbenign lesions, malignant lesions, and non-cancerous growths? The system got the answer right seventy-two per cent of the time. (The actual output of the algorithm is not yes or no but a probability that a given lesion belongs to a category of interest.) Two board-certified dermatologists who were tested alongside did worse: they got the answer correct sixty-six per cent of the time.
Thrun, Esteva, and Kuprel then widened the study to include twenty-five dermatologists, and this time they used a gold-standard test set of roughly two thousand biopsy-proven images. In almost every test, the machine was more sensitive than doctors: it was less likely to miss a melanoma. It was also more specific: it was less likely to call something a melanoma when it wasnt. In every test, the network outperformed expert dermatologists, the team concluded, in a report published in [i]Nature[/i].
about radiology:
Geoffrey Hinton, a computer scientist at the University of Toronto, speaks less gently about the role that learning machines will play in clinical medicine. Hintonthe great-great-grandson of George Boole, whose Boolean algebra is a keystone of digital computinghas sometimes been called the father of deep learning; its a topic hes worked on since the mid-nineteen-seventies, and many of his students have become principal architects of the field today.
I think that if you work as a radiologist you are like Wile E. Coyote in the cartoon, Hinton told me. Youre already over the edge of the cliff, but you havent yet looked down. Theres no ground underneath. Deep-learning systems for breast and heart imaging have already been developed commercially. Its just completely obvious that in five years deep learning is going to do better than radiologists, he went on. It [i]might[/i] be ten years. I said this at a hospital. It did not go down too well.