Generating Patient-like Phantoms Using Fully Unsupervised Deformable Image Registration with Convolutional Neural Networks

The use of Convolutional neural networks (ConvNets) in medical imaging research has become widespread in recent years. However, a major drawback of these methods is that they require a large number of annotated training images. Data augmentation has been proposed to alleviate this. One data augmentation strategy is to apply random deformation to existing image data, but the deformed images often will not follow exhibit realistic shape or intensity patterns. In this paper, we present a novel, ConvNet based image registration method for creating patient-like digital phantoms from the existing computerized phantoms. Unlike existing learning-based registration techniques, for which the performance predominantly depends on the domain-specific training images, the proposed method is fully unsupervised, meaning that it optimizes an objective function independently of training data for a given image pair. While classical methods registration also do not require training data, they work in lower-dimensional parameter space; the proposed approach operates directly in the high-dimensional parameter space without any training beforehand. In this paper, we show that the resulting deformed phantom competently matches the anatomy model of a real human while providing the "gold-standard" for the anatomies. Combined with simulation programs, the generated phantoms could potentially serve as a data augmentation tool in today's deep learning studies.