Machine learning is an important tool for extracting information from medical images. Deep learning has made this more efficient by not requiring an explicit feature extraction step and in some cases detecting features that humans had not identified. The rapid advance of deep learning technologies continues to result in valuable tools. The most effective use of these tools will occur when developers also understand the properties of medical images and the clinical questions at hand. The performance metrics also are critical for guiding the training of an artificial intelligence and for assessing and comparing its tools.