  Transfer learning is a widely used strategy in medical image analysis.
Instead of only training a network with a limited amount of data from the
target task of interest, we can first train the network with other, potentially
larger source datasets, creating a more robust model. The source datasets do
not have to be related to the target task. For a classification task in lung CT
images, we could use both head CT images, or images of cats, as the source.
While head CT images appear more similar to lung CT images, the number and
diversity of cat images might lead to a better model overall. In this survey we
review a number of papers that have performed similar comparisons. Although the
answer to which strategy is best seems to be "it depends", we discuss a number
of research directions we need to take as a community, to gain more
understanding of this topic.
