  Self-supervised learning (SSL) enables label efficient training for machine
learning models. This is essential for domains such as medical imaging, where
labels are costly and time-consuming to curate. However, the most effective
supervised or SSL strategy for transferring models to different healthcare
systems or novel tasks is not well understood. In this work, we systematically
experiment with a variety of supervised and self-supervised pretraining
strategies using multimodal datasets of medical images (chest X-rays) and text
(radiology reports). We then evaluate their performance on data from two
external institutions with diverse sets of tasks. In addition, we experiment
with different transfer learning strategies to effectively adapt these
pretrained models to new tasks and healthcare systems. Our empirical results
suggest that multimodal SSL gives substantial gains over unimodal SSL in
performance across new healthcare systems and tasks, comparable to models
pretrained with full supervision. We demonstrate additional performance gains
with models further adapted to the new dataset and task, using multimodal
domain-adaptive pretraining (DAPT), linear probing then finetuning (LP-FT), and
both methods combined. We offer suggestions for alternative models to use in
scenarios where not all of these additions are feasible. Our results provide
guidance for improving the generalization of medical image interpretation
models to new healthcare systems and novel tasks.
