With the increasingly varied applications of deep learning, transfer learning has emerged as a critically important technique. However, the central question of how much feature reuse in transfer is the source of benefit remains unanswered. In this paper, we present an in-depth analysis of the effects of transfer, focusing on medical imaging, which is a particularly intriguing setting. Here, transfer learning is extremely popular, but data differences between pretraining and finetuing are considerable, reiterating the question of what is transferred. With experiments on two large scale medical imaging datasets, and CIFAR-10, we find transfer has almost negligible effects on performance, but significantly helps convergence speed. However, in all of these settings, convergence without transfer can be sped up dramatically by using only mean and variance statistics of the pretrained weights. Visualizing the lower layer filters shows that models trained from random initialization do not learn Gabor filters on medical images. We use CCA (canonical correlation analysis) to study the learned representations of the different models, finding that pretrained models are surprisingly similar to random initialization at higher layers. This similarity is evidenced both through model learning dynamics and a transfusion experiment, which explores the convergence speed using a subset of pretrained weights.

本文探讨了用自然图像数据集（ImageNet）通过标准大型模型和预训练权重进行迁移学习在深度学习医学成像应用中的方法。然而，此方法在自然图像分类和目标医学任务之间存在根本差异，并且人们对迁移效果的了解甚少。该研究表明，在两项大规模医学成像任务的性能评估中，迁移学习对性能的提升有限，而简单轻量级模型可以与ImageNet架构相当。此外，还探讨了迁移的权重尺度的独立特性，并阐述了对更高效的模型探索的意义。

医学影像迁移学习解析