Background Artificial neural networks have shown promising performance in automatic segmentation of cardiac magnetic resonance imaging. However, initial training of such networks requires large amounts of annotated data and generalization to different vendors, field strengths, sequence parameters, and pathologies is often limited. Transfer learning has been proposed to address this challenge, but specific recommendations on the type and amount of data required is lacking. In this study we aim to assess data requirements for transfer learning to cardiac 7T in humans where the segmentation task can be challenging. In addition, we provide guidelines, tools, and annotated data to enable transfer learning approaches of other researchers and clinicians.
Methods A publicly available model for bi-ventricular segmentation is used to annotate a publicly available data set. This labelled data set is subsequently used to train a neural network for segmentation of left ventricular and myocardial contours in cardiac cine MRI. The network is used as starting point for transfer learning to the segmentation task on 7T cine data of healthy volunteers (n=22, 7873 images). Structured and random data subsets of different sizes were used to systematically assess data requirements for successful transfer learning.
Results Inconsistencies in the publically available data set were corrected, labels created, and a neural network trained. On 7T cardiac cine images the initial model achieved DICELV=0.835 and DICEMY=0.670. Transfer learning using 7T cine data and ImageNet weight initialization significantly (p<10-3) improved model performance to DICELV=0.900 and DICEMY=0.791. Using only end-systolic and end-diastolic images reduced training data by 90%, with no negative impact on segmentation performance (DICELV=0.908, DICEMY=0.805).
Conclusions This work demonstrates the benefits of transfer learning for cardiac cine image segmentation on a quantitative basis. We also make data, models and code publicly available, while providing practical guidelines for researchers planning transfer learning projects in cardiac MRI.