Combining neural networks and image synthesis to enable automatic thoracic cavity segmentation of hyperpolarized Xe MRI without proton scans.

RATIONALE AND OBJECTIVES

Quantification of Xe MRI relies on accurate segmentation of the thoracic cavity, typically performed manually using a combination of H and Xe scans. This can be accelerated by using Convolutional Neural Networks (CNNs) that segment only the Xe scan. However, this task is complicated by peripheral ventilation defects, which requires training CNNs with large, diverse datasets. Here, we accelerate the creation of training data by synthesizing Xe images with a variety of defects. We use this to train a 3D model to provide thoracic cavity segmentation from Xe ventilation MRI alone.

MATERIALS AND METHODS

Training and testing data consisted of 22 and 33 3D Xe ventilation images. Training data were expanded to 484 using Template-based augmentation while an additional 298 images were synthesized using the Pix2Pix model. This data was used to train both a 2D U-net and 3D V-net-based segmentation model using a combination of Dice-Focal and Anatomical Constraint loss functions. Segmentation performance was compared using Dice coefficients calculated over the entire lung and within ventilation defects.

RESULTS

Performance of both U-net and 3D segmentation was improved by including synthetic training data. The 3D models performed significantly better than U-net, and the 3D model trained with synthetic Xe images exhibited the highest overall Dice score of 0.929. Moreover, addition of synthetic training data improved the Dice score in ventilation defect regions from 0.545 to 0.588 for U-net and 0.739 to 0.765 for the 3D model.

CONCLUSION

It is feasible to obtain high-quality segmentations from Xe scan alone using 3D models trained with additional synthetic images.