KEYWORDS: Computed tomography, Magnetic resonance imaging, 3D image processing, Deep learning, Image guided radiation therapy, Deep convolutional neural networks
CT image synthesis from MR images is necessary for MR-only treatment planning, MRI-based quality assurance (QA), and treatment assessment in radiation therapy (RT). For pediatric cancer patients, reducing ionizing radiation from CT scans is preferred for which MRI-based RT planning and assessment are truly beneficial. Recently, deep learning-based synthetic CT (sCT) generation have demonstrated promising results on adult data. Generally, it is challenging to develop a pediatric sCT generation model due to significant anatomical variability and relatively smaller number of available pediatric data compared to adult. In this study, we investigated a 3D conditional generative adversarial network (cGAN)-based transfer learning approach for accurate pediatric sCT generation. Our model was first trained using adult data with augmentation by scaling to simulate pediatric data, followed by fine-tuning on pediatric data. We compared three different training scenarios; (1) training on 50 adult patient data with scaling augmentation, (2) training on combined 50 adult and 50 pediatric patient data, and (3) fine-tuning on 50 pediatric data using the pre-trained model on 50 adult data. 3D cGAN with transfer learning showed significantly better synthesis performance than the other models with average mean absolute error (MAE), peak signal-to-noise ratio (PSNR), and structural similarity (SSIM) index of 51.99 HU, 24.74, and 0.80, respectively. The proposed 3D cGAN-based transfer learning was able to accurately synthesize pediatric CT images from MRI, allowing us to realize pediatric MR-only RT planning, QA, and treatment assessment.
Radiation therapy (RT) planning for pediatric brain cancer is a challenging task. Manual contouring of organs-at-risk (OARs) is particularly difficult due to the small size of brain structures, time-consuming, and shows inter-observer variability. Furthermore, RT plans are typically optimized using CT, thus exposing patients to additional ionizing radiation. MR-only RT planning has recently been actively explored due to its potential to overcome these challenges. While numerous methods have been proposed to solve MR to CT image synthesis or OAR segmentation separately, there exist only a handful of methods tackling both problems jointly, even less specifically developed for pediatric brain cancer RT. We propose a multi-task convolutional neural network to jointly synthesize CT from MRI and segment OARs (eyes, optic nerves, optic chiasm, temporal lobes, hippocampi, and brainstem) for pediatric brain RT planning. The proposed network consists of a modified 3D U-Net architecture with a common encoder for both the synthesis and segmentation tasks combined with two task-specific decoders. The proposed model was trained, validated, and tested on 50, 5, and 15 pediatric brain RT cases, respectively, and achieved a mean±SD structural similarity index of 0.82±0.03 between the synthetic CT and ground truth CT, and dice score for the autosegmentation of 0.92±0.03 (eyes), 0.78±0.06 (optic nerves), 0.690.13 (optic chiasm), 0.91±0.02 (temporal lobes), 0.75±0.09 (hippocampi), and 0.91±0.06 (brainstem) compared to the expert’s manual segmentation. Our proposed multi-task joint synthesis and segmentation network achieves state-of-the-art performance for both tasks for MR-only RT planning.
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