PST-Radiomics: a PET/CT lymphoma classification method based on pseudo spatial-temporal radiomic features and structured atrous recurrent convolutional neural network.

Existing radiomic methods tend to treat each isolated tumor as an inseparable whole, when extracting intratumoral radiomic features. However, they may discard the critical intra-tumor metabolic heterogeneity (ITMH) information, that contributes to triggering tumor subtypes. To resolve this problem and improve tumor classification performance, we propose a pseudo spatial-temporal radiomic method (PST-Radiomics) based on positron emission tomography computed tomography (PET/CT). Specifically, to enable exploitation of ITMH, we first present a multi-threshold gross tumor volume sequence (GTVS). Next, we extract 1D radiomic features based on PET images and each volume in GTVS to create a pseudo spatial-temporal feature sequence (PSTFS) tightly interwoven with ITMH. Then, we reshape them to create 2D pseudo spatialtemporal feature maps (PSTFM), of which the columns are elements of PSTFS. Finally, to learn from PSTFM in an end-to-end manner, we build a light-weighted pseudo spatial-temporal radiomic network (PSTR-Net), in which a structured atrous recurrent convolutional neural network serves as a PET branch to better exploit the strong local dependencies in PSTFM, and a residual convolutional neural network is used as a CT branch to exploit conventional radiomic features extracted from CT volumes. We validate PST-Radiomics on PET/CT volumes with various lymphoma subtypes. Experimental results quantitatively demonstrate the superiority of PST-Radiomics, when compared to the existing radiomics. Feature map visualization of our method shows that it performs complex feature selection and hierarchical feature map extraction, qualitatively demonstrating the superiority of PST-Radiomics.