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Overview: Spectral computed tomography (CT) based on photon-counting detectors, has great potential in material decomposition, tissue characterization, lesion detection, and other applications. During the reconstruction, the increase of the number of channels will reduce the photon number in a single channel, resulting in the decline of the quality of the reconstructed image, which is difficult to meet the actual needs. To improve the quality of image reconstruction, this paper proposes a joint multi-channel total generalized variational based on the unclear norm for spectral CT reconstruction. Firstly, in the reconstruction for spectral CT, the image structure of each channel is highly similar, and the reconstruction of a single channel will ignore the structural information of each channel. Second, gradient information contains a lot of structured information and features of the image. When two images have the same curve, the two images have the same direction gradient and the converse is also true. In order to better utilize the image's structural information between channels, the new regularization function is applied to spectral CT reconstruction. The research shows that if the edges of the two images are aligned, the two images have the same gradient. The image gradients between channels are parallel, which will minimize the nuclear norm. The algorithm will extend total generalized variation to the vector, with the aim of overcoming defects of existing derivative-based regularization. The paper proposed a joint multi-channel total generalized variational for spectral CT reconstruction, employing a vectorial second-order total generalized variation function as joint regularization. The method adopts pixel-by-pixel updating in the image reconstruction, and the multi-channel image coupling is realized by kernel norm and F-norm constraints at the level of first and second derivatives. The nuclear norm and frobenius norm coupling promote joint sparsity of the edge sets and dependence of the gradients. Joint multi-channel total generalized variational is used to promote the linear dependence of the multi-channel image's gradient so that the image edges of each channel are aligned. The structural information of the multi-channel image is shared during the image reconstruction process while unique differences are preserved. The experiment was done on a numerical mouse thorax phantom and clinical mouse data. The quantitative results of peak signal to noise ratio (PSNR), normalized root mean square error (NRMSE) and structure similarity index (SSIM) show that the proposed algorithm greatly improves the image quality. Experimental results show that the proposed algorithm can effectively recover image details and marginal information while suppressing noise.
The reconstruction results of the mouse model. From left to right, the rows are FBP, SART, TV, TDL, TGV and Mutli-NTGV. From top to bottom, the columns are 1st, 2th, 3th and 4th energy channels
Quantitative evaluation index of reconstruction effect of the mouse model. From left to right, the rows are NRMSE, PSNR, SSIM. From (a) to (d), the columns are 1st, 2th, 3th and 4th energy channels
Reconstruction results of the clinical mouse model. From left to right, the methods are FBP, SART, TV, TDL, TGV, and Mutli-NTGV. From top to bottom, the columns are the 1st, 3th, 5th and 7th energy channels