基于改进U-net的自监督低剂量CT图像去噪算法研究

针对低剂量CT(low-dose CT,LDCT)图像去噪过程中配对数据难以获取的问题,本文提出了一种基于注意力机制和联合损失的自监督LDCT图像去噪算法。在该算法中,利用边缘增强后的U-net网络完成LDCT图像的特征提取,在网络框架中引入通道和像素注意力机制,以提高网络对噪声和伪影的抑制能力。同时使用联合损失避免传统损失对图像造成的图像过平滑问题,使得去噪后图像更加接近原图像。实验结果表明：所提出的算法可有效抑制LDCT图像的噪声,保留图像的纹理细节。经过算法处理后的LDCT图像的峰值信噪比(peak signal-to-noise ratio,PSNR)提高了16.40%,结构相似性(structural similarity,SSIM)提高了9.60%。在无配对数据下,该方法可有效保留细节并减少低剂量扫描产生的噪声,为临床LDCT图像去噪提供新思路。     

Effects of adjacent bubble on spatiotemporal evolutions of mechanical stresses surrounding bubbles oscillating in tissues

The cavitation dynamics and mechanical stress in viscoelastic tissues, as the primary mechanisms of some ultrasound therapies, are extremely complex due to the interactions of cavitation bubble with adjacent bubbles and surrounding tissues. Therefore, the cavitation dynamics and resultant mechanical stress of two-interacting bubbles in the viscoelastic tissues are numerically investigated, especially focusing on the effects of the adjacent bubble. The results demonstrate that the mechanical stress is highly dependent on the bubble dynamics. The compressive stress and tensile stress are generated at the stage of bubble expansion and collapse stage, respectively. Furthermore, within the initial parameters examined in this paper, the effects of the adjacent bubble will distinctly suppress the radial expansion of the small bubble and consequently lead its associated stresses to decrease. Owing to the superimposition of two stress fields, the mechanical stresses surrounding the small bubble in the direction of the neighboring bubble are smaller than those in other directions. For two interacting cavitation bubbles, the suppression effects of the nearby bubble on both the cavitation dynamics and the stresses surrounding the small bubble increase as the ultrasound amplitude and the initial radius of the large bubble increase, whereas they decrease with the inter-bubble distance increasing. Moreover, increasing the tissue viscoelasticity will reduce the suppression effects of the nearby bubble, except in instances where the compressive stress and tensile stress first increase and then decrease with the tissue elasticity and viscosity increasing respectively. This study can provide a further understanding of the mechanisms of cavitation-associated mechanical damage to the adjacent tissues or cells.