关键词: 医学图像质量评价, 结构相似度（SSIM）, 梯度方向, 人眼视觉系统

Abstract: Image objective quality assessment has been widely used in image processing to maintain and improve the quality of images being processed.Many researchers have studied the objective quality assessment method based on Human Visual System（HVS）.Recently，the well-known image quality assessment structure similarity（SSIM） is proposed by Zhou Wang，under the assumption that the HVS is highly adapted for extracting structural information from a scene.The simulation results have proved that it is better than PSNR（or MSE）.But，it fails in measuring blurred images.In this paper，improved method is developed which is called as image quality assessment based on gradient direction（GDSSIM）.Experiment results show that GDSSIM gets the best performance for blurred images.Furthermore，GDSSIM has a better performance than SSIM for Fast Fading images.These algorithms are applied on medical image assessment.

Key words: medical image quality assessment, structural similarity（SSIM）, gradient direction, Human Visual System（HVS）