In recent years, the chaos based cryptographic algorithms have suggested some new and efficient ways to develop secure image encryption techniques. In this paper, we propose a novel method to encrypt a gray image based on Arnold transform and Kent mapping. It is a digital medical image encryption algorithm, dual scrambling that based on chaos of pixel position and pixel values. The results of experimental and the statistical analysis show that the histogram of the encryped image is fairly uniform and significantly different from the histogram of the original image. The algorithm improves the traditional image encryption algorithm of image scrambling based on chaos, and it is valid and has good performance.