Recently, radiologists can easily screening with high accuracy based on CTA (Computed Tomography Angiography) images and/or MRI (Magnetic Resonance Imaging) which is supported by medical imaging technology. On the other hand, visual screening makes burden to doctors which cause increasing the images and limited times. To avoid these problems, CAD (Computer Aided Diagnosis or Detection) system is developed by reducing the burdens and to improve the diagnostic accuracy. Problems of diagnosis on legs from CTA are burdens to doctors and detection of blood vessel by manual. In the field for visual screening, it is difficult task to detect blood vessel regions with high accuracy detection rates since blood vessel regions are in contact with bone region on the CTA images. In this paper, we propose a new method for segmentation of arterial area, which are obtained CTA based on image processing by concentration gradients, and satisfactory experimental results are obtained. Some experimental results are shown with discussion.