Abstract: Computed tomography angiography has become an important technology of diagnosing vessel diseases due to its non-invasiveness and acquisition speed.Getting images of optimal quality requires the main scanning is started once the bolus concentration is high enough.At the present time,the most commonly used method of detecting bolus is based on image reconstruction.In this paper,based on fan-beam projections,we have proposed a different method by which we can directly get and reflect the bolus dynamics.Comparing with the image-based method,the method based on fan beam projections have better real-time and lower detecting cost due to its needlessness of image reconstruction.