PURPOSE: Authors studied the injection method of contrast medium in routine chest spiral CT scan to obtain the best image in the mediastinum. MATERIALS AND METHODS: Dynamic static scan had been performed in 5 normal volunteers as a pilot study. In consideration of the result of pilot study, Chest spiral CT was performed in 217 patients by three different methods. We used 100cc nonionic contrast medium. Average attenuation of great vessels in the mediastinum were assessed in various injection methods. Image quality was graded with three levels of score by two radiologists. RESULTS: Peak enhancement time of the great vessels on pilot at dynamic static scan were as follows :52 sec at ascending aorta, 45 sec at pulmonary artery, and 40 sec at SVC. In the study of spiral CT, the highest attenMarion in the great vessels was obtained after injection of 100cc of contrast medium(2cc/sec, with 35-40 second scan delay), althrough artifact from highly enhancing SVC was most common in this method. Image quality were highest in the scans obtained with other methods(3-2-1cc/sec for 10-15-40 seconds, with 40 second scan delay). CONCLUSION: For chest spiral CT, authors recommend that scans should be obtained after infusion of 70--80cc of contrast medium during 35--40sec to obtain maximal vascular enhacement of mediastinum.