Radiation dose comparison between prospectively ECG triggered and retrospectively ECG gated techniques of coronary computed tomography angiography on 256 slice dual source CT scanner 256 kesitli cift tuplu bilgisayarli tomografi cihazinda prospektif EKG tetiklemeli ve retrospektif EKG kapilamali teknikle cekilen koroner BT anjiografi incelemelerinde radyasyon dozu karsilastirmasi