Contrast differentiates blood from soft tissue. The goal is to preferentially opacify the left heart and coronary arteries while avoiding streak from a too-bright right heart. High injection rates of 5–7 cc/sec through at least an 18 G IV in the right arm are required for maximum attenuation.
1 Scan timing techniques
- Test bolus. A small initial injection (≈ 20 cc) paired with a single-slice fluoroscopy scan explicitly measures the patient’s circulation time before the primary scan.
- Bolus tracking. A region-of-interest tracker monitors a target (usually the ascending aorta); once contrast brightness crosses a threshold (typically 100–175 HU), the actual scan triggers automatically.
- Saline chaser. Following the contrast bolus with a saline chaser (biphasic or triphasic protocol) improves arterial enhancement, maintains contrast density, and limits streak artifact in the right heart.