Advanced techniques in cardiac CT imaging – further lowering the effective radiation dose (H. Alkadhi)

Cardiac computed tomography (CT) represents a reliable and accurate modality for the diagnosis and exclusion of coronary artery disease. A huge number of studies have investigated the diagnostic performance of CT coronary angiography among different generations of CT scanners. Nevertheless, radiation doses still remain a matter of concern.

Step-and-Shoot coronary angiography
Various strategies have been developed to reduce radiation exposure. The most important one is the prospectively electrocardiography (ECG)-gated CT coronary angiography, also called step-and-shoot (SAS) mode. The X-ray exposure time of this technique is short, and thus, is associated with low radiation doses ranging between 1.2 to 4.3 mSv  [1]. Most importantly, the SAS method is accurate [2] with a similar performance as the retrospectively ECG-gated spiral cardiac CT protocols.

The high-pitch-mode – the entire heart in a single heart cycle
With the recent advent of the second generation dual-source CT scanner equipped with two 128-slice acquisition detectors (Somatom Definition Flash; Siemens Healthcare, Forchheim, Germany), another low-dose technique, i.e., the high-pitch mode was introduced for cardiac imaging. With this mode, data acquisition is also prospectively triggered with the ECG of the patient. However, the data now is acquired in a spiral mode while the table runs with a very high pitch of 3.4, equaling a table feed of 43 cm/s. Using this high-pitch mode, the entire heart can be scanned within one single cardiac cycle, usually during diastole.

Conclusions
First reports on phantoms, animals, and humans have shown the ability of this scan mode to deliver images of diagnostic quality at a low radiation dose [3, 4, 5]. Most importantly, a recent study has shown that this high-pitch technique of the Flash-Scanner is highly accurate [6]. A prerequisite of this technique is a regular heart rate equal to or below 60–63 beats per minute. Effective radiation dose of a CT coronary angiography study employing the high-pitch mode is on average below 1 mSv. Thus, the high-pitch mode, as compared to the SAS mode, further lowers down the effective radiation doses while maintaining the high image quality and excellent performance characteristics.

1. Stolzmann et al., Radiology 2008;249:71
2. Scheffel et al., Heart 2008;94:1132-7), -80
3. Lell et al., Eur Radiol 2009;19:2576-83;
4. Hausleiter et al., JCCT 2009;3:236-242;
5. Achenbach et al., JCCT 2009;3:117-121
6. Leschka et al., Eur Radiol 2009; Sep 16. (Epub ahead of print)