What is the radiation dose of prospective ECG triggering and sequential scanning for coronary computed tomography angiography (CCTA)?

Updated: Dec 21, 2017
  • Author: Eugene C Lin, MD; Chief Editor: Eugene C Lin, MD  more...
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Coronary CT angiography is usually performed with retrospective ECG gating, where scanning occurs throughout the cardiac cycle and simultaneously acquired ECG data are used retrospectively during image reconstruction. The acquisition of data throughout the cardiac cycle increases radiation dose. In addition, the scan is performed helically with a low pitch, resulting in substantial tissue overlap during scanning, as well as increasing radiation dose. [39]

With prospective ECG triggering, the data are acquired at a specific point in the R-R interval. The scanner acquires data sequentially ("step and shoot") rather than in helical mode. Radiation dose is decreased, as data are not acquired throughout the cardiac cycle, and there is minimal tissue overlap with a sequential scan technique. This technique is standard for coronary artery calcium scoring but can be used to reduce radiation dose substantially during CCTA. Using a prospectively triggered sequential scan technique, Earls et al achieved an 83% reduction in dose as compared to the retrospective gated technique. [3]

The primary disadvantage of this technique is the lack of functional data. In addition, as data are only available from predefined phases of the R-R interval, reconstructions from additional phases to improve image quality are not possible.

This technique is optimal in patients with a low and stable heart rate. [54, 55] High heart rates are not optimal for this technique, as reconstructions at multiple phases during the R-R cycle are sometimes needed. With irregular heart rates, the acquisitions may be triggered at different points in the R-R interval.

Prospective ECG triggering is optimal for 256- or 320-slice CT, where the entire heart could potentially be scanned in one tube rotation and one heartbeat. This obviates the issue of phase misregistration in patients with irregular heart rates. In one study, the median effective radiation dose of 320-slice CT was 4.2 mSv, [56] which was lower than an 8.5 mSv median dose from catheter angiography performed in the same patients. Prospective ECG triggering is also well suited for use with dual-source CT, as the increased temporal resolution may allow the technique to be used at a higher heart rate threshold. [54]

In a meta-analysis of 20 studies [57] in patients with coronary artery disease (CAD) and without tachyarrhythmia, prospectively triggered CCTA provided image quality and diagnostic accuracy comparable to retrospectively gated CTA, but at a much lower radiation dose (3.5 mSv average compared with 12.3 mSv).

SCCT guidelines [53] state that prospective ECG triggering should be used in patients who have stable sinus rhythm and low heart rates (typically < 60-65 beats per minute). The width of the data acquisition window should be kept to a minimum. Retrospective gating is recommended for patients who do not qualify for prospective scanning due to irregular rhythms or high heart rates.

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