European Heart Journal Advance Access originally published online on December 21, 2007
European Heart Journal 2008 29(2):153-154; doi:10.1093/eurheartj/ehm614
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Back to the future: coronary CT angiography using prospective ECG triggering
Imaging Institute and Heart and Vascular Institute, The Cleveland Clinic, 9500 Euclid Avenue, Cleveland OH 44195, USA
* Corresponding author. Tel: +1 216 445 7579. E-mail: schoenp1{at}ccf.org
This editorial refers to ‘Feasibility of low dose coronary CT angiography: first experience with prospective ECG gating’ by L. Husmann et al., on page 191
Footnotes
The opinions expressed in this article are not necessarily those of the Editors of the European Heart Journal or of the European Society of Cardiology.
doi:10.1093/eurheartj/ehm613 Medical imaging with computed tomography (CT) has evolved rapidly during the past few decades, now allowing routine non-invasive coronary angiography1 and also guidance of innovative endovascular procedures of the aorta, and recently the aortic valve.2–4
Initial CT systems, introduced in the 1970s for body imaging, rotated the X-ray tube and detector system (gantry) very slowly around the patient, spanning several heartbeats per rotation.5 After acquisition of a single axial slice, the tube was turned off, and the patient table incremented to the next slice position, where scanning was repeated. Despite gradual improvements in tube rotation time to fractions of the heart cycle, allowing triggering to specific cardiac phases (‘sequential or step-and-shoot acquisition with prospective triggering’), these single-slice systems were too slow to image mobile organs. Therefore, cardiac imaging became feasible only after implementation of two key technical advances, spiral scanning and multislice technology.
Hardware and software advances allowed fast subsecond data acquisition during continuous rotation of the gantry and continuous movement of the patient table. In the resulting spiral acquisition, data are acquired throughout the entire cardiac cycle during simultaneous recording of the ECG signal. Subsequently, data from specific periods of the cardiac cycle (most commonly late diastole) are reconstructed by retrospective referencing to the ECG signal (‘spiral or helical scanning with retrospective ECG gating’).6 Because data are acquired throughout the cardiac cycle, spiral imaging allows reconstruction from multiple cardiac phases into cine-loops, which is required for functional assessment. Although modern scanners reduce the tube current outside the selected phase (dose modulation), the continuous B-ray exposure during the entire cardiac cycle results in an increased patient radiation dose.7
The implementation of spiral imaging coincided with the introduction of multislice technology, allowing the acquisition of more than one image per gantry rotation. Current standard 64-slice scanners cover a few centimetres per rotation. Significant further increases in the number of slices will allow imaging of the entire heart in one rotation, therefore obviating the need to move the patient table. With such a scanner, data acquisition would be performed by selectively turning the X-ray tube on only in the selected phase, triggered by the ECG signal. This basically defines the original concept of ‘prospective triggering’.5 An advantage is the lower radiation dose, because X-ray exposure only occurs during the selected cardiac phase rather than throughout the entire cardiac cycle, unless functional imaging is required. Current multislice scanners can acquire the entire coronary tree with 3–5 gantry rotations, and therefore still require table movement. However, the combination of fast gantry rotation time and large number of slices appears to make sequential imaging with prospective triggering feasible for cardiac scanning with these systems.
Husmann et al.8 describe coronary angiography imaging using prospective ECG gating with a clinical 64-slice system in patients presenting with suspected or known coronary artery disease. In 40 patients a total of 160 vessels and 519 coronary artery segments with a diameter 
1.5 mm were evaluated. The mean radiation dose was very low (2.1 ± 0.6 mSV; range 1.1–3.0 mSV). Overall, non-diagnostic segments were found in 5% of segments and 23% of patients. Importantly, 46% of non-diagnostic segments were related to ‘stair step artefact’ at the junction of sequential image stacks. While such step artefacts and differences of contrast concentration are not specific to prospective triggering, they appear more prominent with this technique. Assessment of lesions at the level of these steps is expected to be a particular diagnostic problem. A technical solution will be data correction by slightly widening the acquisition window. In the study of Hussman et al., in order to demonstrate the minimal possible radiation dose, the authors have chosen the most narrow acquisition window, and therefore have probably compromised diagnostic accuracy. The results were also highly heart rate dependent. In patients with heart rates below and above a statistically determined cut-off (63 bpm), non-diagnostic segments were found in 1.1 and 14.8% of segments and in 7.1 and 58% of patients, respectively. It should be mentioned that this was a selected population with very low frequency of lesion calcification. The presence of calcified lesions would be expected to decrease diagnostic accuracy further.
The data demonstrate the technical feasibility of prospective triggering with significant reduction of radiation exposure. However, diagnostic performance is still limited in comparison with standard retrospective protocols, in particular if aggressive heart rate lowering is not achieved. Future studies will undoubtedly include comparison of diagnostic accuracy with retrospectively gated acquisitions and invasive angiography.
The study is an important contribution to the current scientific debate about cardiovascular CT. It is increasingly obvious that the desired optimal image quality has to be balanced against the potential long-term side effects of radiation exposure.9 The described acquisition mode and several other recent technical developments demonstrate that technical advances and reduced radiation dose are not mutually exclusive. Other novel approaches include further increases in the number of slices (256–320 slice scanner), faster gantry rotation times including dual-source technology, which employs two X-ray tubes to achieve a nearly two-fold improved temporal resolution (83 ms),10,11, dual-energy technology, aggressive dose modulation, more efficient detectors systems, and novel radiation dose shielding. These technical advances are matched by an emerging consensus about appropriate clinical indications, and reasonable utilization of various protocols to answer specific clinical questions.12,13 The choice of the acquisition mode and scanner is influenced by patient characteristics (e.g. age, heart rate, and body mass index) and clinical indication (need for ventricular or valvular functional assessment). The indication for CT and choice of the appropriate protocol should be based on scientific data and considering alternative imaging strategies.14
In summary, the justified concern about the increasing contribution of diagnostic CT scanning to radiation exposure15 is a challenge, but also provides a tremendous opportunity for innovators in the fields of engineering and medical imaging to develop novel scanner technology, and reach consensus about appropriate indications and scan protocols. This will ensure maximal benefit with minimal potential for adverse effects of this exciting technology, allowing innovations in coronary imaging and other emerging developments in cardiovascular medicine,3 and ultimately ensuring improved patient care.
Conflict of interest: none declared.
Footnotes
The opinions expressed in this article are not necessarily those of the Editors of the European Heart Journal or of the European Society of Cardiology.
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Related articles in EHJ:
- Feasibility of low-dose coronary CT angiography: first experience with prospective ECG-gating
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EHJ 2008 29: 191-197.[Abstract] [FREE Full Text]
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