We examined in our study whether the inclusion of CAS-CT into a low-dose, prospectively ECG-triggered CCTA protocol results in a whole-examination radiation dose reduction compared to an alternative approach, where prospectively ECG-triggered CCTA is planned on scout-view images and no CAS-CT is acquired. The essential findings of our study can be summarized as follows: (I) consistently planning the scan length of CCTA on axial non-contrast CAS-CT images results in a shorter scan length than planning on a scout-view. (II) The whole-examination radiation dose of prospective ECG-triggered CCTA planned on CAS-CT is slightly lower than CCTA planned on scout-view, even if no CAS-CT is acquired in the latter scenario. (III) CAS-CT planning of prospective ECG-triggered CCTA results in a radiation dose salvage compared to scout-view planning when individual factors (e.g., high BMI), or technical factors (e.g., high tube voltage, sequential scan with extended ECG-padding) contributing to a higher overall radiation dose are present. (IV) Omitting CAS-CT to save radiation dose of cCTA is not a reasonable strategy because the potential to reduce CCTA scan length and, consequently, the radiation dose cannot be exploited. (V) In that regard, the acquisition of low-dose high-pitch spiral cCTA constitutes an exception.
4.1 Scan length
Our results show that planning CCTA on CAS-CT images results in a shorter scan length than planning on scout-view images (133.7 ± 17.8 mm vs. 114.3 ± 9.7 mm; p < 0.001). These results are confirmatory to a previous study conducted by Leschka et al. using the same CCTA planning approach (their reported result: 139 ± 13 mm for cCTA using scout-view planning vs. 117 ± 9 mm for CCTA using CAS-CT planning (17).
4.2 Radiation dose
We observe a minimal overall radiation dose salvage in prospective ECG-triggered CCTA when the scan-length planning is performed on CAS-CT images compared to scout-view planning of CCTA (mean effective dose reduction, 0.2 mSv, 3.1%). Leschka et al. observed a mean radiation dose reduction of 1.0 mSv, 16% [15]. The differences can be explained as, in their study, only retrospective ECG-synchronized scan protocols were investigated. Retrospective ECG-gating is associated with high radiation exposure, and adjustment of the scan length seems very effective.
In our study, the median overall dose length product of 174 mGy*cm for CCTA is comparable to current data of the PROTECTION VI registry (reported median: 195 mGy*cm) [16]. Subgroup analysis of the specific prospective ECG-triggered acquisition scan mode indicates a crucial finding regarding radiation dose safety: In all high-pitch spiral scans, scout-view planning of CCTA with the omission of CAS-CT would have resulted in a 15.4% radiation dose saving. However, when using sequential scan mode, particularly when ECG-padding exceeds 20% of the R-R interval, the overall radiation dose salvage using CCTA planning on CAS-CT was 10.4% (0.5 mSv).
4.3 Role of Calcium Scoring CT
In addition to being a planning tool, non-contrast CAS-CT itself adds to the benefit of additional diagnostic and prognostic value. In a prospective study on 13.644 individuals and a median follow-up of 9.4 years, it was demonstrated that the use of statins in patients suffering from hyperlipoproteinemia only led to a reduction in serious adverse cardiovascular events if coronary calcification was present [19]. It has also been observed that knowledge of the calcium score increases patients' compliance with statin medication [20]. Therefore, case-by-case, it must be considered whether the omission of CAS-CT seems reasonable for radiation safety reasons. Our study's effective dose of the calcium scoring scan was 0.33 mSv (IQR, 0.25–0.37 mSv), thus slightly lower than in previous comparable studies [21]. One explanation could be that we always acquired the CAS-CT using a high-pitch spiral scan, while in other studies, a sequential scan is obtained if the heart rate exceeds 80 bpm. It should be noted that with the emergence of 3rd generation DSCT and tin filtration, the acquisition of a high-pitch, low-voltage CAS-CT is feasible and results in much lower radiation exposure (0.13 mSv) than in our study [22]. It can be assumed that under such conditions, adjustment of the cCTA z-axis extension on CAS-CT images would result in an even lower whole-examination effective radiation dose.