Prospective Multicenter Registry On RadiaTion Dose Estimates Of Cardiac CT AngIOgraphy IN Daily Practice in 2017

Cardiac CT angiography (CCTA) has emerged to a powerful and commonly used diagnostic tool for the evaluation of coronary artery disease. Advantages of CCTA are its non-invasive approach, widespread availability and high negative predictive value to rule out coronary artery disease. However, radiation exposure during CCTA needs to be considered, due to the potential risk of malignancy induction.

A decade ago, the multicentre observational PROTECTION I study has revealed that the dose-length-product of CCTA ranges between 568-1259 mGy x cm with a median of 885 mGy x cm. This corresponds to an estimated effective dose of approximately 12 mSv. Furthermore, PROTECTION I demonstrated an up to 7-fold variation of radiation dose between different study sites implicating the large potential for dose reduction. Finally, significant differences have been detected in the radiation exposures between CT systems from different vendors and application of dose-saving algorithms, implying that approaches for dose reduction with respect to hardware and software have also been of relevance in 2007.

Since then a variety of techniques have been developed and enhanced in order to reduce radiation exposure during CCTA. Earlier, ECG-controlled tube current modulation has been accepted to reduce radiation dose and was therefore used in ECG-gated retrospective cardiac spiral scanning.

Another dose-saving technique is the reduction of tube potential from conventionally 120 kVp to 100 kVp in non-obese patients. In the PROTECTION II trial we demonstrated that diagnostic image quality is being maintained with a reduction of tube potential to 100 kVp while radiation exposure was significantly reduced by 31%.

Development of prospective ECG-gated axial (sequential) scanning allowed further dose reduction, due to administration of radiation only at the end of diastole. In the PROTECTION III trial we could demonstrate that diagnostic image quality is being maintained with prospective ECG-gated axial scanning in patients with stable and low heart rates while radiation exposure was reduced by 69% when compared to conventional spiral scanning with retrospective ECG gating. Using prospectively ECG-triggered high-pitch spiral CCTA with image acquisition during only one cardiac cycle, additional dosage may be saved when used in selected patients with low and stable heart rates. However, a scan strategy with an attempt of high-pitch scanning first is vulnerable to motion artefacts and bears the risk of repeated scanning.

In the PROTECTION IV trial we compared a scan strategy of high-pitch helical scanning first with a conventional scan first strategy. We could demonstrate maintenance of image quality using the strategy of high-pitch helical scanning first, while radiation exposure was reduced by 57 %, when compared to the conventional scan first strategy.

The introduction of iterative image reconstruction technique with advanced raw data processing let to improvement of image quality compared to traditional filtered back projection. This allowed additional dose saving during image acquisition. In the PROTECTION V trial we demonstrated the feasibility of combining iterative image reconstruction techniques with a reduction of tube currents by 30% leading to maintenance of diagnostic image quality while saving radiation dose linearly to tube current.

In summary, the four randomized clinical trials PROTECTION II-V demonstrated that different techniques for reducing radiation exposure during CCTA can be applied in clinical practice without compromising diagnostic image quality. The studies show that the different approaches can also be applied in combination and that these approaches can be used with different CT platforms from different CT vendors.

Recent studies demonstrated that the effective radiation dose for one CCTA scan may be reduced to 0,3 mSv15 or even below 0,1 mSv in a small amount of carefully selected patients. However, it remains unclear if such dramatically low dosages are being achieved and if diagnostic image quality is being maintained in clinical routine. In order to analyze the magnitude of radiation dose exposure during CCTA in today's clinical practice, we designed the international and multicentric PROTECTION VI study. We will assess the variation of radiation dose with respect to vendors, CT systems and study sites. Furthermore, we will evaluate the usage of dose saving strategies including the above-mentioned in daily practice and analyze their effect on diagnostic image quality. On the basis of the collected data, we will evaluate the potential use of additional dose savings for each site individually.