Low-radiation and high image quality coronary computed tomography angiography in “real-world” unselected patients

ARTICLE HIGHLIGHTS

Research background

Traditionally, coronary angiography has been the gold standard in diagnosing coronary artery disease (CAD). Coronary computed tomography angiography (CCTA), however, is increasingly being used as a rapid and less invasive alternative in diagnosing patients at risk of CAD. A 3D image of the heart and coronary circulation can be rendered with a CT scanner using only an intravenous injection of iodine-rich contrast, thus circumventing the need for insertion of a catheter via an artery of vein. However, imaging coronary arteries presents increased challenges, since it requires both a high temporal resolution to reduce motion artifacts caused by cardiac motion and a high spatial resolution to differentiate small coronary structures. While images obtained with CCTA scanners are now comparable to coronary angiography these CT requirements have previously resulted in higher radiation doses thus increasing the lifetime risk of radiation-induced malignancy. Advances in CCTA scanner hardware and image reconstruction techniques have led to reports of exceptionally low radiation doses, down to 0.06 mSv, while maintaining diagnostic image quality of the coronary arteries. For example, in prospective electrocardiogram-gated acquisition the X-ray tube is switched on only for a reduced percentage of the cardiac cycle rather than the whole cycle. Automatic exposure control uses the lowest possible CT tube current and voltage adjusted to the patient’s body habitus. This works in synergy with novel image IR algorithms that adaptively apply noise correction to offset an increase in image noise caused by a reduced tube voltage. Using pharmacological methods to reduce heart rate, with oral or intravenous beta-blockers, has proven to reduce cardiac motion during the acquisition leading to improved images. An initial scout view of the thorax can also be used to minimize the volume covered per-patient in a single complete acquisition to further reduce the radiation. However, these previous studies were conducted on small cohorts that were pre-selected for low body weight and heart rate, and were limited by a low prevalence of CAD. The feasibility and effect of these low-dose scan modes on both image quality and radiation exposure in a large patient population with various heart rates is currently unknown. In this study, we determined the radiation dose and subjective image quality using a combination of state-of-the-art CCTA acquisition protocols at our institution in consecutive unselected patients undergoing CCTA for suspected coronary disease.

Research motivation

Ultra-low radiation doses of less than one mSv have been reported in other feasibility studies. While these advances in cardiac CT may effectively lower radiation dose, these studies are limited to small cohorts of pre-selected patients with very low and regular heart rates and low body habitus and are thus not representative of the typical population undergoing screening for CAD. This study aims to determine the feasibility of these low-dose CCTA acquisition protocols adopted at our institution in an unselected cohort from a series of consecutive patients who underwent CCTA for suspected CAD. We hope that the outcome may demonstrate that CCTA is a viable, non-invasive alternative to coronary catheterization for screening low-risk populations with suspected CAD.

Research objectives

The primary end points of the study were effective radiation dose and image quality in patients not selected in term of heart rate and body habitus undergoing routine CCTA. Our objective was to demonstrate that low radiation doses were feasible for the majority of real-world patients undergoing routine screening for CAD with CCTA without losing diagnostic image quality.

Research methods

The radiation dose and subjective image quality were analysed over a total of 543 consecutive patients in sinus rhythm who underwent CCTA at our institute for suspected CAD between June 2012 and August 2016. Subjective image quality was assessed by the two trained observers and scored on a four-point scale (4 = excellent, 1 = non-diagnostic). Images were acquired with a 320-row detector CT scanner (Aquilion One, Toshiba Medical Systems, Japan) and a number of integrated packages that have been developed to reduce the radiation dose to as low as reasonably achievable. Prospective electrocardiogram (ECG)-gated acquisition was implemented using the SURE Cardio Prospective Package over an acquisition window of 70%–80% of the interval between two consecutive QRS complexes in ppatients with a heart rate below 65 bpm. The Sure Exposure 3D package (SUREexposure, Toshiba Medical Systems, Japan) automatically adjusted to the lowest possible tube current and voltage in accordance with each patient’s attenuation profile while noise reduction during each acquisition was implemented with the three-dimensional adaptive iterative dose reduction (AIDR-3D) image reconstruction algorithm. If necessary, patients were prepared prior to the scan with rate-limiting oral beta-blockers followed by additional intravenous beta-blocker to achieve a heart rate below 65 bpm unless contraindicated. During the scan, the volume of coverage was reduced to minimal size whilst allowing complete acquisition in a single volume. Contrary to previous studies in which the patients were prospectively selected, the patients were not selected based on age, heart rate, and body mass index. We believe that this is a better representation of real world patients who would be undergoing routine CCTA for diagnosis of CAD.

Research results

The median effective radiation dose was 0.88 mSv, which includes what we believe to be the lowest ever-reported radiation dose for a routine clinical CCTA (0.18 mSv). The mean image quality (± SD) was 3.65 ± 0.61, with a subjective image quality score of 3 (“good”) or above for 93% of patient CCTAs. CAD was confirmed by CCTA in 57 (10%) of patients.

Research conclusions

The median effective radiation dose was 0.88 mSv (IQR, 0.6-1.4 mSv) with a mean subjective image quality score (± SD) of 3.65 ± 0.61 averaged over 500 real-world unselected patients undergoing routine clinical CCTA. This ultra-low radiation dose for CCTA is comparable to the radiation range reported for a chest X-ray in two views. The data also includes what we believe to be the lowest ever-reported radiation dose for a routine clinical CCTA 0.18 mSv with a subjective image quality score of 4 (“excellent”). This demonstrates that low radiation dose CCTA can be used as a routine clinical screening tool for CAD without loss of diagnostic image quality. To date, radiation dose reduction advances in CCTA technology have only been reported in feasibility studies on small cohorts of highly selected patients with low body habitus and heart rate. This study demonstrates that low radiation CCTA with good image quality is possible for most patients undergoing routine screening for CAD with CCTA using a combination of commercially available, state-of-the-art cardiac CT technology advances. CCTA is rapid and non-invasive compared with coronary angiography and has reduced patient recovery time. The reduced risk in radiation-induced malignancy implies that CCTA is a feasible alternative to coronary angiography as a primary screening tool for patients with low risk CAD.

Research perspectives

This study did not include patients with atrial fibrillation or other cardiac CT indications such as evaluation of coronary bypass grafts, evaluation of left atrium anatomy prior to atrial fibrillation ablation, pre-operative assessment for trans-catheter aortic valve replacement or assessment of cardiac function. Feasibility studies with alternative dose-saving strategies have also recorded ultra-low mean effective radiation doses ranging from 0.06 mSv to 0.3 mSv with clinically acceptable diagnostic images. These include techniques such as prospective ECG-triggered high-pitch spiral acquisition but again were limited to carefully selected patients. Extending these techniques to unselected patients could highlight the need for alternative protocols for undertaking routine CCTA for assessment of different patient groups or to incorporate existing technology at other institutions.