In this retrospective study we included patients who underwent CCTA examinations on a third-generation dual-source computed tomography system,
using the so-called step-and-shoot technique.
352 consecutive patients fulfilled these criteria (96 patients with bolus tracking in the left atrium and 256 patients with bolus tracking in the ascending aorta); (Fig. 1).
Propensity score matching,
as described by Baek et al.[2],
was used to select pairs of patients with similar characteristics out of the initial pool of 352 patients.
Matching criteria were sex,
height,
bodyweight and heart rate (Fig. 2 ,Table 1).
Based on propensity score matching results,
96 pairs of patients were selected and included in this study (122 male,
70 female,
mean age 61 years; 96 patients with bolus tracking in the left atrium and 96 patients with bolus tracking in the ascending aorta).
All patients received an intravenous injection of 50ml Iopromide with a flow rate of 5ml/s followed by 50ml saline at the same flow rate using a dual head injector.
The bolus tracking technique with a circular region of interest (ROI),
placed either in the left atrium (Fig. 1 - A) or in the ascending aorta (Fig. 1 - B),
was used to trigger the start of the image acquisition.
A threshold of 120 Hounsfield units (HU) in the respective region of interest was used; the delay time after reaching the threshold was 5 seconds.
Qualitative analysis:
Overall coronary image quality was assessed independently by two radiologists with six and ten years of experience with CCTA.
They were blinded to the position of bolus tracking.
A five point Likert scale with the following scores was used: 5 = perfect; 4 = very good; 3 = good; 2 = poor; 1 = insufficient.
In case of disagreement,
the total score was decided in consensus.
Image quality scores were compared using the Wilcoxon test.
The significance level was set at α = 0,05.
Interrater reliability:
Interrater agreement was evaluated using Cohen´s kappa (κ),
which was interpreted according to the recommendations of Landis and Koch[3] (with κ ≤ 0 being poor agreement; 0.01−0.20 slight agreement; 0.21−0.40 fair agreement; 0.41−0.60 moderate agreement; 0.61−0.80 substantial agreement and 0.81−1.00 almost perfect agreement).
Quantitative analysis:
Signal-to-noise ratio (SNR) was determined in the ascending aorta (AA),
left main coronary artery (LM) and proximal right coronary artery (RCA); (Fig. 3).
It was defined as the quotient of the mean signal intensity and the standard deviation of signal intensity.
Fig. 3: Measurement of SNR using ROIs (after completion of the computed tomography angiography scan). A: ROI in the proximal right coronary artery (RCA); B: ROIs in the ascending aorta (AA) and the left main coronary artery (LM).
SNR was compared using the t-test for paired samples.
Because of multiple comparisons,
the so-called Bonferroni correction was used to set the significance level.
In this case there are three comparisons,
therefore the significance level was set at α = (0,05 / 3) = 0,0167.
Radiation exposure:
Effective dose (in mSv) was calculated by multiplying the dose-length product provided by the scanner with a conversion factor of 18 µSv/mGycm as recommended by Huda et al.[4] Radiation exposure was compared using the t-test for paired samples.