In this study, we evaluated the feasibility of a using low-contrast dose protocol in CCTA assisted by DLIR algorithm. Our results proved that the 24% contrast reduction protocol (0.5 mL/kg) could achieve similar images to the standard contrast protocol. Moreover, the low contrast dose protocol we proposed in this study could be applied to patients with both average HR and high HR.
With the advancement of technology, acquisition time of CCTA has become shorter, and the injection time of contrast medium is also shortened, so the contrast medium dose injection should also be reduced. As a result, there is a clinical trend to reduce the amount of contrast medium[8–10]. However, how to balance image quality and contrast dose is difficult and is the focus of many studies. Our results showed that good image quality could still be obtained when the contrast dose was reduced by 24%, which can greatly reduce the risk of contrast-related diseases such as CIN.
We have noticed that significant efforts have been made to reduce the CM for CCTA. Andreini et al. [20] tried to reduce the CM to 50 mL at 400 mgI/mL for normal-size patients (BMI < 24.9 kg/m2) and for over-weighted patients (BMI ≥ 25 kg/m2), In our study, we personalized the CM injection protocol for patients with an extensive range of body size. Notably, in our study, if the CM was 50mL, the patient's BMI would be 25kg/m2, which would be injected 60mL in the study of Andreini et al. Thus, the CM dosage in our study was much lower than the study of Andreini et al. Furthermore, Wang et al. [14] have tried lower CM protocol to 35mL with 70kVp for normal patients (BMI ≤ 26 kg/m2) and 40mL with 80kVp for over-weight patients (BMI > 26 kg/m2). However, despite using the 100kVp in our study, our study still used less contrast dose in terms of mg-iodine since the contrast medium in Wang’s study had a concentration of 400 mgI/mL, while the one used in our study was 320 mgI/mL. Let us assume an overweight patient who was 1.8m and 90kg (BMI = 27.78kg/m2), then he would require 16g iodine (40mL×400mgI/mL) as described in the study of Wang et al. but only 14.4g (90kg×0.5mL/kg×320mgI/mL) iodine in our study. Furthermore, let us assume a normal patient who was 1.7m and 65kg (BMI = 22.49kg/m2), then he would use 14g (35mL× 400mgI/mL) iodine in the study of Wang et al. and 10.4g iodine (65kg×0.5mL/kg× 320 mgI/mL) in our study. We believe we could further reduce the CM dose if we use lower tube voltages in the future. Moreover, we noticed that the difference in vascular attenuation between the two groups was greater than 100HU in the study of Wang et al. In contrast, our contrast protocol with controlled injection time could ensure the consistency of vascular attenuation values at different CM doses. Besides, we assessed the objective scores for both the major vessels and the myocardium compared with the studies of Wang et al. and Li et al. [10, 14].
One of the interesting or even controversy phenomena we observed in our study was that similar degrees of enhancement were obtained for coronary vessels and myocardium among the three imaging groups, even though same tube voltage was used and two of the groups used contrast volumes as much as 24% less. On the other hand, the same conclusion could not be made for vessels such as pulmonary artery and superior vena cava. As it was shown in our results that CT values of PA, SVC, LA, LV, RA, RV and CV in Group A were higher than the other two low-CM groups (all p < 0.001, Fig. 4). One of the explanations could be that a large amount of contrast was stuck in these vessels at the time of CCTA acquisition, and the additional contrast volume in Group A did not contribute to the imaging in CCTA and the overbrightness of the superior vena cava may also affect our diagnosis of coronary images. Therefore, the 24% contrast reduction protocol was still sufficient to provide adequate enhancement in coronary arteries.
Furthermore, we have conducted subgroups analysis based on HR and we proved that the low contrast dose protocol we proposed in this study could be applied to patients with both average HR and high HR. High HR has been criticized for degraded image quality in CCTA examination, which has raised a significant concern[21–24] in recent years. Compared to patients with regular HR, CCTA images of patients with high heart rates suffer from motion artifacts due to their uneven ratio of systolic to diastolic phases and their excessive motion velocities that exceed the temporal resolution of the CT scanners. In our study, all patients were examined on a new 256-row, 16-cm-wide detector CT scanner with a rotational speed of 280 ms for examinations within a single heartbeat prospective ECG-triggered scan protocol, which took good advantage of the detector's wide coverage and fast rotation speed to significantly reduce the adverse effects of high HR on CCTA image quality. Additionally, this study used the SmartPhase technique to automatically select the optimal reconstruction phase with the SSF2 motion correction algorithm, which could significantly reduce motion artifacts in CCTA images of patients with high HR. Moreover, we have assessed the CT value of the myocardium and our results showed that the myocardium could also be well enhanced with the low CM scanning protocols. CT values of the major vessels and myocardium were consistent in patients with normal HR and high HR.
DLIR is an artificial intelligence reconstruction technique, which integrates high quality FBP images and extracts more key features of the same image. This algorithm can solve the problem that the use of FBP or high intensity IR in low tube voltage CT scans may be either too noisy or affecting the image texture[25, 26]. Previous studies[9, 10, 14–17] have shown that DLIR has more potential to improve image quality and reduce image noise at the same radiation dose and tube voltage. Consistent with previous studies, our study also demonstrated that the excellent diagnostic images of CCTA were acquired with DLIR-H algorithm.
This study has several limitations. First, the sample size of the study was limited. Second, a conventional tube voltage of 100 kV was used in this study, not the lower tube voltages which could further reduce the CM dose. However, we believe the use of 100 kV should not change the basic conclusion that contrast dose could be optimized in CCTA. Third, we did not apply the free-breathing technique in our study, but the use of breath-holding was intended to minimize motion artifacts, and future studies will attempt to allow patients to breathe freely for CCTA examinations. Finally, this contrast protocol has not been validated on other CT scanners. Further study could be established on various CT scanners, using our low-CM protocol as a reference.
In conclusion, contrast medium dose may be reduced by 24% to 0.5 mL/kg (at concentration of 320 mgI/mL) in CCTA to maintain adequate enhancement in coronary arteries. reduction protocol assisted with DLIR could achieve good CCTA image quality in patients with an extensive range of heart rates.