Application of double low-dose mode in left atrial-pulmonary venous computed tomography angiography

This study adopted a 256-slice iCT scanner with the double low-dose mode in left atrial-pulmonary venous computed tomography angiography (CTA) and explored its effect on image quality. 120 patients were included and randomly classified into the Observation group and Control group. Patients in the Control group underwent routine left atrial CTA, while patients in the Observation group performed a double low-dose mode. Other scanning parameters were consistent in the two groups. The Full model-based iterative reconstruction (MBIR) technique was applied to fulfill image reconstruction in observation group. Continuous variables, ordered categorical variables were analyzed by statistical test. The CT values of left atrial in the Observation group were significantly higher than those in the Control group. The exposure doses (ED) and iodine intake were lower in the Observation group, as compared to the Control group. The left atrial-pulmonary venous CTA with the 256-slice iCT scanner in a double low-dose mode can reduce the ED of radiation and iodine contrast while providing high quality images. Comparatively, the ED in the Observation group was reduced by 13% compared with the control, and the iodine intake was reduced by approximately 33%.


Examination methods
All participants were informed of the procedure by an experienced physician before examination, and proper respiratory training was given to make the patients relaxed.CT scans were acquired on a Philips Brilliance 256slice iCT scanner and spital scanning was performed.The scan ranges from tip to bottom of lung.Nonionic contrast agent iohexol (350 mg/mL) was injected through the right median cubital vein using a double-syringe power injector at a rate of 5 mL/s.The Observation group received a dose of 0.7 ml/kg, while the Control group received a dose of 1 mL/kg.Consistently, 30 ml normal saline was injected in both groups with the same rate after iohexol injection.Scanning parameters were set as below: tube voltage: 100 kV for the Observation group and 120 kV for the Control group; detector width: 128 × 0.625; pitch: 0.27; layer thickness: 0.9 mm; layer spacing: 0.45 mm; rotation time: 330 ms.The tube current was auto-regulated.Contrast track-and-trigger technique was applied to depict the regions of interest (ROI) in the LA regions, with the trigger threshold set as 100 HU.The images were processed for image reconstruction with the MBIR technique and then transferred onto a Philips post-processing workstation.The reconstructed images were transferred onto a Philips workstation for post-processing.All images were assessed by two senior radiologists with respect to the LA (8 and 12 years of cardio-thoracic radiology experience, respectively), LAA, and PVs using the volume rendering (VR) and multiple planar reformation (MPR) approaches.Any discrepancy was resolved by discussion.
The subjective criteria for assessing image quality adopted a 5-point scoring system 4 : 1-point: low image quality, unclear anatomical structure, severe artifacts, not acceptable for diagnosis; 2-point: low image quality, unclear anatomical structure, and moderate artifacts; 3-point: moderate image quality, moderate artifacts, clear anatomical structure in the majority, acceptable for diagnosis; 4-point: good image quality, clear anatomical structure, minor artifacts; 5-point: high image quality, no artifact, well-defined anatomical structure.Images scored > 3 points were acceptable for clinical diagnosis.
The objective assessment of image quality was performed as below.The LA, the opening of the PV trunk, the LAA cavity, and the pectoralis major were selected as ROIs from the images of cross-sections, and corresponding enhanced CT values were obtained.ROI areas were confined to 3-8 cm 2 , and the ROIs were draw away from calcification, vessel wall, and the columnar crest in LAA.Image noise was calculated as the standard deviation (SD) of the enhanced CT value of ascending aortic root.Image noise was assessed by signal-noise ratio (SNR) and contrast-to-noise ratio (CNR) with the following formula: SNR = average vascular CT value/SD, and CNR = (vascular CT value − muscle CT value)/vascular SD.
Radiation doses, volume CT dose index (CTD), and dose length product (DLP) were automatically generated by the CT scanner and recorded.Effective radiation dose (ED) was calculated as: ED = DLP × k, where k represents the conversion factor, which is 0.017 mSv/(mGy cm) in chest CTA.Additionally, iodine intake from the contrast was measured as: I (g) = contrast concentration (350 mg/ml) × contrast dose (ml)/1000.

Statistical analysis
SPSS 22 was applied for statistical analysis.All data were initially processed for normality test.The continuous variables meeting normal distribution were analyzed by an independent-sample t test; otherwise, Mann-Whitney U test was used.Dichotomous categories and their proportions were tested by a Chi-square test or a Fisher exact probability test.Intraclass correlation coefficient (ICC) was measured to assess the consistency between the subjective and objective results.P < 0.05 indicated a difference with statistical significance.

Ethical approval
All procedures performed in studies involving human participants were in accordance with the ethical standards of the institutional and/or national research committee and with the 1964 Helsinki declaration and its later amendments or comparable ethical standards.This article does not contain any studies with animals performed by any of the authors.

Subjective assessment
The subjective scores in both the Observation and Control groups were non-normally distributed, and the mean score (95% CI) was 4 (3.72-4.26)and 4.24 (3.96-4.51),respectively.There was no significant difference between the two groups (Mann-Whitney U test: Z = − 1.303, p = 0.193).See Table 1 for more details.

Objective assessment
The enhanced CT values of ROIs, including the opening of the PV trunk, mid LA, and LAA cavity, were significantly higher in the Observation group than the Control group (p < 0.05, Table 2).Besides, the SNR and CNR in the Observation group were slightly lower than those in the Control group, and the differences were not statistically significant (Table 3).The image quality of both groups can meet the requirements of clinical diagnosis (Figs. 2, 3).

Radiation dose and iodine intake
The ED was (5.5 ± 1.1) mSv in the Observation group and (6.3 ± 1.2) mSv in the Control group, while the iodine intake was (14.3 ± 2.1) g in the Observation group and (21.1 ± 3.1) g in the Control group.Both the differences were of statistical significance (p < 0.05).Comparatively, the ED in the Observation group was reduced by 13% compared with the control, and the iodine intake was reduced by approximately 33%.

Discussion
This study shows that the image quality obtained by low voltage, low contrast agent and MBIR technique can meet the diagnostic requirements.The method can effectively reduce the radiation dose and renal function changes caused by contrast medium.This double low-dose mode technique is useful in medical practice.AF is the most common atrial arrhythmia in clinic 8 .It is usually accompanied by reduction in left ventricular systolic function, leading to blood stasis in the LAA and a high susceptibility to cardiogenic emboli formation.In the meantime, dislodging of the emboli can lead to occlusion of the systemic circulation.According to statistics, approximately 60% of the cardiogenic strokes are derived from AF 9,10 .Pulmonary vein ablation is generally applied for the treatment of FA cases that are hard to be managed by drugs, and LAA occlusion is a viable option to prevent the risk of developing stroke induced by dislodging of emboli 1 .An accurate knowledge of the anatomic and pathological conditions of the LA, PVs, and LAA before operation is of paramount clinical significance to guide procedures, shorten the operation time, monitor postoperative complications, and perform a dynamic follow-up.
Depending on the high temporal and spatial resolutions of MDCT, it has been increasingly applied in clinic, especially in the cardiovascular screening with multiple indications 3,5,11 .MDCT can depict the opening width of  the PVs and LAA, and depict the thrombi in the LA system.In addition, it can also provide the three-dimensional locations of the PVs and LAA relative to the esophagus.Therefore, MDCT has promising application prospect in preoperative examination of AF patients 7 .With the extensive application of the CTA technique, the ionizing radiation and iodine contrast have attracted increasing attention for their potential risk of harm to the patients 12,13 .Glands and organs that are sensitive (partially sensitive) to X-irradiation, such as the thyroid and gonads, have an increasing risk of developing cancer when exposed to high-dose ionizing radiation on CT scans.Except the ionizing radiation, the iodine contrast used by CTA also greatly affects the renal function, probably resulting in renal dysfunction 14 .In this context, there is an urgent need to look for new scan strategies that can be effective with a low dose of radiation and iodine contrast while maintaining the image quality 15 .To this end, double low-dose scan technique arises 16,17 .
In the present study, left atrial-pulmonary venous CTA was fulfilled in 120 patients.Both the Observation and Control groups obtained satisfactory CTA images, which clearly showed the relative positions of the LA, PVs, LAA, and the LA system to the esophagus, and the image quality was acceptable for clinical diagnosis.The CT values of the ROIs were significantly higher in the Observation group than the Control group based on the objective assessment, but no statistical differences were noted by subjective assessment.The result demonstrated that the double low-dose scan strategy does not affect the image quality (within the acceptable range for clinical diagnosis) while reducing the radiation and contrast doses.
Reduction in tube voltage can enhance the photoelectric effect of X-ray and increase the X-ray attenuation associated with the iodine-containing tissues, resulting in higher CT value of iodine contrast and enhanced contrast of the atria and PVs with the surrounding tissues 12,14 .Consistently, our study found significant higher CT values in the Observation groups than the Control group.Nevertheless, the reduction in tube voltage simultaneously decreases the X-ray's penetrating power, leading to increased image noise and decreased image quality 18,19 .The MBIR technique provides an optimized statistical model composed of multiple factors, including X-ray beam's width, detector pixel size, and voxel size.It can provide accurate image information, and reduce the noise caused by reduction in tube voltage.With this technique, the decrease in CT value following low-dose contrast administration can be improved to some extent, which makes the double low-dose technique theoretically feasible 20 .Moreover, this study found lower SNR and CNR in the Observation group, which might be due to the increase in image noise caused by low voltage [21][22][23] .However, there were no statistical differences between the two groups.This indicates that MDCT with the MBIR technique can decrease the image noise from double low-dose scans and thus increase the image quality.There are limitations to the study.First, the sample size is small, and high-BMI patients common in clinic are not included in this study.The scan strategy for some individuals requires further exploration.Second, the dose of contrast used in this study adopts a fixed dosing schema and is not individualized by BMI.Thirdly, AI has been widely used to improve the image quality and does reduction nowadays 16,24 , it will be performed and compared in the future studies.Lastly, the lack of individual-specific approaches for different patients were not taken in this study, which may have affected the results, these approaches will be performed in clinical settings in the following research.
To conclude, left atrial-pulmonary venous CTA with the double low-dose technique can reduce the exposure dose of radiation and iodine contrast while maintaining the image quality within an acceptable range for clinical diagnosis, showing a safety profile.

Figure 1 .
Figure 1.The scheme of study design.

Figure 2 .
Figure 2. Images of 65 years old male in observation group.BMI = 23.6,tube voltage was 100 kV, iodine contrast agent dose was 17.5 g.VR and MPR images showed that the left atrium, pulmonary vein and left atrial appendage were well depicted, and the image quality score was 4 points.

Figure 3 .
Figure 3. Images of 52 years old female in control group.BMI = 24.9,tube voltage was 120 kV, iodine contrast agent dose was 20 g, VR and MPR images showed that left atrium, pulmonary vein, left atrial appendage, etc. were clearly developed, and the image quality score was 5 points.

Table 1 .
Baseline characteristics of patients.

Table 2 .
The measured CT value in the observation and control groups.

Table 3 .
Objective scoring in the observation and control groups.