Study on the effect of three-low technique in aortic computed tomography angiography for patients with body mass index <30 kg/m²: a randomized controlled prospective trial

Introduction

Computed tomography (CT) angiography (CTA) can be used for diagnoses, follow-ups, and preoperative preparations of most aortic diseases (1-3). Given the large scanning range of aortic CTA (ACTA) and the need for regular patient follow-up, issues such as radiation dose and contrast agent-induced acute kidney injury have gained widespread attention. Low-dose CTA, which has been demonstrated to reduce radiation damage, represents a key strategy for minimizing radiation dose (4,5).

During the development of CTA, different methods such as increasing pitch, decreasing tube voltage and current, and iterative reconstruction have been proposed for reducing radiation dose (6,7). Decreasing tube voltage remains the most generally employed method to reduce the radiation dose. A study has indicated (8) that the severity of the contrast agent-induced acute kidney injury is dose-dependent. Conversely, the tube voltage can be decreased to be closer to the K-edge of iodine, which in turn enhances the photoelectric effect. The contrast ratio of blood vessels becomes more pronounced as the intrinsic dose attenuation of the iodinated contrast agent increases. Hence, it is possible to minimize both the radiation dose and iodine load by appropriately decreasing the tube voltage, while not compromising the quality of the image (9,10).

Some recent studies have further supported the feasibility and effectiveness of such a low-dose approach. A study on photon-counting detector CT (PCD-CT) demonstrated that low kV imaging can be used flexibly to reduce radiation and contrast agent doses significantly while maintaining excellent contrast-to-noise ratio (CNR) in thoracoabdominal angiography, even in anthropomorphic models (11). The iterative model reconstruction (IMR), a next-generation iterative algorithm, has been proven particularly effective in enabling up to a 75% reduction in radiation dose in patients undergoing pulmonary CTA while maintaining 100% sensitivity to an effective dose (ED) of approximately 0.9 mSv for pulmonary embolism detection, outperforming earlier algorithms such as iDose and filtered back projection (12).

This growing body of evidence further underscores the importance of ongoing refinement of low-dose strategies combining kV reduction, advanced reconstruction algorithms, and optimized contrast protocols to achieve substantial dose savings in vascular CT applications. Building upon this foundation, this study aims to specifically investigate the impact of the three-low technique such as low tube voltage, low contrast agent injection rate, and low contrast agent volume on image quality in ACTA compared to a conventional scanning technique. We present this article in accordance with the CONSORT reporting checklist (available at https://qims.amegroups.com/article/view/10.21037/qims-2025-203/rc).

Methods

Study population

Eighty patients developing chest pain, who underwent CTA examination with a Philips iCT 256-slice scanner in Nantong Third People’s Hospital from August 2021 to November 2023, were selected as subjects in this study, and were randomly assigned into the three-low group (N=40) and the control group (N=40) using simple randomization method according to a random number table for the use of different examination techniques (Table 1). The following patients were included: (I) patients with increased chest pain; (II) those whose body mass index (BMI) was less than 30 kg/m²; (III) those with clear consciousness (who were able to hold their breath). The following patients were excluded: (I) patients with renal damage (creatinine >200 µmol/L); (II) those with iodine contrast allergy; (III) those with hyperthyroidism; (IV) those who were pregnant or preparing for pregnancy. See details about the patient selection in Figure 1. The study was conducted in accordance with the Declaration of Helsinki and its subsequent amendments. The study was approved by the ethics committee of Nantong Third People’s Hospital, Affiliated Nantong Hospital 3 of Nantong University, Nantong, Jiangsu, China (No. EK2022036). All participants had signed an informed consent form.

Figure 1 A flow diagram of patient selection. BMI, body mass index; ACTA, aortic computed tomography angiography.

Examination techniques

All patients underwent CTA examination at an iCT 256-slice scanner (Philips, Best, the Netherlands). The patient was injected with 320 mgI/mL Ioversol (Hengrui Medicine, Lianyungang City, China) via the median cubital vein using a high-pressure syringe. Different examination techniques were used in the two groups: three-low group: tube voltage: 100 kV, contrast agent injection rate: 3 mL/s, contrast agent volume: 50 mL; control group: tube voltage: 120 kV, contrast agent injection rate: 5 mL/s, contrast agent volume: 80 mL. Thereafter, the patients in both groups received intravenous administration of 40 mL of 0.9% normal saline at the same injection rate for rinsing. Scanning range: from the thoracic inlet to the pubic symphysis. Other scanning parameters for both groups: pitch: 0.992; rotation speed: 0.5 s; slice thickness: 5 mm; slice spacing: 2.5 mm. Parameters for aortic lumen localization in CT: 120 kV and 10 mA. After the injection of the contrast agent, the scanning was performed using smart trigger technology, and the descending aorta was set as the region of interest (ROI), the schematic diagram of ROI placement is shown in Figure 2, and the trigger threshold was 150 HU. The above-mentioned scanning parameters are shown in Table 2. All cross-sectional images were reconstructed using the iDose4 iterative reconstruction algorithm. The study data were saved in a cloud server. The multiplanar reconstruction (MPR), maximum intensity projection (MIP), and volume representation (VR) were applied to perform objective and subjective assessments of image quality.

Figure 2 Schematic diagram of ROI placement. ROI, region of interest.

Image analysis and processing

A double-blind method was applied to perform subjective assessment of image quality, and the images were scored by two independent radiologists with more than 10 years of experience. In case of disagreement, a radiologist with 20 years of experience was consulted for further assessment. A five-point Likert scale was used for image scoring, and the aortic images were clearly displayed, with smooth and sharp blood vessel edges and no artefacts; 4 points: the aortic edges were slightly blurred, with fewer artifacts; 3 points: aortic edges were moderately blurred, with mild artefacts, which did not affect the diagnosis; 2 points: the aorta was poorly displayed, with more artefacts, which affected the quality of the images; 1 point: the aorta was poorly displayed, with severe artefacts that could not be evaluated.

In this study, data measurement and evaluation were conducted using the IntelliSpace Portal post-processing software from Philips. The objective assessment of image quality was conducted as follows: a circular ROI was delineated in the thoracic aortic vessels at the 5th thoracic vertebra level, and CT values of the aortic vessels were measured. We placed a maximum ROI while ensuring avoidance of the blood vessel walls and calcified plaques. The CT values and standard deviation (SD) of bilateral erector spinae muscles were detected in the same slices, and their mean values were computed and expressed as mean ± SD, and the schematic diagram of ROI placement is shown in Figure 3. The signal-to-noise ratio (SNR) and CNR were determined respectively according to the following Eqs. [1] and [2] (13), wherein the image noise was expressed as SD_muscle:

SNR=CTaorta/SDmuscle[1]

CNR=(CTaorta−CTmuscle)/SDmuscle[2]

Figure 3 Schematic diagram of ROI placement. The green circle indicates the ROI placement for CT aorta and the white circle indicates the ROI placement for CT muscle and SD muscle. CT, computed tomography; ROI, region of interest; SD, standard deviation.

The value of CT_aorta was directly detected, and the values of SNR and CNR were calculated using the respective formula, all three values were obtained based on the data measured from the transverse section

Radiation dose: the CT machine automatically generated a dose-length product (DLP) record and calculated the ED of radiation according to the formula ED = DLP × k, where k is a conversion factor, and k=0.014 mSv/mGy·cm, which was determined according to the European Guidelines for CT Quality Standards.

Statistical analysis

SPSS 23.0 (IBM) software was applied for statistical analysis. Kolmogorov-Smirnov test was conducted to check the normality. The measurement data with a normal distribution were described as mean ± SD and subjected to statistical analysis using the independent samples t-test. The comparison of categorical data was conducted by Fisher’s exact test. Mann-Whitney U test was applied to compare subjective scores of image quality between the two groups. Kappa test was applied to analyze the consistency in assessing image quality between the two radiologists (excellent: κ≥0.81, good: κ=0.61–0.80, moderate: κ=0.41–0.60, fair: κ=0.21–0.40, poor: κ≤0.20).

Results

Comparative analysis of subjective scores of image quality between the three-low group and the control group

The subjective score of image quality was 4.2±0.9 in the three-low group and 4.4±0.8 in the control group, and the consistency between the two radiologists was good in the three-low group and excellent in the control group. This study found no statistically significant difference in subjective score of image quality between the two groups (P=0.871) and a good to excellent consistency in assessing image quality between the two radiologists (k=0.631–0.902) (Figures 4,5).

Figure 4 CTA images of a 60-year-old man with BMI =24 kg/m² and chest pain in the three-low group. (A-C) VR, MIP, and CPR images of the aorta, respectively, and all images were scored as 5 points. BMI, body mass index; CPR, curved planar reformation; CTA, computed tomography angiography; MIP, maximum intensity projection; VR, volume representation.

Figure 5 CTA images of a 70-year-old man with BMI =25.5 kg/m² and chest pain in the control group. (A-C) VR, MIP, and CPR images of the aorta, respectively, and all images were scored as 5 points. BMI, body mass index; CPR, curved planar reformation; CTA, computed tomography angiography; MIP, maximum intensity projection; VR, volume representation.

Comparative analysis of objective scores of image quality between the three-low group and the control group

Compared with the control group, the CT_aorta was increased by approximately 10%, the image noise was higher, whereas the SNR and CNR were lower in the three-low group, and there were statistically significant differences between two groups (all P<0.05) (Table 3).

Comparative analysis of radiation dose between the three-low group and the control group

Both DLP and ED in three-low group were decreased compared with those in the control group, the ED in the three-low group was reduced by about 76.2% compared with the control group, and there existed statistically significant differences in DLP and ED between the three-low group and the control group (P<0.05). See Table 4 for details.

Discussion

With the rising incidence of thoracoabdominal aortic lesions (aortic aneurysms and dissection) and the increased complexity of surgical treatment options, follow-up-imaging examination is critical for these patients. In patients with untreated thoracoabdominal aortic lesions, the variation in the size and morphology of the aorta should be closely monitored, which may indicate rupture or other complications (14,15). The patients after aortic repair should also undergo follow-up-imaging examinations to assess the complications and recurrent lesions. CTA is the preferred imaging method for most patients with thoracic and abdominal aortic lesions. The extents of thoracoabdominal aortic lesions and their potential complications involve multiple parts of the body and require thoracoabdominal imaging. Hence, it is imperative to investigate new approaches for decreasing the radiation dose and contrast agent volume administered to patients.

The three-low technique reduces radiation dose based on basic physics principles: lowering the tube voltage shifts the X-ray spectrum nearer the iodine k-edge at 33 keV, thereby increasing iodine attenuation and vessel enhancement per unit volume of contrast agent (16). This novel technique allows excellent vascular opacification with less iodine or radiation dose. In practice, reducing kV (e.g., 80–100 kV in non-obese patients) can markedly lower the radiation dose, and cause a proportional reduction in contrast agent volume. However, a lower kV can inherently increase the image noise. To counteract this issue, all cross-sectional images in this study were reconstructed using the iDose4 iterative reconstruction algorithm, which could effectively improve the image quality that is otherwise reduced by low kV and low contrast agent volume, so that the three-low technique causes no unacceptable noise. The efficacy and safety of the three-low technique have been supported by multiple preliminary studies in similar vascular imaging contexts. In previous studies, 100 kV CTA protocol and reduced contrast agent volume have shown promising results in non-obese patients with acceptable image quality and substantial dose reduction (5,11).

In this study, the patients in the three-low group underwent CTA examination with a tube voltage of 100 kV, a contrast agent volume of 50 mL, and an injection rate of 3 mL/s. Compared with the control group, the CT _aorta in the three-low group was greater, with an increase of 42 HU, meanwhile the contrast agent volume in the three-low group was decreased by 33%. However, the image noise in the three-low group was increased, and there was no statistically significant difference between the three-low group and the control group. Shi et al. (17) used a 100 kV tube voltage and 45 mL of iodinated contrast agent in head and neck angiography, which caused an increase in image noise and no statistically significant difference in SNR and CNR compared with the control group (120 kV, 65 mL iodinated contrast agent), not only ensuring the image quality, but also resulting in a significant decrease in iodinated contrast agent volume and radiation dose in the patients at the same time. Other studies (18-21) have also reported the same findings. This study demonstrated that the image noise in the three-low group was increased compared with the control group, while the CNR and SNR in the three-low group were decreased compared with the control group. In this study, the automatic tube current modulation was used to compensate for decreased CNR and SNR caused by reducing the tube voltage, and the radiation dose was significantly decreased (76.2%) in the three-low group (2.46±0.71 mSv) in comparison with the control group (10.32±0.46 mSv).

In clinical practice, lowering the contrast agent volume and injection rate has shown clear benefits for patients. High contrast agent dose is a well-known modifiable risk factor for contrast agent-induced nephropathy or acute kidney injury, especially in at-risk patients (22). By reducing iodine dose, this protocol aligns with guidelines that “reasonable attempts to minimize contrast agent volume” should be made, thereby potentially reducing the incidence of contrast agent-induced nephropathy (22). Meanwhile, low injection rate or smaller boluses may reduce the risk of the occurrence of contrast extravasation. A recent study confirmed that high injection rate (≥3 mL/s) and larger boluses are independent risk factors for contrast extravasation (23). Through careful patient selection and monitoring, this study only enrolled the patients with BMI <30 kg/m², who were most likely to benefit from low-kV imaging and least likely to experience diagnostic compromise. All patients were monitored during and after imaging, and additional imaging was available if diagnostic quality was insufficient, though this was not ultimately performed. No adverse events or compromised diagnoses occurred, which further confirmed the safety of this protocol. Lowering the contrast agent volume and injection rate has shown clear-cut benefits for patients. Therefore, both the low contrast agent injection rate and low contrast agent volume can mitigate the risk for contrast extravasation.

This study found no statistically significant difference in the subjective score of image quality between the three-low group and the control group. Luo et al. (6,18) have also obtained the same result. In addition, this study showed a 100% success rate for CTA examination in both groups, and no patients had decreased image quality due to the use of the three-low technique. The use of an injection rate of 3 mL/s in the three-low group also reduced the risk of contrast agent extravasation in patients, which was consistent with the conclusion of Shi et al. (17). Although the three-low group showed increased image noise and decreased SNR and CNR compared with the control group, the diagnostic value of the images was not compromised. This study confirmed that the three-low technique can meet the clinical diagnostic requirements.

There are some limitations in this study. It only enrolled patients with a BMI of less than 30 kg/m², and this protocol with a three-low technique needs to be used in all patients for better comparisons in further studies (24). It must be determined whether this protocol with three-low technique in patients with a BMI of greater than or equal to 30 kg/m² can meet the diagnostic requirements for image quality in further studies. The contrast agent volume was not calculated individually in this study, and it is required to determine the most appropriate contrast agent volume based on the body size of the patient in further studies.

Conclusions

Under the premise of ensuring image quality, the CTA examination protocol with 100 kV tube voltage, 3 mL/s injection rate, and 50 mL of contrast agent is feasible in patients with a BMI of less than 30 kg/m², and the ED of radiation can be reduced by 76.2%. The findings of this study suggest that three-low technique in ACTA is effective and worthy of promotion in clinical patients with BMI <30 kg/m². However, the generalizability of these results to other populations, such as patients with BMI ≥30 kg/m², remains to be explored.

Acknowledgments

We would like to thank Hanzhen Ji from the library of Nantong Third People’s Hospital for data collection, paper editing and polishing, and submission assistance.

Footnote

Reporting Checklist: The authors have completed the CONSORT reporting checklist. Available at https://qims.amegroups.com/article/view/10.21037/qims-2025-203/rc

Trial Protocol: Available at https://qims.amegroups.com/article/view/10.21037/qims-2025-203/tp

Data Sharing Statement: Available at https://qims.amegroups.com/article/view/10.21037/qims-2025-203/dss

Funding: This study is supported by the Research Project of Nantong Municipal Health Commission (No. MS2022068).

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://qims.amegroups.com/article/view/10.21037/qims-2025-203/coif). The authors have no conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. The study was conducted in accordance with the Declaration of Helsinki and its subsequent amendments. The study was approved by the ethics committee of Nantong Third People’s Hospital, Affiliated Nantong Hospital 3 of Nantong University, Nantong, Jiangsu, China (No. EK2022036). All participants had signed a written informed consent form.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

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