1. Introduction
Cone-beam computed tomography (CBCT) is a valuable three-dimensional (3D) volumetric imaging technique widely utilized in neuroradiology and interventional oncology [
1,
2]. It offers significant advantages in terms of visualizing complex anatomical structures, guiding percutaneous punctures and catheterizations, verifying intracranial stent placement in real time, and promptly monitoring intracranial hemorrhages or infractions [
3,
4,
5,
6,
7,
8]. However, a notable drawback of this technique is the additional degree of radiation exposure, especially when multiple CBCT scans are performed [
9,
10,
11].
While previous studies have explored the role of CBCT in interventional procedures [
3,
6,
10], there has been limited research specifically focusing on the radiation dose associated with CBCT image acquisition. Some studies have employed phantoms to estimate the dose of CBCT, but the corresponding results have shown significant variations due to the different methodologies used for estimating the effective dose (ED) [
12,
13,
14,
15,
16,
17]. Direct measurement of ED may be affected by differences in phantom size or material. ED measured by Monte Carlo simulations may vary when different parameters and assumptions are used. ED values derived from DAP using a conversion factor (CF) are accurate only when the same phantom size and radiation field conditions are used. Furthermore, using phantoms may not accurately represent the actual patient population, as phantoms may have a relatively low body mass index (BMI) and often lack arms, and, thus, doses derived from phantoms tend to be lower than the clinical doses received by patients [
15]. This result underscores the necessity of further investigation into the clinical doses of CBCT. Although patient characteristics, exposure parameters, and field-of-view (FOV) size have been shown to be possible factors that increase the radiation dose from fluoroscopy and 2D angiography [
15,
16,
18,
19,
20,
21], the effects of these factors on the CBCT radiation dose administered to different parts of the body are not well understood due to differences in the modulation of 2D versus 3D imaging parameters by the imaging device. Therefore, there is a need to explore and evaluate the factors affecting CBCT radiation dose and exposure parameters to optimize radiation safety and provide a clinical reference for further research into more dose-efficient imaging protocols.
In this study, we aimed to assess radiation doses and affecting factors with respect to patients undergoing CBCT imaging of the head and abdomen during interventional procedures, analyze the impact of these factors on the exposure parameters, and provide a valuable reference for optimizing radiation doses in interventional procedures.
4. Discussion
The primary objective of this study was to evaluate the clinical radiation dose in patients undergoing CBCT scans of the head and abdomen during interventional procedures and to analyze the independent factors affecting the CBCT radiation dose. The secondary objective was to investigate the influence of relevant factors on CBCT radiation parameters. Since ED cannot be obtained directly during interventional procedures and the results estimated using different methods may vary, the DAP and RAK recommended by the International Commission on Radiological Protection (ICRP) [
24] were used as radiation doses in this study.
Our findings revealed that the DAP and RAK for a single rotation of an abdominal CBCT scan were 47.4 (39.6–54.3) Gy⋅cm
2 and 156 (130.5–179) mGy, respectively. These results are consistent with those from previous studies. Piron et al. [
25] investigated radiation exposure via abdominal CBCT during transarterial chemoembolization (TACE) and reported a DAP of 59.397 (46.290–66.401) Gy⋅cm
2 for a single rotation. Similarly, Berczeli et al. [
3] conducted CBCT imaging for visceral aneurysms and found DAP and skin dose values of 57.03 ± 39.67 Gy⋅cm
2 and 223.6 ± 141.3 mGy, respectively.
At our institution, the protocol used for head CBCT acquisition is 20sDCT-Head, also known as high-resolution CBCT (HR-CBCT). This protocol has proved to be valuable for evaluating intracranial stent expansion in cases of stent-assisted coil embolization as well as for the timely detection of intracranial hemorrhage and early ischemic lesions during procedures [
2,
26,
27]. Compared to 6sDCT-Body, the 20sDCT-Head scans had a higher radiation dose due to having a longer acquisition time (20 s vs. 6 s), more projection images (496 f vs. 397 f), and a higher detector dose (1.200 μGy/fr vs. 0.360 μGy/fr). Our results showed dose values for a single head CBCT scan similar to those in the previous study [
2], with 53.8 (50.5–64.4) Gy⋅cm
2 for DAP and 218 (210–224) mGy for RAK.
Univariate and multivariate analyses were used to analyze the various factors contributing to changes in intraprocedural radiation dose. The results of multiple linear regression showed that patient BMI and gender were independent factors affecting the DAP values in abdominal CBCT scans, while FOV size, gender, BMI, and age were independent factors in head CBCT scans. There was a significant positive correlation between increased patient BMI and higher radiation dose (DAP and RAK) in both abdominal and head CBCT scans. These findings agree with those reported by Suzuki et al. [
28], who noted an increase in DAP values as the BMI of phantoms increased during abdominal CBCT. A similar observation was made by Madder et al. [
21] during coronary angiography, showing that the radiation dose administered to the patient and physician increased with the patient’s BMI. In addition, BMI was confirmed to be an independent predictor of a peak skin dose (PSD) of over 2 Gy among patients treated for internal carotid aneurysms with pipeline embolization devices [
29]. These findings suggest that patient BMI significantly affects the radiation dose in 3D imaging, which is consistent with that of 2D imaging, as dose adjustment automatically keeps the detector signal constant.
However, it should be noted that the influence of BMI on radiation dose varies across different body regions, with there being a more pronounced effect on changes in abdominal CBCT than for head CBCT. Multivariate analysis showed that for each unit increase in BMI, the DAP of the abdominal CBCT scan increased by 1.752 Gy⋅cm2, which was about six times higher than the increase in head dose (0.304 Gy⋅cm2 per BMI unit). This may be due to the uneven distribution of fat throughout the body. As BMI increases, obese patients, especially those with central obesity, are more likely to have fat concentrated in the abdomen, resulting in a significant increase in body thickness and, therefore, greater dose variability. This observation suggests the need for dose-efficient imaging protocols based on a patient’s BMI, especially for abdominal CBCT scans.
Gender was also identified as a factor affecting the radiation dose administered to patients. Our study showed that male patients received significantly higher radiation doses than female patients in abdominal and head CBCT scans. Previous studies have attributed this difference to higher body weight, more complex diseases, and greater technical difficulties that often be seen in male patients [
19,
30]. However, the CBCT dose analyzed in our study was generated by a single rotation and therefore independent of procedural time or the complexity of the disease. Klein et al. [
20] reported lower radiation doses in female patients undergoing fenestrated endovascular aneurysm repair (FEVAR), and this difference was not associated with patient BMI, operative time, or case complexity, a result that is consistent with our findings. While the exact reasons for this gender discrepancy remain unclear, our study suggests that male patients had greater body weight and height than female patients with similar BMIs, which may lead to increased body thickness and, thus, a greater need for dose compensation.
Furthermore, our study highlights the impact of FOV size on radiation dose. A larger FOV size was associated with significantly higher DAP values, which was expected, as DAP is a product of air kerma and irradiation field area. In the head CBCT scans, the DAP value in the FOV48 cm group was 1.28 times higher than that in the FOV42 cm group. Similar findings have been reported by Kawauchi et al. [
16], who found that the DAP value in their FOV27 cm group was 1.44 times higher than that in their FOV22 cm group. However, it is worth noting that their study showed equal RAK values between the two FOV size groups, differing from our results. In our study, the RAK values were higher in the FOV42 cm group than in the FOV48 cm group. This discrepancy may be attributed to differences in the sizes of the patients’ heads, as the cited authors used a phantom for dose evaluation. Additionally, when FOV42 cm was selected, the image was magnified and the input field size was reduced, resulting in decreased brightness. The AEC system then responded by increasing the X-ray dose to maintain constant image brightness. This modulation could explain the increased RAK in the FOV42 cm group. While a large FOV acquisition can visualize an entire object and generate detailed images, a smaller FOV with a smaller voxel size is essential for the accurate and detailed examination of small branches. Therefore, the size of the FOV should be chosen based on the weighing of radiation dose against diagnostic benefits. The use of FOV42 cm reduces the DAP value of a head CBCT scan compared with that for FOV48 cm. Although the RAK value increases with a smaller FOV, the dose dispersion during CBCT rotation limits the deterministic effect on the skin.
Analysis of the exposure parameters with AEC adjustments revealed significant influences of patient BMI and gender on the tube voltage and tube current during CBCT scans. Interestingly, these effects showed different trends during abdominal and head CBCT scans. Although the tube voltage and tube current in scan protocols are set according to application purposes, the AEC dynamically modulates the exposure parameters with reference to the detector entrance dose (DED) [
22]. In our institution, the AEC modulation system was set and calibrated by Siemens Healthineers Limited (Shanghai, China) regularly. This attenuation-based modulation process is expected to exhibit significant variability depending on different body regions, patient cross-sections, and the type and settings of the AEC used [
22,
31]. Our study showed increased tube voltage and tube current for abdominal CBCT and increased tube voltage but decreased tube current for head CBCT for male patients and patients with an increased BMI. This observed variation may be attributed to the modulation priority setting of the AEC controls. Compared to the tube voltage values set in our protocol, the mean kV value for abdominal CBCT scans increased by 3.3% (93 kV compared to 90 kV), while the mean kV value for head CBCT scans increased by 21.4% (85 kV compared to 70 kV). This phenomenon is in accordance with the modulation principles, in that if the initially adjusted tube current fails to reach the desired DED, the tube voltage is adjusted. When the tube voltage increases to a certain level, the tube current decreases. Yel et al. [
32] investigated the radiation dose and image quality of an extremity CBCT system with 55 imaging protocols and identified an optimized parameter setting that resulted in a dose reduction of 18.9% compared to the manufacturer’s recommended protocol. Kirisattayakul et al. [
33] demonstrated that the implementation of a high kV technique may provide an effective reduction in radiation dose. Our findings show the effects of BMI and gender on exposure parameter modulation in head and abdominal CBCT scans. While most of the parameter settings in the protocol cannot be changed by the user, we can communicate with the manufacturer’s experts to provide additional protocol options based on clinical measurements to optimize patient radiation safety.
There are some limitations of this study. It was a single-center retrospective study with radiation dose data primarily collected from patients undergoing interventional procedures for liver cancer and intracranial aneurysms, which may introduce bias due to the specific population sampled. This bias may be a contributing factor to the non-normal distribution of the dataset. Additionally, there may be interactions between variables, such as weight and gender, that were not fully explored in this study. Thus, more advanced statistics such as mixed effects models should be considered in further research to investigate the interactions between variables. This study demonstrated that FOV size has a significant effect on DAP values in head CBCT. Still, for a complete display of the target, only FOV48 cm is currently used in abdominal CBCT at our institution. To optimize the radiation dose and reduce the risk of stochastic effects, further studies should explore the use of smaller FOV sizes in abdominal CBCT for small-sized patients, especially pediatric patients. Furthermore, only two imaging protocols of one angiograph system were used in this study. Other manufacturers offer different types of AEC systems and settings that may have different modulations of the radiation dose. Research using angiography systems and AEC systems from other manufacturers remains to be performed. Moreover, we evaluated radiation dose and parameters but not image quality in this study. To minimize the radiation dose administered to patients, further studies should explore more optimized parameter protocols for CBCT while maintaining image quality and clinical utility.