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Abstract
Background We aimed to investigate the radiation dose to the eye lens (lens dose) during cerebral angiography and to evaluate the effectiveness of the lens dose reduction protocol for 3-dimensional rotational angiography (3D-RA) in reducing overall lens dose exposure.
Methods We conducted a randomized, controlled clinical trial at a tertiary hospital with patients undergoing cerebral angiography. The lens dose reduction protocol in 3D-RA involved raising the table to position the patient’s eye lens away from the rotation axis. The lens dose was estimated by measuring the entrance surface air kerma using a photoluminescent glass dosimeter. The lens doses of 3D-RA, overall examination, and image quality were analyzed and compared between the two groups.
Results A total of 20 participants (mean age, 58±9.4 years; including 12 men [60%]) were enrolled and randomly assigned to either the conventional group or the dose reduction group. The median lens dose in 3D-RA was significantly lower in the dose reduction group compared with the conventional group (1.1 mGy vs 4.5 mGy, p<0.001). The total dose was significantly lower in the dose reduction group (median of 7.5 mGy vs 10.2 mGy, p=0.003). In the conventional group, 3D-RA accounted for 46% of the total lens dose, while in the dose reduction group, its proportion decreased to 16%. No significant differences were observed in the image quality between the groups.
Conclusion The lens dose reduction protocol resulted in a significant reduction in the lens dose of the 3D-RA as well as entire cerebral angiography, while maintaining the image quality.
- Aneurysm
- Angiography
- Technique
- Intervention
Data availability statement
Data are available upon reasonable request. Data are available upon reasonable request to the corresponding author.
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What is already known on this topic
The eye lens, one of the most radiation-sensitive organs, experiences substantial radiation exposure during cerebral angiography. The growing prevalence of 3-dimensional rotational angiography (3D-RA) enhances the ability to detect minute vascular lesions, but it also inevitably augments the radiation dose to the lens. Our previous research indicated that by deliberately off-centering the patient’s head by adjusting the height of the table—a technique we refer to as the 'lens dose reduction protocol'—it was possible to decrease lens radiation exposure by up to 80% during 3D-RA.
What this study adds
In this study, we directly measured the radiation dose beside both eyes using a photoluminescent dosimeter (PLD) to estimate the radiation dose imparted to the lens during cerebral angiography. The overall ‘lens dose’ was determined to be a median value of 10.2 mGy, with 3D-RA contributing approximately 46% of this amount. Notably, employing the 'lens dose reduction protocol' during a 3D-RA procedure resulted in a significant decrease of about 75% in the lens dose, along with a 25% reduction in the total lens dose.
How this study might affect research, practice, or policy
Most radiation dose reduction protocols in the neurointerventional field aim to lower the overall dose by using dose metrics such as the kerma area product (KAP) or air kerma (AK). Contrarily, this study places emphasis on selectively reducing the dose applied to the eye lens, the region in the head most sensitive to radiation. Through this research, we hope to draw more attention from fellow researchers toward understanding and estimating the radiation dose imparted to the lens during neurointerventional surgeries. Additionally, we anticipate this will stimulate innovative research aimed at selectively reducing such exposure.
Introduction
Cerebral angiography is an important diagnostic tool for many types of cerebrovascular lesions, and 3D rotational angiography (3D-RA) plays a crucial role in their meticulous evaluation.1–3 The widespread use of computed tomography angiography (CTA) and magnetic resonance angiography (MRA) has facilitated the detection of smaller aneurysms, further emphasizing the significance of 3D-RA.2 Additionally, due to the multiplicity of cerebral aneurysms, 3D-RA is often necessary not only for the target vessel but also for other vessels.2 4
The eye lens is one of the most radiation-sensitive organs,5–7 and is subject to radiation exposure because of its anatomical proximity to the cerebral vessels during cerebral angiography.8–11 Few studies that have directly measured the radiation dose applied to the lens during cerebral angiography9–11; however, none of them specifically focused on 3D-RA. Based on recent epidemiological evidence, the International Commission on Radiological Protection (ICRP) revised the threshold for absorbed dose to a lower value of 0.5 Gy.5 6 Moreover, it is strongly recommended to optimize radiation doses to ensure they remain below the nominal threshold, as well as to address uncertainties regarding the value or even the existence of a threshold.5
Among the various methods available for reducing the radiation dose during 3D-RA,12–16 one study demonstrated that intentionally off-centering the patient’s head by adjusting the height of the table (lens dose reduction protocol) can reduce the lens dose by up to 80%.17 However, a limitation of the study was that it only measured the lens dose during 3D-RA, and could not accurately estimate the overall impact of the protocol on the lens dose in cerebral angiography. We therefore conducted a randomized controlled trial to investigate the total radiation dose delivered to the lens during cerebral angiography and to evaluate the effectiveness of the lens dose reduction protocol.
Materials and methods
Study design and participants
This study was conducted as a randomized, controlled clinical trial at a tertiary hospital. Participants were randomized in a 1:1 ratio to either the dose reduction group (the lens dose reduction protocol during 3D-RA examination) or the conventional group (conventional 3D-RA protocol). The study protocol and informed consent forms were approved by the Institutional Review Board of Asan Medical Center (IRB No. 2022–0557). Prior to enrollment, we obtained written informed consent from all participants.
Eligible participants included patients with unruptured intracranial aneurysms who were scheduled to undergo diagnostic cerebral angiography and required 3D-RA in both internal carotid arteries (ICAs) due to suspected bilateral cerebral aneurysms or anterior communicating artery (ACoA) aneurysm (online supplemental figure 1). Exclusion criteria included patients who were either younger than 18 or older than 80 years of age, those with unexpected vascular lesions that required additional angiogram, intracranial materials such as coils, clips, or embolic substances, or those who were unwilling to participate in the study (table 1).
Supplemental material
Baseline characteristics of study patients
Lens dose reduction protocol
In contrast to the conventional protocol, which positions the center of the field-of-view (FOV) at the head’s center during 3D-RA, the lens dose reduction protocol raises the table so that the front end of the FOV aligns with the choroidal blush visible in the lateral projection of the digital subtraction angiography (DSA).17 By doing so, the patient’s eye lenses are positioned away from the rotation axis (off-centering), reducing direct exposure of the lenses to the radiation beam (figure 1).
Diagram of 3-dimensional rotational angiography. The tube rotates 200° from 100° on the left lateral side to −100° on the right lateral side during the examination. In the conventional protocol (A), both lenses are directly exposed to the radiation beam throughout a 200 degree rotation in all ranges. In the lens dose reduction protocol (B), there are sections (indicated by double-sided green arrows) where each lens is not exposed to the radiation field. These have a dose-saving effect on the lens by reducing direct exposure to the radiation beam. The ‘+’ signs indicate the center of the field-of-view in each diagram, which is located at the center of the head in the conventional protocol and about 2.4 cm behind it in the lens dose reduction protocol.
Angiography and imaging equipment
The angiography was performed using a 5-French catheter system via the femoral artery approach. The catheter was placed in the proximal cervical ICA or the foraminal segment (V2) of the vertebral artery with the aid of a roadmap and fluoroscopic guidance. Routine DSA involved biplane angiography for both ICAs and the dominant VA and a single plane (frontal projection) for the non-dominant VA. The 3D-RA was performed when an aneurysm or other vascular abnormality was suspected on DSA. The anteroposterior and lateral lengths of each patient’s head were measured on cross-sectional images (CT or MR) at the lens level. A biplane angiography machine (Artis zee biplane; Siemens Healthineers, Forchheim, Germany) was used for image and data acquisition. The detector entrance dose for DSA and 3D-RA was lower than the factory settings of 3.6 and 0.24 μGy/frame, with the routine protocol using 1.82 and 0.24 μGy/frame, respectively.15 18 For DSA and fluoroscopy, copper filters were automatically applied in the range of 0.1 and 0.3 mm, but not for 3D-RA. The angiographic system determined the kVp, milliampere (mA), pulse width, and copper filter automatically in the optimized routine protocol of our angiography suite (online supplemental table 1). The z-axis collimation in 3D-RA was uniformly set at approximately 40%, and after the 3D reconstruction, the region-of-interest was further reduced by approximately 50% in the x and y-axes during the interactive reconstruction process to improve spatial resolution (figure 2).
Application of 3D rotational angiography (3D-RA) protocols. (A) The field-of-view (FOV) of the 3D-RA (white box) in the conventional protocol is centered on the head. (B) The FOV of the lens dose reduction protocol is located behind the eye globe (white circle), which was marked based on the choroidal blush observed during internal carotid angiography. The orange boxes in both protocols represent the standard range of the second reconstruction. Note that the size and position of the FOV in the final 3D image are the same for both protocols.
Dosimetry system
The radiation dose to the eye lens was estimated by measuring the entrance surface air kerma using a photoluminescent glass dosimeter (PLD). In the following text, the term ‘lens dose’ refers to the amount of radiation energy absorbed by the eye lens at the entrance surface, measured in air kerma units. The PLD system consists of a glass dosimeter (GD-352M) and an automatic reader unit (FGD-1000, AGC Techno Glass, Japan).19 The dosimeter holder has a diameter of 4.33 mm and a length of 14.52 mm. The automatic reader unit was used to read the signal, and the dedicated software automatically set each PLD to be read ten times.
The measurement of lens dose was simplified into two components, taking into account the practical aspects of clinical examination: DSA+fluoroscopy and 3D-RA. To reduce variability, three PLDs were combined and applied to the lateral canthus of each eye using a pouch and goggles.17 Radiation dose metrics were obtained using the onboard reference air kerma (mGy) and kerma area product (Gy·cm2) meters. The values displayed in the equipment’s dose report were also compared.
Image quality analysis
Axial source images from 3D-RA were used to analyze image noise, signal-to-noise ratio (SNR), and contrast-to-noise ratio (CNR) for quantitative assessment of image quality.17 Image noise was assessed by measuring the SD of the gray value within a region of interest (ROI; 1 cm2). The ROI was placed in the target aneurysm or, if the aneurysm was too small, in the adjacent parent artery to exclude any artifacts or image quality degradation that could occur as the target lesion shifts off-center while the height of the table is being adjusted. SNR was calculated by dividing the mean gray value of the target lesion by the image noise. The mean gray value and background noise were determined in the surrounding brain parenchyme. CNR was calculated as the difference between the mean gray values of the target lesion and background, divided by the background noise.
Additionally, the image quality was evaluated independently by three blinded neuroradiologists, each with 6 years of expertise. Image analysis was conducted using a five-point Likert scale; excellent (5-point: superior visualization of vasculature); good (4-point: good for visualization; small vessels also visible); fair (3-point: fair vessel visualization; distal parts of small arteries invisible but useful for diagnosis); poor (2-point: small arteries not discernible at all; larger arteries not sharply defined); fail (1-point: unsatisfactory for diagnosis).
Statistical analysis
The calculation of the sample size was based on the result of a previous study,17 which determined that a minimum of 10 participants per group was necessary to achieve a power of 0.8 and an α of 0.05. Continuous variables are expressed as either the mean±standard deviation (SD) or as medians with interquartile ranges. Categorical variables are presented as frequencies and percentages. Continuous variables were analyzed using either Student’s t-test or the Wilcoxon Rank Sum Test, depending on whether the data met the assumption of normality as determined by the Shapiro-Wilk normality test. Categorical variables were analyzed using Fisher’s exact test. A regression correlation test was performed to analyze the relationship between head size and lens dose. P-value <0.05 was considered to indicate statistical significance. Inter-rater reliability for qualitative analysis of image quality was calculated using Gwet’s agreement coefficient with quadratic weighting. All statistical analyses were conducted using R Statistical Software, version 4.2.0 (R Foundation for Statistical Computing, Vienna, Austria).
Results
A total of 20 participants (mean age, 58 years±9.4 (SD); including 12 men [60%]) were enrolled in the study between October 2022 and November 2022 and randomly assigned to either the conventional group (n=10) or the dose reduction group (n=10). The baseline characteristics of both groups were not significantly different (table 1). The average number of rotations in 3D-RA was higher in the dose reduction group, but the difference was not statistically significant (2.5±1.0 vs 2.1±0.3, p=0.23). In the dose reduction group, the table height was increased by an average of 2.4 cm±0.6.
The median lens dose in 3D-RA was significantly lower in the dose reduction group than the conventional group (1.1 [0.9–1.5] mGy vs 4.5 [4.0–5.8] mGy, p<0.001) (table 2 and figure 3A). On the other hand, there was no significant difference in the lens dose between the groups during DSA and fluoroscopy (mean of 6.3 mGy vs 6.2 mGy, p=0.88). The total dose was significantly lower in the dose reduction group (median of 7.5 mGy vs 10.2 mGy, p=0.003). The lens dose of 3D-RA showed no significant difference between bilateral lenses in both groups. However, the lens dose in DSA and fluoroscopy was significantly higher in the left lens for all participants (mean of 9.2 mGy vs 3.3 mGy, p<0.001) (figure 3B). Hence, the median total lens dose was significantly higher in the left eye for both the dose reduction group (11.3 mGy vs 4.6 mGy, p<0.001) and the conventional group (12.6 mGy vs 8.2 mGy, p=0.001).
(A) Box plots showing the lens dose during cerebral angiography. The doses of both protocols were compared according to their subcomponents (3D-RA and DSA+fluoroscopy). (B) Comparison of the dose differences between the right and left lenses.
Lens dose and dose metrics during cerebral angiography
In the conventional group, the average 3D-RA dose accounted for 46% of the total lens dose, whereas the same accounted for 16% of the total lens dose in the dose reduction group (figure 4A). On the other hand, the mean of 3D-RA accounted for 31% and 21% of the total AK and KAP, respectively (figure 4B and C). There were no significant differences in the total air kerma (median of 278.8 mGy vs 281.2 mGy, p=0.55) or kerma area product (median of 35.9 Gy·cm2 vs 37.7 Gy·cm2, p=0.33) between the dose reduction and conventional groups, including their subcomponents.
Values and proportions of the (A) lens dose and dose metrics ((B) air kerma and (C) Kerma area product) presented by the components of cerebral angiography in the two groups.
The AP length of the head showed a significant correlation with the lens dose in conventional 3D-RA regardless of the left-right orientation, with higher doses observed in smaller heads (R=−0.586, p=0.007) (online supplemental figure 2A). In terms of DSA, where both groups followed the same protocol, there was no significant difference in the lens dose between the two groups. However, there was a significant increase in the dose to the left eye lens as head size decreased (R=−0.618, p=0.004), while there was no significant correlation observed between head size and the right eye lens dose (online supplemental figure 2B).
In the analysis of image quality, there was no significant difference observed between the two groups in terms of image noise, SNR, and CNR (online supplemental table 2 and online supplemental figure 3). In the qualitative analysis, there was no significant difference in image quality between the groups (4.20 vs 4.17, p=0.87). Inter-rater reliability was considered acceptable (range, 0.47–0.82).
Discussion
In this randomized controlled trial, we found that the lens dose reduction protocol not only reduced the lens dose during 3D-RA to approximately one-fourth of the conventional protocol but also resulted in a near 25% decrease in the overall dose for cerebral angiography. The lens dose during 3D-RA accounted for approximately 46% of the total dose in cerebral angiography, with an average of 5.2 mGy. The remaining radiation dose was attributed to DSA and fluoroscopy.
There have been few studies that directly measured the radiation dose on a patient’s eye lens during cerebral angiography; the reported doses have shown significant variation, ranging from 7.8 to 267 mGy.8–11 This variability is likely attributed to the heterogeneity in patients, equipment, imaging protocols, and measurements employed across the studies. According to one study that measured the lens dose for each component of cerebral angiography, the dose was 1 mSv for 3D-RA (single rotation) and 4.5 mSv for DSA (biplane) in a phantom model.8 The study estimated that 3D-RA accounts for 25% of the total radiation dose, assuming that one examination includes 3 rotations of 3D-RA and 2-vessel DSA, Our current study demonstrated that 3D-RA contributed more to the lens dose than the previous study, as a single rotation for 3D-RA resulted in an average of 2.5 mGy, and overall 3D-RA (5.2 mGy) accounted for 46% of the total lens dose (11.4 mGy).
Dose metrics provided by angiography machines, including KAP and AK, may provide an indirect estimation of the radiation dose received by the lens; however, they do not accurately represent the actual radiation dose absorbed by the lens.10 Indeed, in our study, both the KAP and AK demonstrated a negative correlation with the lens dose. This can be explained by the variability in head size among individuals. As the head size increases, there is greater attenuation, resulting in higher KAP and AK values. On the other hand, as the head size increases, the lens moves away from the FOV of the lateral projection in 3D-RA, resulting in a decreased dose to the lens. This finding is consistent with an earlier study.17 Interestingly, a seemingly paradoxical phenomenon also occurs in DSA, where higher KAP and AK values correspond to smaller doses to the lens.
The mechanism of the lens dose reduction protocol during 3D-RA involves reducing the number of frames in which the lens is exposed to the primary beam by increasing the table height during tube rotation (figure 1). More importantly, the frames that prevent lens exposure (dose saving sections in figure 1B) correspond to the positions of the tube where the lens is closest to the tube without any attenuation. In participants with larger head sizes, especially those with elongated anteroposterior length, the observed decrease in lens dose can be attributed to a principle that is similar to raising the height of a table.
In DSA, our findings revealed a noteworthy observation that there is a significant inverse relationship between head size and lens dose. We observed that participants with smaller heads were more likely to have their lenses exposed to the primary beam when entering the field of view (FOV) in lateral projections. Additionally, we observed that individuals with smaller and leaner body habitus demonstrated a propensity for anterior head tilting on the same pillow, causing their lenses to be included in the FOV. Hence, appropriate collimation and adjustment of pillow height could be simple yet effective strategies for reducing lens dose during DSA.
In previous research, several attempts have been made to minimize the lens dose in 3D-RA and CT scans by using reduced tube current, wedge filters, or bismuth shielding.12–16 20 While these approaches have demonstrated efficacy, they may have the potential to deteriorate image quality. Utilizing a geometry-based lens dose reduction protocol, similar to gantry tilt in CT scans, can selectively reduce lens dose without compromising the overall beam quality. These could also offer additional benefits when used in conjunction with conventional protocols. Although the lens dose imparted from 3D-RA or CT may not reach the current threshold of 0.5 Gy, it may still have a significant impact on the lens when considering the additional exposure resulting from repeated examinations and procedures.5 21 In fact, a nationwide population-based study showed an association between neurointerventional procedures and an increased risk of cataract occurrence, particularly for patients over the age of 40.22 Furthermore, since there is still debate about the threshold for cataract induction, it is advisable to make every effort to minimize the dose to the lens as much as possible.
There are several limitations to this study. First, we were unable to measure the radiation doses for DSA and fluoroscopy separately, as there were concerns that frequent replacement of the dosimeter might hinder the examination process. However, since fluoroscopy is mainly used in the neck region, its impact on the lens dose was considered insignificant compared with that of DSA. Second, we were unable to measure the frontal and lateral lens doses separately in biplane DSA because it was necessary to administer additional contrast agent to the patient. Third, there is a possibility that the actual radiation dose received by the eye lens may differ from the measured dose, even if the dosimeter was placed as close as possible to the lens.
Conclusion
During a routine diagnostic cerebral angiography, the average absorbed radiation dose to the eye lens was measured to be 11.4 mGy. The proportion of 3D-RA accounted for 46% of this, which is higher than previously thought. In our study, we found that implementing the lens dose reduction protocol resulted in a 76% decrease in lens dose during 3D-RA compared with the conventional protocol. Furthermore, the total lens dose for the entire examination was measured to be 34% lower. Therefore, this study demonstrates that by elevating the table during 3D-RA, radiation exposure to the eye lens can be selectively reduced without compromising radiation beam or image quality.
Data availability statement
Data are available upon reasonable request. Data are available upon reasonable request to the corresponding author.
Ethics statements
Patient consent for publication
Ethics approval
This study involves human participants and was approved by the institutional review board of Asan Medical Center (IRB No. 2022-0557). Participants gave informed consent to participate in the study before taking part.
References
Supplementary materials
Supplementary Data
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Footnotes
J-CR and J-TY are joint first authors.
Contributors YS: Guarantor of the integrity of the entire study; J-CR, JT-Y, DHL, YSS: study concepts/study design or data acquisition or data analysis/interpretation; All authors: manuscript drafting or manuscript revision for important intellectual content; All authors: approval of the final version of the submitted manuscript; All authors: agree to ensure any questions related to the work are appropriately resolved; J-CR, JT-Y, DHL, YSS: literature research; J-CR, JT-Y: clinical studies; J-CR, JT-Y: statistical analysis; and J-CR, JT-Y, BSK, DHL, YS: manuscript editing.
Funding The authors have not declared a specific grant for this research from any funding agency in the public, commercial or not-for-profit sectors.
Competing interests None declared.
Provenance and peer review Not commissioned; externally peer reviewed.
Supplemental material This content has been supplied by the author(s). It has not been vetted by BMJ Publishing Group Limited (BMJ) and may not have been peer-reviewed. Any opinions or recommendations discussed are solely those of the author(s) and are not endorsed by BMJ. BMJ disclaims all liability and responsibility arising from any reliance placed on the content. Where the content includes any translated material, BMJ does not warrant the accuracy and reliability of the translations (including but not limited to local regulations, clinical guidelines, terminology, drug names and drug dosages), and is not responsible for any error and/or omissions arising from translation and adaptation or otherwise.