Background A neurointerventional examination of intracranial aneurysms often involves the eye lens in the primary beam of radiation.
Objective To assess and compare eye-lens doses imparted during interventional and non-interventional imaging techniques for the examination of intracranial aneurysms.
Methods We performed a phantom study on an anthropomorphic phantom (ATOM dosimetry phantom 702-D; CIRS, Norfolk, Virginia, USA) and assessed eye-lens doses with thermoluminescent dosimeters (TLDs) type 100 (LiF:Mg, Ti) during (1) interventional (depiction of all cerebral arteries with triple 3D-rotational angiography and twice 2-plane DSA anteroposterior and lateral projections) and (2) non-interventional (CT angiography (CTA)) diagnosis of intracranial aneurysms. Eye-lens doses were calculated following recommendations of the ICRP 103. Image quality was analysed in retrospective by two experienced radiologists on the basis of non-interventional and interventional pan-angiography examinations of patients with incidental aneurysms (n=50) on a five-point Likert scale.
Results The following eye-lens doses were assessed: (1) interventional setting (triple 3D-rotational angiography and twice 2-plane DSA anteroposterior and lateral projections) 12 mGy; (2) non-interventional setting (CTA) 4.1 mGy. Image quality for depiction of intracranial aneurysms (>3 mm) was evaluated as good by both readers for both imaging techniques.
Conclusions Eye-lens doses are markedly higher during the interventional than during the non-interventional diagnosis of intracranial aneurysms. For the eye-lens dose, CTA offers considerable radiation dose savings in the diagnosis of intracranial aneurysms.
- CT Angiography
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In international clinical guidelines for intracranial aneurysms, CT angiography (CTA) is the preferred follow-up method for a good reason.1–3 Several authors have discussed the limits and strengths of non-interventional imaging techniques, in contrast to interventional angiography, in the neuroradiological setting.4–9 However, previous studies focused primarily on cost-effectiveness, diagnostic accuracy, and procedural complications; few studies discussed radiation doses.10 Recent studies indicate that bone-subtraction CTA may be as accurate as DSA in detecting cerebral aneurysms after subarachnoid hemorrhage at a lower radiation dose.11 Comparison of CTA and interventional angiography, including 3D-rotational angiography, for their eye-lens dose has not yet been discussed. This is of particular interest because radiation doses of 3D-rotational angiography are considerably lower than previously believed.12 Public awareness of radiation doses changes continuously and meanwhile, new thresholds for the eye lens have been introduced by the International Commission on Radiological Protection (ICRP).13 Furthermore, a practical problem of monitoring eye-lens doses is that they do not contribute to the effective dose, and the dose of the brain contributes with a minimal weighting factor of only 0.01.14 Thus, the effective dose does not provide sufficient information about the radiation exposure of the eye lens. Hence, this study aimed to compare eye-lens doses imparted to patients during non-interventional and interventional neuroimaging techniques for diagnosis of intracranial aneurysms assessed with the reference standard of dose measurements, an anthropomorphic phantom, and dosimetry study.
Eye-lens doses were compared during two standardized procedures for the diagnosis of intracranial aneurysms in the (1) non-interventional and (2) interventional, neuroimaging setting. We focused on diagnostic procedures implemented in the clinical routine at our institute, which have been approved by neuroradiologists with many years of proficiency and experience. Therefore, the chosen protocol parameters in the phantom measurements were the same as those used for ‘real-life’ patients undergoing a diagnostic DSA. The non-interventional setting (1) included a CTA examination. The interventional setting (2) encompassed a pan-angiography (depiction of all cerebral arteries; twice a 2-plane DSA (anteroposterior and lateral projections) and triple 3D-rotational angiography) for a complete assessment of intracranial aneurysms. Eye-lens dose assessment was accomplished with phantom measurements and thermoluminescence dosimeters. Descriptive statistics were calculated using SPSS, V.21.0 (IBM Corp., Armonk, New York, USA).
3D-rotational angiography and 2-plane DSA
All neurointerventional examinations were performed in a flat-panel detector and vascular interventional system, Allura Xper (Philips Healthcare, Hamburg, Germany). Clarity technology was not applied. The pan-angiography (depiction of all cerebral arteries) comprised twice a 2-plane DSA (anteroposterior and lateral projections) and triple 3D-rotational angiography. 2-Plane DSA was carried out in anterior/posterior and lateral projections (frame rate 1 fps over 8 s). 3D-rotational angiography comprised altogether 122 images from one single image run around the phantom's head. Table 1 summarizes the technical parameters of both examinations. For sufficient irradiation of the TLDs, each examination was repeated eight times, and an average for one course obtained.
CTA of cranial vessels
CTA examination was performed with the 128-multislice, dual-source CT scanner, SOMATOM Definition Flash (Siemens Healthcare, Erlangen, Germany) (technical parameters, summarized in table 2). CTA was planned manually covering the upper thorax to vertex, including the carotid bulb, the whole circle of Willis, and dural sinuses. The topogram was obtained without TLDs in place. In order to achieve sufficient irradiation of the TLDs, the scan was repeated eight times, and an average for the radiation dose from a single run obtained.
Anthropomorphic phantom and thermoluminescence dosimeters
The anthropomorphic phantom (ATOM dosimetry phantom 702-D; CIRS, Norfolk, Virginia, USA) is a human-like model body (weight 55 kg; height 160 cm; body mass index 21.5 kg/m2). For dose assessment lithium fluoride rods (TLD-type 100; size 1×1×6 mm3) were applied. The average thickness of the eye lens varies from 2 to 6 mm, depending on age, gender, and genetic predisposition. TLD rods were placed at an 8 mm depth in the phantom's orbit. The standard procedure of individual calibration, annealing, and read-out of the TLDs is described in detail in the European Commission report15 and by Lechel et al.16 Eye-lens doses were calculated according to the recommendations of the manufacturer’s user manual for the phantom and organ equivalent doses using the tissue-weighting factors in accordance with ICRP 103.14
Image quality was independently analyzed in retrospect by two experienced radiologists and was based on 50 non-interventional and interventional angiography examinations of patients with incidental aneurysms (aneurysm size >3 mm) at two separate sessions (4-week interval). All patients received both CTA and interventional angiography, after strict clinical indication and interdisciplinary medical consent. Examinations were performed with the same devices and identical technical parameters as used for the phantom measurements (see tables 1 and 2). Image quality was evaluated by considering important aneurysm characteristics (location, size, ratio of the neck to the dome, lobularity, and relationship to adjacent arterial branches) on a five-point Likert scale from 1 to 5 (1 indicating good depiction, all elements can be judged; 2 representing good depiction, but one of the elements cannot be judged; 3 indicating moderate depiction with uncertainty about two elements; 4 representing only partial visibility and three elements cannot be judged; 5 indicating not discernible, none of the five elements can be judged). Consensus reading was performed in cases of discrepancy.
Intermodality comparison shows that eye-lens doses imparted to patients during a standardized diagnostic CT examination of the intracranial aneurysms are threefold lower than with interventional pan-angiography. The following eye-lens doses were assessed in detail: (1) interventional setting (triple 3D rotational angiography and twice 2-plane DSA anteroposterior and lateral projections) 12 mGy; (2) non-interventional setting CTA 4.1 mGy. Table 3 summarizes results in the interventional and non-interventional setting. Image quality for depiction of intracranial aneurysms (>3 mm) was evaluated as good by both readers for both imaging techniques.
This dosimetry phantom study aimed to examine radiation exposure of patients' eye lenses during non-interventional and interventional imaging techniques for the depiction and diagnosis of intracranial vessels. In previous studies authors have focused particularly on image quality and diagnostic accuracy rather than on radiation doses, or applied simplified methods for dose assessment.17–19 Hoh et al9 pointed out that CTA is a non-invasive technique, which can be performed much more quickly and requires fewer resources, but which can also be performed at a considerably lower level of risk of complication and morbidity.
Our study is the first to compare eye-lens doses imparted to patients undergoing non-interventional and interventional imaging techniques assessed by the same method and reference standard of dose measurement in accordance with the latest recommendations of ICRP 103. The results show that eye-lens doses are considerably lower in the non-interventional diagnosis of intracranial aneurysms than in the interventional setting while maintaining high image accuracy for the depiction of aneurysms <3 mm. The results are interesting, particularly in light of the fact that the dose threshold for the eye lens has been revised by the ICRP to a lower level of 0.5 Gy.13 At present, for occupational eye-lens exposure an equivalent dose limit of 20 mSv a year is recommended, averaged over defined periods of 5 years, with no single year exceeding 50 mSv.13 Further research is important to evaluate the radiation sensitivity of eye lenses and potential detriment of the visus. According to the linear model, no threshold exists for cataract development.13 ,20 Hence, eye-lens doses are clinically relevant for a one-time procedure. Because of uncertainty about the potential harm, optimization of all examinations which involve irradiation of the eye lens should be made. It is important to take eye-lens doses more into account when choosing an imaging method in which the eye lens is exposed to a primary beam of radiation.
At present, CTA is an accepted tool for follow-up of intracranial aneurysms. At the same time, some authors argue that CTA is not sufficiently sensitive for aneurysm diagnosis, in particular for a precise estimation of aneurysm size, which is necessary, for example, before coiling. The sensitivity for the detection of an aneurysm by CTA rises with aneurysm size (4–6 mm diameter, sensitivity >90%2). In our study image quality for the depiction of aneurysms >3 mm was evaluated as good for both imaging techniques. These results are in line with recent studies, which highlight the potential of CT diagnosis compared with interventional angiography.11 Therefore, in future for diagnosis of intracranial aneurysms >3 mm we recommend CTA as the preferred imaging method at the lowest eye-lens dose level.
The study has a number of limitations and challenges. Image quality was evaluated on the basis of examinations of patients with an incidental aneurysm finding (size >3 mm). For smaller aneurysms (<3 mm) diagnostic confidence about the presence of an aneurysm may be greater with interventional angiography.11 Therefore, the depiction of aneurysms ≤3 mm may still require diagnostic DSA, resulting in an increase in eye-lens dose, particularly in cases of subarachnoid hemorrhage.
In this study we focused on imaging techniques for the diagnostic depiction of intracranial aneurysms for pretreatment or follow-up. Where available, and in absence of contraindications, MR angiography may be an alternative, radiation-less imaging method. For the purpose of this study, diagnostic DSA should clearly be differentiated from an endovascular aneurysm procedure. An endovascular aneurysm procedure results in accumulation and increase in the eye-lens dose. A phantom study represents an in vitro examination under controlled circumstances. All measurements were performed in one flat-panel detector and vascular interventional system and with one second-generation, dual-source CT scanner. Older-generation CT scanners may impart higher radiation doses.
We focused on imaging procedures implemented in the clinical routine at our institute. The underlying results are in line with preceding studies, which assessed eye-lens doses during comparable imaging protocol settings.12 ,21 Variations in configuration settings result in differences of radiation doses. In order to achieve eye-lens doses exclusively imparted by the neuroimaging techniques examined, the lateral topogram and fluoroscopy time, which are necessary to find the correct phantom position, were excluded by placing the TLDs at the phantom shortly before the main scanning. Considering this, the eye-lens doses are even higher.
A neurointerventional examination of intracranial aneurysms involves considerably higher eye-lens doses than non-interventional CT diagnosis. Further efforts are necessary to reduce eye-lens doses. These results may accelerate the development and help in the choice of the best imaging method with the lowest level of risk.
Contributors NG contributed to the protocol and project development, to the manuscript writing/editing; data analysis; data collection and management. UD contributed to the data analysis and data collection. MF contributed to the manuscript editing. AR contributed to the protocol/project development and the manuscript editing.
Competing interests None declared.
Patient consent Obtained.
Provenance and peer review Not commissioned; externally peer reviewed.
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