Delineation of the central retinal artery using computed tomography: a pilot study

The prospective study aimed to examine the central retinal artery (CRA) using computed tomography (CT). Seventy adult outpatients comprising 32 men and 38 women, at a mean age of 60.6 ± 13.3 years, were enrolled in the study. The patients underwent contrast-enhanced CT. The scan timing was set to start 5.0 s after the circle of Willis began to be delineated. The ophthalmic arteries (OphAs) were comfortably delineated in all. In 97% of the patients, the CRA was delineated from the original site on the OphA to a more distal segment coursing on the optic sheath. Unilateral and bilateral CRA delineations were observed in 44% and 53% of cases, respectively. The delineated CRAs demonstrated highly variable morphologies in terms of the course and length on the optic sheath. In addition, the distance between the original site of the CRA and the posterior limit of the bulb was measured. The mean distance was 18.6 ± 5.0 mm on the right and 17.8 ± 4.3 mm on the left, respectively. No significant right-to-left differences in the measurements (p > 0.05) were observed. If optimal scan timing is adopted, the CRA segments coursing on the optic sheath can be delineated using contrast-enhanced CT. CT may be a useful diagnostic modality for the CRAs and associated pathological conditions.


Introduction
The central retinal artery (CRA) is a small branch of the ophthalmic artery (OphA), commonly arising as the first or second tributary in the orbital apex.After branching from the OphA, the CRA typically courses forward from the inferior or inferomedial surface of the optic sheath (Fig. 1).The CRA then penetrates the optic sheath and reaches the central part of the optic nerve.Subsequently, coursing forward to the anterior limit of the nerve, the arteries distribute over the retina.The CRA measures approximately 0.4-0.6 mm, whereas the diameter of the proximal OphA segment, where the CRA branches, is reported to be approximately 1.69 mm [6,7,10,12,13].Disruption of CRAs commonly results in blindness.Despite their vulnerable structure which can result in blindness, anatomical variability in CRAs has been documented, mainly in cadaver dissection studies [2,9,13].The CRAs have been inconsistently identified on neuroimaging, even with catheter angiography and high-resolution magnetic resonance imaging (MRI) [1,3,4].Recent investigations using thin-sliced, contrast-enhanced MRI have shed light on the delineation of CRAs in vivo [14].
However, to the best of our knowledge, only a few studies have visualized the CRAs using computed tomography (CT) [5,8,11,15,16].Therefore, the present study aimed to explore the CRAs using contrast-enhanced CT.

Materials and methods
The prospective study included consecutive 70 adult outpatients who presented to the hospital between December 2021 and June 2023 with a non-ruptured cerebral aneurysm, brain tumor, intracerebral hemorrhage, subarachnoid hemorrhage, traumatic brain injury, microvascular compression syndrome, venous angioma, or dural arteriovenous fistula.
The population comprised 32 men and 38 women at a mean age of 60.6 ± 13.3 years (range: 31-91 years).Patients with orbital tumors and vascular lesions, stenosis in the intracranial segments of the internal carotid artery, OphAs arising from the external carotid system, or a previous diagnosis of CRA occlusion were excluded from the study.The patients underwent contrast-enhanced CT using the same machine (Aquilion ONE/GENESIS Edition, Canon Corp., Tokyo, Japan), covering the entire intraorbital course of the OphA, with a slice thickness of 0.50 mm.During the examination, 75 mL of Iopamiron 370 Ⓡ was infused from the antecubital vein at a rate of 4.5 mL/s.Preliminary observations using contrast-enhanced CT suggested that the OphAs were most intensely contrasted approximately 3.2 s after the circle of Willis began to be delineated (Fig. 2).Based on this, the scan timing for exploring the CRA, a small branch of the OphA, was set to start 5.0 s after the circle of Willis began to be delineated.Imaging data were displayed as volumetric maximum-intensity projection images in the axial, coronal, and sagittal sections.The slice thickness and interslice gap were 7.0 mm and 2.0 mm, respectively.Due to the low detection performance of the coronal sections confirmed during preliminary observations, they were excluded from the analysis.The imaging data were analyzed independently by two authors (S.T. and H.I.), both of whom have more than 15 years of experience as board-certified neurosurgeons.The CRA was determined when a fine vessel structure arising from the proximal part of the intraorbital OphA and coursing a certain distance on the optic sheath was identified in both the axial and sagittal sections.A fine vessel reaching the posterior pole of the bulb was excluded from the candidate of CRA.
The Wilcoxon signed-rank test was used for the statistical analyses.In this study, differences were considered insignificant at p > 0.05.
The study was conducted according to the guidelines of our institution for human research.Written informed consent was obtained from all patients before their participation.

Results
The OphAs were comfortably delineated in all 70 patients from the original sites on the intracranial internal carotid arteries to the segment branching the anterior ethmoidal artery, leaving the optic sheath (Fig. 2).None of the patients had OphA originating from or supplied by the external carotid system.In 67 patients (97%), the CRA was delineated from the original site of the OphA to a more distal segment coursing along the optic sheath (Fig. 3).Moreover, unilateral delineations were observed in 31 patients (44%) and bilateral delineations in 37 patients (53%) (Table 1).Furthermore, CRA was identified in the right orbit in 49 (70%) patients and the left orbit in 56 (80%) patients.In 2 (3%) patients, all of the CRAs were not contrasted.In addition, in all patients, the CRA segments coursing through the central part of the optic nerve were not detected as vascular structures.The exact CRA-penetrating site into the optic sheath was not identified in any patient.The delineated CRAs demonstrated highly variable morphologies in terms of course and length of the optic sheath (Fig. 4).In extreme cases, the CRA segment coursing through the optic sheath was extremely short (Fig. 5).The distance between the CRA and the posterior limit of the bulb was measured and the mean distance was 18.6 ± 5.0 mm (range, 10.65-31.93mm) on the right and 17.8 ± 4.3 mm (range, 8.54-27.92mm) on the left, respectively (Table 2).No significant right-to-left differences in the measurements (p > 0.05) were observed.

Discussion
In the present study, the CRA segments coursing superficially on the optic sheath were delineated in 97% of the original sites on the OphAs, which were more sensitive than in a previous study using contrast-enhanced MRI [14].Even CRAs with tortuous or short courses were well delineated on the CT images.Furthermore, the measured distances between the original CRA site on the OphA and the posterior limit of the bulb were consistent with those in the previous study [14].Therefore, contrast-enhanced CT was considered sensitive in detecting CRA segments coursing through the optic sheath.The findings have not been documented in previous investigations using CT [5,8,11,15,16].In the study, the scan timing, which was set to start 5.0 s after the circle of Willis began to be delineated, was optimal in most cases.For the two patients without CRA delineation, the timing may not be optimal from the standpoint of circulation time or anatomical characteristics involving the internal carotid artery, OphA, and CRA.
Given the high sensitivity to CRAs and resolution of the obtained images, contrast-enhanced CT can be a potential diagnostic modality for detecting parts of acute CRA occlusions and tumorous lesions affecting the CRA in a more convenient and less invasive manner than fluorescein and catheter angiography.Moreover, during microsurgical maneuvers around the CRA, the surrounding bony structures presenting on CT images can be useful landmarks to maintain orientation and locate the CRA.
The present study has several limitations.The cohort involving only adult subjects was small and comprised inhomogeneous age and sex distributions.The diagnostic accuracy of CRA should be mentioned.In this study, a fine vessel structure arising from the proximal part of the intraorbital OphA and coursing a certain distance on the optic sheath was determined as the CRA.However, such vessel could be a ciliary vessel with common cilioretinal trunk [13].The CRAs were assessed only in the segments coursing through the optic sheath, starting from the original sites on the OphAs.Moreover, the CRA segments lying in the central portion of the optic nerve and CRA-penetrating site into the optic sheath were not assessed as they could not be detected on the CT images.Diameter of the proximal CRA segments coursing superficially on the optic sheath has been documented to be 0.4-0.6 mm [6,13].The CRA segments lying in the optic nerve is more peripheral and much thinner, which may be a reason for the absence even on coronal CT.The optimal scan timing, if determined, may enable consistent visualization of the proximal CRA segments.Despite the

Conclusions
If optimal scan timing is adopted, the CRA segments coursing on the optic sheath can be delineated using contrastenhanced CT.CT may be a useful diagnostic modality for the CRAs and associated pathological conditions.
approval All procedures in the study were performed by the ethical standards of the institutional and/or national research committee as well as the 1964 Declaration of Helsinki and its later amendments or comparable ethical standards.
Informed consent Written informed consent was obtained from all the participants included in the study for their participation in and publication of this article.

Fig. 1 Fig. 2
Fig. 1 Dissected right orbit, viewed from the anterolateral a and lateral b aspects, exhibiting the central retinal artery presenting serpentine course (arrows) on the optic sheath (OS).In b, the extraocular muscles are resected for better exposure of the ophthalmic and central retinal arteries.IDOMN: inferior division of the oculomotor nerve; IObM: inferior oblique muscle; IRM: inferior rectus muscle; SRM: superior rectus muscle; Dashed arrows: ophthalmic artery.a and b are obtained by one of the authors (S.T.) after microsurgical dissection of a cadaveric right orbit

Fig. 3
Fig. 3 Contrast-enhanced axial a and sagittal b, c computed tomography of an 87-year-old man with non-ruptured anterior cerebral artery aneurysm, scanned 5.0 s after the circle of Willis began to be delineated, demonstrating a satisfactory delineation of the bilateral central retinal arteries (arrows) from the original site on the ophthalmic artery to a more distal segment coursing on the optic sheath.Dashed arrows: ophthalmic artery

Table 1
Classification of identified central retinal arteries.CRA: central retinal artery