A Review on the Extraction of Quantitative Retinal Microvascular Image Feature <s> Retinal Vascularization <s> A major physiologic role for the high flow choroidal vasculature is to help maintain a stable temperature in the macula. The choroidal vasculature modulates tissue temperature in the macula via both active and passive mechanisms. The active mechanisms involve a reflexive increase in choroidal blood flow in response to light. The neuro-anatomical pathways mediating this reflective mechanism have been demonstrated to involve the supra-chiasmatic and the Edinger-Westphal nuclei. The importance of the thermal environment for the retina has been neglected in looking for causes of retinal disease. The observations on the ability of the choroidal circulation to modulate the thermal environment of the macula should excite further study of the role of temperature and the ability of the eye to dissipate light-generated heat on macula disease. <s> BIB001 </s> A Review on the Extraction of Quantitative Retinal Microvascular Image Feature <s> Retinal Vascularization <s> The retinal vasculature is a complex, layered network of vessels and capillary beds that permeate the retina. They provide a steady supply of nutrients and dispose of cellular byproducts, both essential for maintaining retinal health and function. Compromise of this support system has severe consequences to the retina and often underlies retinal abnormalities, in particular diabetic retinopathy, a leading cause of blindness in the Western world. Fluorescein angiography (FA)1–7 and entoptic viewing8–15 are established methods for observing fine details of the retinal vasculature and early pathologic changes. Both techniques, however, are not without shortcomings. FA requires injection of a toxic fluorescent dye and fails to detect a significant fraction of capillaries, especially those with small diameters, because of its poor depth sectioning capability and sensitivity to quality of the angiogram (optical quality of the eye). In spite of these shortcomings, FA has been used to measure the retinal microcirculation (e.g., capillary blood flow velocity5–7), area between capillaries,6 and FAZ size.1–4,6 ::: ::: Entoptic viewing is a psychophysical method that allows self-visualization of the retinal vasculature or, more specifically, the shadow patterns the vasculature casts during illumination of the retina. Unlike FA, entoptic viewing is noninvasive, unaffected by the eye's optics, and limited to the fovea for viewing capillaries. In this localized area, the method has been used to study foveal capillary details, FAZ size and shape,8–10 foveal vessels,9–11 macular blood flow,12,13 and capillary density around the FAZ.14 It has also been compared with fluorescein angiography and shown to contain more foveal capillary detail.15 ::: ::: More recently, retinal imaging instruments equipped with adaptive optics (AO) have garnered considerable interest for imaging the retinal microvasculature. AO increases imaging resolution and sensitivity, both advantageous for detecting small, weakly reflecting capillaries. AO scanning laser ophthalmoscopes (AO-SLOs) have been used to map the local 2D distribution and flow dynamics of the retinal microvasculature in living eyes.16–18 AO retina cameras based on flood illumination have also demonstrated similar imaging capability.19 AO has been incorporated into optical coherence tomography (OCT).20–27 The major advantage of OCT is its substantially higher axial resolution28,29 (∼3–8 μm compared with >60 μm for AO-SLO and AO flood-illumination retina cameras), which, when combined with AO, offers the potential to map the microvasculature in all three dimensions to differentiate, for example, the multilaminar networks of capillaries. ::: ::: There have been reports of AO-OCT imaging of retinal capillaries,23–25,30–32 in particular that by Hammer et al.,24 who compared the diameters of capillaries of patients with retinopathy of prematurity with those of persons without it. The vasculature detail realized with these instruments is impressive, yet the qualitative nature of these observations along with the quantitative analysis of Hammer et al.24 do not address whether the capillaries are faithfully imaged. This raises a fundamental issue about the capacity of AO-OCT to image capillaries, whose size varies considerably (2.5–7 μm33; 2.5–10 μm34) and that network with vessels of increasingly larger size. Certainly the AO-SLO mapping of the small capillaries near the rim of the FAZ suggests AO-OCT is capable of the same, but this has yet to be demonstrated. Moreover, AO-SLO and AO-OCT instruments are fundamentally different. The interferometric nature of AO-OCT results in high-contrast speckle that permeates the AO-OCT image and confounds the detection of structures that approach the size of speckle (e.g., comparable to the diameters of small capillaries). The much slower image acquisition of AO-OCT compared with AO-SLO (100–1000 times slower) also leads to larger and more disruptive motion artifacts that degrade the image and obstruct visualization of capillaries. ::: ::: Given these unknowns, we investigated the capability of an ultrahigh-resolution AO OCT system (UHR-AO-OCT) to image capillaries in a 3° × 3° region centered on the fovea. This region35–37 is occupied by the FAZ and the network of capillaries that define its terminal rim. Use of the rim capillaries enabled us to compare the UHR-AO-OCT images with entopic viewing, an independent and established psychophysical method for mapping foveal capillaries8–15 on the same seven subjects. Comparisons were made in terms of presence or absence of a FAZ, gross dimensions of the FAZ, and qualitative characteristics of capillary patterns unique to specific eyes. In addition, UHR-AO-OCT measurements of the capillary diameter and depth variation with retinal eccentricity are reported. <s> BIB002 </s> A Review on the Extraction of Quantitative Retinal Microvascular Image Feature <s> Retinal Vascularization <s> PURPOSE ::: The aim of the study was to compare the diameter of the central retinal artery (CRA) between patients with diabetes mellitus and healthy subjects in vivo. CRA diameters were calculated from retinal haemodynamic parameters measured with noninvasive measurement techniques. ::: ::: ::: METHODS ::: Sixteen patients with type 1 diabetes with no or mild diabetic retinopathy and 16 age-matched healthy controls were included in this study. Total retinal blood flow was assessed by combining bidirectional laser Doppler velocimetry in all veins entering the optic disc with measurements of retinal vessel diameters using a Dynamic Vessel Analyser. Blood flow velocity in the CRA was measured with colour Doppler imaging (CDI). The diameter of the CRA in the optic nerve was then calculated for each subject using the individual ocular haemodynamic data. ::: ::: ::: RESULTS ::: Calculated CRA diameters were significantly larger in patients with diabetes (180 ± 13 μm) compared with healthy controls (166 ± 10 μm, p = 0.001). No significant differences were found in retinal blood flow, retinal artery or vein diameters or mean flow velocity of the CRA. ::: ::: ::: CONCLUSION ::: The data of the present study show increased calculated CRA diameter in patients with type 1 diabetes with no or mild retinopathy. Blood flow velocity measurements in retrobulbar vessels using CDI need to be interpreted with caution in terms of retinal blood flow, because no information about vessel diameters is obtained by this technique. <s> BIB003
The retina is a highly vascular tissue, which receives supply from two sources, namely, central and choroidal circulatory system ( Figure 1 ). The central blood vessel supplies the inner retina which made ∼30% of the retinal blood flow  . Central retinal artery (CRA) runs through the optic disc and enters the inner layer of the retina. CRA branches into superior-and inferior-arteries with diameters of 150 m BIB003 . Ultimately it forms a network of capillaries with diameters of 5 m BIB002 . On the other hand, blood from the retina drains into the central retinal vein (CRV) . Approximately 70% of the retinal blood flow is supplied by the choroidal blood vessel which nourishes the outer retina and the photoreceptorretinal pigment epithelium (RPE) complex located adjacent to Bruch's membrane  . Apart from nourishing the region, the choroidal circulatory system serves as a heat sink which removes generated metabolic heat due to light photons strike on the photopigments and the melanin of the RPE BIB001 . Figure 1 shows the schematic diagram of the retinal vasculature.