*2.3.1 Medicine basis of vein structure*

Retinal vasculature occurs mainly by *angiogenesis*. Its formation and the factors that regulate the development of superficial retinal vascular plexus in humans are already quite known. The cell–cell signaling that occurs between different cellular components affects the regression of the vessels, the sprouting angiogenesis, the vascular remodeling, and vessel differentiation events that are involved. These cellular components include neurons, glia, endothelial cells, pericytes, and immune cells.

An invasion of migrating astrocytes coming from the optic nerve and going into the retina precedes the development of the retinal vasculature. They begin from the optic nerve head and extend to the retina's inner surface in a centrifugal fashion as a cell population that is proliferating. They form a cellular mesh-like network which provides the blood vessels a template in their wake. Astrocytes experience hypoxia and express the vascular endothelial growth factor (VEGF) strongly before blood vessels cover it. VEFG is the key stimulus for angiogenesis. It induces the endothelial cells' migration and the nascent vascular network expansion over then retina's inner surface. Bit by bit, the astrocytes start to downregulate the VEGF expression right after a perfused vascular network has formed. There emerges a typical stellate morphology of the retinal blood vessels. Sprouting angiogenesis are also believed to have formed the deeper networks of the retinal vasculature where new vessel sprouts are formed by proliferating endothelial cells and the vascular network is extended from pre-existing vessels [1].

The presence of astrocytes is related to the blood vessels in the retina. Retinal astrocytes and blood vessels are covering the entire retina in primates, except for the primate fovea, which does not have retinal astrocytes and blood vessels. Combined together, these observations imply that the retinal astrocyte network and the retinal vasculature are linked developmentally and are evolutionary.

The process of *retinal angiogenesis* explains why the pattern of retinal vascular network appears quite uniform in the population. Near the disc, the arterioles are more heavily concentrated in the outer choroid, especially nasal and temporal to the disc. The choroidal arteries are wavy or rippled, some like corkscrews, some with one or more 360° loops, and many with rather tightly twisted S-shaped turns.

**81**

*Retina Recognition Using Crossings and Bifurcations DOI: http://dx.doi.org/10.5772/intechopen.96142*

laries only a short distance from the parent trunk.

**2.4 Tools for medical examination and biometric scanning**

smaller branches [10].

*2.4.1 Medical devices*

visual examination of the retina.

area and need cooperation of patients [11].

When the outer vessels are gently removed, the smaller vessels are visible. Posteriorly, most prominently in the submuscular region, the vessels in the middle layers are highly complicated. Branches are usually not equal in length. The angles formed by two twigs range from 30° or 40° to 180°, and T-shaped branchings are common. After the bifurcation, a branch may continue in a straight path, making a sweeping C-shaped curve of 240° to almost 360° and diving inward to enter capil-

Most of the bifurcations of the larger choroidal arteries are dichotomous and the very acute angle formed by the offshoots points toward the disc. Second and third branchings may occur almost immediately so that a parent vessel seems to break up, fanlike, into four or six radiating branches after the first branching. Anastomoses between the larger choroidal arteries are not common but are frequent in the

An image is acquired in the retina, like how a camera captures it. The beam first passes through the pupil of the eye then appears in the focus of the lens on the retina, resembling a film. Specialized optical devices are used in medicine for the

We will first describe the existing medical devices used for retinal acquisition and examination, followed by biometric devices. High-quality scans of the retina are provided by specialized medical devices. However, two significant disadvantages cause their failure in the biometric market. First is the very high cost, ranging from thousands (used devices) to tens of thousands of euro. Second, medical personnel is needed for data acquisition since medical devices only have the manual or semi-automatic mode. Up to now, there is still no device in the market that can work in fully-automatic mode (without user interaction). The device is still in development; however, its price is already estimated too high for the biometric market.

A frequently used ophthalmologist's device for examining the human retina is a *direct ophthalmoscope*. The doctor examines the retina through the pupil at only a few centimeters. The disadvantage of this device is the relatively small observed

For a more thorough observation of the retina, it is appropriate to use a so-called *fundus camera* (as be seen at **Figure 3**). After a relatively long acquisition and focus, the camera takes a picture of the back of the eye in a short time and the doctor can go through it for any length of time, or look for changes in the retina since the last examination. By rotating the eye, a large area of the retinal area can be examined. A *slit lamp* allows examining the eye's anterior segment using *biomicroscopy.* This, along with direct and indirect ophthalmoscopy, are the main ophthalmoscopic examination methods for the eye's anterior and posterior parts, where the *slit lamp* is the most widely used. A *fundus camera*, also known as a *retinal camera*, is a special device to display the optic nerve's posterior segment, the yellow spots, and the retina's peripheral part. It works by indirect ophthalmoscopy where the instrument has a builtin source of primary white light, which can be modified by different types of filters. The optical system focuses on the human eye where the light reflects from the retina and bounces back to the lens of the fundus camera. If mydriasis has to be first applied to the eye, then a mydriatic fundus camera is used. Its intention is to enlarge the "inlet opening" of the pupil, which allows scanning a larger portion of the retina. Non-mydriatic fundus cameras are favored as no procedure is done that

*Retina Recognition Using Crossings and Bifurcations DOI: http://dx.doi.org/10.5772/intechopen.96142*

*Applications of Pattern Recognition*

each other, the presence of bifurcations (division points of a single vessel), crossings (intersection points of two or multiple vessels), ending points of vessels etc., which

*Slit lamp example (left) [6]; example of a non-mydriatic fundus camera (middle) [7]; (right) fundus camera* 

Retinal vasculature occurs mainly by *angiogenesis*. Its formation and the factors that regulate the development of superficial retinal vascular plexus in humans are already quite known. The cell–cell signaling that occurs between different cellular components affects the regression of the vessels, the sprouting angiogenesis, the vascular remodeling, and vessel differentiation events that are involved. These cellular components include neurons, glia, endothelial cells, pericytes, and immune cells. An invasion of migrating astrocytes coming from the optic nerve and going into the retina precedes the development of the retinal vasculature. They begin from the optic nerve head and extend to the retina's inner surface in a centrifugal fashion as a cell population that is proliferating. They form a cellular mesh-like network which provides the blood vessels a template in their wake. Astrocytes experience hypoxia and express the vascular endothelial growth factor (VEGF) strongly before blood vessels cover it. VEFG is the key stimulus for angiogenesis. It induces the endothelial cells' migration and the nascent vascular network expansion over then retina's inner surface. Bit by bit, the astrocytes start to downregulate the VEGF expression right after a perfused vascular network has formed. There emerges a typical stellate morphology of the retinal blood vessels. Sprouting angiogenesis are also believed to have formed the deeper networks of the retinal vasculature where new vessel sprouts are formed by proliferating endothelial cells and the vascular network is

The presence of astrocytes is related to the blood vessels in the retina. Retinal astrocytes and blood vessels are covering the entire retina in primates, except for the primate fovea, which does not have retinal astrocytes and blood vessels. Combined together, these observations imply that the retinal astrocyte network and

The process of *retinal angiogenesis* explains why the pattern of retinal vascular network appears quite uniform in the population. Near the disc, the arterioles are more heavily concentrated in the outer choroid, especially nasal and temporal to the disc. The choroidal arteries are wavy or rippled, some like corkscrews, some with one or more 360° loops, and many with rather tightly twisted S-shaped turns.

the retinal vasculature are linked developmentally and are evolutionary.

are all in different locations and in various numbers.

*2.3.1 Medicine basis of vein structure*

*used in the biometric laboratory at FIT BUT (right).*

**Figure 3.**

extended from pre-existing vessels [1].

**80**

When the outer vessels are gently removed, the smaller vessels are visible. Posteriorly, most prominently in the submuscular region, the vessels in the middle layers are highly complicated. Branches are usually not equal in length. The angles formed by two twigs range from 30° or 40° to 180°, and T-shaped branchings are common. After the bifurcation, a branch may continue in a straight path, making a sweeping C-shaped curve of 240° to almost 360° and diving inward to enter capillaries only a short distance from the parent trunk.

Most of the bifurcations of the larger choroidal arteries are dichotomous and the very acute angle formed by the offshoots points toward the disc. Second and third branchings may occur almost immediately so that a parent vessel seems to break up, fanlike, into four or six radiating branches after the first branching. Anastomoses between the larger choroidal arteries are not common but are frequent in the smaller branches [10].

#### **2.4 Tools for medical examination and biometric scanning**

An image is acquired in the retina, like how a camera captures it. The beam first passes through the pupil of the eye then appears in the focus of the lens on the retina, resembling a film. Specialized optical devices are used in medicine for the visual examination of the retina.

We will first describe the existing medical devices used for retinal acquisition and examination, followed by biometric devices. High-quality scans of the retina are provided by specialized medical devices. However, two significant disadvantages cause their failure in the biometric market. First is the very high cost, ranging from thousands (used devices) to tens of thousands of euro. Second, medical personnel is needed for data acquisition since medical devices only have the manual or semi-automatic mode. Up to now, there is still no device in the market that can work in fully-automatic mode (without user interaction). The device is still in development; however, its price is already estimated too high for the biometric market.

#### *2.4.1 Medical devices*

A frequently used ophthalmologist's device for examining the human retina is a *direct ophthalmoscope*. The doctor examines the retina through the pupil at only a few centimeters. The disadvantage of this device is the relatively small observed area and need cooperation of patients [11].

For a more thorough observation of the retina, it is appropriate to use a so-called *fundus camera* (as be seen at **Figure 3**). After a relatively long acquisition and focus, the camera takes a picture of the back of the eye in a short time and the doctor can go through it for any length of time, or look for changes in the retina since the last examination. By rotating the eye, a large area of the retinal area can be examined.

A *slit lamp* allows examining the eye's anterior segment using *biomicroscopy.* This, along with direct and indirect ophthalmoscopy, are the main ophthalmoscopic examination methods for the eye's anterior and posterior parts, where the *slit lamp* is the most widely used. A *fundus camera*, also known as a *retinal camera*, is a special device to display the optic nerve's posterior segment, the yellow spots, and the retina's peripheral part. It works by indirect ophthalmoscopy where the instrument has a builtin source of primary white light, which can be modified by different types of filters.

The optical system focuses on the human eye where the light reflects from the retina and bounces back to the lens of the fundus camera. If mydriasis has to be first applied to the eye, then a mydriatic fundus camera is used. Its intention is to enlarge the "inlet opening" of the pupil, which allows scanning a larger portion of the retina. Non-mydriatic fundus cameras are favored as no procedure is done that

**Figure 4.** *Principle for obtaining an eye background image [12].*

will alter the normal sight of the subject. Mydriasis, on the other hand, is required for some subjects. The costs of these devices are calculated by medically specialized workplaces and are in tens of thousands of euros.

The optical device has a complex mechanical construction. The scanning device works based on the concept of medical eye-optic devices. These retinoscopes or fundus cameras are complex devices which are also quite expensive.

The reflection of a part of the light that came from a beam and hit the retina is scanned by the CCD camera. This concept is similar to retinoscope, where the eye lens concentrates on the retina's surface due to the adjustment made to the beam of light that is coming from it. The ophthalmic lens receives back the reflection of only a part of the transmitted light beam and readjusts it. The beam leaves the eye below the angle where it entered the eye (return reflection). An image showing the eye's surface can be obtained at roughly 10° surrounding the visual axis, as in **Figure 4**. A circular snapshot of the retina is captured by the device from the reflection of light coming from the cornea, which would be useless in raster scanning [13].
