Scanner for corneal mapping

The optical system introduced here is aimed to map both external and internal corneal surfaces. It is also designed to measure the corneal thickness and irregularity in Three Dimensions (3D).

Because the system is an optical system, it has numbers of benefits:
1. It is a non-invasive operation.
2. It is a non-contact process.
3. It is transpupillary.
4. It can achieve high resolution, up to one micron.
5. it can be performed pre-, intra- and post-operatively
6. It is miniature, light and easy to operate.

The marketing applications of such a device are:
1. The medical profession. Real-time and high scanning abilities are required during surgery.
2. The medical diagnostic. It was concluded that a wide range of eye illnesses can be diagnosed by measuring accurately the cornea.
3. The specialised contact-lenses companies. Some eye related illnesses are corrected by customised contact-lenses.
4. The opticians and vision companies in the high-street. The device will assist in fitting off-the-shelf contact-lenses requiring cornea shape assessment.

Hence, the system has additional advantages compared with the preceding devices in the market. This document highlights the following:
1. It offers comprehensive introduction of the object to be measured, the cornea, its properties and essentials.
2. It examines all existing cornea measuring devices (up to date), explaining their working concept, their market place and their disadvantages/ advantages.
3. It proposes the market exploitation and product opportunities.
4. Finally, this document provides a novel and an innovative solution for mapping the cornea. Detailed description of the system design and insights into the integration concept are also given.

The cornea
It is a transparent dome located in the anterior part of the human eye as part of the ocular tunic. The cornea protrudes slightly beyond the scleral globe (the white of the eye) because of the different curvatures of the two structures.

The cornea is a vital eye structure. It has an array of essential functions:
1. preserves clarity.
2. comprises ocular defence mechanisms.
3. acts as a powerful converging lens system.
4. provides two-thirds of the eye's refractive power and must have smooth surfaces and a high degree of transparency for orderly refraction of light rays with minimal light scattering at the visible wavelength region. (the corneal index of refraction is about 1.376)
5. performs as a filter, screening out some of the most damaging ultraviolet (UV) wavelengths in sunlight.

Normally, up to 90% of the incident light is transmitted through the cornea. This is achieved by the cornea’s physical factors such as:
1. a smooth anterior surface.
2. uniform and regular arrangement of the epithelial cells, closely packed stromal lamellae of uniform size, and the absence of vasculature.
3. and corneal hydration.

Cornea physical dimensions
Externally, the cornea appears elliptical with its vertical chord shorter than its horizontal chord. For example, approx. 10.6mm versus 11.7 mm for males and 9.6mm versus 10.7 mm for females. This difference arises from opaque scleral tissue extending over the anterior corneal margin slightly more along its superior and inferior aspects.

When viewed from within the dissected globe, the posterior cornea appears circular with a diameter of about 11.7 mm. The external corneal surface area is approximately 1.3 mm2 or about 1/14th of the total area of the globe.

Still in these days, a schematic eye calculation is used as an approximate tool. The anterior and posterior surfaces of the human cornea can be calculated by radii of curvature of 7.8mm and 6.5 mm, respectively, compared to the external surface of the scleral globe, which has a radius of approximately 11.5 mm.

Peripherally, the cornea's anterior surface tends to become slightly flatter compared with its central curvature, giving the cornea a prolate shape. This flattening reduces, but does not entirely eliminate, the corneal contribution of spherical aberration to the optical system of the eye.

Nevertheless, the difference in curvature between the anterior and posterior corneal surfaces results from the central cornea being relatively thinner at its periphery.

Optically, the radius of curvature of the anterior surface translates into a vergence power of approximately 48.8 D (diopters), which accounts for roughly three quarters of the total refractive power of the eye's optical system.

Corneal Pachymetry
Primarily, corneal thickness measurements (Corneal Pachymetry) have been used in the evaluation of persons with corneal diseases. A development in the Corneal Pachymetry techniques leads to other useful assignments such as:
1. assessing candidates for penetrating keratoplasty (corneal transplant), and graft failure.
2. considering the need for regrafting in corneal transplant recipients by aiding in the early diagnosis and treatment of graft rejection.
3. taking into account the response to treatment of corneal transplant rejection.
4. and considering a progression of disease in patients with certain corneal dystrophies and degenerative diseases. (The cornea may be thinner than usual in Keratoconus patients)

However, with the increasing popularity of corneal plane refractive surgery such as:
1. Excimer laser procedures, Photo-Refractive Keratoplasty (PRK)
2. Laser assisted in-situ keratomeliuis (LASIK), known to the masses as ‘laser vision correction treatment’.
3. and Intra-Corneal rings (Intacts / Ferrara rings) corneal pachymetry.
It is essential in pre-operative evaluation for correct patient selection and avoiding possible further complications.

Surgeons wish to take every possible safeguard to avoid excessive ablation and consequent keratectasia (in the case where the laser removes too much tissue during LASIK or the flap made too deep) in their LASIK patients. Many surgeons also believe that intra-operative pachymetry is mandatory, if the thin residual cornea has been associated with post refractive surgery corneal ectasia.

Other studies such as ‘the Ocular Hypertension Treatment Study; Kass, et al;, 2002; Gordon, et al., 2002’ links the central corneal thickness as a predictor of development of glaucoma (eye-condition with increased pressure in the eyeball and gradual loss of sight).

Hence, the American Academy of Ophthalmology prefers a practice pattern on evaluation of the glaucoma suspect patients and it recommends the intra-operative pachymetry as the preferred method.

The cornea thickness measurement also demonstrated the importance of at the Ocular Hypertension Treatment Studies where individuals with thicker corneas may be misclassified as having ocular hypertension.


Corneal Pachymetry techniques
Intensive research shows that there are only four established techniques for performing intra-operative corneal pachymetry. (Some of the techniques listed below are named as the device itself)
1. Corneal optical pachymetry, a technique using an optical technology.
2. Orbscan II, a device name, using an imaging technology.
3. Ultrasonic Pachymetry, a technique using a sound technology.
4. Optical Coherence Topography (OCT) Pachymetry, an optical technique.

Corneal optical pachymetry
Corneal optical pachymetry is the measurement of the thickness of the optical cross section of the cornea viewed in a slit lamp. The slit lamp using oblique illumination observes an oblique slice of the cornea.

The apparent thickness is a physical measurement made by moving an optical marker from the front to the back of the cornea. The optical pachymeter produces two images of the oblique section. Rotating the top image (front surface) and aligning it to the bottom image (back surface) moves an optical marker from the front to the back of the cornea.

The observation microscope is set at an angle with the illumination system. The cornea thickness is the hypotenuse of the triangle in which the oblique optical section is the base. The cornea thickness equals the oblique optical section divided by the sine of the observation angle.

Orbscan II
The orbscan uses visible white light slits in order to optically measure the corneal thickness. The use of 20 slits scanning the cornea enables the instrument to calculate a thickness map of the cornea which makes it a good screening tool for pre-operative evaluation. The instrument is large and cannot be used for intra-operative measurements.


Ultrasonic Pachymetry
Ultrasonic corneal pachymetry is performed by placing an ultrasonic probe on the central cornea, after the cornea has been anesthetized with a topical anesthetic.

A technician can operate the pachymeter and it normally takes less than 30 seconds per eye to complete measurements. The probe measures the corneal thickness at a certain point only and the measurement can not be repeated in exactly the same spot. Corneal thickness maps are not routinely available with ultrasonic devices. Ultrasonic pachymetry is considered the gold standard today.


Optical Coherence Topography (OCT ) Pachymetry
The OCT is able to measure the corneal structure by obtaining the time delay of reflected or backscattered light using low-coherence interferometry. It uses light in the IR range (843 nm). Recent reports have shown good agreement between OCT corneal thickness measurements and ultrasonic pachymetry.

OCT devices are still very bulky, have a rather prolonged acquisition time and are reported to be very expensive so only a few institutions can afford them. There are no commercially available OCT pachymeters for intra-operative measurements.

Market exploitation

The marketing applications of such a high-speed laser scanning device are huge. The hardware design of the optical concept introduced here will remain the same, whereas, the user end application will be different. This will allow reaching several solutions by employing a single development cost. For example, a mechanical structure of the instrument can be manufactured as a probe or as rest head rig.


The proposed system
The system proposed here implements the recent imaging technology integrated with a bespoke opto-mechanical arrangement. A software application, part of the integrated system, displays the scanning output in real-time. The software also implements novel mapping techniques and provides tools for further medical diagnoses.

The optical system uses a monochrome light, a laser (or LED) with a unique wavelength. The laser (or LED) beam is projected perpendicularly to the corneal centre through a thin cross slits disc. That produces a laser projection in a shape of a cross on the corneal sphere, whereas the centre of the cross is aligned (with approximation) to the corneal centre sphere.

The laser wavelength should produce maximum optical backscattering only on the anatomic layer of the cornea. It was suggested that the optical backscattering would not take place within the visible light (400nm-760nm) and not within the IR- A range (176-1400nm).
1. It could take place below 400nm within the UV-A range,
2. or it could take place above the IR-A, 1400nm up to 1mm.

The exact laser wavelength will differentiate the cornea from the rest of the scanning surroundings because corneal anatomic layer will be the most scattered layer during projection.

The three neighbouring CCDs, part of the optical system, will capture only the backscattering occurring on the corneal anatomic layer. High ratio signal-to-noise (up to 95%) will be achieved by using:
1. bypass filter ±2nm FWHM to the laser wavelength
2. polarised filter, again, with relations to the laser polarisation set-up.

The high-speed scanning will be achieved by the CCDs high acquisition rate and by the CCDs high frame rate (up to 60fps ´ 3 CCD = 180 fps in total). A computing system will manage the CCDs system and other hardware events during the scan.

The software application, part of the computing system, will accumulate in real-time the images from the CCDs system into a 3D model. Hence, the virtual 3D cornea model will be displayed simultaneously on a high-resolution screen.

The 3D mapping procedure will be accomplished by rotating (continually) the thin cross slits. The rotating element will rotate the projected cross shape on its centre, which is set to be on the cornea sphere centre (with approximation). This process will illustrate for the three stationary CCDs, a 3D view of the cornea sphere. As a result, it will generate a full cycle of images to the computing system for constructing the 3D virtual cornea model.

The lens element design is dependant upon the object size (i.e. cornea diameter and height), the CCD number of pixels, the CCD pixels size and the distance between object to image (i.e. the distance between the cameras from the eye).

A ring stepper motor with a resolution of 0.5° will drive the only opto-mechanical element in the system, the rotating thin disc, whereas the frame grabber will grab a frame every pulse event. Simultaneously, a pulse generator will generate a synchronised pulse for:
1. the driving of the stepper motor,
2. and for the triggering of the CCD frame grabber.

The slits thickness (laser beam width) should be as narrow as possible. It will improve object resolution and will reduce time in post-processing the data.


Regulations
A laser scanner of a human eye must comply with health and safety regulations. Below are listed the international regulations required for such as device.
• FDA 510K (regulation)
• EN 60601-1 // EN 60601-1-1 // EN 60601-1-2
• EN 60825-1
• EN 1441 // EN 1041
• EN 980
A health and safety regulation expert should examine these regulations and suggest any additional concerns that may impact.