Automatic Diagnosis of

Astigmatism for Pentacam

Sagittal maps

By: Sarah Ali Hasan & Dr. M.D Singh

Outline: Background

Human Eye & facts about Cornea

Astigmatism physics & types

Tools

Corneal Topography & PENTACAM

Characterizing Astigmatic Corneas

Methodology

Results

Comparison to Related previous work

Future work

Anatomy of human eye

External structures:

eyelids, eyelashes, tears

and fat glands, extra ocular

muscles, conjunctiva.

Internal structures:

cornea, sclera, iris, ciliary

body, choroid, retina, lens,

anterior and compartment,

optic nerve.

Spheroid structure with an average diameter of 24mm.

Cornea scatters almost

10% of the incident light.

Cornea’s average of

curvature is 7.8 mm.

Cornea is accounting for

about 43-44 diopters at

corneal Apex (that is: 2/3 of

total dioptric power of

human eye).

Corneal Geography:

Apex: central zone

(4mm), overlies the pupil &

responsible For HD vision.

Paracentral Zone:

Where the cornea begins

to flatten.

Peripheral zone.

Limbal zone.

Facts about Cornea

Astigmatism physics & types

Either both or one surface of

the cornea or the lenses is

not spherical, but cylindrical

Which results in no distinct

point of focus inside the eye;

rather a smeared or spread

out foci.

Astigmatism is of two types;

regular & irregular.

One eye can has two

refractive errors in the

perpendicular meridians;

each one is a resultant of

spherical error + astigmatic

error.

Steep meridian is the one of

higher degree of curvature

and the flat meridian is the

one with less degree of

curvature

Position of meridians

decides the type of regular

astigmatism case.

Continued..

Corneal Topography & PENTACAM

A non-invasive medical

imaging technique for

mapping the surface

curvature of the cornea.

The corneal topography

and color coded maps

derived from quantitative

analysis of numerous

surface points.

Physicians interpret the

color coded images to

diagnose & treat patients

with eye refracting issues.

Corneal topographers is of two types:

Projection based corneal

topography, from which Curvature

maps like Saggital maps are driven.

Reflection Based corneal

topography, from which Elevation

maps are driven.

PENTACAM is a combined device

consisting of a slit illumination system

and a Scheimpflug camera which

rotates around the eye.

This camera rotates around the camera 360° in 2seconds to capture 50

images for the surface of cornea, all

are centered at the cornea center.

Continued..

Characterizing Corneas

Continued..

Methodology The aim is to suggest a computerized technique to help

making this decision on diagnosing Astigmatic corneas.

Sample image

Data &Results

Data base is a selected

group of (84) PETACAM

topographic images.

All the samples are for

patients of Dr.G.S

Randhawa Eye Hospital.

Correctly diagnosed &

missed cases were

distributed over

confusion matrix for

accuracy calculation.

Classification accuracy

is approximately 80%.

Continued..

Comparison to Related previous work

Neural Networks classification.

Zernike polynomials analysis .

Also many ophthalmologists provided similar criterion

for keratoconus detection.

We used image processing techniques such as

morphological operations and cross correlation to

classify and identify the cases.

Astigmatism diagnosis and classification is not

discussed as much as Astigmatism treatment in the

literature , it is still done manually by interpreting the

corneal topographic and tomographic images.

The algorithm presented in this paper is novel.

Future work

Pixel values can be included as they represent the

diopter values at the specific meridians and the last have

such an important rule on deciding the diagnosis of the

refractive error.

Performing all the image processing techniques on the

colored images.

Including the scale provided beside the color coded

topographic image.

THANK YOU !