How to obtain the correct data points to fit circle for optic disk detection

Please help, I am struggling to get this right:

I have obtained a thresholded image, which is in binary image format. My problem is from this image I want to locate the round optic disk by means of the kasa method. Optic disk is the biggest and the lightest circle in the retina image. I want to illuminate this, because this is normal for every eye. I know about bw label and region props. But Please in exact coding, how must I get the the xy datapoints to give to the function of KASA to draw circle. The circle must be drawn around where the most ones (maximumx) in the binary image are present. I want a circle be drawn effectively around the optic disk for every retina image.

Here is my MATLAB code and a image.

figure,imshow(a),'title binary image';

[L,num]=bwlabel(a);

[y x] = find(a);

XY=[y x] ;

figure,imshow(a),'title binary image';

hold on

 Par = CircleFitByKasa(XY)

 r=Par(3);

 x=Par(2);

 y=Par(1);

th = 0:pi/50:2*pi;

xunit = r * cos(th) + x;

yunit = r * sin(th) + y;

h = plot(xunit, yunit);

hold off

% [L,num]=bwlabel(a);

% blobMeasurements = regionprops(L, 'all')

% [r, c,v] = find(L);

% rbar=mean(r);

% cbar=mean(c);

% datapoints=[r,c,v];

% s = regionprops(L,'area');

% CC = bwconncomp(a)

[Valeska Pearson]

http://imgur.com/PIgeATD LAB colourspace image

After filter and threshold image will look like this http://imgur.com/ziU7IxL

[Image Analyst]

Which cluster of dots is the optic disc? Does it have a solid contiguous outline, or is it broken?

[gojiya]

plz i want to find optical disc .... help me