NURBS curve extraction from an image

I am trying to extract the edge of an image and convert it to a NURBS curve. But it ended in vain. I am using NURBS toolbox for matlab. I am attaching the code i used below. The image i used is a simple circle image. Please help me out.

Code:

close all; clear all;
im=imread('im\1.bmp');
%imshow(im);
imBW=im2bw(im,.8);
%figure; imshow(imBW);
imE=edge(imBW);
imE=not(imE);
%figure; imshow(imE);
imSIZE=size(imE);

%NURB Curve Formation
imNRB=[];
KNT=[];
n=1;
tmp=[];
cnt=1;
Kcnt=1;
for i=1:imSIZE(1)
    for j=1:imSIZE(2)
        if imE(i,j) == 0
            tmp=[tmp;i j];
            if cnt == 2
                imNRB(:,:,n)=tmp;
                KNT{1,Kcnt}=[0 0 1 1];
                n=n+1;
                tmp=[];
                cnt=0;
                Kcnt=Kcnt+1;
            end         
            cnt=cnt+1;
        end
    end
end

NRB=nrbmak(imNRB,KNT);
clear n;
nrbplot(NRB,[600 -600]);

And i have to attach this note. I am not well versed in NURBS.

Thank you :)