Something like this will do it:
A=imread('https://www.mathworks.com/matlabcentral/answers/uploaded_files/524729/image.png');
%convert image back to logical mask
L=im2double(im2gray(A))>0.5;
width=zeros(size(L,2),1);
for n=1:size(L,2)
tmp=L(:,n);
tmp=diff([find(tmp,1,'first') find(tmp,1,'last')]);
tmp=sort([0 tmp]);tmp=tmp(end);%catch empty case
width(n)=tmp;
end
width=median(width);
disp(width)
