How to calculate the radius of a ring in MATLAB?

Question

I have a stack of cortical bone images, high resolution and binarization. How do I calculate the average internal and external radii for each image? Here is an example of the images I need to process:

Answer 1

This could be one approach -

%// Read in image and conert to binary
im = im2bw(imread('http://s9.postimg.org/aew1l7tvz/4_COPY_copy.png'));
figure, imshow(im), title('Original Image')

%// Fill holes giving us the "outer blob"
outer_blob = imfill(im,'holes');
outer_blob = biggest_blob(outer_blob); %// remove noise
figure, imshow(outer_blob), title('Outer Blob')

%// Get the inner filled blob by "removing" the original image from outer blob
inner_blob = outer_blob & ~im;
inner_blob = biggest_blob(inner_blob); %// remove noise
figure, imshow(inner_blob), title('Inner Blob')

%// Find the equivalent/mean inner and outer radii 
inner_dia  = regionprops(inner_blob,'Equivdiameter');
inner_radius = inner_dia.EquivDiameter/2

outer_dia  = regionprops(outer_blob,'Equivdiameter');
outer_radius = outer_dia.EquivDiameter/2

Associated Function Code -

function out = biggest_blob(BW)

%// Find and labels blobs in the binary image BW
[L, num] = bwlabel(BW, 8); 

%// Count of pixels in each blob, basically this should give the area of each blob
counts = sum(bsxfun(@eq,L(:),1:num)); 

%// Get the label(ind) cooresponding to blob with the maximum area 
%// which would be the biggest blob
[~,ind] = max(counts);

%// Get only the logical mask of the biggest blob by comparing all labels 
%// to the label(ind) of the biggest blob
out = (L==ind);

return;

Code execution and debugging images -

inner_radius =
  211.4740
outer_radius =
  267.8926