Steve on Image Processing with MATLAB

Image processing concepts, algorithms, and MATLAB

Determining point positions in MRI peg phantom

Blog reader Jonathan from St. Jude Children's Research Hospital sent me an image derived from an MRI peg phantom:

bw = imread('');

Jonathan wanted to know how to determine the position of each point. The functions bwlabel andregionprops do the trick.

The function bwlabel takes a binary image and figures which groups of white pixels are connected to each other.

L = bwlabel(bw);

The output L is called a label matrix. It has the same size as bw and contains nonnegative integers. Each positive integer value corresponds to a particular object. For example, to display the 10th object, just compare L to 10:

imshow(L == 10)

The function regionprops computes a number of different geometric properties of all the different regions contained in a label matrix. All we need for this application is the centroid:

s = regionprops(L, 'Centroid')
s = 

205x1 struct array with fields:

The size of the structure array s tells you the number of labeled objects: 205. The centroid of the 10th object is:

centroid_10 = s(10).Centroid
centroid_10 =

   44.1687   77.8072

Here's a simple way to superimpose the centroid locations onto the original image:

hold on
for k = 1:numel(s)
    plot(s(k).Centroid(1), s(k).Centroid(2), 'x')
hold off

Thanks for letting me show this image in the blog, Jonathan.

Published with MATLAB® 7.2

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