how to apply a contour to an image

Hello,

Does anyone knows how to apply a red contour to the lungs in this image?

I want something like this:

Thank you so much!

Answer 1

[Image Analyst]

Try something like this (untested)

hold on;
boundaries = bwboundaries(grayImage > 0);
numberOfBoundaries = size(boundaries, 1);
for k = 1 : numberOfBoundaries
  thisBoundary = boundaries{k};
  plot(thisBoundary(:,2), thisBoundary(:,1), 'r-', 'LineWidth', 2);
end
hold off;

[Ana SIlva]

First of all, thank you for the answer :)

unfortunately , the output of that code is:

the objective is to fuse this two outputs