Feature extraction using SURF with ROI as one input argument

Can someone help me to give one example for how to specify ROI of the image so that it can be feed as one input argument to SURF.

My prgram is,

I=imread('ncp.png');
points = detectSURFFeatures(I);
imshow(I); hold on;
plot(points.selectStrongest(100));

This is my input image. How can I mention ROI as the 2 lungs only so that SURF will gives features from lungs only.

Please help.

Answer 1

[Image Analyst]

See my attached lung segmentation demo. Adapt as needed.

[SILPA C S]

Thank You for the response @Image Analyst, but I can't set the threshold value manually. I have large number of CT images, so how I could set threshold value automatically.