Extracting a region of interest from a contour map of gradient values

I've used the imgradient3() function on one of my test images in order to subsequently establish a contour map that will graphically demonstrate the areas of interest. For instance, after running the imgradient3 and contour functions, I achieve the following image:

The brightest regions of my original image will correlate to the minimum gradient value i.e. the white contours in the image. My issue is being to extract the white regions in the form of a mask on which I can run further analysis and processing. At the present I only have the gradient magnitudes.