As the region of interest is clearly distinct, you can directly obtain the results by binarization of the input image. You may have to set thrshold limit accordingly. Considering the input as a gray image, if not, please do convert the image to a gray image, using the rgb2gray function.
bwImage=imbinarize(grayImage,0.1);
%Clean all the small blobs connenetd with 100 pixels
bwImage=~bwareaopen(~bwImage,100);
%.............................^Do Modify as per requirements
figure,imshow(bwImage);
