To segment a specific region in an image

Hi all, i need help with this task. I'm very new to Matlab and currently trying to do some image segmentation work here. I have an image in DICOM format. I have now able to read it, and convert it to binary image using bwlabel function. But now i'm stuck at how to segment and get only the specific region that i want in the image.. My code as in below. How if i only want to segment and obtain the round shape object (lumen, at the middle of the image)? Need your help and advice so that i can proceed. Thanks a lot.

info = dicominfo('C:\Users\aho3\Desktop\Matlab\CT Data\IM-0001-0001.dcm');
i = dicomread(info);
HU = i*1 - 1024;
[nx ny] = size(HU);
for i = 1:nx 
    for j = 1:ny 
        if (HU(i,j) < 230) 
            ucut(i,j) = 0; 
        end 
        if ((HU(i,j) >= 230) && (HU(i,j) <= 580)) 
            ucut(i,j) = 1; 
        end
    end
end
Lumen = bwlabel(ucut); 
figure; imshow(Lumen);