Edge detection using morphology in Matlab

I'm trying to figure out how to detect edges using morphology in Matlab. I have this image:

and I'm trying to get the following results:

but I can't get anything close to that. The only thing I was able to do is this:

a = imread("mri.png");
SEedge = strel('square',5);

b = imdilate(a, SEedge);
c=b-a;

imshow(c),title("1");

And I got the following results:

But this is not good enough. I know about built-in functions in Matlab but I don't want to use them, I want to do it with erosion or dilation and my own structuring element. Could anyone help me figure this out or provide some useful resources?

[Mary Gh]

what are built-in functions? Did you find any answer? If you found please share it here I need it. Thank you