Yes it is

If you have no contrast, you will not have any edges to detect. However, the thing that you really want to maximize is your contrast to noise ratio.

Thank you for the reply.

I m trying to get a clean edge detection using canny operator. I've found some references talking about supervised Learning of edges. I don't know if it can be applied to bone detection in MRI.

Whether you need contrast enhancement really depends the quality of your images. I have got away without doing any enhancement in some MR images.

I think you might want to normalize the image intensities though. Otherwise you may have to retune your edge detection parameters each time for a new image, especially if they are acquired using different machines.