How do I segment the square inside the square?

Hi

How do I segment the square/rectangle inside the square image. All the rest should be masked. I want only the gray square in the middle.

[Rik]

This isn't your first question. What have you tried so far? And is that yellow line in your original data as well?

[Stelios Fanourakis]

No, the yellow line are indicators, to see where the scanning should start and where it should stop. I only want the grayscale square in the middle of the image. In the rect.jpg image I attach is the yellow rectangle guide that tells the area needs to be segmented. The height of the rectangle is determined by the vertical ruler at the right side of the image. That's why it goes down capturing some black area as well.

[Rik]

Closed as duplicate of this question.