How to extend this boundary line?

Dear all,

I have this boundary line

and I would like to extend this blue line (boundary points). Is there any option for my problem?

I attach my code.And original image.

[Image Analyst]

I don't know what "extend" means to you. Do you mean like you want to enlarge the region, like as if you used imdilate() or something?

[Veronika]

Yes, this is actually I want. I want enlarge the region, because as you can see this region (thorax) isn´t accurate, there is small space to enlarge this blue boundary line.

Answer 1

[Image Analyst]

The threshold value of 180 probably needs to be adjusted for this image.

[Veronika]

But I don´t know how....Do you think, that I can´t change only the boundary blue line or points somehow?