I had an ultra sound done and apparently I have dead tissue about 3cmx3cm. What could be causing it?

It is not clear to me what context you are talking about.  Did you have a breast reduction or a tummy tuck or something else entirely?  Or didn't you have any surgery at all?Sometimes tissue that is not alive needs to be removed, but sometimes it can just be left alone (if it doesn't have bacteria living in it), or your body will reabsorb it.Anything that is not alive won't regrow.  However if it is a tissue like your omentum, a fatty apron that covers the abdominal organs, it isn't entirely needed.Without more information I'm afaid I can't give you much more insight than that. 

I'm sorry but I can't answer your question. We just don't have enough information. You would have to be much more specific of the exact area, the MRI report, the type of tissue that is dead, did you have a surgery or an injury, your age, health status, etc,etc