Since you are asking for a way in which you can get the non- NaN elements, I suppose you are having a matrix which corresponds to the image. Let me know if that is not the case.
Now, I would suggest you to make use of the function "isnan". isnan(A) returns an array with the same size as A containing logical 1 (true) where the elements of A are NaNs and logical 0 (false) where they are not.
So if it's just a 1D vector ( say y), you can simply do something like this:
You can read about this in detail in the below link:
Hope it helps!!