[R] taking a small piece of large tiff

[Louisa Reynolds]

Dear Forum 
I am trying to cut out a small section of a very large 2-dimensional grayscale image as a tiff in R, but it is having difficulty handling such large files.  I have looked at bigmemory and ff packages but it is unclear how I can use these packages with tiffs. Can anyone please suggest something? I have tried tiff and rtiff libraries.
Thanks in advance.
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