advice on anlysing the spacing of growth rings

Hello,

I have an image of a fatigue crack that I want to analyse. It is simular 
to a photograph of the cross section of a (non-circular) tree showing the 
tree rings that indicate the growth of the tree. I would like to determine 
the spacing of the repeated growth patterns without actually picking off 
individual points (there are too many). Could someone give me an 
indication of how I could tackle this. It may be similar to a geographical 
mapping approach. Can anyone recommand any R packages that I should look 
at ?

Thanks
Paul

Paul White
Defence Science and Technology Organisation
Department of Defence
Melbourne, Australia