Mukherjee,Jayanta, Das,P.P and Chatterji,B.N.(1989):
Thinning of 3-D Images Using Safe Point Thinning Algorithm (SPTA),
Pattern Recognition Letters, 10, pp.167-173.  Abstract. 
In this paper the Extended Safe Point Thinning Algorithm (ESPTA) is presented for thinning 3D images.
It is developed on a 2D algorithm by Naccache and Singhal (1984). It is shown that ESPTA
preserves the 18-connectivity of the image. Some experimental results with ESPTA will also be discussed.