The Fourier slice theorem is derived by taking the one-dimensional Fourier transform of a parallel projection and noting that it is equal to a slice of the two-dimensional Fourier transform of the original object. It follows that given the projection data, it should then be possible to estimate the object by simply performing the 2D inverse Fourier transform.
Start by defining the 2D Fourier transform of the object function as:
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http://www.eng.warwick.ac.uk/oel/courses/undergrad/lec13/fourier_slice.htm
Feng Li's Homepage (李冯) 
Andy，你这blog开的很学术，不是OpenGL，就是C++，或是Fourier transformation。 我那blog里面就是美女帅哥+数学人生价值观。呵呵
hehe, this might be useful:
Click to access CT_ch6_part1.pdf
Click to access CT_ch6_part2.pdf
ethel, thanks. o:)
老板说，没有一个小时还看不懂的SIGGRAPH paper，可是我为了查paper里用的一个小小定理，理解它就花了很多时间。。。