There are two technologies to get CT images; Fan-beam and Cone-beam. Our CT scanner is using cone-beam to take a x-ray image. The basic algorithm to reconstruct the image cross-sections is based on FDK (Feldkamp-Davis-Kress).
There are couple of open-source projects about this algorithm based on matlab.
http://www.mathworks.com/matlabcentral/fileexchange/23314-multithreaded-mex-fdk-conebeam-ct-reconstruction-algorithm
http://www.cs.toronto.edu/~nrezvani/OSCaR.html
Oscar overview
http://www.cs.toronto.edu/~nrezvani/PosterMITACS2008.pdf