You are using a very old Slicer version. Upgrade to at least the latest stable version. It might not solve application freezing, because that is most likely due to that you are running out of memory. You may need downsample your input image (but then you may lose some small details), increase virtual memory setting in your operating system (but then the software may slow down when your memory usage increases), or add more RAM to your computer (but it costs money and if you computer is old then it might not be feasible, and you may be better off buying a new computer).
Once you took care of the memory usage issue, you can segment the spine very quickly. You can either use classic methods, such as “Grow from seeds” effect with intensity masking followed by hole filling; or deep learning based methods, for example in MONAILabel. See more information in this post: