A bit-stream image is a sector-by-sector / bit-by-bit copy of a hard drive. A bit-stream image is actually a set of files that can be used to create an exact copy of a hard drive, preserving all latent data in addition to the files and directory structures.  A bit-stream image can be read by the majority of the tools used by the Center for Computer Forensics to analyze the hard drive such as Encase, FTK, ProDiscover and many others.  By utilizing the bit-stream image,  the Center for Computer Forensics takes no risk of contaminating the original evidence.