How are ketone bodies used?
First, there are three ketone bodies: Acetone (top), acetoacetic acid (middle), and beta-hydroxybutyric acid (bottom), see the illustration from the Wikipedia:



The second and the third are taken up by heart and brain cells and then converted into Acetyl-CoA which is fed into the citric acid cycle where it is further metabolized. Acetone is mostly excreted. See here for some more information.

Aceto acetic acid is either converted into acetone and removed from the body or enzymatically converted into beta-hydroxybutyric acid by the beta-hydroxybutyrate dehydrogenase.
The third possibility is that it is converted into Acetyl-CoA. See the figure for the scheme:



For a more detailed explanation of this process (both, the generation and the metabolization of the ketone bodies) see here.