This is a tittle of some paper I found online.
A quantitative theory of the Hounsfield unit and its application to dual energy scanning
There are two energies in this CT system. But why the author used "Dual Energy" instead of "Dual Energies"?
Another example: Multi-energy CT imaging, multi panel canvas set, etc.
"Dual energy" is also commonly written as "dual-energy".
This is not the first time I notice this kind of naming method. The title above is just an example I found recently.