> "Whenever a high energy photon strikes the CZT, it mobilises an electron and this electrical signal can be used to make an image. Earlier scanner technology used a two-step process, which was not as precise."
I understand the unnamed alternative is the scintillation-type detector, where high-energy photons induce fluorescence, emitting secondary photons of lower energy. Detecting the secondary photons (converting them to electrons) is the second step.
https://en.wikipedia.org/wiki/Scintillator