coherent scattering 

This effect is not very important in Radiotheray / Radiology . When the interacting photon passes through the orbital electron of the target atom, it starts occluding. This results in the emission of a recurring energy similar to the incident photon. 

Here the incident beam is scattering without any energy loss, that is, the photon changes its direction. This phenomenon is more in the state of high atomic number and low photon energy. 

coherent scattering 

Note  

Bound electron

When the energy of the photon is equal to or slightly higher than that of the interating electron, it is called bound or bound electron. 

Free electron

When the energy of photon is much higher than the interacting electron, it is called free electron.