If an exterior output vibration signal/wave is created to target a hollow bone cavity under the skin surface, and if the hollow space within that bone cavity is empty vs. fluid filled, would there be a dissimilar vibration return signal to differentiate between the empty vs. fluid filled cavity space, and if so, why?
Yes, it is possible to differentiate between an empty vs. fluid-filled cavity space by comparing the amplitude of the return signal from the bone cavity to a known reference signal.
The reason is that the vibration signal/wave is a mechanical wave. When the vibration wave encounters an empty cavity, it will produce a return signal with low amplitude because air is a poor conductor of mechanical waves and the vibration wave will not interact with the air strongly. In contrast, fluids are good conductors of mechanical waves. When the vibration wave encounters fluids, it will interact strongly with the fluids, producing a strong return signal with a high amplitude. Therefore, by comparing the amplitude of the return signal, it is possible to differentiate between an empty cavity and a cavity filled with fluid.
It is worth noting that the feasibility and accuracy of the above approach depend on many factors, such as the frequency and amplitude of the vibration wave, the shape and size of the cavity, the properties of surrounding materials etc. It may need further investigation.