Present digital radiography utilizes digital X ray detectors of several types. Well know flat panel detector was developed in recent years and it is already employed in many branches. Although this detectors overcame some obstacles of the X ray detectors before still some troubles like dark current remained. Properties of the flat panel will be compared with nowadays semiconductor single photon counting detector of the Medipix type which is still under development. It will be shown, that concept of the single photon counting detector is more appropriate for the small biological object imaging thanks to better spatial resolution and sensitivity on low energy photons.