It looks like solarization but that is a phenomenon if film photography. I am not sure how you create that in digital, but I suspect overloading of the ADC or corruption of the ADC data. You could try using a more predictable object to image. I am not sure how easy it would be to do with X-Rays but when dealing with cameras I would normally be using calibrated test charts with a grey scale, colour chart and resolution chart. Maybe there is something similar for testing/characterizing X-Ray cameras?
Keith