You always need some time to find out the best exposure values for a photo. Same idea holds in X-Ray imaging.
Today I did an x-ray series with my biggest Nautilus shell on a conventional radiography sensor, not a film. Starting from the lowest possible value 40kV an increment of 10 kV up to 70 kV can be seen in the images:
Black regions in the image a transparent, white are opaque. The center of the Nautilus has a loss of structure.
With 50 kV the structure in the center of the Nautilus is better depicted wheras the edge gets more transparent:
Same effect for the center and the edge can be seen with 60 kV:
With 70 kV it’s an exaggeration for the edge and best depiction for the center:
Higher kV means more transparency for denser structures but a loss of structure in transparent areas.
At fixed energy, X-Ray imaging behaves like a shadow related to visible light. When photographing, there is not chance to look through an opaque object. With higher energies, x-rays go through opaque objects and can be collected on a sensor.
Composing the images obtained at different energies is an X-Ray HDR image:
The representation of an X-Ray with white on black is a reminiscence of the film era. Radiologists just looked at the negatives ! Inverting black and white shows the positive image, like a print. Here I show the same image as positive, but rotated and flipped horizontally. Look how ethereal it appears now: