Sensitivity in an RT image is demonstrated by the image quality indicator (IQI) I am familiar with wire type and hole type. The specifications of the code or your client will tell you what hole or wire must be visible in the image. IQIs come in a large variety of materials and sizes depending on how thick/dense your subject material is.

I am not sure what usage contrast has in your context. I have only seen that term used in applications where there is a difference in thickness in your image. In that case the definition would be much the same as contrast in your television how much difference there is between light and dark.

Density in lay terms is how dark or light the image is. There is a required darkness (density) of the base metal so that the light and dark spots in the weld or area of interest can be interpreted with some idea of what they mean. If an image is too dark or too light indications will be missed.