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Attenuation: Atomic Number, Density and Thickness
Thicker, denser and higher-Z tissue removes more of the beam. Air looks dark because it is about 770 times less dense than soft tissue.
What controls attenuation
Attenuation is the reduction in beam intensity as photons are absorbed or scattered. Four things control it: thickness, mass density, effective atomic number (Z) and photon energy. Thickness acts exponentially. Each half-value layer of tissue removes half of what is left, so thick parts need much more technique; a common rule is to double the mAs for every 4 to 5 cm of added thickness. Mass density is how tightly matter is packed, in grams per cubic centimeter (g/cm³). Attenuation rises in proportion to it, because more atoms sit in the beam's path. Higher Z raises photoelectric absorption steeply, about as Z³. Higher photon energy lowers attenuation overall.