Figure 1 .

Comparison of density, as defined in physics, and radiological (areal) bone density, as commonly used in the clinical setting. The two bones have the same physical density, but the larger bone appears denser when projected on to a screen. The reason is that it absorbs more radiation because of the longer path length of the radiation beam through the bone. This is analogous to comparing the shadows of two differently sized bottles placed in the sun. The larger bottle will have a darker shadow, even if the two bottles are made of the same material and contain the same liquid. Similarly, the areal bone density of a premature infant with a birth weight of 1000 g is lower than that of a healthy term newborn, even if the physical density of the bones is identical.