Hypodense

Radiodensity (or radiopacity) refers to the relative inability of electromagnetic radiation, particularly X-rays, to pass through a particular material. Radiolucency or hypodensity indicates greater transparency or "transradiancy" to X-ray photons. Materials that inhibit the passage of electromagnetic radiation are called radiodense or radiopaque, while those that allow radiation to pass more freely are referred to as radiolucent. The term refers to the relatively opaque white appearance of dense materials or substances on radiographic imaging studies, compared with the relatively darker appearance of less dense materials.

X-rays are part of the electromagnetic spectrum, with photon energies above those of visible (and ultraviolet) light. X-rays are distinguished from gamma rays in that they are produced not by transitions within the atomic nucleus, but either by deceleration of a charged particle or by the transition of state of orbital electrons. Diagnostic X-rays are produced using X-ray tubes. The radio waves portion of the electromagnetic spectrum represents much lower energy/frequency photons than visible light. Thus, referring to the property of X-ray density as radiodensity appears contradictory by current knowledge, but is still used as a historical artifact.

Though the term radiodensity is more commonly used in the context of qualitative comparison, radiodensity can also be quantified according to the Hounsfield scale, a principle which is central to X-ray computed tomography (CT scan) applications. On the Hounsfield scale, distilled water has a value of 0 Hounsfield units (HU), while air is specified as -1000 HU.

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