Radiodensity

Radiodensity is the degree to which a material or tissue attenuates X-ray or gamma radiation, thus influencing its appearance on radiographic images. In X-ray imaging, substances with higher radiodensity absorb more photons and manifest as lighter, whiter regions, while less dense materials transmit more radiation and appear darker.

The physical basis of radiodensity lies in the attenuation of photons as they pass through matter. The transmitted intensity I is related to the incident intensity I0 by I = I0 exp(-μx), where μ is the linear attenuation coefficient and x is the thickness of the material. The coefficient μ depends on the material's atomic composition, electron density, physical density, and the energy of the incident photons. Higher atomic numbers, greater density, and greater thickness increase μ, and thus radiodensity. Lower-energy photons are more readily absorbed, so the same material can have higher radiodensity at lower X-ray energies.

In medical imaging, radiodensity is commonly quantified in different ways. Conventional radiographs use an optical density or digital radiography gray level to reflect attenuation; computed tomography uses Hounsfield units, where air is approximately -1000, water is 0, and dense bone is about +700 to +1000. Radiodensity informs diagnostic interpretation by differentiating tissues, detecting pathologies such as fractures, tumors, edema, or calcifications.

Limitations include dependency on the X-ray spectrum, beam hardening, detector response, and exposure settings, which can affect measured radiodensity. Proper calibration and standardization are essential for comparisons across imaging modalities and time.