Question

Anatomy absorbs x-rays based on the density of the structure (subject thickness). Considering your understanding of how black, grey, clear and white tones (radiolucent and radiopaque) are created on an x-ray film, Why do some anatomical structures appear black, and others clear or white? Why do some structures allow more x-rays to pass through and energize the film crystals, while others do not?

Answer 1

X-rays are radiations with wavelengths of 0.01 to 10nanometers called electromagnetic waves. It creates an image in X-ray film during X-ray imaging in black, grey, clear and white tones. There are some factors that influence the appearance of anatomical structures during X-ray imaging. The first factor is the density of the structure. If the structure has more density then the more X-ray beam is absorbed and hence less beam reaches the detector. Structures that are less dense absorb less X-ray beam. This radio opacity differs in various anatomical tissues and structures. For example calcium in bones absorbs more x-ray beam and it appears white in an X-ray film. That means it is radio opaque. Fat and other soft tissues appear gray in color. Gas is a radiolucent material and appears black in X-ray film. It also depends on the atomic number, if atomic number is higher, higher the radiopacity and thicker the object. So it appears white in an image.If there is any abnormality in the structure this known densities of the structure can help find the increase or decrease of density and it helps find problem with that particular anatomical structure.

X-Ray film contains mainly a film base, an adhesive layer, a film emulsion and a protective layer. The transparent film base is adhered on both sides with adhesive layer to attach the emulsion layer that has silver halide crystals sensitive to the X-rays. A gelatin protective layer helps protect the emulsion layer. Silver halide crystals are chemical compound that are sensitive to radiation. So during radiograph some structures like a radiolucent structure allows greater penetration of x-ray beams that allows more radiation to reach the film. Thus it exposes more silver halide crystals which energizes the crystals and deposits more black metallic silver. It appears in image as black which shows a translucent structure like air in lungs. Another example is X-ray film positioned behind amalgam filling can appear white in color as it is dense and absorbs the radiation. This exposes no or very little radiation to the film and so no silver halide crystals are energized and no or less deposits of metallic silver.