Principles of diagnostic radiology

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PHYSICAL BASIS OF IONIZING RADIATION

  • X-rays or roentgen rays are a form of electromagnetic radiation of extremely short wavelength
  • The shorter the wavelength of the electromagnetic radiation form, the greater its energy and the greater
  • its ability to penetrate materials or tissues.
  • All electromagnetic waves travel through space at speed of light.

PRODUCTION OF X-RAYS

Production of X-rays for diagnostic purposes requires a vacuum tube and the presence of a high potential difference between a cathode and an anode.

q    In the basic x-ray vacuum tube, electrons are boiled off the cathode (filament) by heating it to a very high temperature. To move these electrons toward the anode at energy sufficient to produce x-rays, a high potential difference, up to 125.000 volts (125 KV) is used.

q    When the accelerated electrons strike the tungsten anode, x-rays are produced.

q    The x-ray tube is completely surrounded by lead on all sides, except for a small exit port. The lead absorbs most of the emitted x-rays except for the portion that escapes through port.

Radiologic density

There are two types of density encountered in X-rays recording media:

1.    Physical density: a density of material is the number of grams per cubic centimeter. The greater the physical density, the greater the ability of material to absorb and scatter x-rays (greater attenuation).

2.    Radiographic density: radiographic density refers to the degree of blackness of a film. The radiographic density of a substance is related to its physical density. The effect on film or other recording media occurs “paradoxically”; structures of high physical density produce less radiodensity (less blackening or more whitening) and vice versa.

q    Structures that produce more blackening on film are referred to as being radiolucent, the one that produce less blackening is called radiopaque or radiodense.

q    Generally, structures are of four types of radiographic densities; these are, in increasing order of physical density:

Ø     Gas (air): appears black due to its low density that does not attenuate the x-rays.

Ø     Fat appears gray black

Ø     Water (soft tissues) appears gray

Ø     Bone: appears white due to its high density and the great attenuation of x-rays as they pass through it.

RECORDING MEDIA

X-ray film:

q    This is the most familiar recording media.

q    X-ray film consists of a plastic sheet coated with a thin emulsion of silver bromide and a small amount of silver iodide. This emulsion is sensitive to light and radiation.

q    When film is exposed to ionizing radiation then developed, chemical changes take place within the emulsion resulting in the deposition of metallic silver, which is black.

q    The amount of blackening on the film depends entirely on the amount of radiation reaching the film and, therefore on the amount of attenuated or removed x-rays from the beam by tissues.