Electromagnetic Waves- Radiography

Electromagnetic Waves - Radiography

Introduction

The production of shadow photographs (radiographs) of the internal structure of bodies opaque to visible light by the radiation from x-rays, or by gamma rays emitted from radio-nuclides (see SPECT). The quality of radiographs formed using X-radiation depends on the degree of attenuation of the incident X-ray beam. This is usually characterized by the linear attenuation coefficient, the density, and the thickness of the structure under investigation.

Body

In the medical application of radiography, bone is the medium that offers the greatest degree of attenuation, followed by soft tissue and then pockets of air. Conventional radiography cannot easily distinguish between different types of soft tissue (e.g. liver and kidney tissues) owing to their similar attenuation characters and densities. Bone, however, is very easily distinguished from neighbouring soft tissue, such as muscle. In situations in which better contrast is required, a contrast medium, such as barium sulphate, is introduced because it preferentially absorbs X-rays, enabling otherwise radiotransparent anatomical features to be observed. Kidney, arterial, and intestinal functions are examined by this method. [1]

When an X-ray interacts with an atom in the medium through which it is passing, there are many possible mechanisms that can attenuate the X-ray energy. The attenuation mechanism that the X-ray undergoes depends on its energy. The table below shows the thresholds for the various attenuation mechanisms and their variation with the ATOMIC NUMBER (Z) of the medium material and the energy (E) of the X-ray.[2]

The optimum X-ray energy for diagnostic radiography is in the range of photoelectric dominance. At about 30 keV, the photoelectric effect dominates the attenuation of the X-radiation and leads to a better contrast between media of different Z. The Z3 variation of the attenuation within the regime dominated by the photoelectric effect means that the distinction between media of different ...