Radiography: Radiation Physics

In this article termed as radiation physics, we shall talk about the ways, the x-rays are produced, the events that occur at atomic level during their production and how can one save oneself and others.

Production of X Rays  

Electrons that travel from the filament to the tungsten convert part of their kinetic energy into x-ray photons. This phenomenon occurs by the formation of bremsstrahlung and characteristic radiation.  

Bremsstrahlung  

Bremsstrahlung interaction is the primary source of x-ray photons that are produced from an x-ray tube. These photons are produced when falling electrons from the filament suddenly stopped or slowed down at the target. Electrons from the filament falling out on the tungsten target either hit the nuclei or they nearly miss it. When they directly hit the nucleus, a photon with energy equal to that of the falling electron is generated and an x-ray is produced with the same energy.  

When the falling electron does not hit the nucleus and passes beside the nucleus, it decelerates and deflects towards the tungsten nucleus. In this process it releases some kinetic energy and produces photons.  

  

Characteristic radiation  

Characteristic radiation occurs when an electron from the filament displaces an electron from a shell of a tungsten target atom thereby ionizing the atom.  

Bremsstrahlung radiation constitutes 70% whereas characteristic radiation constitutes 30% of total radiation.

 

Photoelectric Effect 

The phenomenon where the photons are absorbed by the matter being exposed is known as photoelectric effect. 

Mostly, photoelectric absorption occurs in the k shell due to higher electron density. The falling electron from the source must have equal or a bit higher energy than that of the k shell electron to eject it. The recoil electron travels short distance in the medium and then stops. 

The photoelectric absorption occurs more in bone than soft tissues. It is directly proportional to the cube of the atomic number of the absorbing medium. The approximate atomic numbers of soft tissues and the bones are 7 and 12 respectively. 

The atomic number (Z) of Lead  is 82. Therefore, it’s ability to absorbs the radiation is extremely well as you can see below in the table. Because of this factor, it is used in lead apron for radiation protection . 

 

Comparison of photoelectric absorption by various materials 

Soft tissue                  Z = 7                   Z 3  = 343 

Bone                           Z = 12                 Z 3  = 1728 

Lead                           Z = 82                 Z  = 551,368 

 

Effect of High Potential Difference Between Filaments: Shades of Gray Scale

When there is an increase in voltage across filament and the target, it produces photons with higher mean energy and provides many shades of gray rendering the image quality of less diagnostic value. This is called long gray scale or low contrast or long scale contrast. The overall radiographic contrast depends on several factors.

  1. Film contrast 
  2. Subject contrast
  3. Quality of radiation

Film contrast: It depends on the size of the silver halide crystals along with other factors and thus inherent property of the film.

Subject contrast: It depends on the thickness and density of the organ of the subject. E. g. bone and soft tissues will have different contrasts based on their densities.

Quality of Radiation: Photons with high energy give decreased contrast.

At this point, it would be prudent to provide a list of various factors that affect the image quality of a radiograph. See below.

 

Practice test paper with explanation of questions is present HERE.

 

 

 

 

Read on radiation biology HERE.

Ref: White & Pharoah Oral Radiology Principles and Interpretation 5th Ed.

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