Medical Imaging Physics

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Submitted By mq23
Words 1552
Pages 7
Medical Imaging Physics- W. R. Hendee
Chapter 7- Interaction of X and γ Rays in the Body
A Summary submitted by Miles Que

I. Introduction
The dominant interaction of X and γ rays in the body depend of three main factors: electron density, effective atomic number and the photon energy. All of which, cause different reactions in the body which is divided into four regions: fat, muscle, air- filled cavities and bones.

II. ƒ Factor As discussed in the previous chapters, an exposure of 1 Coulomb/ kilogram provides and absorbed Dose in air of 33.85 gray, provided by the equation: where X is the amount of exposure, in Coulombs/ Kilogram.

The dose absorbed in a certain tissue can be computed by multiplying the dose absorbed in air and the ratio of energy absorption in the medium to that of air. Deriving this equation, we can get:

The ƒ factor is important in getting the absorbed dose of radiation in a given medium. It is also directly proportional to the amount of dose absorbed in a particular tissue. The ƒ factor plot is given for the four regions of the body.

Looking at this plot, it is important that we note two things. First is that the values beyond 3MeV cannot be considered accurate. This is due to the fact the measuring devices cannot are not capable of providing accurate results at higher photon energies. Also, we have to know that the plot of fat is merely an approximation, since the composition of fat varies from person to person.

III. Attenuation of X and γ Rays in Tissue
A simplified model of the body divides the tissues into three: muscle, fat and bone. There are also structures that are filled with air such as the lungs, sinuses and the gastrointestinal tract. It is important for us to know how X and γ rays are absorbed in each of these. The attenuation of these rays in tissues…...

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