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Medical Imaging Physics

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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 is mainly affected by three major factors. Each of which will be discussed.

A. Photon Energy
Photon energy is the amount of energy released by the source (e.g. X-ray machine). For this chapter, we can divide photon energies into two main categories: high- voltage and low- voltage with 28keV as the baseline. Energies that exceed 28keV will be considered as high voltage radiography while anything under 28keV will be categorized to low- voltage radiography. Knowing this will help us determine what type of interactions occurs at certain energy levels. It is important to remember that at high- energies, Compton scattering is more likely to occur and at low- energies, photoelectric absorption is most likely to occur.

Photoelectric absorption is the type of interaction wherein the atom absorbs all the energy as an electron is released from its orbital. The photoelectric effect can only eject bounded electron out of its orbit.

On the other hand, Compton scattering occurs at high- energies. It is when an electron is ejected out of its orbit, and absorbs only part of the photon energy. This then scatters the incident ray at a lower energy.

Knowing how and when these interactions occur will be useful as we study the following factors that affect the attenuation of X and γ rays in the body. B. Effective Atomic Number
The effective atomic number of a certain compound can be calculated from the following equation:

To be able to compute for this, we must also know the elemental composition of the tissues we want to study. This is given in the table below.

From the given values, we are able to get the effective atomic numbers for the four types of tissues in out body. The elemental composition is also important in knowing what substitutes can be used to replace certain tissues for testing.

By knowing this, we can now relate the effective atomic number of a tissue to its physical density. As we can see, the effective atomic number is directly proportional to the physical density of a tissue. Tissues that have a higher physical density have more atoms that can absorb the energy it is exposed to. Based on our previous knowledge that photoelectric absorption occurs at low energies, we can now say that at low energies, a greater effective atomic number leads to a higher attenuation. Contrary to that, at low energies, a lower effective atomic number leads to less rapid attenuation of the tissues.

C. Electron Density
The third factor that affects attenuation in the body is the electron density of the tissue. Electron density is the amount of electrons for every given mass. It is commonly expressed in electrons/kg. Recalling the interactions at different photon energies, we can say that at high- energies, more Compton interactions occur. By knowing the process of Compton interactions, we can conclude that the more electrons there are, the more chances there would be of Compton interactions happening. Also, the more Compton interactions, more energy will be absorbed, and thus, energy will attenuate more quickly in tissues that have a higher electron energy at high energy x-rays.

As we look at table 7-2, we can see that bones have the lowest electron density. This would mean that at high energies, bones would attenuate X and γ rays the least. On the other hand, fat, which has the highest electron density, will attenuate X and γ rays more rapidly at high- energies.

Effect/ Photon Energy | Low Energy (<28keV) | High Energy (>28keV) | Increased Attenuation | Higher Zeff | Higher Electron Density | | Higher Physical Density | | Decreased Attenuation | Lower Zeff | Lower Electron Density | | Lower Physical Density | | This table summarizes the attenuation of X and γ rays in tissues based on the three factors.

IV. Dose to Soft Tissue Beyond Bone
Many times, we conduct X-rays in order to see internal organs, rather than just bones. There are basically two factors that affect the dose absorbed by soft tissues beyond the bone.

A. Increased attenuation of primary photons in bone
Bones have a higher effective atomic number and physical density as compared to other tissues. This is why the bone is able to absorb the most energy at low energy radiographs. If the bone already absorbed the energy, the tissues beyond the bone can no longer absorb it. B. Changes in the amount of energy scattered to the soft tissue
The amount of energy scattered to the soft tissue affects how much energy is absorbed. This can differ because of the quality of radiation, distance between the bone and soft tissue, and the depth of the soft tissue itself.

The effect of the bone on the dose absorbed and the exposure on soft tissues is illustrated in the following figure.

V. High- Voltage Radiography
Like previously mentioned, at high voltages, tissues with higher electron density will be attenuating the X-rays more. Thus, it is more often used for structures that have high- intrinsic contrast (Chest). Filters can also be used in order for the bones to attenuate less X- rays. Other common uses of high- voltage radiography is as follows: * Myelography * Pneumoencephalography * Study of Air- filled Structures * Study of Gastro Intestinal Tract

VI. Low- Voltage Radiography
In contrast to high- voltage radiography, low- voltage radiography is commonly used for structures that have low intrinsic contrast. It is commonly used for areas that have tissues that have similar effective atomic numbers and densities. For example, low- voltage is used in mammography because the breast is composed of glandular tissues and fat, which have relatively close effective atomic numbers and physical densities. By using low- voltage radiography, a higher contrast can be expected in these kinds of structures.

VII. Contrast Agents
Contrast agents are used to either increase or decrease the attenuation of structures that would normally be not so visible to the X-ray. More often than not, it is used to increase the attenuation. Typical Contrast agents, such as Iodine and Barium are effective because they have much greater effective atomic numbers than the tissues in the body. One of the most important considerations in selecting contrast agents is safety. Contrast agents must non- toxic. They must not have any immediate or long- term effects on the patient. Barium and iodine are non- toxic and can be used in a wide variety of radiographic examinations.

This figure shows the very noticeable difference of the attenuation of iodine and lead as compared to tissues in the body. From this we can see how high the contrast will be when a contrast agent is used for radiographic examinations.

VIII. Summary * The f factor relates the radiation exposure (C/kg) in air to the absorbed dose (Gy) in a medium. * Attenuation in the body is affected by three major factors: * Photon Energy * Effective atomic number (Zeff) * Electron Density (Electrons/ Kilogram) * The dose to soft- tissue beyond bone is mainly affected by: * Increased attenuation of primary photons in bone * Changes in amount of energy scattered to the soft tissue * High- Voltage radiography is commonly used to structures with high intrinsic contrast. * Low- voltage radiography is used to give higher contrast in structures that have similar effective atomic numbers and physical densities. * Contrast agents are used to increase or decrease the attenuation in a particular tissue.

Reference:
Medical Imaging Physics (Fourth Edition) by William R. Hendee, Ph. D.…...

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