Radiation Protection - Chapter 3 (Interaction of X-Radiation with Matter)

Why do we focus on the Interaction of X-Radiation with Matter?

It provides a basic understanding on how to select technical exposure factors such as Peak kilovoltage (kVp) and Milliampere-seconds (mAs)

What is kVp?

Peak kilovoltage (kVp) is the highest energy level of photons in the x-ray beam, equal to the highest voltage established across the x-ray tube.

What does kVp control?

It controls the quality, or penetrating power, of the photons in the X-ray beam and, to some degree, also affects the quantity or number of photons in the beam.

What is Milliampere-seconds (mAs)?

Milliampere-seconds (mAs) is the product of milliamperes (mA) , which is electron tube current, and the amount of time in seconds that the X-ray tube is activated.

What does mAs control and equation?

Strictly controls quantity. mA * s = mAs

What may x-rays do when they enter a material such as human tissue?

1.Interact with the atoms of the biological material in the patient and be absorbed

2.Interact with the atoms of the biological material and be scattered, causing some indirect transmission (interact and leave the patient in a different direction)

3.Pass through without interaction

What is absorption?

When interaction occurs electromagnetic energy is transferred from the X-rays to the atoms of the patient's biological tissue

What is absorbed dose?

the amount of energy absorbed per unit mass

Why is absorption is important in image formation?

Without absorption and the differences in the absorption properties of various body structures, it would not be possible to produce diagnostically useful images in which different anatomic structures could be perceived and distinguished. (Can only have images if the xray loses energy by interacting the atoms)

What are the risk when more electromagnetic energy is received by the patient?

The more electromagnetic energy that is received by the atoms of the patient's body, the greater possibility of biologic damage in the patient.

The smallest amount of radiation should be used.

What is primary radiation/ primary beam?

Are x-rays leaving the tube and entering the patient

How is a diagnostic x-ray beam produced?

A diagnostic X-ray beam is produced when a stream of very energetic electrons bombards a positively charged target in a highly evacuated glass tube.

Why is Tungsten and tungsten rhenium (anode) are used as the target material?

Because it has a high melting point and high atomic number of 74.

(Tungsten rhenium - a metal alloy)

What are the properties and characteristics of the energy of photons in a diagnostic x-ray beam?

Not all photons in a diagnostic x-ray beam have the same energy.

-The most energetic photons in the beam can have no more energy than the electrons that bombard the target.

How is the energy of electrons inside the x-ray tube specified?

•The energy of the electrons inside the x-ray tube is specified in terms of electrical voltage applied across the tube. •In diagnostic radiology, the voltage is expressed in thousands of volts, or kilovolts (kV).

Why is voltage of the electrons are characterized by kilovolt peak value (kVp)?

Because the voltage across the tube fluctuates

What is attenuation?

When an X-ray beam passes through a patient

What is absorption and attenuation and how are they related?

Absorption is the transfer of x-ray energy with biological matter. For there to be absorption their has to be a transfer of energy. Absorption can have absorption or scatter. Attenuation is when it passes through the body which can include absorption, scatter, and passing through the body. Absorption happens with attenuation

What happens when a x-ray is absorbed?

x-ray photons that interact with the atoms of a patient such that they give up all of their energy and cease to exist

What happens when a x-ray scatters?

x-ray photons that interact with the atoms of the patient, but only surrender part of their energy. (They continue to exist but emerge from the interaction at a different angle)

Attenuation vs. Transmission

Attenuated photon that have undergone either absorption or scatter do not strike the IR.

Transmitted photons that strike the IR are transmitted.

(Attenuated is the process that the x-rays when x-rays enter the body. Transmitted photons is what hits the IR and creates the image)

What is Direct Transmission?

Some primary photons traverse the patient without interacting and reach the image receptor. (does not interact with the body and goes straight through the PT and hits the IR and creates a black spot in the image)

What is indirect transmission?

Other primary photons can undergo Compton and/or coherent interactions and, as a result, may be scattered or deflected, potentially causing a loss of energy.

Does indirect transmission create the image?

No indirect are not the photons that create the image. Indirect is bad for the image

Which type of photons play a pivotal role in x-ray imaging?

Exit or image formation (Remnant beam) photons play a pivotal role in x-ray imaging. These photons pass through the patient being radiographed and directly reach the radiographic image receptor, forming the image we see.

What is the probability of photon interaction with matter?

The interaction of photons with biological matter is random

Is it possible to predict with certain what will happen to a single photon when it enters human tissue?

It is impossible to predict

What are the five types of interaction between x-radiation and matter?

Coherent scattering, Photoelectric Absorption, Compton scattering, Pair production, and photodisintegration

What are the only two interactions between x-radiation and matter that are important in diagnostic radiology?

Compton Scattering and Photoelectric absorption

As a rad tech what type of absorption would you want?

Photoelectric absorption

What are the types of photons that create the image?

The photoelectric photons that come from photoelectric absorption

What is Coherent scattering also known as?

Classical, Elastic, and unmodified

Does Coherent scattering result in energy loss?

It is a simple process that results in no loss of energy as X-rays scatter.

What type of energy does coherent scattering occur with?

It occurs with low-energy photons, typically less than 10keV

Why no net energy has been absorbed by the atom during coherent scattering?

Because the wavelengths of both incident and scattered waves are the same

What are the steps of Coherent Scattering?

1.Low energy x-ray photon interacts with tissue atoms passing through the atom

2.as it passes through it transfer its energy by causing all or some of the electrons to vibrate

3.the electrons then releases energy as of a result of the vibration

4.The release energy cause out as electromagnetic wave, which combines with other waves to form a one scattered wave.

5.The scatter wave has the same power as a scattered photon.

In coherent scattering what does the vibration cause?

The vibration causes scatter to come out as an electron wave. It can be one big scatter or a bunch of small scatters

For Photoelectric absorption what is the diagnostic radiology energy range?

Diagnostic radiology energy range: 23 to 150 kVp

Why is Photoelectric absorption important?

This is the most important mode of interaction between x-ray photons and the atoms of the patient's body for producing useful images.

What happens if an image has scatter?

It becomes non-diagnostic/ non useful

What is the process of Photoelectric Absorption?

x-ray photon is absorbed within the tissue and ejects an inner shell electron. (For this to occur the photon needs as large as or larger than the amount of energy that holds the electron in its orbit.) As a result of the vacancy in the -Shell, an electron from a higher energy level shell fills the vacancy. Due to this cascade effect a characteristic ray photon is emitted

Wha is the difference between characteristic and photoelectric?

Characteristic is in the tube and photoelectric is in the body

What type of effect is part of photoelectric absorption?

Auger effect (pronounced awzhay)

-Discovered by Pierre Victor Auger in 1925

-Produces an Auger electron

-is a radiationless effect

What is the process involved in Auger effect?

1.X-ray photon is absorbed within the tissue and interact with an inner shell electron.

2.Instead of ejecting the Inner-shell electron, the energy of the X-ray photon transfers to the outer electron

3.As a result of the transfer of energy the outer shell is ejected

4.Due because this process does not require a vacancy to be filled, no x-ray emission is required, causing a radiationless effect

What are the by-products of photoelectric absorption?

Photoelectrons, Characteristic x-ray photons, and sometimes Auger electron

What does the probability of occurrence of photoelectric absorption depend on?

Depends on energy of the incident x-ray photons and atomic number (Z) of the atoms comprising the irradiated object.

How does the probability of photoelectric absorption increase?

Increases the possibility as the energy of the incident photon decreases and the atomic number of irradiated atoms increases

What happens as density of atoms increases?

As density increases (# of atoms in given volume) the more chance of X-ray photons to interact with increasing attenuation

What is the atomic number of compact bone?

Compact bone has a higher atomic # than soft tissue. Has an effective atomic number (Z) of 13.8

What is the thickness factor?

Approx. linear. If two structures have the same density and atomic # but one is twice as thick as the other. The thicker structure will absorb twice as many photons

What are the effects of attenuation on radiographic images?

The less a structure attenuates radiation, the darker the image will be and vice versa

Why is it that when the less the structure attenuates radiation the darker the image?

X-rays need to have some interaction with matter to produce an image.

What happens if there's no absorption when creating a radiographic image?

Without absorption and the differences in the absorption properties of various body structures, it would not be possible to produce diagnostically useful images in which different anatomic structures could be perceived and distinguished. (You need absorption to create an x-ray image)

What is the impact of photoelectric absorption on radiographic contrast?

The more significant the difference in the amount of photoelectric absorption, the greater the contrast in the radiographic image between adjacent structures of differing atomic numbers (Z).

What occurs as absorption increases?

The potential for biological damage also increases

How to maximize the radiographic image quality and patient safety?

To ensure both radiographic image quality and patient safety, choose the highest-energy X -ray beam that permits adequate radiographic contrast for computed, digital, or conventional radiography. But deliver the smallest dose to the patient.

What is the best technique to get best image quality and patient safety?

The best technique is to use High kVp and Low mAs. (mAs effects patient dose)

How does using the technique of high kVp and low mAs help the patient?

-Higher kVp increases the penetrating power of the X-ray photon

-Low mAs decrease the number of photons used = less patient dose

What are the structures in the body that have low atomic numbers and may be difficult to distinguish on a x-ray?

The stomach, intestines, kidneys, bladder, lungs, etc., have low atomic numbers (Z) and look grayish. In that case, appropriate contrast media may be needed to ensure visualization of those tissues or structures in the radiographic image.

What does contrast do?

Contrast media is a substance that increases the atomic number of a low atomic organ to visualize the anatomy more profoundly and can be injected, oral contrast, or rectal contrast.

What does the use of positive contrast medium do?

brightens anatomy/ makes the image look bright white. Oral/ IV contrast

What does negative contrast medium do?

Darkens the anatomy/ blackens the images. Air/ Gas

What is Compton scattering also known as?

Incoherent scattering

Inelastic scattering

Modified scattering

What is Compton scattering responsible for?

It is responsible for most of the scattered radiation produced during radiologic procedures.

How is Compton different from photoelectric?

It is the opposite of photoelectric instead of hitting the inner shell it hits the outer electron (weakest electron) and creates a free radical

Why is Compton scattering bad?

Scatter radiation is what increases exposure to occupational personnel.

What is the process of Compton scattering?

High energy x-ray photon interacts with tissue atoms giving up some of their energy to eject an outer shell electron. As a result of the collision, the X-ray photon is deflected with some reduced energy. The x-ray photon leaves the body as an energetic scattered photon

What is the by-product of compact scattering?

Recoiled electrons

At what energy level does pair production occur?

Occurs at an energy level of at least 1.022 million electron volts (MeV). Nowhere near diagnostic radiography

What is the process of pair production?

Incoming X-ray photon interacts with the electric field surrounding the nucleus of an atom of irradiation tissue and disappear. The photon's energy is absorbed and transformed into a negatron and a positron. This is considered an antimatter annihilation reaction

Why does the nucleus emit negatron and positron during pair production?

Because the particles of the nucleus keep the radiation inside the nucleus and it decays releasing

At what energy level does photodisintegration occur?

An interaction that occurs at more than 10 MeV in high-energy radiation therapy treatment machines

What interaction is photodisintegration considered?

It is considered the interaction that makes a nucleus partially break apart

What is the process of photodisintegration?

An incoming high-energy photon collides with the nucleus of the atom of the irradiated object and absorbs all the photon's energy.

This energy excess in the nucleus creates an instability that is usually alleviated by the emission of a neutron

What other types of emissions can occur if sufficient energy is absorbed during photodisintegration?

In addition, if sufficient energy is absorbed by the nucleus, other types of emissions will be possible, such as a proton or proton-neutron combination (deuteron), or even an alpha particle.