chapter 9 bushong - Interaction of X-Radiation with Matter, Physics / X-Ray Production / Bushong Chapter 7, Physics PSC Radiography/ Bushong Chapter 8 / X-Ray Emission

Bremsstrahlung Radiation =

"Bending radiation"

5. In which of the following x-ray interactions with matter is the energy of the incident photon partially absorbed?

A. Compton.

7. Which of the following characteristics primarily differentiates the probability of occurrence of the various interactions of x-radiation with human tissue?

A. Energy of the incoming photon.

___________________ x-rays result from the interaction between a projectile electron and a target nucleus. The electron is slowed, and its direction is changed.

Bremsstrahlung

Heat production increases _________ with increasing kVp, at least in the diagnostic range.

Directly

The production of heat in the anode increases ________ with increasing x-ray tube current.

Directly

Small-angle scatter

Photons (2) that on their path before they hit the IR. They degrade the appearance of a completed radiographic image by blurring the sharp outlines of dense structures.

3. Which of the following defines attenuation?

A. Absorption and scatter.

Summary of All Interactions (Just to Practice)

*Coherent*: - Occurs at energies less than 10 keV (less than diagnostic range). - Doesn't usually occur in diagnostic x-ray, but if it does, the amount of noise it produces is extremely minimal. - The incoming low-energy electron interacts with the atom, causing it to become excited and vibrate one or all of the electrons. - This excitation causes a scattered photon of the same wavelength, energy, and frequency as the incident photon, just traveling in a different direction. *Compton (incoherent, inelastic, modified) scatter*: - Occurs at diagnostic range energies (approx. 35 keV). - The main source of image noise and occupational dose due to backscatter. - The incoming incident photon interacts with a loosely-bound outer electron. - The electron is ejected as a Compton Scattered electron (secondary or recoil electron). - A lower-energy scattered photon is also released. *Photoelectric Absorption*: - Occurs at diagnostic range energies. - The interaction that makes x-ray possible. - The incoming incident photon interacts with a tightly-bound inner shell electron, transferring all of its energy to the electron and disappearing. - The electron is ejected as the photoelectron. - The vacancy left by the photoelectron is then filled by an electron from the outer shells, which releases a characteristic photon (also called secondary radiation. it does not contribute to image noise and does not hit the IR). The energy of the characteristic photon is directly related to the shell structure. - If characteristic photon is not released, Auger effect may occur. - Electrons will keep dropping down until equilibrium is regained (characteristic cascade). - The greater the difference is in the amount of photoelectric absorption, the greater the contrast in the radiographic image will be between adjacent structures of differing atomic numbers. However, as absorption increases, so does the potential for biologic damage. *Pair Production*: - Occurs at energies of 1.02 MeV. - Useful in PET scanning. - High-energy incident photon interacts with the nucleus and releases a positron and negatron. - The negatron recombines with another atom that needs an electron. - The positron (a form of antimatter) reacts destructively with a nearby electron to cause an annihilation reaction that results in two photons (half of the original energy each, so about 0.511 MeV) traveling in different directions. *Photodisintegration*: - Occurs at energies above 10 MeV. - Useful in radiation therapy. - High-energy photon collides with the nucleus. - The energy is completely absorbed. - The excess energy instability is relieved by the ejection of a neutron from the nucleus.

List two types of x-ray photon transmission, and explain the difference between them.

*Direct*: When photons transverse the patient without interacting and still reach the image receptor. *Indirect*: When photons transverse the patient, undergo Compton and/or coherent interactions that may cause them to scatter or deflect with a potential loss of energy before striking the IR.

Radiographic fog

*Undesirable, additional exposure* that degrades the appearance of a completed radiographic image. (Example A = fog, B = less fog because of better collimation)

Differentiate between peak kilovoltage (kVp) and milliampere-seconds (mAs) as technical exposure factors.

*kVp* is the highest energy level of photons in the x-ray beam. It controls the penetrating ability of the x-ray beam. *mAs* is the the product of electron tube current and the amount of time in seconds that the x-ray tube is activated.

At 60 kVp, only ____% of the electron kinetic energy is converted to x-rays ; At 100 kVp,approx. ______% is converted to x-rays; and at 20 MV ______% is converted.

.5 ; 1 ; 70

Only approximately ____% of projectile electron kinetic energy is used for the production of x-radiation.

1

The distance between the filament and the x-ray tube target is only approximately __________.

1 cm

The anode target is made of the metal tungsten or a metal alloy tungsten rhenium because...

1. High melting point 2. High atomic number (Tungsten: 74, Rhenium: 75)

In summary, the by-products of photoelectric absorption include the following:

1. Photoelectrons (those induced by interaction with external radiation and the internally generated Auger electrons). 2. Characteristic x-ray photons (fluorescent radiation).

Approximately _______ % of the kinetic energy of projectile electrons is converted to heat.

99

Coherent scattering

A simple process that results in *no loss of energy* as x-rays scatter. The *incoming low-energy x-ray photon (<10 keV) interacts* with an atom and *transfers its energy* by causing some or all of the e*lectrons of the atom to vibrate momentarily*. The electrons then radiate energy in the form of *electromagnetic waves*. These waves nondestructively combine with one another to form a *scattered wave*, which represents the scattered photon. Its wavelength and energy, or penetrating power, are the *same as those of the incident photon*. Generally, the emitted photon may change in direction less than 20 degrees with respect to the direction of the original photon."

Photoelectric absorption

A, On encountering an inner-shell electron in the K or L shells, the incoming x-ray photon surrenders all its energy to the electron, and the photon ceases to exist. B, The atom responds by ejecting the electron, called a photoelectron, from its inner shell, thus creating a vacancy in that shell. C, To fill the opening, an electron from an outer shell drops down to the vacated inner shell by releasing energy in the form of a characteristic photon. Then, to fill the new vacancy in the outer shell, another electron from the shell next farthest out drops down and another characteristic photon is emitted, and so on until the atom regains electrical equilibrium. There is also some probability that instead of a characteristic photon, an Auger electron will be ejected.

9. A decrease in contrast of the image by adding an additional, unwanted exposure (radiographic fog) results from which of the following interactions between x-radiation and matter? 1. Compton scattering. 2. Pair production. 3. Photoelectric absorption.

A. 1 only.

Photodisintegration

An interaction that occurs at *more than 10 MeV* in high-energy radiation therapy treatment machines. *High-energy photons collide with the nucleus* of an atom, which directly *absorbs the photon's energy*. This energy *excess in the nucleus creates an instability* that in most cases is *alleviated by the emission of a neutron* by the nucleus.

Discuss the way x-rays are produced, and explain the range of energies present in the x-ray beam.

An x-ray beam is produced when a stream of high-speed electrons hit a positively charged tungsten target, known as an anode, in a vacuumed glass tube. As the electrons interact with the atoms of the target, x-ray photons emerge with a broad range of energies and leave through a glass window. The glass window permits passage of all but the lowest-energy components of the x-ray spectrum. Aluminum is also placed within the collimator assembly to assist in intercepting the x-rays before they hit the patient.

4. In the radiographic kilovoltage range, which of the following structures undergoes the most photoelectric absorption?

B. Compact bone.

1. Exit, or image-formation, radiation is composed of which of the following?

B. Non-interacting and small-angle scattered photons.

6. When a high atomic number solution is either ingested or injected into human tissue or a structure to visualize it during an imaging procedure, which of the following occurs?

B. Photoelectric interaction becomes significantly enhanced, leading to an increase in the absorbed dose in the body tissues or structures that contain the contrast medium.

Discuss the probability of photon interaction with matter.

Because the interaction of photons with biologic matter is random, it is impossible to predict what will happen to a single photon when it enters human tissue. When dealing with a large number of photons, however, it is possible to predict what will happen on the average.

____________ x-rays are produced when a projectile electron is slowed by the nuclear field of a target atom nucleus

Brehmsstrahlung

In the diagnostic range, most x-rays are ___________ x-rays

Bremsstrahlung

____________ is a German word for "slowed down radiation"

Bremsstrahlung

2. Which of the following contributes significantly to the exposure of the radiographer?

C. Compton-scattered photons.

10. The interactions of x-ray photons with any atoms of biologic matter are:

C. Random, and therefore the effects of such interactions cannot be predicted with certainty.

The type of x-radiation is called ____________ because it is characteristic of the target element.

Characteristic

This chapter explains the interactions of the projectile electrons that are accelerated from the cathode to the x-ray tube target. These interactions produce two types of x-rays; _____________ and _______________.

Characteristic ; Bremsstrahlung

If the projectile electron interacts with an inner shell electron of the target atom rather than with an outer shell electron, __________ _________ can be produced.

Characteristic x-rays

__________ __________ are emitted when an outer-shell electron fills an inner shell void.

Characteristic x-rays

The atomic numbers of compact bone, soft tissue, and air...

Compact bone: 13.8 Soft tissue: 7.4 Air: 7.6 Air has a slightly higher effective atomic number (7.6) than soft tissue (7.4); however, the density of air is approximately 1,000 times smaller than that of soft tissue. Therefore, air absorbs far fewer x-ray photons, and this permits more radiation to reach the IR.

Regardless of what mA is selected, the efficiency of x-ray production remains ____________.

Constant

8. Which of the following influences attenuation? 1. Effective atomic number of the absorber. 2. Mass density. 3. Thickness of the absorber.

D. 1, 2, and 3. Mass density: For example, if radiography is performed on an equal thickness of bone and soft tissue, the bone, which is approximately twice as dense as soft tissue, will absorb about nine times as many photons in the diagnostic energy range as will the soft tissue. Thickness: If two structures have the same density and atomic number but one is twice as thick as the other, the thicker structure will absorb twice as many photons.

Radiographic density

Degree of overall blackening on a radiographic film. (Image A = substantial quantities of calcium. B = demineralized bones that are not as dense)

The x-ray has energy equal to the ________ in the binding energies of the orbital electrons involved.

Difference

Radiographic contrast

Differences in densities and the absorption properties among different body structures.

Doubling the x-ray tube current ____________ the heat produced.

Doubles

A projectile electron that completely avoids the orbital electrons as it passes through a target atom may come sufficiently close to the nucleus of the atom to come under under the influence of its ___________ ___________.

Electric field

The closer the projectile electron gets to the nucleus, the more it is influenced by the __________ __________ of the nucleus. This field is very strong because the nucleus contains many ___________ and the distance between the nucleus and the projectile electron is very small.

Electric field; protons

In an x-ray tube, the projectile is the _______________.

Electron

These interactions result in the conversion of __________ __________ __________ into __________ _________ (heat) and electromagnetic energy in the form of ____________ __________ (also heat) and x-rays.

Electron kinetic energy ; thermal energy ; infrared radiation

Because the electron is negatively charged, and the nucleus is positively charged, there is an ____________ _________ of attraction between them.

Electrostatic force

Exit, or image-formation, photons

Photons (1 and 2) that emerge from the tissues and strike the radiographic IR below it.

Most of the kinetic energy of projectile electrons is converted into ___________.

Heat

Describe and illustrate by diagram the x-ray photon interactions with matter that are important in diagnostic radiology.

In *photoelectric absorption*, an incoming *x-ray photon hits an inner-shell electron, gives up all its energy, and ceases to exist*. The atom responds by *ejecting the photoelectron*, which creates a vacancy. To fill the opening, an *electron from an outer shell drops down* by releasing energy in the form of a photon. To fill the new vacancy in the outer shell, another electron from the next farthest shell drops down. This process continues until the atom regains equilibrium. In *Compton scattering*, the incoming *x-ray photon encounters a loosely bound outer-shell electron and surrenders a portion of its kinetic energy to dislodge the electron* from orbit. The *energy-degraded x-ray photon then continues on its way in a new direction*. The high-speed *electron ejected from orbit is called a Compton-scattered electron*, or secondary "recoil" electron.

Compton scattering

In the Compton process, an incoming x-ray *photon interacts with a loosely bound outer electron* of an atom of the irradiated object. On encountering the electron, the incoming x-ray photon *surrenders a portion of its kinetic energy to dislodge the electron from its outer-shell orbit*, thereby ionizing the biologic atom.

The effective energy of characteristic x-rays ________ with increasing atomic number of the target element.

Increases

The efficiency of x-ray production is _____________ of tube current.

Independent

As sophisticated as it is, the x-ray imaging system is very ____________.

Inefficient

The outer shell electrons immediately drop back to their normal energy level with the emission of ________ ____________.

Infrared radiation

Similar characteristic x-rays are produced when the target atom is __________ by the removal of electrons of shells other than the K shell.

Ionized

X-rays are called ___ _________ because they result from the electron transitions into the K-shell.

K x-rays

Objects in motion have ___________ energy proportional to their ________ and to the _________ of their velocity

Kinetic ; Mass ; Square

The energy of motion

Kinetic energy

X-rays resulting from the electron transitions to the L shell are called ____ _________, and have much less energy than ____ _________ because the binding energy of an L-shell electron is much lower than that of a K-shell electron.

L X-rays ; K X-rays

All electrons have the same ________; Therefore electron kinetic energy is raised by raising the ______.

Mass ; kVp

Mass density

Measurement of different body structures denoted in grams per cubic centimeter.

A third type of interaction in which the projectile electron can lose its kinetic energy is an interaction with the ________ of the target atom. In this type of interaction, the kinetic energy of the projectile electron is also converted into _____________ energy.

Nuclear field ; Electromagnetic energy

Radiographic image receptor (IR)

Object that catches exit radiation. - Phosphor plate - Digital radiography receptor - Radiographic film

The projectile electrons interact with the _______ ________ _______ of the target atoms but do not transfer sufficient energy to these outer shell electrons to _______ them. Rather, the outer shell electrons are raised to an excited, or higher energy level.

Outer shell electrons ; ionize

List the x-ray photon interactions with matter that occur above the energy range used in diagnostic radiology. [ADD MORE DETAIL]

Pair production, and photodisintegration are x-ray photon interactions that occur above the energy range used in diagnostic radiology.

Describe the impact of positive contrast media on photoelectric absorption, and identify its effects regarding absorbed dose in the body structure that contains it.

Positive contrast media has a high atomic number that significantly enhances the occurrence of photoelectric interaction relative to similar adjacent structures that do not have contrast media. It also leads to an *increase in absorbed dose* in the body structures that contain it.

Differentiate among the following: primary radiation; exit, or image-formation, radiation; and scattered radiation.

Primary radiation: The emerging x-ray photon beam. Exit, or image-formation radiation: Noninteracting and small-angle scattered photons that pass through the patient and hit the IR. Scatter radiation: photons that change directions, involve a partial loss of energy, and do not hit the IR.

The ___________ electron interacts with the _____________ electrons or the nuclear field of target atoms.

Projectile ; Orbital

The production of heat and characteristic x-rays involves interactions between the _________ electrons and the electrons of x-ray tube _______ _________.

Projectile ; Target atoms

Electrons traveling from from cathode to anode constitute the x-ray tube current and are sometimes called _______________ _____________.

Projectile Electrons

Add in the fifth interaction: Coherent Scattering

Sometimes called: •Classical scattering •Elastic scattering •Unmodified scattering It is basically a relatively simple process that actually results in no loss of energy as x-rays scatter. It is of no importance in any energy range. When the low-energy x-ray photon interacts with an atom of human tissue, it does not lose kinetic energy. The emitted photon merely changes direction by 20 degrees or less. No ionization of the biologic atom occurs. Rayleigh and Thompson scattering are forms of coherent scattering.

Although many characteristic x-rays can be produced, these can be produced only at ________ ________, equal to the differences in electron binding energies for the various electron transitions.

Specific energies

__________ objects have no kinetic energy.

Stationary

Fluorescent radiation

Synonymous with characteristic x-rays.

When the projectile electron ionizes a target atom by removing a K-shell electron, a ________ ________ ________ is produced in the K shell. This is a highly unnatural state for the target atom , and it is corrected when an outer shell electron falls into the void in the K shell.

Temporary electron void

Image receptor (IR) exposure

Term that *replaces density* in the digital environment. It is used because radiographic film is no longer used as the primary IR.

Characteristic x-ray

The "released" energy that is carried off in the form of a photon. It's energy is directly related to the shell structure of the atom from which it was emitted. Characteristic x-rays are *also known as fluorescent radiation*.

Characteristic photon

The *"released" energy that is carried off in the form of a photon*. It's energy is *directly related to the shell structure* of the atom from which it was emitted.

Peak kilovoltage (kVp)

The *highest energy level of photons* in the x-ray beam.

Window level

The *midpoint of the range of densities* visible on the image. Adjusting the window level, also known as windowing, refers to *changing the brightness*, either to be increased or decreased *throughout the entire range of densities*.

Attenuation

The *reduction in the number of primary photons* in the x-ray beam through *absorption* (a total loss of radiation energy) and *scatter* (a change in direction of travel that may also involve a partial loss of radiation energy) as the beam passes through the patient in its path.

Absorbed dose (D)

The amount of *energy absorbed per unit mass*.

Photoelectron

The ejected orbital electron that possesses kinetic energy equal to the energy of the incident photon less the binding energy of the electron shell.

Primary radiation

The emerging x-ray photon beam.

Compton scattered electron, or secondary, or recoil electron

The freed electron that possesses *excess kinetic energy* and is capable of ionizing other atoms.

Pair production

The incoming photon (equivalent in energy to at least 1.022 MeV) strongly interacts with the nucleus of the atom of the irradiated object and disappears. In the process, the energy of the photon is transformed into two new particles: a negatron (electron) and a positron. The negatron eventually recombines with any atom that needs another electron. The positron interacts destructively with a nearby electron. During the interaction, the positron and the electron annihilate each other, with their rest masses converted into energy, which appears in the form of two 0.511-MeV photons, each moving in the opposite direction.

Methods that have been devised to limit the effects of indirectly transmitted x-ray photons...

The most common methods include the following: •Air gap techniques •Radiographic grids

Milliampere-seconds (mAs)

The product of *electron tube current* and the *amount of time in seconds* that the x-ray tube is activated.

Only the K characteristic x-rays of __________ are useful for imaging.

Tungsten

In determining the magnitude of the kinetic energy of a projectile, _________ is more important than ________.

Velocity ; Mass

Absorption

When *electromagnetic energy is transferred* from the x-rays to the atoms of the patient's biologic material.

Describe the effect of kVp on radiographic image quality and patient absorbed dose.

When kVp is increased, the patient receives a lower dose, but image quality may be compromised.

List the events that occur when x-radiation passes through matter.

When x-rays pass through matter, they are either attenuated (meaning they interact with the atoms of the patient's biologic tissue and are absorbed or scattered), or they pass through the patient as small-angle scatter photons or exit, image-forming radiation that hits the image receptor.

The transition of an orbital electron from an outer shell to an inner shell is accompanied by the emission of an ______.

X-ray

Describe the process of absorption, and explain the reason why absorbed dose in atoms of biologic matter should be kept as small as possible.

X-ray photons can interact with atoms of the patient's body and transfer energy to the tissue. The amount of energy absorbed per unit mass is referred to as the absorbed dose (D). This dose should be kept low to prevent the possibility of biologic damage in the patient.

The efficiency of x-ray production increases with increasing ________.

kVp ;

Some examples of unstable nuclei used in PET scanning are as follows:

• Fluorine-18 (18F) • Carbon-11 (11C) • Nitrogen-13 (13N)

Compton scattering is also called...

• Incoherent scattering • Inelastic scattering • Modified scattering It results in all-directional scatter. The scatter created may be directed onward as small-angle scatter, backward as backscatter, and to the side as sidescatter. The intensity of radiation scatter in various directions is a major factor in planning the protection for medical imaging personnel during a radiologic examination.