Ch_3_Interaction of X-Radiation With Matter

5. In which of the following x-ray interactions with matter is the energy of the incident photon partially absorbed? A. Compton B. Photoelectric C. Coherent D. Pair production

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 B. Direction of the incident photon C. X-ray beam intensity D. Exposure time

A. Energy of the incoming photon

small-angle scatter

Photons that pass through the patient being radiographed, interact with the atoms of the body, and are deflected at such a small angle that they can reach the image receptor, thereby degrading the completed radiographic image by producing small amounts of radiographic fog.

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

1) Direct (transmissions) - xray photons that pass through the pt and reach the IR without any interactions 2) Indirect (transmissions) - photons that undergo Compton and/or coherent interactions and result in a scattering and loss of energy from the original beam

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

1) primary radiation is high energy photons exiting the tube 2) exit, or image formation beams are those that hit the IR 3) scatter is the result of attenuation in which photons are diverted from the original trajectory and lose energy.

Processes of Interaction

1. *Coherent scattering* 2. *Photoelectric absorption* 3. *Compton scattering* 4. *Pair production* 5. *Photodisintegration*

effective atomic number (Zeff)

A composite Z value for when multiple chemical elements comprise a material.

Contrast media (positive)

A liquid solution containing an element with a higher atomic number than surrounding tissue (e.g., barium or iodine) that is either ingested or injected into biologic tissues or structures to be visualized.

characteristic photon *(characteristic x-ray)*

A quantum or quantity of radiant energy given off by an atom when an electron from an outer- shell drops down to fill an inner-shell vacancy. The energy of a characteristic photon is equivalent to the difference in energy level between the two electron shells. Also known as characteristic x-ray or fluorescent radiation.

3. Which of the following defines attenuation? A. Absorption and scatter B. Absorption only C. Scatter only D. Weakened only

A. Absorption and scatter

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

Absorption is the process of electromagnetic energy interacting with cells. These interactions create a possibility of biologic damage to the pt.

exit, or image-formation, photons

All the x-ray photons that reach their destination (the image receptor) after passing through the patient being radiographed; previously known as *remnant radiation.*

Compton scattered electron, or secondary, or recoil, electron

An energetic electron dislodged from the outer shell of an atom of the irradiated object as a result of a Compton interaction with an incoming x-ray photon. *Also known as a secondary, or recoil, electron*.

Photodisintegration

An incoming high-energy photon collides with the nucleus of an atom 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. 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.

Compton scattering

An interaction between an incoming x-ray photon and a loosely bound outer-shell electron of an atom in the irradiated object. The photon surrenders a portion of its kinetic energy to dislodge the electron from its outer-shell orbit, thereby ionizing the atom, and then continues in a new direction. This process accounts for most of the scattered radiation produced during diagnostic procedures. *Also known as incoherent scattering, inelastic scattering, or modified scattering.*

photodisintegration

An interaction that occurs above 10 MeV in high-energy radiation therapy treatment machines. In this interaction, a high energy photon collides with the nucleus of an atom, which directly absorbs all 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 from the nucleus. Also, if sufficient energy is absorbed by the nucleus, another type of emission is possible, such as a proton or proton-neutron combination (deuteron) or even an alpha particle.

4. In the radiographic kilovoltage range, which of the following structures undergoes the most photoelectric absorption? A. Air cavities B. Compact bone C. Fat D. Soft tissue

B. Compact bone

1. Exit, or image-formation, radiation is composed of which of the following? A. Primary photons and wide-angle Compton scattered photons B. Non-interacting and small-angle scattered photons C. Very low energy photons D. Auger electrons

B. Non-interacting and small-angle scattered photons

10. The interactions of x-ray photons with any atoms of biologic matter are: A. Able to be preplanned to selective atoms to limit radiation exposure to those atoms B. Important only in therapeutic radiology C. Random, so the effects of such interactions cannot be predicted with certainty D. Unimportant in diagnostic radiology, thus making radiation protection unnecessary

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

radiographic contrast

Differences in gray levels between adjacent anatomic structures on a completed image. Image receptor contrast and subject contrast combined produce radiographic contrast.

Summary of the Process of Compton Scattering

Compton scattering is important in the energy range used in diagnostic radiology. Because the scattered x-ray photon produced from the interaction of the incoming photon with an outer-shell electron of an atom of human tissue results only in a partial transfer of kinetic energy to that biologic atom, the scattered photon now traveling in a different direction can become a potential health hazard for imaging personnel by increasing their occupational radiation exposure

8. Which of the following influences attenuation? 1. Effective atomic number of the absorber 2. Mass density 3. Thickness of the absorber A. 1 and 2 only B. 1 and 3 only C. 2 and 3 only D. 1, 2, and 3

D. 1, 2, and 3

2. Which of the following contributes significantly to the exposure of the radiographer? A. Positrons B. Electrons C. Compton scattered photons D. Compton scattered electrons

D. Compton scattered electrons

pair production

Interaction between an incoming photon of at least 1.022 MeV and an atom of irradiated biologic tissue in which the photon approaches, strongly interacts with the nucleus of the atom of the irradiated tissue, and disappears. In the process, the energy of the incoming photon is transformed into two new particles—a negatron and a positron—after which these particles exit from the atom and carry away some of the momentum of the absorbed photon when the photon's energy is greater than 1.022 MeV.

Pair Production

Pair production occurs when incoming x-ray photons with energy of at least 1.022 MeV interact with the nucleus of an atom. The end result of this interaction is the annihilation of a positron and an electron with their rest masses converted into energy, which appears in the form of two 0.511-MeV photons, each moving in the opposite direction.

• Explain the meaning and significance of peak kilovoltage (kVp) and milliampere-seconds (mAs) as technical exposure factors.

Peak kilovoltage 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. The product of milliamperes (mA), which is the x-ray tube current, and time (seconds [s] during which the x-ray tube is activated) is the main determinant of how much radiation is directed toward a patient during a selected x-ray exposure

radiographic image receptor

Phosphor plate, digital radiography receptor, or radiographic film.

Summary of the Process of Photoelectric Absorption

Photoelectric absorption is the most important mode of interaction between x-radiation and the atoms of the patient's body in the energy range used in diagnostic radiology because this interaction is responsible for both the patient's dose and contrast in the image

photoelectric absorption

Process whereby the kinetic energy of the incident photon is completely absorbed as it interacts with an atom and ejects an inner-shell electron in its orbit.

mass density

Quantity of matter per unit volume. It is generally specified in units of kilograms per cubic meter (kg/m3) or grams per cubic centimeter (g/cc).

primary radiation

Radiation that emerges directly from the x-ray tube collimator and moves without deflection toward a wall, door, viewing window, and so on. Also called direct radiation or the useful beam.

absorbed dose (D)

The amount of energy per unit mass absorbed by an irradiated object (e.g., the patient's body tissue). This absorbed energy is responsible for any biologic damage resulting from the tissues being exposed to radiation. The gray (Gy) is the SI unit of this radiation quantity.

photoelectron

The electron ejected from its inner-shell orbit during the process of photoelectric absorption. It possesses kinetic energy and can ionize other atoms it encounters until its energy is spent.

peak kilovoltage (kVp)

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

fluorescent yield

The number of x-rays emitted by an atom per inner-shell vacancy.

radiographic fog

Undesirable additional darkness on a completed radiographic image caused by scattered radiation reaching the image receptor.

Summary of the Process of Coherent Scattering

The process of coherent scattering 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.

coherent scattering

The process wherein a low-energy photon (typically less than 10 keV) interacts with an atom of human tissue and does not lose kinetic energy. The atom responds by releasing the energy it has received in the form of a scattered photon that has the same wavelength and energy as the original incident photon. The emitted photon changes direction by 20 degrees or less. No ionization of the biologic atom occurs. Also known as classical scattering, elastic scattering, unmodified scattering, and Rayleigh scattering. Thompson scattering is also another type of coherent scattering.

milliampere-seconds (mAs)

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

Contrast media (negative)

The use of air or gas to enhance visualization of body structures during a radiologic procedure.

absorption

Transference of electromagnetic energy from an x-ray beam to the atoms or molecules of the matter through which it passes (e.g., the patient's biologic material).

Auger effect

When an inner-shell vacancy occurs in an atom, the energy liberated when this vacancy is filled can be transferred to another electron of the atom, thereby ejecting the electron. The process is known as the Auger effect, and the emitted electron is known as an Auger electron.

attenuation

results when, through the processes of absorption and scatter, the intensity of the primary photons in an x-ray beam decreases as it passes through matter.

permanent inherent filtration

the combination of the glass tube and added aluminum within an x-ray tube which filters low energy un-useful diagnostic photons prior to reaching the pt (primary radiation)