Chapter 3: Interaction of X-Radiation with Matter Flashcards
The processes of interaction between radiation and matter are emphasized because a basic understanding of the subject is necessary for radiographers to optimally select the following technical exposure factors:
- milliampere-seconds (mAs)
- peak kilovoltage (kVp)
no dose is a
safe dose
the highest energy level of photons in the x-ray beam, equal to the highest voltage established across the x-ray tube
peak kilovoltage (kVp)
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.
peak kilovoltage (kVp)
is your penetration and quality
kvp
polygenetic heterogenous beam
kvp
90 kvp average energy is
1/3 so is 30
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
Milliampere-seconds (mAs)
is considered pt dose
mAs
decrease pt dose
What technical factors?
increase kvp and decrease mAs
if mAs is decrease too much
an grainy image will appear called quantum mottle
is your current and quantity
mAs
Selects technical exposure factors that control beam quality and quantity
- is actually responsible for the dose the patient receives during an imaging procedure
radiographer
what are carriers of manmade electromagnetic energy
x-rays
With a suitable understanding of these factors, radiographers will be able to select appropriate so they can
that can minimize that dose to the patient while producing optimal-quality images.
If x-rays enter a material such as human tissue, they may:
- Interact with the atoms of the biologic material in the patient and be absorbed
- Interact with the atoms in the biologic material and be scattered, causing some indirect transmission
- Pass through without interaction
If an interaction occurs, electromagnetic energy is transferred from the x-rays to the atoms of the patient’s biologic tissue. This process is called
absorption
the amount of energy absorbed per unit mass is referred to as
absorbed dose
The more electromagnetic energy that is received by the atoms of the patient’s body,
the greater is the possibility of biologic damage in the patient
without absorption and the differences in the absorption properties of various body structures,
it would not be possible to produce diagnostically useful images, that is, images in which different anatomic structures could be perceived and distinguished
gives you the different shades of gray black / white
absorption
photoelectric is
absorption
deposited into patient body
absorption
absorbed dose is measured in
milligray (mGY)
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.
atomic number of tungsten
74
the target known as the anode is made up of
tungsten rhenium
why is tungsten used
High melting points
* High atomic numbers
atomic number of rhenium
75
As the electrons interact with the atoms of the tube target
x-ray photons are produced
Photons are particles associated with electromagnetic radiation that have neither mass nor electric charge and travel at the speed of light. X-ray photons exit from the tube target with a broad range, or spectrum, of energies and leave the x-ray tube through a glass window. The glass window permits passage of all but the lowest-energy components of the x-ray spectrum. It therefore acts as a filter by removing diagnostically useless, very-low-energy x-rays.
important
is the x-ray photon beam that emerges from the x-ray tube and is directed toward the image receptor
primary radiation
are particles associated with electromagnetic radiation that have neither mass nor electric charge and travel at the speed of light
photons
exit from the tube target with a broad range, or spectrum, of energies and leave the x-ray tube through a glass window.
xray photons
permits passage of all but the lowest-energy components of the x-ray spectrum. It therefore acts as a filter by removing diagnostically useless, very-low-energy x-rays. In addition to this, a certain thickness of added aluminum is placed within the collimator assembly to intercept the emerging x-rays before they reach the patient.
glass window
what is the average energy
1/3 of the kvp
whole voltage on the x-ray tube
kvp
individual energy of specific x-rays
kev
In diagnostic radiology, the voltage is expressed in thousands of volts, or:
kilovolts (kv)
because the voltage across the tube fluctuates, it is usually charcterized by
kilovolt peak value (kvp)
True or False:
Not all photons in a diagnostic xray bean have the same energy
true
True or False :
The most energetic photons in the beam can have no more energy than the electrons that bombard the target
true
Photons that strike the image receptor are called
Transmitted photons
are the photons that have undergone either absorption or scatter and do not strike the image receptor
attenuated photons
X-rays sometimes interact with atoms of a patient such that they give up all of their energy and cease to exist. These photons are said to be
absorbed
Other photons interact with atoms of the patient, but only surrender part of their energy. They will continue to exist but will emerge from the interaction at a different angle (somewhat like a billiard ball colliding with another billiard ball). These photons are said to be
scattered
Some primary photons will traverse the patient without interacting. These noninteracting x-ray photons reach the radiographic image receptor (e.g., phosphor plate or digital radiography receptor).
what kind of transmission
direct transmission
Decrease in amount of photons reaching IR
attenuation
Direct and indirect transmission of x-ray photons
When an x-ray beam passes through a patient, it goes through a process called
attenuation
If they interacted but still happened to strike the image receptor, they are termed
- as a result of scattered
indirect transmission of photons
If photons pass through the patient without interacting with the atoms of the patient, they are referred to as
direct transmission photons
Other primary photons can undergo Compton and/or coherent interactions and as a result may be scattered or deflected with a potential loss of energy. Such photons may still traverse the patient and strike the image receptor
indirect transmission
Primary, exit, and attenuated photons are photons that emerge
from the x-ray source
are photons that pass through the patient being radiographed and reach the radiographic image receptor
exit or image formation photons
are photons that have interacted with atoms of the patient’s biologic tissue and have been scattered or absorbed such that they do not reach the radiographic image receptor.
attenuated photons
interaction of photon is random or normal
random with biological matter
primary beam going into
patient
photoelectric
- absorbed in the body
absorption
not diagnostic what type of interaction
coherent interaction
diagnostic what types of interaction
photoelectric and compton
scattered
compton
When x-rays interact with human tissue electromagnetic energy is transferred from the x-rays to the atoms of the patient’s biologic material (absorption), and the amount of energy absorbed per unit mass
is the absorbed dose
Keep the amount of electromagnetic energy transferred to the patient’s body as small as possible to
minimize the possibility of biological damage
Diagnostic radiology
photoelectric absorption
Not significant in any energy range
Coherent scattering
diagnostic radiology and therapeutic radiology
Compton scattering
interactions that interact with tube
Bremsstrahlung and characteristics
interactions that interact with matter
photoelectric absorption, Compton scattering and coherent
Coherent scattering is sometimes also called by the following names:
- classical scattering
- elastic scattering
- unmodified scattering
- Thomson scattering
- Rayleigh scattering
A relatively simple process that results in no loss of energy as x-rays scatter
coherent scattering
It occurs with low-energy photons, typically less than 10 keV.Because the wavelengths of both incident and scattered waves are the same, no net energy has been absorbed by the atom (see Appendix E in textbook).Rayleigh and Thompson scattering play essentially no role in radiography
coherent scattering
scattering comes in with the same energy and leaves with the same energy
- no ionization
- decrease contrast b/c of scattered occurs
coherent scattering
. The incoming low-energy x-ray photon interacts with an atom and transfers its energy by causing some or all of the electrons of the atom to momentarily vibrate. 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
coherent scattering
one coming in and one coming out
- photon coming in and photon coming out
coherent scattering
Diagnostic radiology energy range: 23 to 150 kVp
This is the most important mode of interaction between x-ray photons and the atoms of the patient’s body for producing useful images
photoelectric absorption
inner shell (k-shell)
photoelectric absorption
one incoming and one leaving
photoelectric absorption
cascade effect
- without it no shades of gray
photoelectric absorption
To dislodge an inner-shell electron from its atomic orbit, the incoming x-ray photon must be able to transfer a quantity of energy as large as or larger than the amount of energy that holds the electron in its orbit. On interacting with an inner-shell electron, the x-ray photon surrenders all its energy to the orbital electron and ceases to exist. The electron escapes from its inner shell, thus creating a vacancy. The now unbound orbital electron, called a photoelectron, possesses energy equal to the energy of the incident photon minus the binding energy of its electron shell. This photoelectron may interact with other atoms in the vicinity, thereby causing excitation (promotion of electrons from lower energy shells to higher energy shells) or ionization (complete ejection of the electron from an atom), until all of its energy has been spent.
photoelectric absorption
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.
photoelectric absorption
come in with same energy, leave with same energy
coherent scattering
photon coming in and photoelectron leaving
photoelectric absorption
a vacancy is created in an inner shell of the target atom. For the ionized atom, this represents an unstable energy situation. The instability is alleviated by filling the vacancy in the inner shell with an electron from an outer shell, which spontaneously “falls down” into this opening. To do this, the descending electron must lose energy, that is, must pass from a less tightly bound atomic state (farther from the nucleus) to a more tightly held status (closer to the nucleus). The amount of energy loss involved is simply equal to the difference in the binding, or “holding,” energies associated with each electron shell. For a large atom such as an atom of the element lead, this energy can be in the kiloelectron volt range, whereas for the small or low atomic number atoms that make up most of the human body, the energy is on the order of 10 eV. The “released” energy is carried off in the form of a photon that is called a characteristic photon, or characteristic x-ray, because its energy is directly related to the shell structure of the atom from which it was emitted. Those photons generated from photoelectric interactions within human tissue are low enough in energy that they are predominantly absorbed within the body. In general, ensuing vacancies in other electron shells are successively filled, and associated characteristic photons are emitted until the atom achieves an electronic equilibrium.
photoelectric absorption
discovered by Pierre Victor Auger in 1925
- Produces an Auger electronIs - a radiationless effect
Auger effect (pronounced awzhay)
When an inner electron is removed from an atom in a photoelectric interaction, thus causing an inner-shell vacancy, the energy liberated when this vacancy is filled can be transferred to another electron of the atom, thereby ejecting that electron, instead of emerging from the atom as characteristic radiation. Such an emitted electron is called an Auger electron. Its energy is equal to the difference between that released by an outer electron in filling the initial created vacancy and the binding energy of the emitted or Auger electron. Because this process does not include any x-ray emission, it is called a radiationless effect. It reduces the total amount of characteristic radiation produced by photoelectric interactions.
Auger effect
refers to the number of x-rays emitted per inner-shell vacancy
fluorescent yield
the by-products of photoelectric absorption include the following:
- Photoelectrons (those induced by interaction with external radiation and the internally generated Auger electrons)
- Characteristic x-ray photons (fluorescent radiation)
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. During the process of photoelectric absorption, the total energy of the incident photon is completely absorbed as it interacts with and ejects an inner-shell electron of an atom within human tissue or bone from its orbit. The newly ejected photoelectron has appreciable energy and thus can subsequently ionize other atoms it encounters until its energy is sufficiently depleted. After losing an electron, the original ionized atom is unstable and attempts to re-stabilize. This occurs as an electron from a higher shell drops down and fills the vacancy in the inner shell by releasing energy as a characteristic photon. This cascading effect of electrons dropping down to fill existing shell vacancies continues until the original atom regains its stability.
Photoelectric absorption
The probability of occurrence of photoelectric absorption per atom within a particular material depends on
- the energy (E) of the incident x-ray photons
- the atomic number (Z) of the atoms comprising the irradiated object
Probability of occurrence of photoelectric absorption increases
as the energy of the incident photon decreases and the atomic number of the irradiated atoms increases.
Aluminum atomic number and symbol
13 and Al
copper atomic number and symbol
29 and Cu
lead atomic number and symbol
Pb 82
(mass density measured in grams per cubic centimeter) of different body structures influence
attenuation
brighter
more absorption, less transmission
bone
less absorption
- more transmission
soft tissue
tightly packed they are
density
least to most attenuated
air, muscle, fat, organ, bone, metal
- air
- fat
- muscle
- organs
- bone
- metal
higher atomic number
higher absorption
considered positively charge
protons
tungsten binding energy
69.5 in k -shell or 70
electrons in outer shell possess
kinetic energy
electrons in inner shell ( k, l, m) have
lower kinetic energy but higher binding energy
contrast, window depth, dynamic range
photoelectric absorption
higher atomic number higher absorption less transmission
more white
fat atomic number
6.3
muscle atomic number
7.4
bone atomic number
13.8
air atomic number
7.6
iodine atomic numbr
53
barium atomic number
56
lead atomic number
82
radiation that originates from irradiated material outside tube
secondary radiation
Occupational dose is measured in
Sievert
The amount of radiation on object
Exposure
Amount of energy per unit mass absorbed
Absorbed dose
Measurement of overall risk of exposure to humans
Effective dose
Body part thickness
The thickness factor is approximately
Linear
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
As absorption increases
So does the potential for biological damage
The greater the difference in the amount of photoelectric absorption,
The greater the contrast in the radiographic image will be between adjacent structures of differing atomic numbers
To ensure both radiographic image quality and patient safety
Choose the highest-energy x-ray beam that permits adequate radiographic contrast for computed radiography, digital radiography, or conventional radiography
Use of positive contrast medium (barium or iodine)
Containing elements having a higher atomic number than surrounding soft tissue. Appear lighter higher absorbed dose
Use of a negative contrast medium (air or gas)
Are easier to penetrate result in areas of decreased brightness on the radiographic image
1 contrast agent
Air
Less attenuation
The darker the image
Compton scattering is also known as
- incoherent scattering
- inelastic scattering
- modified scattering
- occupational dose
Responsible for most of the scattered radiation produced during radiographic procedures
Compton scattering
Best place to stand is at
A 90 degree angle
You should stand
6 feet away
Don’t Point Tube at
Console
Outer shell, modified, scattered
Photon coming in interacting with outer shell electron
Compton scatter
One coming 2 leaving
Compton scatter
Degrading your image because of the fog
Compton scatter
Fog on image
Decreases contrast
How many times can it scattered before loosing its energy
2 times
Everytime x-ray photon scatters it leaves with
. 1% original intensity. One one thousand
On encountering the electron, the incoming x-ray photon surrendes a portion of its energy in dislodging the electron from its outer - shell orbit, thereby ionizing the biological atom. The freed electron called a Compton scattered electron or secondary or recoil, electron, possesses excess energy and thus is potentially capable of ionizing other biological atoms
Compton scatter
Annihilation radiation is used in an imaging modality employed in Nuclear Medicine called
positron emission tomography (PET)
Attenuation (loss of photons) , absorption, transmission
Increased thickness
^ attenuation ^ absorption decrease transmission
Attenuation (loss of photons) , absorption, transmission
Decreased thickness
decreased attenuation, decreased absorption, ^ transmission
Attenuation (loss of photons) , absorption, transmission
Increased atomic number
^ attenuation ^ absorption decrease transmission
Attenuation (loss of photons) , absorption, transmission
Decrease atomic number
Decrease attenuation, decrease absorption ^ transmission
Attenuation (loss of photons) , absorption, transmission
Increased tissue density
^ attenuation ^ absorption decrease transmission
Attenuation (loss of photons) , absorption, transmission
Decreased tissue density
Decrease attenuation, decrease absorption ^ transmission
Attenuation (loss of photons) , absorption, transmission
Increased beam quality
Decrease attenuation, decrease absorption ^ transmission
Attenuation (loss of photons) , absorption, transmission
Decreased beam quality
^ attenuation ^ absorption Decrease transmission
are emitted from nuclei of very heavy elements, such as uranium and plutonium, during their radioactive decay.
Alpha particles,
Each contains two protons and two neutrons.
Are simply helium nuclei (i.e., helium atoms minus their electrons)
Have a large mass (approximately four times the mass of a hydrogen atom) and a positive charge twice that of an electron
Alpha Particles
are less penetrating than beta particles
- They lose energy quickly as they travel a short distance in biologic matter
- Considered virtually harmless
- As an internal source of radiation, they can be very damaging
- If emitted from a radioisotope deposited in the body, such as in the lungs, alpha particles can be absorbed in the relatively radiosensitive epithelial tissue and are very damaging to that tissue
- superficial of skin
- more biological damage
alpha particles
emitted from nuclei/ nucleus
- if you ingest it is very damaging to your internal organs
Alpha particles
Identical to high-speed electrons except for their origin
8000 times lighter than alpha particles and have only one unit of electric charge (−1)
negative charge
beta particles
two units of electric charge (+2)
alpha particles
Number of protons in the nucleus of an atom constitutes its
atomic number
make up the nucleus of an atom
positively charge
proton
the lesser a structure attenuates
the darker the image will be
the more a structure attenuates
the brighter the image will be
what is remaining in the beam after it passes through matter
remnant beam
electron in
tube interactions
photon in
matter interactions
are photons that emerge from the x-ray source
Primary, exit, and attenuated photons
what comes in and what leaves in a photoelectric absorption
photon coming in, photoelectron leaving
is a composite Z value by weight for a material that is composed of multiple chemical elements.
Effective atomic number [Zeff]
