UNIT 1: Interactions with Matter Flashcards
Absorption
an interaction occurring within the patient in which electromagnetic energy is transferred from the x-rays to the atoms of the patient’s biological tissue.
Absorbed dose
amount of energy absorbed per unit mass
Primary radiation
well confined 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 and useful beam.
Attenuation
any process decreasing the intensity of the primary photon beam directed toward a particular destination (reduction in the # of photons as it passes through matter)
Direct transmission
photons pass through the patient without interacting with the atoms of the patient
Indirect transmission
photons interact with the atoms of the patient, but still happen to strike the IR, always the result of scatter
Radiographic fog
additional, undesirable exposure from scatter. It interferes with the radiologists ability to distinguish different structures in the image.
Primary photon
the photons that are produced before they enter enter human tissue
Exit photons
photons that emerge from the subject’s tissue and strike the x-ray detector
What outcome(s) may occur when a primary photon interacts with human material?
- 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, direct transmission
Summarize how a diagnostic x-ray photon is produced
a stream of very energetic electrons bombard a positively charged target in a highly evacuated glass tube, as the electrons interact with the material of the target x-ray photons are produced
What are the purposes of the glass window?
it permits passage of all but the lowest energy photons, acting as a filter by removing diagnostically useless, very low energy x-rays
Explain how the beam becomes “hardened”. What impact does this have on the radiographic beam? How does this impact the patient dose?
Certain thickness of added aluminum is placed within the collimator assembly to intercept the emerging x-rays before they reach the patient. This aluminum “hardens” the x-ray beam (i.e., raises its effective energy) by removing low-energy components that would serve only to increase patient dose
Identify the components of the permanent inherent filtration
•Glass envelope encasing the x-ray tube
•Insulating oil surrounding the tube
•Glass window in the tube housing
How is the energy of an x-ray photon derived?
-kVp
-for a diagnostic x-ray unit, the mean photon energy in the x-ray beam is about one third the energy of the most energetic photon
What is the average energy of the radiation beam?
about ⅓ of the energy of the most energetic photon.
What leads to an optimal image?
The use of correct technical factors, correct collimation, and the reduction of scatter
How might images become degraded?
radiographic fog/scatter
How can radiographic fog be reduced?
- by reducing scatter
- reducing the amount of tissue irradiated (collimation)
Photoelectric Interaction summary
Within the energy range of diagnostic radiology (23-150 kVp). interaction between an inner shell electron and an x-ray photon. The photon knocks out the inner shell electron (k-shell), creating a photo electron that can interact with surrounding tissue and cause ionization until all of its energy is spent. The k-shell vacancy causes a cascade effect to occur releasing secondary photon energy that is absorbed within the body.
*Auger effect can also occur: when an inner electron is removed from an atom, the energy liberated when the vacancy is filled can be transferred to another electron of the atom, thereby ejecting the electron, instead of emerging from the atom as characteristic radiation. The emitted electron is now called an auger electron
*Biggest reason for patient dose but is needed to create the x-ray image
Photoelectric Interaction results
-increased patient dose responsible for the latent image
-auger effect: radiationless effect (reduces the total number of characteristic radiation produced by photoelectric interactions)
-byproducts:
-photoelectrons
-characteristic x-ray photons (fluorescent radiation)
-possible auger electron
Photoelectric Interaction: Probability of occurrence
-When photon has lower energy and the material has a higher atomic number and an increased mass density
-More atoms in more dense tissue = increases the probability of this interaction
-Thicker body structure will have more interactions
-Photoelectric interactions take place within the contrast media due to higher atomic number of contrast
Which appendix describes electron shell structure of an atom?
appendix F
What factors help determine imaging contrast?
- density
- atomic number
How can structures of similar atomic number & mass density be distinguished from one another?
contrast media
How does the atomic number of contrast relate to the incidence of photoelectric interactions?
it significantly increases
How does the use of contrast media impact the patient’s absorbed dose?
positive contrast media leads to increased absorbed dose
Identify the common photon energy (range) to produce this interaction.
25-150 kEv
Photoelectric Interaction Picture
Compton Scattering Interaction other names
-incoherent
-inelastic
-modified
Compton Scattering Interaction summary
an incoming photon interacts with a loosely bound outer shell electron of an atom. The incoming photon surrenders a portion of its energy in dislodging the electron from its orbit, thereby ionizing the atom. The freed electron (scattered, secondary, or recoil electron: outer shell electron that is ejected from its orbit), has excess energy and is capable of ionizing other biological atoms. The incident photon continues on its way, but in a new direction (Compton scattered photon)
Compton Scattering Results
-increased patient dose
-the reason for occupational exposure
-reduced image quality
Differentiate between a Compton scattered electron and a Compton scattered photon
- Compton scattered electron: interacts with surrounding tissue causing ionization and biological damage in the subject
- Compton photon: scatter radiation that is the cause of occupational dose
Explain the factors that influence the probability of Compton interactions?
- density of the patient
- photon energy (beam energy)
How can the radiographer reduce Compton scattering?
- Collimation
- Depressing tissue
What is the significance of Compton scattering as it relates to the healthcare worker?
It’s the cause of occupational exposure
At what energy range/ level does Compton Scattering Interaction occur?
> 60 kEv
Compton Scattering Interaction picture
Coherent Scattering Interaction other names
-classical
-elastic
-unmodified
Coherent Scattering Interaction summary
a low energy photon interacts with the entire atom, energy is transferred as the electrons of the atom momentarily vibrate. The vibrating electrons radiate energy in the form of electromagnetic waves forming scatter photons. Because the wavelength of both incident and scattered waves are the same, no net energy has been absorbed by the atom.
Coherent Scattering Interaction Results/Image and dose significance
-unmodifying/non-ionizing
-Not much significance on patient dose/Image
-Has a noticeable effect on visible light
At what energy range/ level does Coherent Scattering Interaction occur?
< 10 KEV
In what modality is Coherent Scattering most often seen?
mammography
Coherent Scattering Interaction picture
Pair Production Interaction summary
the incoming photon strongly interacts with the electric field surrounding the nucleus of an atom of irradiated biological tissue and subsequently disappears. The energy of the photon is absorbed and transformed into matter composed of two particles
-negatron (ordinary electron)
-positron (positively charged electron)
The positron interacts destructively with the negatron causing them to anihalate each other.
Pair Production Results
-Negatron interacts with other matter around it until it runs out of energy.
-Positron is extremely destructive to nearby ordinary matter.
-Annihilation event: destroys both positron and electron it interacted with, energy is converted into two x-ray photons with 0.511 meV energy EACH
At what energy range/ level does Pair Production Interaction occur?
1.022 million electron volts (MeV)
In what modality and imaging tool is the Pair Production Interaction concept utilized?
- nuclear medicine
- positron emission tomography (PET)
Pair Production Interaction picture
Photodisintegration Interaction summary
a high energy photon collides with the nucleus of an atom, which directly absorbs all the photons energy. The energy excess in the nucleus creates an instability that in most cases is alleviated by the emission of a neutron by the nucleus. In addition, if sufficient energy is absorbed by the nucleus, other types of emission will be possible, such as a proton or proton-neutron combination (deuteron), or even an alpha particle
Photodisintegration Interaction results
Nucleus becomes radioactive
At what energy range/ level does Photodisintegration Interaction occur?
exceeding 10 MeV
In what modality is Photodisintegration Interaction most often seen?
Radiation therapy
Photodisintegration Interaction picture
