3.0 Interactions of X-ray with Matter Flashcards
Change in direction of a photon without a loss of energy
Scattering
Deposition of energy, i.e. removal of energy from the beam (total loss of energy)
Absorption
Reduction in the intensity of the main X-ray beam caused by absorption and scattering
Attenuation
Interactions of X-rays at the Atomic level (3)
Photoelectric absorption
Compton scattering
Coherent Scattering
Percentage of
No Interaction,
coherent scattering,
Photoelectric absorption, and
Compton scattering
No Interaction 9%
Coherent scattering 7%
Photoelectric absorption 27%
Compton scattering 57%
This process occurs when an incident photon interacts with an electron in an inner orbital of an atom in the patient.
The incident photon loses all its energy to the electron and ceases to exist.
The energy absorbed by the electron is expended to overcome the binding energy, and the remainder energy remains as the kinetic energy of the electron as it escapes the confines of its orbital.
Photoelectric Absorption
The electron that was interacted by the incident photon
recoil electron or photoelectron
It is the basis of image radiographic information.
Photoelectric absorption
_______________________ - Ejects inner electron and ceases to exist; releases characteristic photon.
___________________ - Ejects outer electron, both scatter
_________________ - Scatters from atom.
_________________ - Passes through patient.
Photoelectric absorption
Compton scattering
Coherent scattering
No interaction
__________________ (a) = The fate of scattered photons resulting from primary Compton, photoelectric, and coherent interactions.
______________ (b) = The sum of the total number of photoelectric interactions and photons that exit the patient equals the total number of incident photons.
Scattered photons
Total photons
» (A) Photoelectric absorption occurs when an incident photon gives up all of its energy to an inner electron, which is ejected from the atom (a photoelectron). The incident electron ceases to exist at this point.
(B) The ionized atom now has an electron vacancy in the inner orbital.
(C) An electron from a higher energy level fills the vacancy and emits characteristic radiation.
(D) All orbitals are subsequently filled, completing the energy exchange.
» Stage 1: X-ray photon interacts with an ________________.
» Stage 2: Inner-shell electron is _______ and the X-ray photon is totally absorbed.
» Stage 3: _________________ cascade in to till the inner- shell vacancy and energy is given out in the form of very low energy radiation (e.g. light)
inner-shell electron
ejected
Outer-shell elections
- The probability of photoelectric interaction is directly proportional to the ____________________________________ of the absorber, and inversely proportional to the ____________________________ (E).
third power of the atomic number (Z)
third power of the energy of the incident photon
(1) * it occurs when a photon interacts with an outer orbital electron.
* Approximately ____ of interactions in a dental x-ray beam exposure involve it.
* it results in the loss of an electron and ionization of the absorbing atom.
* _______________ - ejected electron.
» ____________________ occurs when an incident photon interacts with an outer electron, producing a scattered photon of lower energy than the incident photon and a recoil electron ejected from the target atom. The new scattered photon travels in a different direction from the incident photon.
COMPTON SCATTER
57%
Recoil electron
Compton scattering
» (A) Diagram showing the angle of scatter θ with (i) high- and (il) low-energy scattered photons.
» (B) Typical scatter distribution diagram of a ______ X-ray set. The length of any radius from the source of scatter indicates the relative amount of scatter in that direction. At this voltage, the majority of scatter is in a forward direction.
70 kV
» __________________ that exit the patient can expose the operator.
» Scattered photons travel varying distances within the patient’s tissues and cause ionizations. This internal scatter increases the patient radiation dose and often exposes organs and tissues outside of and distant from the path of the primary beam.
Scattered photons
- Coherent scatter (also known as _____________________________) may occur when a low-energy incident photon (<10 keV) interacts with a whole atom.
- No energy is transferred to the biologic atom and __________________ are caused.
- The biologic effects of coherent scatter are insignificant.
- Has minimal impact on image degradation.
» _____________________ results from the interaction of a low-energy incident photon with a whole atom, causing it to be momentarily excited.
» After this interaction, the atom quickly returns to the ground state and emits a scattered photon of the same energy but at a different angle from the path of the incident photon.
Rayleigh, classical, or elastic scatter
no ionizations
Coherent scattering
- Frequently used units of radiation and radiation detriment.
Summary of Radiation Quantities and Units:
(1) Amount of ionization of air by x- or y-rays; its SI Unit; its Traditional unit; its Conversion
(2) Kinetic energy transferred to charged particles; its SI Unit; its Traditional unit; its Conversion
(3) Total energy absorbed by a mass; its SI Unit; its Traditional unit; its Conversion
(4) Absorbed dose weighted by biologic effectiveness of radiation type used; its SI Unit; its Traditional unit; its Conversion
(5) Sum of equivalent doses weighted by radiosensitivity of exposed tissue or organ; its SI Unit; its Traditional unit; its Conversion
(6) Rate of radioactive decay; its SI Unit; its Traditional unit; its Conversion
DOSIMETRY
(1) Exposure; coulomb/ kg (C/kg); Roentgen (r); 1 C/kg = 3876 R
(2) Kerma; gray (Gy); NA; NA
(3) Absorbed dose; gray (Gy); rad; 1 Gy = 100 rad
(4) Equivalent dose; sievert (Sv); rem; 1 Sv = 100 rem
(5) Effective dose; sievert (Sv); NA; NA
(6) Radioactivity; becquerel
(Bq); Curie (Ci); 1 bq = 2.7 x 10-11 Ci
_________________
o The photon directly interacts with and ionizes a biologic macromolecule.
_________________
o Photons and secondary electrons interact with water and the products of water ionizations cause biologic damage.
Direct actions
Indirect actions
- Biologic molecules absorb energy from ionizing radiation and within 10-10 seconds form unstable free radicals.
- These free radicals quickly re-form into stable configurations by _____________ (breaking apart) or ____________ (joining of two molecules).
- Because the altered biologic molecules differ structurally and functionally from the original molecules, the consequence is a biologic
- What does this pertain to? __________
dissociation; cross-linking
DIRECT ACTION
- In indirect actions, the initial interaction of a photon occurs with a water molecule-which constitutes approximately 70% of mammalian cells. Indirect action are the predominant mode of x-radiation-induced biologic damage.
- Ionizing radiation initiates a complex series of chemical changes in water, collectively referred to as _______________.
- The initial series of interactions of x-ray photon with water produce ____________ (H) and ____________ (OH) free radicals that interact with biologic macromolecules.
- The _______ radical is highly reactive and is estimated to cause two-thirds of the biologic damage to mammalian cells from X-rays.
- free radicals in indirect action (2)
radiolysis of water
hydrogen; hydroxyl
hydroxyl
» Hydroperoxyl
» Hydrogen Peroxide
Deoxyribonucleic Acid and Chromosomal Damage and Damage Response (4)
Base damage
Single-strand breaks
Double-strand breaks
DNA-DNA and DNA-protein cross links
- Most important damage type
- Repaired by nonhomologous end-joining or homologous recombination
- ________________________ - accounts for many of the ionizing radiation-induced mutations.
DOUBLE-STRAND BREAKS
Nonhomologous end-joining
» A single photon may cause multiple ionizations in DNA, resulting in a ________________________________. In this instance, an _______________ causes ionization of a water molecule, and the _______________ causes a cluster of damage to multiple sites in a DNA molecule.
» Such cluster damage is difficult to repair and is believed to be responsible for most radiation cell killing, carcinogenesis, and heritable effects.
cluster of DNA damage/ clustered DNA damage
incident photon
recoil electron
Double-strand breaks
» ___ – Cellular contents excluding the chromosomes, are duplicated.
» ___ – Cell cycle arrest.
» ___ – each of the 46 chromosomes is duplicated by the cell.
» ___ – The cell “double checks” the duplicated chromosomes for error, making any needed repairs.
» Mitosis
» Cytokinesis
G1
G0
S
G2
- _______________________ - prior to chromosomal duplication
- _______________________ - after chromosomal duplication
- ______________________ - cells are most radiosensitive
- _________ - cells are less radiosensitive (latter part)
- _________ - cells are least radiosensitive.
Chromosome aberration
Chromatid aberration
G, and mitosis phase
G, phase
S phase
CLASSIFICATION OF BIOLOGIC EFFECTS (2)
a) Stochastic Effect
b) Deterministic Effect
Caused by: Sublethal DNA damage
Threshold dose: No. There is no minimum threshold dose. Effect can be caused by any dose of radiation.
Severity of clinical effects and dose: Severity of clinical effects is independent of dose; all-or-none response an individual either manifests effect or does not.
Relationship between dose and effect: Frequency of effect proportional to dose; the higher the dose, the higher the risk of manifesting the effect.
Caused by doses used in diagnostic radiology: Yes
Examples: Radiation-induced cancer, Heritable effects, Radiation-induced skin cancer
Stochastic Effects
Caused by: Cell killing
Threshold dose: Yes. Effect occurs only when the threshold dose is exceeded.
Severity of clinical effects and dose: Severity of clinical effects is proportional to dose; the higher the dose, the more severe the effect.
Relationship between dose and effect.: Probability of effect independent of dose; most individuals manifest effect when threshold dose is exceeded.
Caused by doses used in diagnostic radiology: No
Examples: Osteoradionecrosis, Radiation-induced cataract formation, Radiation-induced skin burns
Deterministic Effects
(1) * It is the consequence of such sublethal changes in the DNA of an individual cell.
* NO threshold dose.
* A single x-ray photon has the potential to cause a DNA mutation. Thus even the smallest radiation dose could cause a heritable effect or cancer.
* As radiation dose increases, the number of radiation induced DNA damage sites in increased and the risk of subsequent disease-causing mutation is higher. Thus, the probability of a stochastic effect increases with dose.
(2) * Its 2 subdivisions:
o Radiation-Induced Cancer
o Heritable effects
Stochastic effects
General Features of ____________________________
* These are clinically and histologically indistinguishable from sporadically occurring or chemical-induced cancers.
* Certain tissues, for example the female breast and the thyroid gland, are more sensitive to the carcinogenic effects of ionizing radiation.
* There is a long latent period, ranging from years to decades, between radiation exposure and the occurrence of cancer.
* The risk of radiation-induced tumor induction is approximately threefold higher in children than in adults.
Radiation-Induced Cancer
Examples:
Leukemia
Thyroid cancer
Salivary gland tumors (Warthin tumor)
Breast cancer
Brain and nervous system cancers
- Changes seen in the offspring of irradiated individuals
- Consequence of DNA damage in germ cells
- Studies of the children of patients who received radiotherapy show no detectable increase in the frequency of genetic diseases
HERITABLE EFFECTS
(1) * Caused by cell killing and are the consequences of cell death on the function of a tissue or organ.
* Manifest only when the radiation exposure to an organ or tissue exceeds a threshold level.
* The severity of this effect is ____________ to the dose: the higher the dose, the more severe the effect.
* Dental radiography - low to no effect
* 2 Types of cell killing:
o Mitotic death
o Apoptosis
(1) DETERMINISTIC EFFECT (TISSUE REACTIONS)
proportional
- Most common means of cell killing.
- The law of Bergonié and Tribondeau.
- Cell radiosensitivity to killing = Mitotic rate / Degree of differentiation
- This rule predicts that rapidly dividing cells will be more radiosensitive and postmitotic specialized cells will be most radioresistant.
MITOTIC DEATH
- (1) Programmed cell death.
- _________ are an exception to Bergonié and Tribondeau’s law.
- __________ are the most radiosensitive mammalian cell.
- _____________________________ are highly radiosensitive.
(1) APOPTOSIS
Lymphocytes
Lymphocytes
Serous acini of the salivary glands
Relative Radiosensitivity of Various Organs
High (5)
Intermediate (7)
Low (2)
High (5)
Lymphoid organs
Bone marrow
Testes
Intestines
Mucosal lining
Intermediate (7)
Fine vasculature
Growing cartilage
Growing bone
Salivary glands
Lungs
Kidney
Liver
Low (2)
Neurons
Muscle
- Embryos and fetuses are considerably more radiosensitive than are adults because most embryonic cells are relatively _____________ and ____________.
- The effects are dependent on the dose and gestational age during irradiation.
undifferentiated
rapidly mitotic
- _____________________________ (0 to 9 days in humans) causes embryonic death. The threshold for this effect is estimated to be 100 mGy-approximately 14,000 times more than the fetal dose from dental radiographic examinations. In comparison, the dose to an embryo and fetus from natural background radiation is approximately 0.5 to 1 mSv during the 9 months of gestation.
- In humans, fetal irradiation is associated with ____________ (irradiation at 8 to 15 weeks gestation) and ____________________ (irradiated at 8 to 25 weeks’ gestation). The threshold dose for these effects is 0.3 Gy-approximately 42,000 times higher than fetal doses from dentomaxillofacial radiographic examinations.
Irradiation during preimplantation
microcephaly
mental retardation
ORAL MUCOSA
* Basal layer composed of rapidly dividing, radiosensitive progenitor cells
* By the end of the second week of therapy, the cell death induces an inflammatory response and the mucous membranes begin to show areas of redness and inflammation (_____________)
* it is most severe after radiotherapy
* Secondary yeast infection by ___________________ is a common complication and may require treatment.
mucositis
Candida albicans
- Sensitive to radiation.
- Lost of taste occurs in the second or third week of radiotherapy.
- Taste loss is reversible.
TASTE BUDS
(1) * Highly sensitive to radiation.
* Likely due to the radiation-induced apoptosis
of salivary acinar cells.
(2) * ___________ - dry mouth.
SALIVARY GLANDS
Xerostomia
- Rampant form of dental decay.
- Aggravated by xerostomia.
RADIATION CARIES
- Radiation affects tooth development
- Irradiation affects the tooth bud
- Malformations
- Retard or abort root formation.
TEETH
- “An area of exposed irradiated bone tissue that fails to heal over a period of three months, without residual or recurrent tumor; and when other causes of osteonecrosis have been excluded”.
- Manifests 6 to 12 months following radiation treatment.
- 5% to 7% incidence for convention radiotherapy.
- More frequent on the mandible than maxilla.
OSTEORADIONECROSIS (ORN)
(1) * Contracture and trismus of the muscles of mastication.
- _____________ or _______________ muscles.
MUSCULATURE
Masseter
pterygoid
