1 Radiation Physics and Biology

what is energy in transit called?

radiation

what is the removal of electrons from an atom resulting in the formation of an ‘ion pair’ called?

ionization

what is the the attractive force that keeps electrons bound to the nucleus in their orbitals?

electron binding energy

describe the structure of atoms

orbital electrons: S, P, D, F, G, H

what is the relationship between Z # and binding energy?

higher Z # = higher binding energy

what are characteristics of neutrons?

• no charge (neutral)

• mass is approx same as proton (1.68 × 10^-24 g)

what is Z-number?

• # of protons (aka atomic #)

  • in a neutral atom, # of protons = # of electrons

• determines the identity of an element

what is A-number?

# of protons + neutrons (aka atomic mass #)

movement of energy through space as a combination of electric and magnetic fields is known as…?

electromagnetic radiation (gamma rays, xrays, UV, visible light, infrared, microwaves, radiowaves)

what does the quantum theory suggest?

energy transfer in the form of “bundles” (or packets) of energy called photons (or “quanta”) that move at the speed of light and w a specific amount of energy

what are some important properties of x-rays?

• no mass

• no electric charge

• travel in waves

• travel w specific frequency (speed of light)

• highly penetrating

• travel in straight lines (diverging from central focus)

• able to ionize

• range of wavelengths (0.1A to 0.5A)

xrays are able to ionize. what does this mean?

• affect photographic film

• able to produce biological/chemical changes

what are the 2 mechanisms of X-ray production?

1. Bremsstrahlung (electron to nucleus interaction)

2. characteristic radiation (electron to electron interaction)

of the 2 mechanisms of x-ray production, which is the primary source of radiation generated by an x-ray tube?

Bremsstrahlung mechanism

describe how the Bremsstrahlung mechanism produces x-ray.

X rays are produced when high velocity electrons are suddenly decelerated when they pass close to or directly hits the nuclei of high Z # absorbing material.

• direct hit → entire kinetic energy of electron gets transferred to xray photon → high/max energy xray photon

• indirect/near miss → only part of kinetic energy gets transferred → lower energy xray photon is produced

what types of xray photons are produced via bremsstrahlung mechanis?

photons w varying energy levels (within a specific range)

describe how the characteristic mechanism produces x-ray.

1. high velocity electrons interact with an inner shell electron and knocks it out of the orbit

2. This vacancy( gap) is filled out by an outer shell electron.

3. The difference in the energy levels is released and an x-ray photon

what types of xray photons are produced via characteristic mechanism?

photons with specific energy level that is characteristic of the atom produced

of the 2 mechanisms of x-ray production, which contributes a small fraction of photons in the beam?

characteristic mechanism

what are the key components of xray generating tubes?

1. electron source (cathode)

2. concentration of electrons (focussing cup)

3. mechanism to accelerate electrons (potential difference or tube voltage kVp)

4. suitable target to stop electrons (anode)

describe the process of thermionic emission.

why is conversion of kinetic energy in a xray tube an inefficient process?

>99% is lost as heat

only <1% becomes xradiation

what are the 2 components that make up the anode of a xray tube ?

• tungsten target

• copper stem (good thermal conductor)

why is tungsten a good anode target?

• high atomic #

• high melting point

• low vapor pressure

• high degree of thermal conductivity

what factors control the xray beam? (6)

1. tube voltage (kVp)

2. exposure time (S)

3. tube current (mA)

4. filtration

5. collimation

6. distance of xray tube from pt/receptor

what factor of the xray beam controls the acceleration of electrons from cathode to anode?

Tube Voltage (kVp)

what does filtration control in an xray beam?

removes lower energy xray photons

what does collimation control in an xray beam?

adjusts shape/size of xray beam

how does distance of xray tube from patient/receptor affect xray beam?

controls intensity of xray beam

how does tube current (mA) affect xray beams?

controls heating of cathode to produce electrons

how does exposure time (S) affect xray beams?

controls heating of cathode to produce electrons

what 2 factors control heating of the cathode to produce electrons in xray beam?

exposure time (S) and tube current (mA)

in an x-ray generating tube, what material makes up the anode vs cathode?

• cathode → tungsten filament

• anode (tungsten targe in a copper block)

what does this graph tell us about what happens when kVP of a beam is increased?

increased kVP →

1. total # of photons increase

2. max energy of beam increase

3. mean energy of beam increase

Movement of energy through space as a combination of electric and magnetic fields is referred to as…?

electromagnetic radiation (gamme rays, x-rays, ultraviolet, visible light, infrared, microwaves, radio waves)

what category of organs and cells are MOST sensitive to radiation (highest radiosensitivity)?

• Bone marrow (lymphoblasts, lymphocytes, plasma cells, erythroblasts)

• intestines (epithelial stem cells)

• oral mucous membrane (basal cells )

• Spermatogenic cells

what cell types are an exception to the law of Bergonie and Tribondeau?

small lymphocytes & oocytes (mature in differentiation yet sensitive to radiation)

what are the teratogenic effects (deterministic, safe doses) of radiation on an embryo/fetus?

• intrauterine death (1st week of pregnancy)

• intra-uterine growth retardation

• congenital malformations

• developmental abnormalities

what are the stochastic effects (no safe dose) of radiation on an embryo/fetus?

cancer in childhood

what factors influence the probability of radiation effecting an embryo?

• dose to embryo/fetus

• stage of gestation at time of exposure

A dose threshold of ______ to the fetus is required to produce x-ray induced birth defects

100-250 mSv

This is orders of magnitude above the fetal dose from dental radiographs (<.01 mSv)

what are possible radiogenic effects at a gestational age of 0-9 days (preimplantation stage) ?

all or none

what are possible radiogenic effects at a gestational age of 10 days - 6 weeks (organogensis stage) ?

congenital anomalies, growth retardation

what are possible radiogenic effects at a gestational age of 6-40 weeks (fetal stage) ?

growth retardation, microcephaly, mental retardation

t/f: Risk of Stochastic effects (childhood cancer/ genetic mutation) occur throughout pregnancy

true

acute radiation syndromes occur usuaully as a result of…?

whole body irradiation (accidental, nuclear plant disasters, etc.) → never happens any other way

acute radiation syndromes are ___-dependent.

dose

• Prodromal symptoms (1-2 Gy)

• Hematopoietic syndrome (2-7Gy)

• Gastrointestinal syndrome(7-15Gy)

• Central nervous system syndrome (50Gy)

acute radiation syndrome: prodromal symptoms

• dose?

• time of onset?

• manifestation?

• dose: 1-2 Gy

• time of onset: minutes-hours after exposure

• manifestation: anorexia, nausea, vomiting, fatigue

acute radiation syndrome: hematopoietic symptoms

• dose?

• time of onset?

• organs affected?

• manifestation?

• dose: 2-7 Gy

• time of onset: days to months

• organs affected: hematopoietic stem cells in bone marrow and spleen

• manifestation: infection, hemorrhage, anemia (death 10-30 days)

acute radiation syndrome: gastrointestinal symptoms

• dose?

• time of onset?

• organs affected?

• manifestation?

• death?

• dose: 7-15 Gy

• time of onset: few weeks to months

• organs affected: small intestine

• manifestation: nausea, vomiting, diarrhea, electrolyte imbalance, circualtory collapse

• death 3-10 days

acute radiation syndrome: cardiovascular and CNS symptoms

• dose?

• time of onset?

• organs affected?

• manifestation?

• dose: 50 Gy

• time of onset: death in 1-2 days

• organs affected: brain

• manifestation: lethargy, tremors, convulsions, ataxia, coma

radiation therapy in the oral cavity is used to target malignant oral lesions that are radiosensitive. what is the total dose used?

64-70 Gy in 6-7 weeks

how does radiation affect salivary glands?

• radiation caries

• reduced secretion (xerostomia)

• pH altered (decalcification of enamel)

how does radiation affect taste buds?

• decrease taste acquity

• recovery to almost normal 60-120 days post irradiation

how does radiation affect teeth? (pre-calcification vs post-calcification vs post-eruption)

• prior to calcification → tooth bud destroyed

• post calcification → malformations, arrested growth

• erupted teeth → radioresistant

• severity is dose dependent

how does radiation affect bone?

osteoradionecrosis

• damage to vasculature of periosteum, cortical bone

• destruction of osteoblasts

how does radiation affect oral mucous membrane?

-Desquamation

-Inflammation/pain

-white/yellow pseudomembrane

-Secondary Fungal Infections

-Long term: atrophic, thin, avascular mucosa

-healing –2months after

what is the Removal of low energy photons from the x ray beam while allowing passage of high energy photons called?

filtration

why is filtration important?

1. Low energy photons do not contribute to radiograph formation and contributes to patient risk

2. Filtration reduces intensity of x-ray beam (hence requires compensatory increase in exposure time)

what is the total filtration in xray tube?

1. inherent filtration

   1. glass and oil

2. added filtration (by manufacturer)

   1. aluminum disk

total filtration = aluminum + inherent

by law, when operating a tube at 50-70 kVp, ___ mm aluminum equivalent of total filtration is required. what about at above 70 kVp?

50-70 kVp = 1.5 mm aluminum equivalent

above 70 kVp = 2.5 mm aluminum equivalent

when filtration is increased, how is exposure time and pt skin exposure affected?

• increase in exposure time

• decrease in pt skin exposure

how does filtration affect the beam?

• decreased number of photons

• increased mean energy of photons

• same max energy of photons

Although filtration requires an increase in exposure time up to % to compensate for loss of intensity, it reduces patient skin exposure by as much as %

50%

80%

what is a useful way to characterize the penetrating quality (mean energy) of an x ray beam?

Half Value layer (HVL)

HVL is defined as the thickness of an absorber that reduces the quantity of photons by …?

1/2

as average energy of beam (kVp) increases, what happens to HVL?

increases

how does changing tube current (mA), affect beams?

increasing tube current = increasing number of photons

no affect on quality (mean and max energy stays the same)

how does changing exposure time (S) affect beams?

increasing exposure time = increasing number of photons

no affect on quality (mean and max energy stays the same)

Collimator is a metallic barrier with an aperture that:

1. Reduces size and modifies shape of beam

2. Reduces volume of tissue irradiated

3. Improves image quality

what law describes the effect of distance from xray source to receptor on intensity?

inverse square law (For a given beam, the intensity is inversely proportional to the square of the distance from the source)

• increased distance = decreased intensity/quality

• Clinical Significance: Changing the distance between the x-ray tube and patient has marked effect on beam intensity, so position the cone of x ray tube as close to the patient (not touching) as possible

what are 3 types of interactions (scattering an absorption events) that can occur?

1. coherent scattering

2. photoelectric effect

3. compton scattering

what happens during coherent scattering?

low energy photon passes near an outer electron and releases that same energy in a different direction

what % of total interactions are coherent scattering?

7%

what interaction is the primary contributor to image formation?

photoelectric effect

what happens in photoelectric effect?

1. Incident photon collides with an inner-shell electron in an atom of the absorbing medium

2. The electron is ejected (ionization)

3. Transfers all energy to the electron; incident photon ceases to exist

4. An electron from higher energy level fills the vacancy

5. Characteristic radiation is emitted (low energy, absorbed in the patient)

6. The recoil electron (photoelectron) travels a short distance before giving up all its energy

what % of total interactions are photoelectric effect ?

27%

how is photoelectric effect different from characteristic mechanism?

photoelectric is triggered by photon colliding with inner-shell electron

characteristic is triggered by a high energy electron colliding

what is the clinical significance of photoelectric effect?

Z (atomic #) of bone is higher than Z of soft tissue- differential photoelectric absorption within different types of tissues, makes production of a radiographic image possible!

what happens in compton scattering?

1. Incident photon interacts with an outer orbital electron

2. Electron recoils and is ejected from the target atom

3. Incident photon is deflected as scattered photon with a lower energy

what % of total interactions are compton scattering?

57%

scattered photons caused by compton scattering travel in all direction. what % of scattered photons exit the pt’s head? what does this causee?

30%

Causes fogging (darkening) of the film/receptor while carrying no useful information due to altered path

what term refers to A Measure of comparison of Biological Effectiveness of Different types of radiation?

equivalent dose (eg. xray vs alpha radiaiton)

what unit is equivalent dose reported in?

Sievert Sv

what is used to estimate risk in humans by comparing different exposure types (Panoramic Vs chest radiograph)?

effective dose

what factors are taken into account when determining effective dose?

1. volume/radiosensitivity of tissue irradiated

2. biologic effectiveness of radiation

what unit is effective dose reported in?

Sievert (Sv)

what is the study of effects of ionizing radiation on living systems?

radiation biology

what types of ionizing radiation is uncharged and non-particulate (electromagnetic)?

xrays, gamma rays

what types of ionizing radiation is uncharged and particulate?

neutrons

what types of ionizing radiation is charged and particulate?

beta particles, alpha particles, protons, heavy ions

Radiation Injury /Damage is a result of…?

Ionizations within tissues/cells

what are the 2 mechanisms via which radiation causes cell injury?

1. direct effect

2. indirect effect

what proportion of all radiation-induced biologic effects is caused by x-rays by direct effect?

1/3

what is the direct effect (mechanism of radiation induced cell injury)?

ionization of biologic macromolecules directly by a photon of ionizing radiation

Majority of all radiation-induced biologic effects caused by x-rays is by ..?

indirect effect

what is the indirect effect? (mechanism of radiation induced cell injury

1. step 1: Photon absorbed by water (in living tissues),causing ionization of some water molecules to form free radicals (radiolysis of water)

2. Free radicals interact with macromolecules to produce biologic changes

what proportion of all radiation-induced biologic damage caused by x-ray exposure is by indirect effects?

2/3

what is the primary target for cell damage from ionizing radiation?

DNA