AIM: Ch 3: Interaction of Radiation with Matter

Question 1

It is the transfer of some of the incident particles’ energy to electrons in the absorbing material, promoting them to electron orbits farther from the nucleus (i.e., higher energy level)

Answer 1

Excitation

Question 2

T/F: In excitation, the energy transferred to an electron does not exceed its binding energy.

Answer 2

True

Question 3

Following excitation, the electron will return to a lower energy level, with the emission of the excitation energy in the form of electromagnetic radiation or Auger electrons. This process is referred to as ____

Answer 3

De-excitation

Question 4

If the transferred energy exceeds the binding energy of the electron, ____ occurs, whereby the electron is ejected from the atom

Answer 4

Ionization

Question 5

After ionization, sometimes, the ejected electrons possess sufficient energy to produce further ionizations called ____. These electrons are called ____

Answer 5

Secondary ionization, delta rays

Question 6

T/F: As electron energy decreases the probability of energy loss via excitation decreases.

Answer 6

False

As electron energy decreases the probability of energy loss via excitation increases.

Question 7

The average number of primary and secondary ion pairs produced per unit length of the charged particle’s path is called

Answer 7

Specific ionization

Expressed in ion pairs (IP)/mm

Question 8

T/F: Specific ionization decreases with the square of the electrical charge (Q) of the particle and increases with the square of the incident particle velocity (v)

Answer 8

False

Specific ionization INcreases with the square of the electrical charge (Q) of the particle and DEcreases with the square of the incident particle velocity (v)

A larger charge produces a greater coulombic field; as the particle loses kinetic energy, it slows down, allowing the coulombic field to interact at a given location for a longer period of time.

Question 9

As the alpha particle slows, the specific ionization increases to a maximum (called the ____), beyond which it decreases rapidly as the alpha particle acquires electrons and becomes electrically neutral, thus losing its capacity for further ionization

Answer 9

Bragg peak

Question 10

The ____ of a particle is defined as the distance the particle travels

Answer 10

Path length

Question 11

The ____ of a particle is defined as the depth of penetration of the particle in matter

Answer 11

Range

Question 12

The path length of the ____ almost always exceeds its range, whereas the typically straight ionization track of a heavy charged particle results in the path length and range being nearly ___.

Answer 12

Electron, equal

Question 13

While specific ionization reflects all energy losses that occur before an ion pair is produced, the ____ is a measure of the average amount of energy deposited locally (near the incident particle track) in the absorber per unit path length.

Answer 13

Linear energy transfer (LET)

Question 14

Unit of linear energy transfer (LET)

Answer 14

keV or eV per μm

Question 15

T/F: The LET of a charged particle is proportional to the square of the charge and inversely proportional to the particle’s kinetic energy

Answer 15

True

Question 16

It refers to an interaction that deflects a particle or photon from its original trajectory

Answer 16

Scattering

Question 17

A scattering event in which the total kinetic energy of the colliding particles is unchanged is called ____

Answer 17

Elastic

Question 18

When scattering occurs with a loss of kinetic energy (i.e., the total kinetic energy of the scattered particles is less than that of the particles before the interaction), the interaction is said to be ____

Answer 18

Inelastic

Question 19

Elastic ionization: KE of ejected electron is ____ KE lost by the incident electron

Answer 19

Equal

Question 20

Inelastic ionization: KE of ejected electron is ____ KE lost by the incident electron

Answer 20

Less

Question 21

The radiation emission accompanying electron deceleration is called

Answer 21

Bremsstrahlung

Question 22

T/F: Total bremsstrahlung emission per atom is proportional to Z2, where Z is the atomic number of the absorber, and inversely proportional to the square of the mass of the incident particle, that is, Z2/m2

Answer 22

True

Question 23

There are four major types of interactions of x-ray and gamma-ray photons with matter, the first three of which play a role in diagnostic radiology and nuclear medicine:

Answer 23

(a) Rayleigh scattering
(b) Compton scattering
(c) Photoelectric absorption
(d) Pair production

Question 24

In this type of interaction, the incident photon interacts with and excites the total atom

Answer 24

Rayleigh scattering

Question 25

This interaction occurs mainly with very low energy x-rays, such as those used in mammography (15 to 30 keV).

Answer 25

Rayleigh scattering

Question 26

T/F: In general, the average scattering angle decreases as the x-ray energy increases in Rayleigh scattering

Answer 26

True

Question 27

Which statement is false about Rayleigh scattering:
a. Also referred to as “coherent” or “classical” scattering
b. In medical imaging, detection of the scattered x-ray will have a deleterious effect on image quality. However, this type of interaction has a low probability of occurrence in the diagnostic energy range
c. In this interaction, electrons are ejected, and thus, ionization does occur.
d. This interaction occurs mainly with very low energy x-rays, such as those used in mammography (15 to 30 keV).

Answer 27

c. In this interaction, electrons are NOT ejected, and thus, ionization does NOT occur.

Question 28

Also called inelastic or nonclassical scattering, is the predominant interaction of x-ray and gamma-ray photons in the diagnostic energy range with soft tissue

Answer 28

Compton Scattering

Question 29

T/F: As the incident photon energy increases, both scattered photons and electrons are scattered more toward the forward direction. In x-ray transmission imaging, these photons are much more likely to be detected by the image receptor.

Answer 29

True

Question 30

For a given scattering angle, the fraction of energy transferred to the scattered photon ____ with increasing incident photon energy.

Answer 30

Decreases

Question 31

When Compton scattering occurs at the lower x-ray energies used in diagnostic imaging (15 to 150 keV), the majority of the incident photon energy is transferred to the scattered ____.

Answer 31

Photon

Question 32

What factors are being degraded by the following interaction:
a. Rayleigh scattering
b. Compton scattering

Answer 32

a. Image quality
b. Image contrast and increase in random noise

Question 33

T/F: The laws of conservation of energy and momentum place limits on both scattering angle and energy transfer.

Answer 33

True

Question 34

The maximal energy transfer to the Compton electron (and thus, the maximum reduction in incident photon energy) occurs with a ____-degree photon scatter (backscatter)

Answer 34

180

Question 35

T/F: In contrast to the scattered photon, the energy of the ejected electron is usually absorbed near the scattering site.

Answer 35

True

Question 36

The scattering angle of the ejected electron cannot exceed ____ degrees, whereas that of the scattered photon can be any value including a ____-degree backscatter.

Answer 36

90, 180

Question 37

T/F: The relative probability of a Compton interaction increases, compared to Rayleigh scattering or photoelectric absorption, as the incident photon energy increases.

Answer 37

True

The incident photon energy must be substantially greater than the electron’s binding energy before a Compton interaction is likely to take place.

Question 38

T/F: Hydrogenous materials have a higher probability of Compton scattering than anhydrogenous material of equal mass.

Answer 38

True

Compared to other elements, the absence of neutrons in the hydrogen atom results in an approximate doubling of electron density. And, the probability of Compton scattering also depends on electron density = # of e-/g x density

Question 39

T/F: The probability of photoelectric absorption per unit mass is approximately proportional to Z3/E3, where Z is the atomic number and E is the energy of the incident photon.

Answer 39

True

Question 40

T/F: The benefit of photoelectric absorption in x-ray transmission imaging is that there are no scattered photons to degrade the image.

Answer 40

True

The fact that the probability of photoelectric interaction is proportional to 1/E3 explains, in part, why image contrast decreases when higher x-ray energies are used in the imaging process

Question 41

For every element, the probability of the photoelectric effect, as a function of photon energy, exhibits sharp discontinuities called ____

Answer 41

Absorption edges

Question 42

The ____ process predominates when lower energy photons interact with high Z materials

Answer 42

Photoelectric

Question 43

____ is the primary mode of interaction of diagnostic x-rays with image receptors, radiographic contrast materials, and radiation shielding

Answer 43

Photoelectric absorption

Question 44

When diagnostic energy photons interact with materials of low atomic number (e.g., soft tissue), the ____ process dominates.

Answer 44

Compton

Question 45

____ can only occur when the energies of x-rays and gamma rays exceed 1.02 MeV

Answer 45

Pair production

Question 46

It is the removal of photons from a beam of x-rays or gamma rays as it passes through matter.

Answer 46

Attenuation

Question 47

Predict what interaction predominates in the following cases of attenuation processes in soft tissues:
1. At low photon energies (less than 26 keV) interacting with high Z materials
2. When higher energy photons interact with low Z materials (e.g., soft tissue)
3. Occurs in medical imaging with low probability comprising about 10% of the interactions in mammography and 5% in chest radiography
4. At very high photon energies (greater than 1.02 MeV)

Answer 47

1. Photoelectric
2. Compton scattering
3. Rayleigh scattering
4. Pair production

Question 48

The fraction of photons removed from a monoenergetic beam of x-rays or gamma rays per unit thickness of material is called the

Answer 48

Linear attenuation coefficient

Typically expressed in units of inverse centimeters (cm^-1)

Question 49

T/F: The linear attenuation coefficient is the sum of the individual linear attenuation coefficients for each type of interaction

Answer 49

True

Question 50

In the diagnostic energy range, the linear attenuation coefficient ____ with increasing energy except at ____

Answer 50

Decreases, absorption edges (e.g., K-edge)

Question 51

T/F: The linear attenuation coefficient is inversely proportional to the density of the material

Answer 51

False

The linear attenuation coefficient is proportional to the density of the material

Question 52

The linear attenuation coefficient, normalized to unit density, is called the

Answer 52

Mass attenuation coefficient

Unit: cm2/g

Question 53

T/F: The mass attenuation coefficient is independent of density.

Answer 53

True

Question 54

The product px is called the

Answer 54

Mass thickness or areal thickness

Question 55

It is defined as the thickness of material required to reduce the intensity (e.g., air kerma rate) of an x-ray or gamma-ray beam to one half of its initial value.

Answer 55

Half-value layer (HVL)

Question 56

The HVL of a beam is an indirect measure of the photon energies (also referred to as the ____) of a beam, when measured under conditions of ____-beam geometry.

Answer 56

quality, narrow

Question 57

Determination of the HVL in diagnostic radiology is a way of characterizing the penetrability of the polyenergetic x-ray beam and can be converted to a quantity called

Answer 57

Effective energy

Question 58

T/F: The effective energy of an x-ray spectrum is the energy of a mono-energetic beam of photons that has the same penetrating ability (HVL) as the polyenergetic spectrum of photons.

Answer 58

True

Question 59

Effective energy of an x-ray beam from a typical diagnostic x-ray tube is ____ the maximal value.

Answer 59

1/3 to 1/2

Question 60

Average distance traveled before interaction, calculated from the linear attenuation coefficient or the HVL of the beam

Answer 60

Mean free path

Question 61

Lower energy photons of x-ray beams are removed from the beam while passing through matter, causing a shift of the x-ray spectrum to higher effective energies as the beam transverses matter. This selective attenuation of lower (soft) energy photons is also referred to as

Answer 61

Beam filtration

Question 62

____ is commonly used in mammography (which uses low-energy photons) for beam filtration

Answer 62

Molybdenum

Question 63

FDA regulations require a minimum total filtration of ____ of aluminum equivalent for x-ray tubes operating above 70 kV

Answer 63

2.5 mm

Question 64

The number of photons/particles passing through a unit cross-sectional area is referred to as the ____

Answer 64

Fluence

Question 65

Defined as fluence per unit time

Answer 65

Flux

Question 66

The amount of energy passing through a unit cross-sectional area is referred to as

Answer 66

Energy fluence

Question 67

The mass attenuation coefficient multiplied by the fraction of the energy of interacting photons transferred to charged particles as kinetic energy.

Answer 67

Mass attenuation transfer coefficient

Question 68

The kinetic energy transferred to charged particles by indirectly ionizing radiation per unit mass

Answer 68

Kerma

Question 69

1 J/kg = ?

Answer 69

1 Gy

Question 70

It is the energy (E) imparted by ionizing radiation per unit mass (m):

Answer 70

Absorbed dose

Question 71

Charged particles deposit imparted energy locally, when bremsstrahlung production is negligible, kerma will be ____ to the absorbed dose.

Answer 71

Equal

Question 72

The absorbed dose is expressed in either ____

Answer 72

Energ unit per mass (J/kg) or in units of gray (Gy)

Question 73

The amount of electrical charge (Q) produced by ionizing electromagnetic radiation per mass (m) of air is called ____

Answer 73

Exposure, X

Question 74

Give the SI unit for the following:
a. Length
b. Mass
c. Time
d. Electric current
e. Temperature
f. Luminous intensity
g. Amount of substance

Answer 74

a. Meters
b. Kilogram
c. Second
d. Ampere
e. Kelvin
f. Candela
g. Mole

Question 75

The shift of the x-ray spectrum to higher effective energies as the beam transverses matter is called ____

Answer 75

Beam hardening

Question 76

T/F: Low-energy (soft) x-rays will not penetrate the entire thickness of the body; thus, their removal reduces patient dose without affecting the diagnostic quality of the exam

Answer 76

True

Question 77

It is the ratio of the first to the second HVL and describes the polyenergetic character of the beam.

Answer 77

Homogeneity coefficient

The first HVL is the thickness that reduces the incident intensity to 50%, and the second HVL reduces it to 25% of its original intensity (i.e., 0.5 3 0.5 5 0.25)

Question 78

For most of diagnostic x-ray imaging the homogeneity coefficient of the x-ray spectrum is between ____

Answer 78

0.5–0.7

Question 79

Homogeneity coefficient of:
a. Mammography
b. Monoenergetic source of gamma rays

Answer 79

a. 0.97
b. 1

Question 80

The SI unit of Kerma

Answer 80

Gray (Gy) or milligray (mGy)

Question 81

SI unit of absorbed dose

Answer 81

Gray

Question 82

Older unit of absorbed dose

Answer 82

rad (radiation absorbed dose)

1 rad = 10 mGy

Question 83

If the energy imparted to charged particles is deposited locally and the bremsstrahlung produced by the energetic electrons is negligible, the absorbed dose will ____ relative to the kerma

Answer 83

Be EQUAL

Question 84

T/F: The mass energy absorption coefficient will be the same as the mass energy transfer coefficient when all transferred energy is locally absorbed.

Answer 84

True

Question 85

The difference between the calculation of kerma and dose for air is that kerma is defined using the mass energy ____ coefficient, whereas dose is defined using the mass energy ____ coefficient.

Answer 85

transfer, absorption

Question 86

The historical unit of exposure is the ____

Answer 86

Roentgen (R)

1 R = 2.58 x 10^-4 C/kg (exactly)

Question 87

The total amount of energy deposited in matter, called the ____ is the product of the dose and the mass over which the energy is imparted.

Answer 87

Imparted energy

Question 88

The product of the absorbed dose (D) and the radiation weighing factor is the ____

Answer 88

Equivalent dose (H)

Question 89

SI unit for equivalent dose

Answer 89

Sievert (Sv) = 1 J/kg

Question 90

Which type of radiation has the highest radiation weighting factor?
a. Xray, gamma rays, beta particles
b. Protons
c. Neutrons
d. Alpha particles, and other multi-charged particles

Answer 90

d. Alpha particles and, other multi-charged particles

Question 91

Which type of radiation has the lowest radiation weighting factor (=1)?
a. Xray, gamma rays, beta particles
b. Protons
c. Neutrons
d. Alpha particles, and other multi-charged particles

Answer 91

a. Xray, gamma rays, beta particles

Question 92

To modify the dose to reflect the relative effectiveness of the type of radiation in producing biologic damage, ____ was established by the ICRP as part of an overall system for radiation protection.

Answer 92

Radiation weighting factor (wR)

Question 93

____ were also established by the ICRP as part of their radiation protection system to assign a particular organ or tissue (T) the proportion of the detriment from stochastic effects

Answer 93

Tissue weighting factors (wT)

Question 94

The sum of the products of the equivalent dose to each organ or tissue irradiated (HT) and the corresponding weighting factor (wT) for that organ or tissue is called the ____.

Answer 94

Effective dose (E)

Question 95

SI unit for effective dose

Answer 95

Sievert (Sv) or rem

Question 96

Define, give SI unit and traditional unit (if there is) of the following:
a. Exposure
b. Absorbed dose
c. Kerma
d. Air Kerma
e. Imparted energy
d. Dose equivalent
e. Equivalent dose
f. Effective dose equivalent
g. Effective dose

Answer 96

See below