Week1

Question 1

What is the only harmful radiation?

Answer 1

ionizing radiation

Question 2

Classification of radiation (3):

Answer 2

1. Electromagnetic radiation
2. Particulate radiation
3. Ionising and non-ionising radiation

Question 3

Characteristics of electromagnetic waves (4):

Answer 3

1. amplitude (trough/crest from straight line)
2. wavelength (b/w 2 crests/troughs) λ
3. frequency f
4. speed v

Question 4

For X-rays, λ and f are expressed:

Answer 4

λ - nm
f - Hz (1 Hz = 1 cycle/s = 1s^-1)

Question 5

γ-rays:

Answer 5

• commonly referred as photons
• bundle or particle of radiation

Question 6

diff b/w light photons and γ-ray photons?

Answer 6

their energy (E) and f

E = hf = hc/λ

where h - Planck’s constant = 6.626*10^-34 Js

Question 7

unit f/ denotation of photon E -

Answer 7

eV - electronvolt
1 J = 6.241509*10¹⁸ eV

Question 8

particulate radiation in diagnostic radiology and its rest m and E:

Answer 8

electron particulate radiation
rest m = 9.109*10^-31 kg
rest E = 511 keV

Question 9

Radiation is classified as ionizing/non-ionizing depending on:

Answer 9

its ability to ionize matter:
- EM radiation of frequency higher than near-UV region of EM spectrum - ionizing
- EM radiation w/ E below the far-UV region (visible light, infrared, radiofrequency) - non-ionizing

Question 10

Interactions of photons w/ matter (3):

Answer 10

• photoelectric absorption
• Compton scattering
• pair production

Question 11

characteristic photon has E

Answer 11

characteristic of the absorbing material

Question 12

Photoelectric absorption:

Answer 12

• photon is absorbed by transferring all of its E to an inner orbital electron => electron is ejected & photon disappears => atom fills vacant orbit w/ outer electrons => atom releases a characteristic X-ray photon
• the most likely form of absorption f/ incident photons w/ lower E (especially below 25 keV)
• likelihood of photoelectric absorption increases as the cube of the atomic # Z³ => heavy metals like lead (Z=82) are good absorbers of X-ray photons

Question 13

All interactions b/w photons & matter are probabilistic:

Answer 13

odds that a proton is absorbed by photoelectric absorption depend on chem elements in the absorbing material

Question 14

Attenuation of an X ray beam in the air:

Answer 14

negligible

Question 15

Attenuation of an X ray beam in the bone:

Answer 15

significant due to relatively high density (atom mass number of Ca)

Question 16

Attenuation of an X ray beam in the soft tissue (muscle, …):

Answer 16

similar to water

Question 17

Attenuation of an X ray beam in the fat tissue:

Answer 17

less than in water

Question 18

Attenuation of an X ray beam in the lungs:

Answer 18

weak due to density

Question 19

Compton effect:

Answer 19

• interaction w/ the outer e that isn’t tightly bound to an atom: photon collides w/ an e and gives some of its E to it
• in the “head-on” collision, photon has its direction of travelling reversed and loses max E
• in the glancing collision, E given to the recoil e is much less
• the Compton eff is dominant f/ photon E above 200 keV up to 2 MeV
• a single photon may undergo several collisions, loosing some E and eventually be absorbed by the photoelectric eff

Question 20

the actual loss of E as a result of Compton scattering depends upon

Answer 20

the angle through which the photon is scattered

Question 21

depending on E of photon, what is the most dominant effect:

Answer 21

• <25 keV - photoelectric effect
• 60-90 keV - both photoelectric & Compton effects can occur
• 200 keV up to 2 MeV - Compton effect
• above 5 MeV - pair production

Question 22

why is the Compton effect relatively insensitive to variations in anatomy, compared to photoelectric effect?

Answer 22

most soft tissues have similar densities

Question 23

Compton effect depends on - ?

Answer 23

photons interact w/ electrons as though they are not bound to an atom => only total # of electrons in a block of material matters => only thickness of an absorber & its density are important f/ photon absorption

Question 24

What contributes to contrast in x-ray imaging?

Answer 24

diff in density & thickness of a tissue

Question 25

What effect plays the dominant role in x-ray imaging?

Answer 25

photoelectric absorption effect plays a dominant role => contrast is affected more by chemical composition of materials and photon E

Question 26

Pair production:

Answer 26

- only happens f/ very high-E protons: photon can be absorbed by an atomic nucleus in the absorber => **electron + positron** - **E is being converted to m!!!**

Question 27

how much E is needed to produce the pair of particles, based on mass of electron and positron?

Answer 27

1.02 MeV, if the incident photon has more E, the excess increases the velocity of electron & positron

Question 28

does positron live long?

Answer 28

no, it will not live very long, b/c if it meets an electron it can combine with it to produce 2 photons of 0.51 MeV - ***annihilation radiation***

Question 29

PET (positron emission tomography) basis:

Answer 29

synchronous detection of the two 0.51 MeV γ photons to localize the emitter

Question 30

ABSORPTION OF PHOTONS AS A FUNCTION OF ENERGY

Answer 30

Question 31

Absorption processes are **NOT** the only processes that reduce the intensity of a beam of photons:

Answer 31

*point source*: intensity falls off as you move away from the source *if radiation from the source can spread in all directions*: its intensity will fall off in **INVERSR PROPORTION TO THE DISTANCE SQUARED**

Question 32

The inverse square law states that:

Answer 32

the energy twice as far from the source is spread over four times the area thus we have one-fourth of the intensity

Question 33

In measuring radiation, we:

Answer 33

can't detect it directly. Thus, we rely on it to interact w/ smth else that allows it to be detected

Question 34

dosimeter -

Answer 34

instrument that measures ionizing radiation; comprises of a measuring assembly and one or more detector assemblies (may or may not be an integral part of measuring assembly)

Question 35

examples of dosimeters:

Answer 35

- low levels of dose rate: **Geiger-Muller (G-M) tubes** / **scintillation counters** - high dose rate: **ionisation chambers** are more accurate & less affected by radiation E

Question 36

Film dosimeters' advantages (5):

Answer 36

- not expensive - provides a permanent record - very reliable - very simple personal device - used to measure and record radiation exposure due to **γ rays, X-rays, beta particles**

Question 37

Film dosimeters' disadvantages (3):

Answer 37

- can't be read on site, need to be sent for developing - not reusable - exposures of less than 0.2 mSv (millirem) can't be accurately measured

Question 38

TLDs dosimeters' advantages (3):

Answer 38

- measure a great range of doses, compared to film dosimeters - doses can easily be obtained, b/c can be read on site - reusable

Question 39

TLDs dosimeters' disadvantages (2):

Answer 39

- each dose can't be read out more than once - readout process "zeroes" the TLD

Question 40

Electronic personal dosimeter:

Answer 40

- high range - alarming - designed to be worn by occupational radiation workers in **planned exposure situations** f/ regulatory compliance (industrial, medical) - displays dose & rate - high level of radiation sensitivity

Question 41

Electric current is simply a flow of electrons / ions, BUT

Answer 41

if some of the atoms in the air are ionised, then free electrons are produced => electric current can flow *ex: flash of lightning - high potential gradient b/w cloud & ground is sufficient to ionise the air => electric current*

Question 42

Ionisation chamber's principle of work:

Answer 42

ionising radiation frees electrons in the air => electrons fill the chamber => electric current when the chamber is exposed to ionising radiation, positive or negative ions are produced => they are attracted to either negative or positive plate => current flows through the chamber => current is measured by a sensitive ammeter (often of 10^-9 A, which **corresponds to 6*10⁹ electrons per second**)

Question 43

Where are ionization chambers used?

Answer 43

- to measure the ionising radiation output of therapeutic & diagnostic ionising radiation generators - to make accurate measurements of patient radiation dose

Question 44

G-M counter picture

Answer 44

Question 45

G-M counters what it is and principle of work:

Answer 45

- very sensitive form of ionisation chamber: can detect single ionising particles which enter the tube - construction is similar to ionisation chambers: central wire electrode inside a hollow metal tube; **difference**: tube is filled w/ gas (Ar, Ne), which is at 1/5 atmospheric P => incident ionising radiation will produce free electrons within the tube => they are attracted to the central electrode (held at positive potential) => electrons are accelerated by potential => gain sufficient E => further ionisation => chain reaction => all electrons hitting the central anode can cause photons to be emitted => visible light / UV radiation => more ionisation

Question 46

what is G-M used for?

Answer 46

- detection & measurements of all types of radiation: α , β, γ - NOT recommended f/ diagnostic radiology, typically designed to detect isotope emissions

Question 47

2 main difficculties in diagnostic radiology for G-M counters:

Answer 47

- response time of several s - strong E dependence at low photon levels