Week 5- Interactions Of X-Rays

Question 1

How does EMR behave

Answer 1

Like streams or packets or energy called photons that have no mass or electric charge

Question 2

How does EMR energy travel

Answer 2

Straight lines

Question 3

What is E(photon energy) proportional to

Answer 3

Wave frequency (F)

Question 4

How many fates are there for X-Ray photons when they pass through matter

Answer 4

1) penetration
2) absorption
3) scattered with energy loss
4) scattered without energy loss

Question 5

Describe penetration

Answer 5

The photon penetrates the matter withoutinteracting with it

Question 6

Describe absorption

Answer 6

The photon interacts with the matter and iscompletely absorbed by depositing ALL of its energy into thematter

Question 7

Describe scattered

Answer 7

The photon interacts with the matter and isscattered (deflected) from its original direction, depositing eitherPART or [4] NONE of its energy into the matter

Question 8

What is attenuation

Answer 8

the attempt to numerically describe the collectivefates of a large number of photons as they interact with

Represents deduction in beam intensity , reduction in energy content, reduction in number of photons

Question 9

Name the 2 main components of attenuation

Answer 9

Scatter

Absorption

Question 10

How many processes are involved in attenuation

Answer 10

5

Question 11

List 1,2,3 attenuation processes

Answer 11

-elastic or unmodified scatter (no energy loss)

2) Compton or modified scatter (energy loss)

3) photoelectric absorption

Question 12

Describe elastic scatter

Answer 12

occurs at very low X-Ray energies and involvesscattering (deflection) of photons without loss of energy.§

This type of scatter is of little practical relevance to diagnosticimaging; but is discussed for the sake of completenes

Energy of photon is less than energy gap. Doesn’t excite to shell so electron doesn’t absorb energy.

Question 13

Does elastic scatter have energy loss

Answer 13

No

Question 14

What is in elastic scatter

Answer 14

involvesscattering (deflection) of photons with an associated loss of photonenergy.§

main scattering process important from a diagnosticimaging perspective is Compton scatter

Energy loss

Energy from photon is transferred to atomic electron, raising to next shell. Electron falls back to original shell, emitting radiation

Question 15

The greater the angle of scatter, the…

Answer 15

Greater the energy and range of the recoil electron

Question 16

Does a side or forward scatter photon have more energy

Answer 16

Forward

Greater loss of energy of scattered photon

Question 17

What does scattered radiation make up

Answer 17

50-90% of total number of photons emerging from patient

Question 18

What is photoelectric absorption

Answer 18

an inelastic collision of a photonwith an orbiting electron in an atom of an absorbing material, withall the energy of the photon given up to the electron.

photon disappears (absorbed). There is recoil of theelectron, emission of characteristic radiation, and the productionof Auger electrons

Energy of photon is greater than energy gap and binding energy of inner shell electron. All energy is transferred to electron, allowing ionisation of atom. Electron vacancy is filled by an electron from another shell, EMITTIJNG RADIATIOMN

Question 19

How is they’re a signal difference (contrast) in soft tissue and bone

Answer 19

Due to different atomic numbers

Tissue- 7
Bone- 14

Magnitude is 8x greater

Question 20

Explain cascade effect

Answer 20

INCIDENT PHOTON ISCOMPLETELY ABSORBEDLEAVING A VACANCY INTHE K SHELl

2) VACANCY IS FILLED BY AN ELECTRON FROM HIGHER ENERGY LEVEL

3) RADIATION IS EMITTED AS HIGHER ENERGY ELECTRON DROPS TO LOWER LEBEL

Question 21

What is the Auger effect

Answer 21

the emission of an electronfrom an atom that accompanies the filling of an innerelectron shell vacancy

Inner shell electron is removed. Ionised and left in excited energy state. Relaxation to ground occurs rapidly. Vacancy filled by outer shell electron, releasing energy. If energy released is small it is absorbed immediately with ejection of very low energy AUGER ELECTRON

Question 22

What is the linear attenuation coefficient

Answer 22

is the fraction of photons removedfrom the beam per unit thickness of the attenuating material

Question 23

What is photon fluence

Answer 23

Photons per unit area

Question 24

What is energy fluence

Answer 24

Energy per unit area

Question 25

When dopes intesnity of the beam decrease

Answer 25

As thickness of absorbing material increases Linear attention coefficient is affected by pressure or temperature

Question 26

What is linear attenuation coefficient affected by

Answer 26

Pressure and temperature

Question 27

As pressure increases and vol decreases what happens to density

Answer 27

Increases

Question 28

As temp increases and vol increases what happens to density

Answer 28

Decreases

Question 29

When is photoelectric coefficient high

Answer 29

When photon energy is greater than binding energy of electrons within a shell The more tightly the electron is bound to the atom, and the nearerthe photon energy is to its binding energy

Question 30

When does probability of photoelectric absorption decrease

Answer 30

asphoton energy increases. It increases markedly as the atomicnumber [ Z ] increases

Question 31

When does scatter increase

Answer 31

When kv, thick body parts, field size increases

Question 32

Why is scatter a problem

Answer 32

It can cause foggy image Affects radiology workers

Question 33

Does lead have high or low attenuation

Answer 33

HIGH

Question 34

Give symbol for tube voltage

Answer 34

KV

Question 35

What does tube voltage determine

Answer 35

thephoton energies within the beam. The greater the tube voltage, the greater the range of photon energieswithin the beam, and the greater the penetrating power of the beam. rise in penetration through all tissues – a decreasein attenuation differences – resulting in a lower contrast radiograph and alarger range of shades of grey

Question 36

What is increased if photon energy and kV Is reduced

Answer 36

Photoelectric absorption Scatter photons reduced

Question 37

When is scatter generally lower

Answer 37

At lower kV values

Question 38

Name 2 factors which reduce amount of scatters

Answer 38

1] thin body tissue is easier for photons topenetrate; [2] the presence of bone [high Z] means thatmost interactions are photoelectric

Question 39

What does a higher tube voltage mean

Answer 39

Decrease in attenuation differences Lower contrast

Question 40

When is a high kV technique good

Answer 40

produces a more uniformly dense image to betterdemonstrate lung markings. There is a reduction in dose, too As decrease in attention differences so lower contrast

Question 41

what are anti-scatter grids

Answer 41

placed between the patient and the image receptor to reduce the scattered photons reaching the receptor produced by Compton scatter; thus, improving image quality. GRIDS are made of parallel strips of high attenuating material [ Lead ]with an interspace filled with low attenuating material scatter increases withincreased tissue thickness andlarger field sizes; thus, GRIDS arerequired to improve image contrast.

Question 42

what is scatter air gap

Answer 42

an additional distance between a patient and an image detector. The gap decreases the likelihood for scattered x-ray radiation to reach the detector, as radiation is partially absorbed and scattered in the air.

Question 43

what is the amount of scattered radiatipn related to

Answer 43

tube voltage [controlledby kV], body part thickness [controlled using grids], and field size