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*FRCR1 Physics > X Ray Physics > Flashcards

X Ray Physics Flashcards

1
Q
  1. CATHODE produces -e’s via thermionic emission
  • high melting point
  • high atomic number
A

Tungsten = high electric resistance = heats with current = excitation of valence -e’s

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2
Q

-e’s attracted towards the POSITIVE anode and strike it

A

Accelerated through

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3
Q

Two tungsten filaments and focussing cup

A
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4
Q
  1. ANODE is also made of tungsten - collision site is where x-rays are generated
A

But only using 1% - rest wasted as heat

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5
Q

Anode angle beam can be change to alter xray geometry

A
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6
Q

Anode generates a lot of heat - it spins to distribute the heat

A
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7
Q

Vacuum in the x-ray machine to avoid electron flow disruption

A

Lead housing to absorb anyone who should not be irradiated

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8
Q
  1. FOCAL SPOT

You have the bombarding electron beam,
ACTUAL focal spot (red square)and EFFECTIVE focal spot (by patient)

A
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9
Q

Line focus principle, ANODE ANGLE will change the EFFECTIVE focal spot

A

Smaller angles or smaller filament = narrower effective focal spot

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10
Q

Larger anode angles = larger FIELD SIZE

A
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11
Q

Effective focal spot changes due to geometry:

Closer to anode side = gets narrower
Closer to cathode side = gets larger

A
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12
Q
  1. ANODE HEEL EFFECT
  • The variation in x-ray beam intensity between the anode and cathode side
  • When x-rays are produced 360 degrees / isotropic / all directions
  • x rays cathode side travel a shorter distance
  • those travelling through the HEEL of the anode (bottom part of blue triangle)
A
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13
Q

Larger anode angles = heel effect / distal through anode (blue triangle) is LESS

A
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14
Q

Source to image distance

if you move detector away = increasing distance = less variation

A
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15
Q

Changing field size = collimation

A

The effect can be exploited so that you have better penetration of denser structures

e.g. pelvic images or mammography

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16
Q

Xrays of varying energy are produced
- most photo electrons are LOW energy - contributing to patient dose and NOT image as absorbed

A
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17
Q
  1. Filtration process via PHOTOELECTRIC effect
    tau = likelihood of photoelectric effect

p = density
Z = atomic number
E = energy of x-ray photo

A
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18
Q
  1. INHERENT FILTRATION
    - cannot control - due to components glass, conducting oil and x-ray window within x-ray tube - all will attenuate x-ray tube

Beryllium Z = 4
Aluminium Z = 13

Or mirror during collimation

A

Equivalent to 0.5 - 1.5mm of Aluminium

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19
Q
  1. ADDED FILTRATION

You could add more sheets to filter

  • Notice you never reduce the maximum energy
  • Reduces x-ray beam quantity
A
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20
Q

Compensation filters give more even exposure

You want inherent + added filtration to be around the equivalent of 2.5mm Aluminium

A
  1. Wedge
  2. Bowtie filter
  3. Trough filter
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21
Q
  1. COLLIMATION

Lead sheets which will attenuate x-rays
Bulb and mirror allows you to see effective collimation

A

xrays
1. Transmitted - go straight through
2. Attenuated = absorption via photoelectric effect
3. Scatter = adds noise and REDUCES CONTRAST

Collimation reduces the amount of scatter

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22
Q

8 XRAY CIRCUIT - OVERVIEW

You need to
1. Change potential to 100,000 Volts
2. Convert it to direct current to provide a constant stream
3. Adjust voltage depending on the density being imaged

A
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23
Q

8A. PRIMRARY CIRCUIT

  1. Line monitor
  2. Autotransformer - allows you to select kVp
  3. Exposure timer
  4. Red square - circuit breaker if current too high
A
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24
Q

Primary circuit - forms first part of step up transformer

A
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25
Q

8B SECONDARY CIRCUIT

Changes current to flow in one direction

Use diodes = semiconductors only allowing current in one direction

A
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26
Q

Tube current = no. electrons flowing from anode to cathode manipulate by

  1. Increases kVp Increases tube current
  2. If you increase kVp by 15% you need to reduce tube current by 50%
A
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27
Q
  1. FILAMENT CIRCUIT
    Controls thermionic emission from cathode

V = IR
Voltage is the same so -> Varying resistance to vary current
Step down transformer then increases current

A
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28
Q

Filament current directly influences tube current

A
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29
Q
  1. BREMSSTRAHLUNG Radiation

ATTRACTIVE FORCE BETWEEN ELECTRON AND NUCLEUS
(German for BREAKING)

A
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30
Q
  1. BREMSSTRAHLUNG Radiation
  2. Cathode produced -e’s accelerated to anode focal spot
    (<1% energy converted into x-rays)
  3. Energy of electron = tube potential accelerating it
  4. Energy released as it breaks and changes direction due to nucleus
A

Further away - loses less!

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31
Q

This produces a BREMSSTRAHLUNG Spectrum

The pic is UNFILTRERED

A

FILTERED:
Preferentially remove lower energy photos via PHOTOELECTRIC effect

Inherent will remove anything below 12

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32
Q

Filtered BREMSSTRAHLUNG Spectrum

  1. Max is determined by kVp
  2. No. XRs = area under curve - determined by kVp, target material AND filtration
A
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33
Q
  1. CHARACTERISTIC RADIATION
  • Accelerated electron interacts with the inner shell electron
  • K shell electron will be ejected IF it exceeds binding energy
  • Released K shell -e = photoelectron
A

-e from a higher shell will drop down releasing characteristic radiation with the energy = difference between binding energies

34
Q

Difference between K and L = Kα Peak
Difference between K and M = Kβ Peak

Specific for the element

A
35
Q
  1. X-RAY SPECTRUM

Combining characteristic radiation peak with BREMSSTRAHLUNG curve = GIVES X RAY SPECTRUM

A
36
Q

X-Ray beam quality = AVERAGE Energy = Green Line

Quantity = Area under the curve

A
37
Q

6A. X-RAY SPECTRUM + Filament Current

Current through tungsten filament on CATHODE

A

Higher current = hotter = more -e’s released

So increases QUANTITY not quality

38
Q

6B. Affect of tube potential

    • Pulls more -e’s away from cathode = so increases TUBE CURRENT - increases area under curve

Tube potential and tube current has an exponential relationship

A
39
Q

6B. Affect of tube potential

Average ENERGY also increased - shifts curve to the right (as well as up due to current)

A
40
Q

6C. FILTRATION

  • REDUCES QUANITITY AND DOSE
  • INCREASES AEVERAGE ENERGY (by filtering out the lower ones)
A
41
Q

6D. Changing target material

Bremstraughlung radiation = EXPONENTIALLY related to atomic number

More attractive force from larger nuceli = more breaking radiation

A

Characteristic xrays will also change

42
Q

6E. Generator waveform

Ripples = reduction in photons and average energy

So reduces quantity and quality

A
43
Q

3 ways x-rays can interaction with matter

A
44
Q
  1. PHOTOELECTRIC EFFECT

All incident x-ray energy is deposited into the -e

Releasing a PHOTOELECTRON

with energy, E = E of the original x-ray - binding energy

CREATING AN ION

A

Outer shell -e’s within the patient will drop down releasing characteristic radiation, but energy is very low 1-2 kEV and will be attenuated.

45
Q

Probability of photoelectric interaction τ

p = Density
Z = Atomic number ^3
E = 1/ Energy of Xray ^3

A

K Edge = above an energy threshold to overcome binding energy = new pool of available -e’s

Iodine atomic number Z = 53
Tissue Z = 7

46
Q
  1. COMPTON SCATTER

Contributes to patient dose

Not congruent with incident XR
= Increases noise within the image

A
47
Q

NOT ALL XR energy is conferred to electron, but enough to free it = photoelectron

Energy of photoelectron proportional to SCATTER ANGLE

Larger angle = LARGER energy conferred

A
48
Q

Compton scatter - largely depending on electron density of the tissue

Atomic number going up - number of electrons go up - so electron density similar

Not really affected by photoenergy

A

INCREASING photon energy

Compton Scatter predominates as it levels as photoelectric interactions drop.

49
Q
  1. Rayleigh Scatter = Elastic Scatter

Does not infer dose to patient
Generally occurs are lower energies / longer wavelengths

Generally if atom is SMALLER than wavelength

ALL -e’s in the atom are excited, oscillate and energy released equal to energy of incident x-ray.

A

BUT energy is INDEPENDANT OF angle

NO DOSE TO PATIENT

50
Q

Rayleigh scatter drops with increasing energies

A
51
Q
  1. LINEAR ENERGY TRANSFER

When CHARGED particles travel through TISSUE

Smaller = electron = lower LET (lower)

A

LET is proportional to
Charge, Q ^2
Energy, 1/E

As the electron loses energy MORE energy is transferred.

52
Q
  1. LINEAR ATTENUATION COEFFICIENT
    Determines no. XRs either ATTENUATED or SCATTERED

μ

Combo = photoelectric + compton + rayleigh

A

Dependant on 3 Main things:

Photon Energy
Material Density
Atomic Number

53
Q

LINEAR ATTENUATION COEFFICIENT

Same number proportion of xrays reduced with a set distance.

As you increased distance, its not linear it is actually exponential

A

LINEAR ATTENUATION COEFFICIENT
determines HALF VALUE LAYER

Tissue thickness to HALVE no. XRs

HIGHER LINEAR ATTENUATION COEFFICIENT = SMALLER HALF VALUE LAYER

54
Q

INCREASING photon ENERGY
REDUCES the Linear attenuation coefficient (less interactions at higher energies)

Linear attenuation coefficient increases with DENSITY.

A

So higher energies gives you LESS CONTRAST as degree of transmissions through different densities become similar

55
Q
  1. MASS ATTENUTATION COEFFICIENT
  • Takes into consideration the MASS and AREA being exposed to XRAYS

it ACCOUNTS for density

A

MAC = linear attenuation coefficient / density (ρ)

56
Q
  1. Back to Half Value Layer (cm)

INVERSELY PROPORTIONAL to linear attenuation coefficient

A

HVL = 0.693 / μ

57
Q

Photon reduction factor = 2 ^ no. Half Value Layers

A
58
Q
  1. DIGITAL RADIOGRAPHY

Indirect digital radiography requires:
Scintillation = converting XR energy into light

Computed Radiography and Direct Digital Radiography does not require Scintallation

A
59
Q
  1. COMPUTED RADIOGRAPHY uses a cassette and an analogue to digital converter

Detector = Barium Flurobromide crystals doped with Europium -> disrupts crystal structural integrity -> creating F centres = Positively charged Flouride ions

A

Disrupted structure generates

  1. Valence Band = low energy state with lots of electrons available
  2. Conduction Band = Theoretical energy level in order to reach it
60
Q

XRs oxidise Europium, released electron REDUCES F+ to F

Latent image is the F and Eu 3+

A

The last bit is the READOUT

Using 700nm red light = oxidises F to F +

Release of energy is in BLUE light spectrum 400-450nm

Blue light detected with a PHOTOMULTIPLIER and RED FILTER

61
Q

This therefore uses PHOSPHORESCENCE

Delay in XRAY to light conversion

A

IMMEDIATE XRAY to light conversion would be FLUORSCENCE

62
Q

Back 15. DIGITAL RADIOGRAPHY

  1. Indirect - XRs need light conversion - SCINTILLATION LAYER
  2. Direct - XRs directly into digital signal
A
63
Q

Uses Cesium Iodide - photon converted into light funnelled down a colour to the film = better spatial resolution

A
64
Q

15A. INDIRECT Radiography = Charged Coupled Device = CCD chip

3 Layers

  1. Scintillation layer made of CSI
  2. COUPLING LAYER - using fibre optics down to CCD chip or lens that can focus
  3. Crystalline Silicon with dexals = same as pixels with voltage gates

Signal drop off = secondary quantum sink, loss of energy as it goes down, also when it is focused onto a smaller area.

A

The more electrons that accumulate in each dexel = the DARKER it will be.

65
Q

15B. INDRECT Radiography = THIN FILM TRANSISTOR ARRAYS

3 Layers again:

  1. Scintillation layer made of CSI
  2. PHOTODIODE LAYER (hydrogenated amorphous silicon)
  3. Thin film transistor (TFT) array
A

Making up a “DEL” = Detector Element

Bottom right = capacitor - STORES CHARGE
Top left = switch = CAN RELEASE CHARGE

Capacitor charged stored proportional to electrons released from scintillation.

66
Q

Wasted electrons - Fill factor about 80% better resolution and less signal loss

A
67
Q

15C. DIRECT DIGITAL RADIOGRAPHY

Also uses a DEL layer

But has an AMORPHOUS SELENIUM SEMICONDUCTOR LAYER on top

  • Striking -e’s generate ION PAIRS
  • charge differential is created with a positive anode end at the TOP

-negative CATHODE adjacent to TFT array

A

Moving -e’s generate an ELECTRON HOLE where other -e’s move in

THIS CURRENT FROM THE HOLE is measured by the TFT array

EXCELLENT SPATIAL resolution
But cannot deal with HIGH ENERGIES

So good for mammography - compressed breast tissue with low energies = you can use lower energy x-rays with good contrast

68
Q
  1. SCATTER
  • Decreases contrast in image
  • Increases background noise
  • Increases occupational exposure
A

Scatter to primary ration
Or Fraction of scatter

Measures of scatter = doesn’t take into account photelectric effect so NOT a direct measure of image quality

69
Q

Increasing tissue thickness increases scatter

A

Collimating to reduces the scatter and patient dose

70
Q

Tissue thickness and field size both increase scatter

A

INCREASING ENERGY Increases scatter = less photoelectric effect and Compton scatter predominates

71
Q
  1. SCATTER REDUCTION
  2. Primary XR Beam enters
  3. EXIT XR Beam leaving patient
A

Collimation using lead shields

Reduced field Size
Reduce Patient Dose
Eliminates Scatter

72
Q

High proportion of scatter = blurs the imae

A
73
Q

Tissue compression gives better contrast and reduces scatter

A

AIR GAP =
Increasing object distance from detector means some of the scattered photons would not reach detector

BUT
1. You magnify
2. Get geometric blurring

74
Q

Anti-Scatter Grids
- Multiple septa - usually lead
- Scattered photos NOT PARALLEL to the grid will be attenuated

A

Non-linear grids in line with divergent beam

75
Q

Issues with grids!

A
76
Q

h = Height = Grid Thickness
W = Between septa = interspace material
t = SEPTAL THICKNESS

GRID RATIO = H / W - iNCREASES IT reduces photons getting through

A
77
Q

BUT with these grids - in order to keep the exposure the same YOU INCREASE dose to patient

A
78
Q
  1. XRays released in isotropic manner
  2. Focal spot is not a single spot but has some size
A

Reducing source to object distance REDUCES geometric blurring

(Remember geomtric blurring is because the focal spot is FAT)

79
Q

Magnification = SID / SOD

Source to image distance / Source to object distance

SID / SOD

A

How to decrease magnification

  1. Increase source to object distance
  2. OR DECREASE SOURCE TO IMAGE DISTANCE

Decrease SID
iNCREASE SOD

80
Q

Reducing anode angle reduces focal spot
SO REDUCES GEMETRIC blurring and the smaller the penumbra

A

Geometric Blurring = f x (OID/SOD)

f = Focal spot
OID = Object to image distance
SOD Source to image distance

*FRCR1 Physics (19 decks)

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