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FRCR Part 1 Physics > Xray (Erin) > Flashcards

Xray (Erin) Flashcards

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1
Q

What happens to the quantity of x-ray photons when the kV is increased?

A

Quantity of xray photons increases

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2
Q

What happens to the average energy of xray photons when the kV is increased?

A

Average energy increases

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3
Q

What happens to the maximum xray photon energy when the kV is increased?

A

Maximum energy increases

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4
Q

What happens to the average photon energy in a beam when mA is increased?

A

No change

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5
Q

What happens to the minimum photon energy in a beam when mA is increased?

A

No change

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6
Q

What happens to the maximum photon energy in a beam when mA is increased?

A

No change

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7
Q

What effect does filtration have on the average energy of photons?

A

Increases the average energy (removes the lower energy photons)

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8
Q

What effect does filtration have on the total number of photons?

A

Decreases the total number of photons (as lower energy photons are removed)

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9
Q

What happens to wavelength as frequency increases?

A

It decreases (they are indirectly proportional to each other)

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10
Q

What happens to frequency as wavelength increases?

A

It decreases (they are indirectly proportional to each other)

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11
Q

What effect does increasing the atomic number of the anode target have on the quantity of x-ray photons?

A

Increases the quantity of x-ray photons

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12
Q

What effect does increasing the atomic number of the anode target have on the energy of the characteristic x-rays produced?

A

Increases the characteristic energies (higher atomic number = more positive nucleus = higher electron binding energies)

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13
Q

Does the photoelectric or compton effect predominate at lower kVp?

A

Photoelectric

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14
Q

Does the photoelectric or compton effect predominate at higher kVp?

A

Compton

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15
Q

What effect does decreasing the anode angle have on the focal spot size?

A

Decreases the focal spot size

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16
Q

What effect does increasing the anode angle have on the focal spot size?

A

Increases the focal spot size

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17
Q

What effect does decreasing the anode angle have on the anode heel effect?

A

Larger anode heel effect

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18
Q

What effect does increasing the anode angle have on the anode heel effect?

A

Smaller anode heel effect

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19
Q

What effect does increasing mA have on noise?

A

Decreases noise

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20
Q

What effect does making the image receptor thicker on noise?

A

Decreases noise

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21
Q

What effect does decreasing kV have on image contrast?

A

Increases contrast

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22
Q

What effect does increasing kV have on image contrast?

A

Decreases contrast

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23
Q

What effect does increased scatter have on image contrast?

A

Decreases image contrast

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24
Q

What effect does increasing the atomic number difference in objects have on image contrast?

A

Increases contrast

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25
Q

What effect decreasing the atomic number difference in objects have on image contrast?

A

Decreases image contrast

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26
Q

What is the photoelectric effect equation?

A

ρZ3 / E3

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27
Q

What is the effect of using an image receptor with a greater attenuation coefficient on noise?

A

Decreases noise (more photons are absorbed and converted into signal)

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28
Q

What is the effect of using an image receptor with a lesser attenuation coefficient on noise?

A

Increases noise (less photons are absorbed and converted into signal)

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29
Q

What is the effect of using a thicker image receptor on noise?

A

Decreases noise (as more photons are absorbed)

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30
Q

What is the effect of using a thinner image receptor on noise?

A

Increases noise (as less photons are absorbed)

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31
Q

What effect does a smaller focal spot size have on the energy of the xrays emitted?

A

No effect

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31
Q

Does compton scattering usually involve outer or inner shell electrons?

A

Usually outer shell electrons because they are weakly attached to the atom

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32
Q

In compton scatter as the scattering angle increases what happens to the energy of the scattered xray?

A

Decreases the energy of the scattered xray

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33
Q

The anode disc has an angle of around…?

A

5-15 degrees

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34
Q

What 3 parameters can be changed to preserve tube rating?

A

kV
mA
Exposure time

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35
Q

What is the equation for linear attenuation co-efficient?

A

μLAC = 0.693* / HVL

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36
Q

What is the equation for mass attenuation co-efficient?

A

μMAC = μLAC/ρ

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37
Q

How is magnification increased?

A

Decreasing focus to film distance
Increasing object to film distance

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38
Q

How is magnification decreased?

A

Increasing focus to film distance
Decreasing object to film distance

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39
Q

What is the effect of using flat plate detectors for magnification views in fluoroscopy on patient dose?

A

No change in dose - as images are just digitally magnified

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40
Q

What is the effect of using flat plate detectors for magnification views in fluoroscopy on spatial resolution?

A

No improvement in spatial resolution
As images are just digitally magnified (the same image is taken)

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41
Q

How is GRID RATIO calculated?

A

Interspace channel height divided by channel width

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42
Q

How is grid selectivity calculated

A

Primary radiation (non-scattered photons)/secondary radiation (scattered photons)

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43
Q

How is grid factor calculated

A

exposure necessary with grid/exposure necessary without grid

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44
Q

What is a moving grid often called

A

Often referred to as a Bucky

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45
Q

In fluoroscopy where in the image is signal used to guide the adjustment of kV and mA

A

Signal from the centre is used

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46
Q

What is spatial resolution measured in?

A

Line pairs per mm
lp/mm

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47
Q

Describe vignetting

A

Term used to describe the phenomenon whereby the centre of an uncorrected image appears brighter than the peripheries

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48
Q

What causes vignetting

A

Front of image intensifier is dome-shaped however the screen we view the image on is flat. The transposition between these two causes a pincushion type appearance and results in loss of brightness at the edges of the image

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49
Q

What is the fixed focus-film distance in mammography (in cm)

A

Fixed around 65/66cm

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50
Q

What is the average dose per film in mammography?

A

1.5-3mGy

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51
Q

Through what process are electrons emitted from the filament at the cathode

A

Thermionic emission

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52
Q

What is the filament in an X-ray tube commonly made of

A

Tungsten

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53
Q

Why does Tungsten make a good filament (4)

A

Good thermionic emitter
High atomic number
Can be manufactured into thin wire
High melting temperature

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54
Q

Why is molybdenum used for the focusing cup in an X-ray tube (2)

A

High melting point
Poor thermionic emitter (electrons won’t be released that could interfere with filament electrons)

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55
Q

What is the focusing cup made of in an xray tube

A

Molybdenum

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56
Q

What charge does the focusing cup have

A

Negative charge

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57
Q

What is the anode typically made of

A

Tungsten with rhenium added

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58
Q

Why is rhenium added to tungsten at the anode

A

to prevent cracking

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59
Q

What charge does the anode have

A

Positive to attract electrons

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60
Q

What is the actual focal spot

A

Physical area of the focal track that is impacted

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61
Q

What is the effective focal spot

A

The area of the focal spot that is projected out of a tube

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62
Q

Where are stationary anodes still used

A

Dental radiology and radiotherapy systems

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63
Q

Why is a rotating anode now used (compared to a stationary anode)

A

Because it rotates it overcomes heating by having different areas exposed to the electron stream over time

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64
Q

Why is molybdenum used as the stem in the anode

A

Poor conductor of heat
High melting point

Conducts very little heat to prevent damage to the metal bearings

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65
Q

By what process is heat removed from the anode

A

Radiation

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66
Q

What is the effect of lengthening exposure time on tube rating

A

Decreases tube rating

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67
Q

What is the effect of decreasing focal spot size (at the anode) on tube rating

A

Decreases tube rating

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68
Q

What is the effect of decreasing disc rotation speed on tube rating

A

Decreases tube rating

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69
Q

What is the effect of decreasing anode tube diameter on tube rating

A

Decreases tube rating

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70
Q

What is the window in an xray tube Made from

A

Beryllium with aluminium
Or
Copper

71
Q

What must be total filtration be equal to (terms of aluminium thickness)

A

> 2.5mm aluminium equivalent

72
Q

What is total filtration a combination of

A

Inherent filtration and added filtration

73
Q

How much current is usually passed through the filament at the cathode

A

10A

74
Q

What is the effect of wave rectification on mean photon energy

A

Increased mean photon energy
As fever photons of lower energy released

75
Q

What is the effect of wave rectification on xray output

A

Increased xray output
As stays closer to the maximum for longer

76
Q

What is the effect of wave rectification on exposure time

A

Shorter exposure time
As output higher, can run exposure for shorter time to get same output

77
Q

What is the effect of wave rectification on patient dose

A

Lower patient dose
As increased mean energy photons means fewer low energy photons that contribute to the patient dose but not to the final image

78
Q

What % of energy is converted to heat and photons at the anode

A

99% heat
1% xray photons

79
Q

What is the purpose of the glass envelope in an xray tube

A

Contains vacuum

80
Q

What 2 methods can full wave rectification be achieved by

A

Three phase supply
High frequency generator

81
Q

What % of X-rays are emitted via Bremsstrahlung radiation

A

80%

82
Q

What is meant by beam QUALITY

A

the ability of the beam to penetrate an object or the energy of the beam

83
Q

What is meant by beam QUANTITY

A

The number of xray photons in the beam

84
Q

What is the effect of a more uniform waveform current on average xray energy

A

Increased average xray energy

85
Q

What is the effect of a more uniform waveform current on quantity of xray photons

A

Increased quantity of xray photons

86
Q

What is the effect of a more uniform waveform current on maximum keV

A

Same maximum keV

87
Q

What 3 processes contribute to attenuation

A

Compton effect
Photoelectric absorption
Elastic scatter

88
Q

Does Compton scatter mostly occur with inner or outer shell electrons

A

Outer shell/loosely bound electrons

89
Q

What is the equation for the probability of the Compton affect (Compton attenuating co-efficient)

A

Compton attenuating co-efficient = density/energy

90
Q

What effect does increased material density have on the probability of the Compton effect

A

Increases probability

91
Q

What effect does increased photon energy have on the probability of the Compton effect

A

Decreases probability
However note at higher energies the proportion of Compton to photoelectric effect is higher
Hence in CT Compton effect predominates
But overall Compton goes down as energy increases

92
Q

What effect does increasing atomic number have on the probability of the Compton effect

A

NO EFFECT
Compton effect is INDEPENDENT OF ATOMIC NUMBER

93
Q

What is the effect of decreased xray energy on the probability of the Compton effect

A

Increases probability of the Compton effect
(although note photoelectric effect does predominate at lower keV)

94
Q

Does the photoelectric effect occur due to interaction with a inner or outer shell electron

A

Typically inner shell (bound) electron

95
Q

How much of the incoming photon energy is transferred to the bound electron in the photoelectric effect

A

All the energy

96
Q

What is the equation for the probability of the photoelectric effect

A

τ = ρZ3 / E3

97
Q

As the energy of the photon increases what happens to the probability of the photoelectric effect

A

Decreases

98
Q

As the mass density of a material increases what happens to the probability of the photoelectric effect

A

Increases

99
Q

As the atomic number of a material increases what happens to the probability of the photoelectric effect

A

Increases

100
Q

When is the probability of the photoelectric effect the highest in relation to binding energies

A

When the xray photon energy is slightly above the electron binding energy (L edge and K edge)

101
Q

What is the k edge of iodine

A

33 keV

102
Q

As the mass density of a material increases what happens to the probability of the photoelectric effect

A

Increases

103
Q

What is HVL a measure of

A

The penetrating power of the xray beam

104
Q

What is the equation for calculating the factor of reduction by HVL

A

2 (to power of number of HVL)
E.g if beam has gone through 4 HVL is have been attenuated by a factor of 16
(2 to power of 4 = 16)

105
Q

What is the linear attenuation coefficient (LAC)

A

The probability of the material to attenuate the beam

106
Q

How is the LAC calculated

A

0.693/HVL

107
Q

What unit is LAC measured in

A

cm-1

108
Q

What is MAC (mass attenuation coefficient) a measure of?

A

Rate of energy loss by a photon beam as it travels through an area of material

109
Q

How is MAC calculated

A

MAC = LAC/density

110
Q

What is MAC independent of

A

Density

111
Q

What is the unit for MAC

A

cm2 g-1

112
Q

What 2 assumptions does attenuation make about a beam

A

Monoenergetic beam
X-rays coming from a point source

113
Q

What causes beam hardening

A

The lower energy photons are attenuated proportionally more than the higher energy photons and are removed
Leaving behind higher energy photons (i.e beam hardening)

114
Q

What is the effect of beam hardening on the HVL value?

A

Increased HVL

115
Q

Why does a wider xray beam increase the measured HVL

A

Due to increased scatter

116
Q

What is the equation for image subject contrast

A

c ∝ (μ1 – μ2) x t
where:
c = contrast
μ = attenuation coefficient of object 1 and 2 in the material being imaged
t = thickness of the structure

117
Q

What is the effect of a thicker structure being image on contrast

A

Increases contrast

118
Q

What is the contrast in an image due to

A

Difference in attenuation between tissues

119
Q

What is the effect of a greater attenuation difference between two objects on image contrast

A

Improves image contrast

120
Q

What is the effect of decreasing tube potential kV on image contrast

A

Improves image contrast

121
Q

What is the effect of increasing the difference between atomic number (Z) between two objects on image contrast

A

Improves image contrast

122
Q

What is the effect of increasing the difference in density between two objects on image contrast

A

Improves image contrast

123
Q

What 5 factors improve image contrast

A

Decreasing kV
Thicker structure
Greater attenuation between two objects
Increasing difference in Z of objects
Increasing difference in density of objects

124
Q

What is the effect of scatter on contrast

A

Reduced contrast

125
Q

What is the effect of overlying tissue on image contrast

A

No effect

126
Q

What is spatial resolution a measure of

A

How far apart two objects must be before they can be seen as separate details in the image

127
Q

What is Modulation Transfer Function (MTF)?

A

How well a system is able to represent the object with all its detail (contrast, resolution etc). Basically how perfect is the imaging system.

128
Q

What is meant by an MTF = 1

A

Same range is obtained in the image

129
Q

What is meant by an MTF <1

A

Lower range in the image

130
Q

What is meant by an MTF = 0

A

No information in the image

131
Q

What is the Nyquist frequency

A

The highest frequency that can be imaged

132
Q

What does Nyquist criterion state

A

The sampling frequency must be at least twice the highest signal frequency
Sampling frequency = 2 x Nyquist frequency

133
Q

What effect does increasing mA have on noise

A

Decreases noise

134
Q

What is the effect of using an image receptor with a greater attenuation coefficient on noise

A

Decreases noise

135
Q

What is the effect of making the image receptor on noise

A

Decreases noise

136
Q

What is the effect of using larger detector elements on noise

A

Decreases noise

137
Q

What are the 4 causes of unsharpness

A

Geometrical unsharpness
Movement unsharpness
Edge unsharpness
Image receptor unsharpness

138
Q

What 3 things improve geometrical unsharpness

A

Fine focal spot size
Increased focus to detector distance
Reduced object to detector distance

139
Q

What is the cause of image unsharpness

A

If the detector element lies across the border between a light and a dark area the pixel displayed will be an average of these two values creating a blurred border

140
Q

What causes movement unsharpness

A

If object moves during acquisition
Edges will be blurred

141
Q

What causes edge unsharpness

A

If an object has a tapering edge the attenuation will gradually decrease along the object

142
Q

What 3 things worsen geometrical unsharpness

A

Larger focal spot size
Decreased focus to detector distance
Increase object to detector distance

143
Q

What is the effect of smaller detector elements on noise and SR

A

Improved SR
Increased noise

144
Q

What is the effect of smaller distance between detector elements on SR

A

Improved SR

145
Q

Likelihood of Bremstrahlung radiation equation?

A

Probability = EZ (energy and atomic number)

146
Q

Tungsten atomic number?

A

74

147
Q

For tungsten targets, X-rays tube voltage is set at above…?

A

70kVp

148
Q

Tungsten characteristic X-rays are what energies?

A

58keV and 68keV

149
Q

The X-ray tube has a ??? Shield to protect from ??? Radiation

A

Lead
Leaking

150
Q

Rayleigh scatter has how much energy transfer?

A

No energy is transferred

151
Q

At about ???kV, in soft tissue, the number of PE interactions = the number of Compton interactions

A

25kEv

152
Q

At about ???kEV, in bone, the number of PE interactions = Compton interactions

A

45keV

153
Q

LAC equation

A

pZ / E

154
Q

At a K edge, X-rays with energy above the k edge are preferentially…

A

Attentuated

155
Q

At a K edge, X-rays with energy below the k edge are preferentially…

A

Transmitted

156
Q

In iodine contrast studies, typically the k edge is 33 so a tube voltage of ??? Is used

A

75kEv
So that the average photon has a higher chance of being attenuated

157
Q

How does CR work? Step by step

A
  1. X-rays stored in phosphor
  2. Red light used to read, which releases blue light
  3. PMT tubes convert light into electric signal
  4. White light used to “clean” the plate
158
Q

Computer radiography: how is the sample digitised?

A

An ADC samples at regular intervals

159
Q

Computed Radiography: what is sampling pitch? What is sampling frequency?

A

Pitch - pixel size
Frequency- samples per mm,
ie 1/pixel size

160
Q

For Computed radiography: spatial resolution depends on…? (4)

A
  1. Small pixels/high frequency
  2. Thinner readout laser
  3. Thinner phosphor layer
  4. Smaller crystals (less scatter)
161
Q

Computed radiography: typical spatial resolution? Highest resolution?

A

Typical = 2-3 lp /mm
Highest is 5 lp/mm

162
Q

Indirect Digital radiography: how does it work

A
  1. Caesium iodide scintillation layer turns X-rays to light
  2. Light is amplified and detected by photodiodes
  3. This is converted into voltage by TFT switches
  4. The voltage is digitised for images
163
Q

Indirect Digital radiography: spatial resolution typically?

A

2-3.5 lp/ mm

164
Q

Direct Digital radiography: how does it work steps?

A
  1. X-rays directly hit a photoconductor to release electrons (no light)
    The conductor is amorphous selenium
  2. The electron charges are stored and read out to make an image
165
Q

Direct Digital radiography: pixel size depends on..?

A

Size of the transistors in the array

166
Q

Advantage of direct vs indirect Digital radiography?

A

Better spatial resolution in direct

167
Q

4 ways to reduce scatter in X-rays

A
  1. Reduce the volume of tissue hit with X-rays (compression or collimating)
  2. Reduce the kvP
  3. Use anti scatter grid
  4. Increased air gap between patient and image receptor
168
Q

How is intensity related to kvP?

A

Intensity proportional to kVP SQUARED

169
Q

How is mA related to intensity?

A

Intensity is proportional to mA

170
Q

What is the definition of spatial resolution?

A

The ability to perceive fine detail in an image
OR
The ability to resolve objectd as separate entities

171
Q
A
172
Q

In an X-ray machine what are the rotor bearing lubricated with?

A

Silver

173
Q

What is a typical scatter grid ratio?

A

8:1

174
Q

Thermal rating has units of…?

A

kW per 0.1 second

175
Q

Intensity is proportional to…? (Equation)

A

mA x kVP squared

176
Q

High pass filters have what effect in spatial resolution? Why?

A

High pass filters are edge enhancement filters that improve resolution (but increase noise)

FRCR Part 1 Physics (20 decks)

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