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RAD 380 - Physics of Radiation Therapy > Test 2 > Flashcards

Test 2 Flashcards

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1
Q

Diagnostic x-ray operates at voltages between _____ to _____ kVp and currents between ____ to ____ mA; average energy is 1/3 max = ____ to _____ keV average

A

Diagnostic x-ray operates at voltages between 25 to 150 kVp and currents between 100 to 1200 mA; average energy is 1/3 max = 10 to 50 keV average

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2
Q

Therapeutic radiology operates from ____ keV (superficial) to _____ MeV (linacs)

A

Therapeutic radiology operates from 50 keV (superficial) to 25 MeV (linacs)

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3
Q

3 main components of x-ray tubes

A

X-ray tube
Operating console
High voltage generator

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4
Q

The x-ray tube is held in a _____ contained in a glass or metal enclosure which allows for more efficient x-ray production; glass enclosure is made of ______ glass and can withstand high temperatures

A

Vacuum

Pyrex

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5
Q

Cobalt machines operate at _____ MeV and ____ MeV; averaging at ____ MeV (two different decays)

A

Cobalt machines operate at 1.17 MeV and 1.33 MeV; averaging at 1.25 MeV (two different decays)

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6
Q

Natural gamma photons from decay; have one energy

A

Monoenergetic

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7
Q

Manmade x-ray photons from Brems and characteristics interactions, multiple energies

A

Polyenergetic

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8
Q

Positive side of the x-ray tube

A

Anode

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9
Q

Negative side of the x-ray tube

A

Cathode

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10
Q

Normal use of a rotating anode will eventually vaporize sufficient target focal track material to roughen the target area

A

Pitting

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11
Q

Area of glass or metal about 5 cm^2 that is thin and emits useful x-ray beam

A

Tube window

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12
Q

As tungsten vaporizes it coats the inside of the glass and tube current strays and interact with the glass enclosure (photons may exit glass envelope)

A

Arcing and tube failure

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13
Q

2 primary parts of the cathode

A

Filament

Focusing cup

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14
Q

Coil of wire 2 mm in diameter and 1-2 cm long in cathode, emits electrons when heated

A

Filament

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15
Q

When current is high enough, outer-shell electrons are “boiled-off” and ejected from the filament

A

Thermionic emission

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16
Q

Filament is made of thoriated tungsten which has a melting point of ______°C; high melting point does not vaporize easily, less likely to burn out the filament

A

3370°C

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17
Q

Houses the filament and is negatively charged so that electron boiled off are focused to the anode

A

Focusing cup

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18
Q

Most rotating anode x-ray tubes contain 2 filaments

A

Small focal spot used for better spatial resolution but heat is focused on a smaller area
Large focal spot can handle more heat/technique (large body parts) but gives less/worse detail and has more magnification

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19
Q

3 fixed stations of the cathode that correspond to discrete connections on filament transformer

A

100 mA
200 mA
300 mA

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20
Q

Product of current and time, number of photons

A

mAs

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21
Q

Selection of large or small focal spot is used with mA selector on operating console, but larger selection of about _____ mA, only chooses larger focal spot

A

400 mA

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22
Q

2 types of anodes

A

Stationary (RT)

Rotating (x-ray)

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23
Q

Anode used in low energy systems (dental x-rays, some portable imaging and other special purpose units, RT)

A

Stationary anode

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24
Q

Anode that provides high intensity x-ray beams

A

Rotating anode

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25
Q

3 functions of a rotating anode

A

Thermal dissipater
Electrical conductor: anode receives electrons emitted by cathode and conducts them through the tube to the connecting cables and back to high-voltage generator
Mechanical support for the target

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26
Q

During x-ray production, over ____% of kinetic energy is converted to heat

A

99%

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27
Q

Area of anode struck by electrons from the cathode

A

Target

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28
Q

2 types of anode targets

A

Stationary

Rotating

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29
Q

Tungsten alloy embedded in copper anode

A

Stationary target

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30
Q

Entire rotating disc is the target

Allows electron beam to interact with larger target area, heating anode not confined to one small spot

A

Rotating target

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31
Q

Alloying the target with ________ gives added mechanical strength during high-speed rotation and thermal expansion and contraction; high-capacity systems have ______ or _______ layered under tungsten target to allow for lighter anodes and ease of rotation

A

Rhenium; graphite or molybdenum

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32
Q

3 main reasons tungsten is the material of choice for the target for general radiography

A

High atomic number of 74 results in high-efficiency x-ray production and in high-energy x-rays
Thermal conductivity nearly equal to that of copper and is therefore an efficient metal for dissipating the heat produced
Higher melting point of 3400°C and therefore can stand up under high tube current without pitting or bubbling

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33
Q

Most anodes rotate at ______ rpm; some high-capacity anodes up to _______ rmp (dissipates heat)

A

3400 rpm; 10,000 rpm

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34
Q

Shaft between anode and rotor
Narrow to reduce thermal conductivity
Made of molybdenum since it is a poor heat conductor

A

Stem of anode

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35
Q

While the anode rotates and has no mechanical connection to the outside, it can still rotate due to electromagnetic field

A

Induction motor

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36
Q

The _______ is located outside the glass and contains electromagnets equally spaced around the rotor (static); inside glass enclosure is the _____ which rotates via electromagnetic induction
When pushing the exposure button, there is a delay which allows rotor to accelerate to designated rpm while filament gets heated

A

Stator; rotor

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37
Q

Time it takes the rotor to rest after use

After exposure, the rotors comes to stop, induction motor is put in reverse; as bearings wear out it increases

A

Coast time (60 sec/1 min)

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38
Q

Area of target from which x-rays are emitted

A

Focal spot

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39
Q

Angling the target makes the effective area of the target smaller than actual area of electron interaction

A

Line-focus principle

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40
Q

2 focal spots

A

Effective focal spot/target area

Actual focal spot

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41
Q

The area projecting onto the patient and IR

A

Effective focal spot/target area

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42
Q

Area on the anode target that’s exposed to electrons from the tube current

A

Actual focal spot

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43
Q

When target angle is decreased, effective focal spot is made smaller also; most diagnostic tubes have target angles ranging from ____° to ____°

A

5° to 20°

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44
Q

Used to produce two focal spot sizes due to two different target angles on the anode

A

Biangular targets

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45
Q

Radiation intensity on cathode side of x-ray field is greater than the anode side
The x-rays emitted on anode side must travel through a thicker part of the target than those on the cathode side, anode attenuates beam
Intensity emitted through the anode side are reduced due to the longer path of travel (increased absorption)
Important in imaging structures of differing thickness or density
More attenuation for photons produced at greater depths in the target, attenuation is more pronounced at anode

A

Anode heel effect

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46
Q

Under which side of the tube do you want the thicker body part?

A

Thicker body part under the cathode

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47
Q

Due to the anode heel effect, the difference in intensity can vary as much as ____%

A

45%

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48
Q

When electrons bounce off of the focal spot and interact outside of the focal spot where they produce x-rays
Extends size of focal spot, increases skin dose, and reduces image contrast
Geometric solution: reduce with fixed diaphragm in tube housing
Can be removed by metal enclosure of x-ray tube
X-rays produced in anode but not at the focal spot

A

Off-focus radiation

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49
Q

Process of converting alternating current (AC) to direct (DC); flips inverse to top so it can be used all the time

A

Rectification

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50
Q

Frequency of incoming power is 60 cycles per second = ? Hz

A

60 Hz

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51
Q

2 main methods of x-ray production

A

Bremsstrahlung

Characteristic

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52
Q

Electrons interacting with a nucleus (polyenergetic because it depends on how close electron gets to nucleus)
A high speed negative electron passes near a positive nucleus and is deflected from its path after being acted upon by Coulomb forces; they lose energy, slow down, and propagates the energy through space as a photon
Part or all of the electron’s energy may be given off; electron can engage in multiple interactions
Direction depends on energy; increase energy = more forward directed radiation (high energy = more forward peaked, shoot electron straight through so they don’t deflect)
Continuous energy spectrum
Breaking radiation

A

Bremsstrahlung

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53
Q

Electrons interacting with orbital electrons (monoenergetic)
Occur at levels of BE
An electron interacts with an atom by ejecting an orbital electron which leaves the atom ionized, loses electrons and becomes positively charged
A vacancy is created and an outer orbital electron will fall down to a closer orbital
When an electron moves down there is an energy potential, which results in x-rays
Energy potential between orbital levels results in discrete energies because of its difference in binding energies
Sharp peaks on graph occur when vacancies are produced and electrons drop down to fill the gap
Energy levels are unique to each atom and act as ‘fingerprints’ for various elements
Favored in the higher Z range

A

Characteristic/cascade

Fluorescence

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54
Q

Filtration of useful x-ray beams provided by the permanently installed components of an x-ray tube housing assembly and the glass window of the x-ray tube (ex: oil, window); blocks low energy photons
Absorption in target, glass of tube, beryllium window

A

Inherent filtration

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55
Q

Average energy of a photon beam

A

Average E = maximum E/3

Avg E = 1/3 max photon E

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56
Q

Targets used in MV x-ray machines in RT

A

Transmission type targets

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57
Q

Probability of brems production varies with ____ of the target

A

Z^2

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58
Q

Why are characteristic interactions named so?

A

They’re characteristic of the certain elements BEs

These energy patterns for x-ray emission are characteristic for each particular energy level and element

59
Q

2 competing modes of production

A

Characteristic

Auger

60
Q

After an inner shell electron is ejected an outer shell electron takes its place
An energy potential is therefore created which is going to be released, but it has two routes or effects that it can cause
Most often it will release the energy in form of a photon
Less often this energy will be used to eject a second (Auger) electron

A

Auger effect

61
Q

Energy from brems and characteristic are superimposed when considering how x-ray machines produce energies and are therefore considered heterogeneous
Without considering filtration, the energy spectrum will be straight line; not really a true unfiltered representation in practice as we have inherent filtration
Filtration primarily affects (eliminates) low energy photons and scatter
Higher energy beam is more forward peaked after going through the transmission target, higher energy has a beam that goes more forward

A

Energy spectra

62
Q

Filtration that enriches the beam by eliminating low energy/unuseful energies

A

Added filtration

63
Q

Increased filtration results in a higher average energy (polyenergetic beam) and more penetrating power; filter out low energy rays

A

Beam hardening

64
Q

Thickness of a specified material that’s necessary to reduce/attenuate the intensity of a radiation beam to half its original value; describe beam quality

A

Half-value layer

65
Q

What happens to the HVL thickness of a monoenergetic beam as it passes through?

A

Stays the same (equal) because energy stays the same the whole time

66
Q

What happens to the HVL thickness of a polyenergetic beam as it passes through?

A

Thickness has to increase each time because beam hardens and average energy increases each time (low energy photons got knocked out) it passes through an HVL

67
Q

Amount of ionization produced in air affected by tube and filament current

A

Output

68
Q

Emission of electrons from filament

A

Filament current

69
Q

Linear relationship with relative exposure rate

A

Tube current

70
Q

Measure of ionization per unit mass of air

A

Exposure

71
Q

How much is attenuated per unit length, must be inverse difference
Varies with energy

A

Linear attenuation coefficient (LAC) (u) (cm^-1 or m^-1)

72
Q

HVL as a thickness formula

A

HVL = 0.693/u

73
Q

Number of HVLs formula

A

Ix/Io = (1/2)^n

74
Q 
Mass attenuation (mass per unit volume) formula
Affected by Z and energy
A

Mass attenuation coefficient = u/p

u = LAC
p = density
75
Q

Attenuation depends on the ______ of photons and the ________ of matter

A

Energy, density

76
Q

Biggest increase for machine output is increasing ____; increasing _____ also increases output because of higher energy

A

mA; kVp

77
Q

1 cGy = ? rad = ? Gy

A

1 cGy = 1 rad = 0.01 Gy

78
Q

1 Gy = ? rad

A

1 Gy = 100 rad

79
Q

Inverse square law (ISL)

A

I1/I2 = D2^2/D1^2

I can be Roentgen, cGy, photons, etc.

80
Q

Production efficiency formula

A

Production efficiency = (9 x 10^-10)zv

z = 74 (z of tungsten target)
v = energy in voltage
81
Q

During large field abdominal radiography, image is _____ in contrast ____ scale due to Compton scatter; high scatter due to large area x-ray beam and image degraded due to superimposition of anatomical structures in abdomen

A

Low, long

82
Q

Plane of image is parallel to long axis of body which results in sagittal and coronal images

A

Axial tomography

83
Q

Perpendicular to long axis of body, resulting in a CT slice; coronal and sagittal images are reconstructed from transverse image set

A

Transverse image

84
Q

When the source tube and detector assembly makes one sweep across the patient; internal structures attenuate x-ray beam according to mass density and atomic number

A

Translation

85
Q

Intensity profile across the patient while the gantry angle is at a given point

A

Projection

86
Q

Estimating a value between two known values; image reconstruction at any z-axis is possible by this mathematical process

A

Interpolation

87
Q

Estimating a value beyond known values

A

Extrapolation

88
Q

Computed tomography in which the x-ray tube continuously revolves around the patient, who is simultaneously moved longitudinally; computer interpolation allows reconstruction of standard transverse scans or images in any preferred plane
Has several parallel detector arrays with thousands of individual detectors

A

Helical/spiral CT

89
Q

Special computer that interpolates data

A

Interpolation algorithm

90
Q

Relationship between patient couch movement and x-ray beam width

A

Pitch

91
Q

Helical pitch ratio formula

A

Pitch = couch movement each 360° or rotation of gantry/beam width

92
Q

_____ interpolation algorithms allow imaging at a pitch over 1

A

180°

93
Q

5 things increasing pitch greater than 1 does (volume, dose, time, data,)

A

More volume imaged per unit time
Less patient dose
Faster scan time leads to less motion artifact, especially during breath hold (10-30 sec, as low as possible)
Less data obtained, lower quality image
Reduced z-axis resolution due to wide section sensitivity profile

94
Q

5 things decreasing pitch less than 1 does (volume, dose, time, data, quality)

A

Less volume imaged per unit time, smaller slice thickness
More patient dose
Slow scan time
More data obtained; higher quality image, more detail
Better z-axis resolution

95
Q

Head to foot axis

A

Z-axis

96
Q

3 parts of the CT imaging system

A

Operating console
Computer
Gantry

97
Q

Includes x-ray tube, detector array, high-voltage generator, patient support couch, and mechanical support
Rotates around patient

A

Gantry

98
Q

X-ray tubes are expected to last at least how many exposures?

A

50,000 exposures

99
Q

Collimation ___________ patient dose by restricting volume irradiated and ___________ image contrast by limiting scatter

A

Reduces, improves

100
Q

2 collimators CT uses

A

Pre-patient

Pre-detector

101
Q

Collimator mounted on x-ray tube housing or adjacent to it and functions to limit area of patient that receives useful beam

A

Pre-patient collimator

102
Q

Collimator that restricts beam viewed by detector array and functions to reduce scatter incident on detector array

A

Pre-detector collimator

103
Q

All multislice helical CT uses high-frequency power; small so it can be mounted on rotating gantry, uses ______ kW power

A

50 kW

104
Q

Functions to hold patient
Must be made of low Z material to transmit
Should be smoothly and accurately driven for precise patient positioning

A

Support couch

105
Q

Devices that conduct electricity through rings and brushes from a rotating surface onto a fixed surface, allows gantry to rotate continuously without interruption; stops cables from becoming twisted
Brushes made of conductive silver graphite which are the sliding contact; replaced every year or so while ring lasts for life of imaging system

A

Slip-ring technology

106
Q

Current CT systems have a matrix size of _____ x ______; resulting in ________ cells

A

512 x 512; 262,144 cells

107
Q

An individual cell which contains numerical information (CT number or HU); 2D representation of a corresponding tissue volume

A

Pixel (picture element)

108
Q

Takes the pixel a step further and considers depth, 3D pixel
Determined by multiplying pixel size by slice thickness
Z average between corkscrews

A

Voxel (volume element)

109
Q

Voxel size formula

A 
Voxel size (mm^3) = pixel size (mm^2) x slice thickness
Voxel = length x width x slice thickness
110
Q

Image matrix size provided by digital x-ray systems
Diameter of image reconstruction
How big screen is how big anatomy appears on it

A

Field of view (FOV)

111
Q

Increase FOV with fixed matrix size = ________ pixel size

A

Increase

112
Q

Increase matrix size with fixed FOV = ________ pixel size = ________ resolution

A

Decreased, better

113
Q

Pixel size formula

A

Pixel size = FOV/matrix size

114
Q

Each pixel is displayed as a brightness level corresponding to numbers ranging from -1000 to 3000
The linear attenuation coefficient has to do with how much radiation is transmitted through a medium and corresponds to these numbers
Based on water (corresponding value of 0)

A

CT numbers

115
Q

Scale of CT numbers used to assess the nature of tissue

A

Hounsfield unit (HU)

116
Q

CT numbers formula

A

CT number = (ut-uw/uw) x 1000

ut = linear attenuation coefficient of the tissue in the voxel under analysis
uw = x-ray attenuation coefficient of water
117
Q

While actual dynamic range of image is _______ shades but displayed on a monitor in _______ shades, information might be wasted; however window level and window width allow entire range to be made visible

A

4096, 32

118
Q

Specific number of gray levels or digital image numbers assigned to an image; determines the grayscale rendition of the imaged tissue and therefore contrast

A

Window width (WW)

119
Q

Brightness; location on a digital image number scale at which the levels of gray are assigned that regulates the OD of the displayed image and identifies the type of tissue to be imaged

A

Window level (WL)

120
Q

CT number of air, lungs, fat, water, blood, muscle, bone, and metal

A 
Air = -1000
Lungs = -600 to -800
Fat = -10 to -20
Water = 0
Blood = 20
Muscle = 50
Bone = 200 (low density) to 1000 (high density)
Metal = 3000
121
Q

Increase window width = ______ shades of gray

A

More

122
Q

After projections are acquired by CT detectors and stored in memory, the image is reconstructed; filter refers to a mathematical algorithm

A

Filtered back projection

123
Q

More recent, robust reconstruction algorithm which results in improved contrast resolution at lower patient dose

A

Iterative reconstruction

124
Q

Stacking transverse images to form a 3D dataset; take contrast from vessel or intestine and reconstruct it removing bone

A

Multiplanar reformation (MPR)

125
Q

3 common MPR algorithms

A

Maximum intensity projection (MIP)
Shaded surface display (SSD)
Shaded volume display (SVD)

126
Q

Reconstructs image by selecting highest pixel value along an arbitrary line and showing only those pixels; provides excellent differentiation of vasculature (with contrast)
Best for tumors in thorax/lung; practical for lungs
Maximum intensity on a breathing cycle
Lungs show up black, tumor white; blurs white edges of tumor during breathing cycle to show full extent of tumor
Can be rotated to show 3D features

A

Maximum intensity projection (MIP)

127
Q

Borrowed from a computer aided detection and manufacturing applications which identifies a narrow range of values belonging to the object and displays those ranges

A

Shaded surface display (SSD)

128
Q

Ability to image small objects that have a high subject contrast; if an image has a sharp interface, the image at the interface will be somewhat blurred
Most important component of the QC program
Esures proper performance of detector array, reconstruction electronic, and mechanical components

A

Spatial resolution (SR)

129
Q

4 factors that control SR

A

Limitations of CT system
Pixel size
Slice thickness
Collimation

130
Q

Unit of SR

A

Line pair per millimeter (LP/mm)

131
Q

The smaller the pixel size the ______ the SR; CT imaging systems can reconstruct images after data is acquired, followed by post-processing tasks which affects SR

A

Better

132
Q

Thinner CT slices yield ________ SR; smaller slice thickness ________ SR

A

Better, increases

133
Q

Anatomy outside the slice thickness may not be allowed; therefore pixel volume also affects SR

A

Partial volume

134
Q

Pre-patient and post-patient collimation affects level of scatter, which influence SR by affecting contrast; tighter collimation = ________ SR

A

Better

135
Q

Ability to distinguish between and image similar tissues
100% is black and white
Digital is better than screen-film imaging; CT superior to radiography, ability to image low-contrast object is limited by size and uniformity of object and by noise of the system

A

Contrast resolution (CR)

136
Q

Grainy or uneven appearance of an image caused by insufficient number of primary x-rays or uniform signal produced by scattered x-rays
Variation in CT number above or below average
Appears as graininess (low appears smooth, high appears blotchy)

A

Noise

Standard deviation

137
Q

5 things noise depends on

A 
Patient dose (controls most)
Number of x-rays (mA) (controls most)
kVp and filtration
Pixel size
Slice thickness
Detector efficiency
138
Q

Increase mA, _______ noise

A

Decrease

139
Q

Increase kVp, _______ noise because more photons transmit through (15% rule)

A

Decrease

140
Q

Decrease pixel size and slice thickness to _________ noise

A

Decrease

141
Q

Constancy of pixel values in all regions of the reconstructed image

A

Uniformity

142
Q

CT contrast resolution should be capable of resolving 5-mm objects at _____% contrast; assessed ____________

A

0.5%; semiannually

143
Q

Intensity after HVL formula

A

Ix = Ioe^-ux

Ix = intensity after filtration
Io = original intensity
u = linear attenuation coefficient (per unit length)
x = thickness of filter
144
Q

What generation CT scanner is commonly used today?

A

3rd generation

RAD 380 - Physics of Radiation Therapy (3 decks)

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