Test 2 Flashcards
(144 cards)
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
2
Q
Therapeutic radiology operates from ____ keV (superficial) to _____ MeV (linacs)
A
Therapeutic radiology operates from 50 keV (superficial) to 25 MeV (linacs)
3
Q
3 main components of x-ray tubes
A
X-ray tube
Operating console
High voltage generator
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
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)
6
Q
Natural gamma photons from decay; have one energy
A
Monoenergetic
7
Q
Manmade x-ray photons from Brems and characteristics interactions, multiple energies
A
Polyenergetic
8
Q
Positive side of the x-ray tube
A
Anode
9
Q
Negative side of the x-ray tube
A
Cathode
10
Q
Normal use of a rotating anode will eventually vaporize sufficient target focal track material to roughen the target area
A
Pitting
11
Q
Area of glass or metal about 5 cm^2 that is thin and emits useful x-ray beam
A
Tube window
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
13
Q
2 primary parts of the cathode
A
Filament
Focusing cup
14
Q
Coil of wire 2 mm in diameter and 1-2 cm long in cathode, emits electrons when heated
A
Filament
15
Q
When current is high enough, outer-shell electrons are “boiled-off” and ejected from the filament
A
Thermionic emission
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
17
Q
Houses the filament and is negatively charged so that electron boiled off are focused to the anode
A
Focusing cup
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
19
Q
3 fixed stations of the cathode that correspond to discrete connections on filament transformer
A
100 mA
200 mA
300 mA
20
Q
Product of current and time, number of photons
A
mAs
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
22
Q
2 types of anodes
A
Stationary (RT)
Rotating (x-ray)
23
Q
Anode used in low energy systems (dental x-rays, some portable imaging and other special purpose units, RT)
A
Stationary anode
24
Q
Anode that provides high intensity x-ray beams
A
Rotating anode
25
3 functions of a rotating anode
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
26
During x-ray production, over ____% of kinetic energy is converted to heat
99%
27
Area of anode struck by electrons from the cathode
Target
28
2 types of anode targets
Stationary
| Rotating
29
Tungsten alloy embedded in copper anode
Stationary target
30
Entire rotating disc is the target
| Allows electron beam to interact with larger target area, heating anode not confined to one small spot
Rotating target
31
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
Rhenium; graphite or molybdenum
32
3 main reasons tungsten is the material of choice for the target for general radiography
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
33
Most anodes rotate at ______ rpm; some high-capacity anodes up to _______ rmp (dissipates heat)
3400 rpm; 10,000 rpm
34
Shaft between anode and rotor
Narrow to reduce thermal conductivity
Made of molybdenum since it is a poor heat conductor
Stem of anode
35
While the anode rotates and has no mechanical connection to the outside, it can still rotate due to electromagnetic field
Induction motor
36
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
Stator; rotor
37
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
Coast time (60 sec/1 min)
38
Area of target from which x-rays are emitted
Focal spot
39
Angling the target makes the effective area of the target smaller than actual area of electron interaction
Line-focus principle
40
2 focal spots
Effective focal spot/target area
| Actual focal spot
41
The area projecting onto the patient and IR
Effective focal spot/target area
42
Area on the anode target that's exposed to electrons from the tube current
Actual focal spot
43
When target angle is decreased, effective focal spot is made smaller also; most diagnostic tubes have target angles ranging from ____° to ____°
5° to 20°
44
Used to produce two focal spot sizes due to two different target angles on the anode
Biangular targets
45
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
Anode heel effect
46
Under which side of the tube do you want the thicker body part?
Thicker body part under the cathode
47
Due to the anode heel effect, the difference in intensity can vary as much as ____%
45%
48
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
Off-focus radiation
49
Process of converting alternating current (AC) to direct (DC); flips inverse to top so it can be used all the time
Rectification
50
Frequency of incoming power is 60 cycles per second = ? Hz
60 Hz
51
2 main methods of x-ray production
Bremsstrahlung
| Characteristic
52
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
Bremsstrahlung
53
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
Characteristic/cascade
| Fluorescence
54
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
Inherent filtration
55
Average energy of a photon beam
Average E = maximum E/3
| Avg E = 1/3 max photon E
56
Targets used in MV x-ray machines in RT
Transmission type targets
57
Probability of brems production varies with ____ of the target
Z^2
58
Why are characteristic interactions named so?
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
2 competing modes of production
Characteristic
| Auger
60
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
Auger effect
61
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
Energy spectra
62
Filtration that enriches the beam by eliminating low energy/unuseful energies
Added filtration
63
Increased filtration results in a higher average energy (polyenergetic beam) and more penetrating power; filter out low energy rays
Beam hardening
64
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
Half-value layer
65
What happens to the HVL thickness of a monoenergetic beam as it passes through?
Stays the same (equal) because energy stays the same the whole time
66
What happens to the HVL thickness of a polyenergetic beam as it passes through?
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
Amount of ionization produced in air affected by tube and filament current
Output
68
Emission of electrons from filament
Filament current
69
Linear relationship with relative exposure rate
Tube current
70
Measure of ionization per unit mass of air
Exposure
71
How much is attenuated per unit length, must be inverse difference
Varies with energy
Linear attenuation coefficient (LAC) (u) (cm^-1 or m^-1)
72
HVL as a thickness formula
HVL = 0.693/u
73
Number of HVLs formula
Ix/Io = (1/2)^n
74
```
Mass attenuation (mass per unit volume) formula
Affected by Z and energy
```
Mass attenuation coefficient = u/p
```
u = LAC
p = density
```
75
Attenuation depends on the ______ of photons and the ________ of matter
Energy, density
76
Biggest increase for machine output is increasing ____; increasing _____ also increases output because of higher energy
mA; kVp
77
1 cGy = ? rad = ? Gy
1 cGy = 1 rad = 0.01 Gy
78
1 Gy = ? rad
1 Gy = 100 rad
79
Inverse square law (ISL)
I1/I2 = D2^2/D1^2
I can be Roentgen, cGy, photons, etc.
80
Production efficiency formula
Production efficiency = (9 x 10^-10)zv
```
z = 74 (z of tungsten target)
v = energy in voltage
```
81
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
Low, long
82
Plane of image is parallel to long axis of body which results in sagittal and coronal images
Axial tomography
83
Perpendicular to long axis of body, resulting in a CT slice; coronal and sagittal images are reconstructed from transverse image set
Transverse image
84
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
Translation
85
Intensity profile across the patient while the gantry angle is at a given point
Projection
86
Estimating a value between two known values; image reconstruction at any z-axis is possible by this mathematical process
Interpolation
87
Estimating a value beyond known values
Extrapolation
88
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
Helical/spiral CT
89
Special computer that interpolates data
Interpolation algorithm
90
Relationship between patient couch movement and x-ray beam width
Pitch
91
Helical pitch ratio formula
Pitch = couch movement each 360° or rotation of gantry/beam width
92
_____ interpolation algorithms allow imaging at a pitch over 1
180°
93
5 things increasing pitch greater than 1 does (volume, dose, time, data,)
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
5 things decreasing pitch less than 1 does (volume, dose, time, data, quality)
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
Head to foot axis
Z-axis
96
3 parts of the CT imaging system
Operating console
Computer
Gantry
97
Includes x-ray tube, detector array, high-voltage generator, patient support couch, and mechanical support
Rotates around patient
Gantry
98
X-ray tubes are expected to last at least how many exposures?
50,000 exposures
99
Collimation ___________ patient dose by restricting volume irradiated and ___________ image contrast by limiting scatter
Reduces, improves
100
2 collimators CT uses
Pre-patient
| Pre-detector
101
Collimator mounted on x-ray tube housing or adjacent to it and functions to limit area of patient that receives useful beam
Pre-patient collimator
102
Collimator that restricts beam viewed by detector array and functions to reduce scatter incident on detector array
Pre-detector collimator
103
All multislice helical CT uses high-frequency power; small so it can be mounted on rotating gantry, uses ______ kW power
50 kW
104
Functions to hold patient
Must be made of low Z material to transmit
Should be smoothly and accurately driven for precise patient positioning
Support couch
105
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
Slip-ring technology
106
Current CT systems have a matrix size of _____ x ______; resulting in ________ cells
512 x 512; 262,144 cells
107
An individual cell which contains numerical information (CT number or HU); 2D representation of a corresponding tissue volume
Pixel (picture element)
108
Takes the pixel a step further and considers depth, 3D pixel
Determined by multiplying pixel size by slice thickness
Z average between corkscrews
Voxel (volume element)
109
Voxel size formula
```
Voxel size (mm^3) = pixel size (mm^2) x slice thickness
Voxel = length x width x slice thickness
```
110
Image matrix size provided by digital x-ray systems
Diameter of image reconstruction
How big screen is how big anatomy appears on it
Field of view (FOV)
111
Increase FOV with fixed matrix size = ________ pixel size
Increase
112
Increase matrix size with fixed FOV = ________ pixel size = ________ resolution
Decreased, better
113
Pixel size formula
Pixel size = FOV/matrix size
114
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)
CT numbers
115
Scale of CT numbers used to assess the nature of tissue
Hounsfield unit (HU)
116
CT numbers formula
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
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
4096, 32
118
Specific number of gray levels or digital image numbers assigned to an image; determines the grayscale rendition of the imaged tissue and therefore contrast
Window width (WW)
119
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
Window level (WL)
120
CT number of air, lungs, fat, water, blood, muscle, bone, and metal
```
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
Increase window width = ______ shades of gray
More
122
After projections are acquired by CT detectors and stored in memory, the image is reconstructed; filter refers to a mathematical algorithm
Filtered back projection
123
More recent, robust reconstruction algorithm which results in improved contrast resolution at lower patient dose
Iterative reconstruction
124
Stacking transverse images to form a 3D dataset; take contrast from vessel or intestine and reconstruct it removing bone
Multiplanar reformation (MPR)
125
3 common MPR algorithms
Maximum intensity projection (MIP)
Shaded surface display (SSD)
Shaded volume display (SVD)
126
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
Maximum intensity projection (MIP)
127
Borrowed from a computer aided detection and manufacturing applications which identifies a narrow range of values belonging to the object and displays those ranges
Shaded surface display (SSD)
128
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
Spatial resolution (SR)
129
4 factors that control SR
Limitations of CT system
Pixel size
Slice thickness
Collimation
130
Unit of SR
Line pair per millimeter (LP/mm)
131
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
Better
132
Thinner CT slices yield ________ SR; smaller slice thickness ________ SR
Better, increases
133
Anatomy outside the slice thickness may not be allowed; therefore pixel volume also affects SR
Partial volume
134
Pre-patient and post-patient collimation affects level of scatter, which influence SR by affecting contrast; tighter collimation = ________ SR
Better
135
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
Contrast resolution (CR)
136
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)
Noise
| Standard deviation
137
5 things noise depends on
```
Patient dose (controls most)
Number of x-rays (mA) (controls most)
kVp and filtration
Pixel size
Slice thickness
Detector efficiency
```
138
Increase mA, _______ noise
Decrease
139
Increase kVp, _______ noise because more photons transmit through (15% rule)
Decrease
140
Decrease pixel size and slice thickness to _________ noise
Decrease
141
Constancy of pixel values in all regions of the reconstructed image
Uniformity
142
CT contrast resolution should be capable of resolving 5-mm objects at _____% contrast; assessed ____________
0.5%; semiannually
143
Intensity after HVL formula
Ix = Ioe^-ux
```
Ix = intensity after filtration
Io = original intensity
u = linear attenuation coefficient (per unit length)
x = thickness of filter
```
144
What generation CT scanner is commonly used today?
3rd generation
