test 3 Flashcards
(143 cards)
1
Q
what is tube current
A
the flow of electrons from cathode to anode
2
Q
what is tube current measured in
A
milliamps
3
Q
what is filament current
A
electron flow through the cathode
4
Q
what is filament current measured in
A
amperes
5
Q
how are tube current and filament current related
A
tube current is adjusted by controlling the filament current
6
Q
what is the cathode
A
negative side of tube; 2 parts: filament and focusing cup
7
Q
what is the anode
A
positive side of tube; conducts electricity and radiates heat and contains target
8
Q
what does the mAs timers do
A
automatically picks high mA and short exposure time
9
Q
what is rectification
A
convert AC to near DC
10
Q
what is resiprocity
A
when the mAs are the same
11
Q
1/2 wave, single phase= _____ pulses
A
60
12
Q
full wave, single phase=______pulses
A
120
13
Q
how far does the supply to the tube fall away from maximum (peak) between impulses/cycles for single phase, half wave
A
100%
14
Q
single phase, full wave
A
100%
15
Q
three phase, 6 pulse
A
14%
16
Q
three phase, 12 pulse
A
4%
17
Q
high frequency
A
1%
18
Q
what is an example of a capacitor discharge unit
A
portable
19
Q
the source of energy from a capacity discharge unit is
A
battery
20
Q
what declines as the exposure is made on a capacitor discharge unit
A
kvp
21
Q
what is a regular x-ray tube called
A
diode
22
Q
why use a grid controlled tube
A
because photon emission could continue after the exposure is terminated for a capacitor discharge unit
23
Q
What does a grid controlled tube do
A
terminates exposure precisely (pt. dose and image quality related)
24
Q
how does a “grid” work
A
a grid is placed between the cathode and anode and is negatively charged to terminate the flow of electrons at the precise time photon generation should stop.
25
how do you calculate power ratings
(mA*kVp)/1000= kW
26
how many electrodes does a diode have
2
27
why is the glass or metal envelope thin
for least attenuation possible
28
what does the protective house do
1. protect from leakage radiation
| 2. protect from electric shock
29
what is isotropic x-ray emission
when x-rays are emitted equally and in all directions
30
what is the window of the x-ray tube
only use x-rays emitted throught the window
31
there can be no leakage greater than what
100 mR/hr at 1 meter
32
what are filaments made of and why
thoriated tungsten; high melting point and low vaporization
33
what is the most common cause of tube failure
1. abrupt tube failure
| 2. chronic use
34
once you go above saturation current, increasing kVp does what to tube current or quantity of x-rays produced
nothing
35
what does mAs control
quantity
36
what does kv control
quality
37
the rotation of the anode is based on what
induction motor
38
what is the stator
part of induction motor; outside the glass or metal enclosure; series of electromagnets equally spaced around the neck of the tube
39
what is the rotor
part of induction motor; inside glass; shaft made of bars of copper and soft iron fabricated into one mass
40
how does an induction motor work
1. stators are energized in sequence
2. rotor attempts to align with magnetic fields sequentially
3. when exposure terminates order of stator activation is reversed to slow dow rotation
41
what is space charge effect
cloud of negatively charged electrons around filament discourages further thermionic emission by electrostatic repulsion (makes it harder for electrons to boil off wire)
42
what is the focusing cup
negatively charged to condense/repel electrons toward anode
43
what are the two types of anodes
stationary and rotating
44
which type of anode gives more surface area for heat loading by providing an increased target area
rotating.
45
what material is the anode typically made of
needs to be a good electrical and thermal conductor-copper
46
what is the target
specific area of the anode where electrons interact to produce x-rays
47
why is tungsten metal a good target
1. high atomic number (so high energy x-rays)
2. thermal conductivity (dissipates heat produced)
3. high melting point (can stand high tube current without melting)
48
what is saturation current
the point at which all electrons go
49
what is the line focus principle
get effective focal spot size smaller than actual electron beam due to angled anode.
50
the smaller the anode angle, the _____effective focal spot
smaller
51
what is the anode heel effect
reduced intensity on anode side of beam due to absorption of photons in 'heel' of anode.
52
in theory the _____should be placed over the thick/dense portion of the patient to produce more uniform density on resulting image
cathode
53
an an anode heel increases, there is ____ significant anode heel effect
less
54
the more electrons sent off the cathode, the ______ x-rays recieved
more
55
what is the anode cooling for a H.U. production
mA*time*kVp
56
what is the anode cooling for a three phase/six pulse
mA*time*kVp*1.35
57
what is the anode cooling for a three phase/twelve pulse
mA*time*kVp*1.41
58
what can be determined from an anode cooling chart
1. tells maximum heat for tube and how long to completely cool
2. calculates required "wait" between exposures
3. partial cooling between exposures
59
tube "warm up" prevents what
damage to cold anode from high heat unit exposures
60
what does a radiographic rating chart determine
the safety of a single exposure for a specific x-ray tube.
61
what does the radiographic rating chart contain
proper filament size, anode rpm, target angle, rectification method
62
kVpl time, mA combinations which intersect ______ curve in question are safe
below
63
Even though quantity does not change above saturation current, what happens to density
it goes up as kVp goes up
64
when tube current increases, quantity of x-rays ____, quality ______, when all electrons go
increase, increase
65
what is saturation current
specific kVp at which all space charge electrons are immediately propelled to anode every exposure
66
what happens above saturation current
mA is a function solely of filament temp. (increasing kV doesn't increase tube current)
67
What happens below saturation current
space charge limited region- not all electrons at cathode went to anode on exposure so changes in kV change the tube current (mS is influenced by kVp)
68
what is the only thing that changes above saturation current
mAs
69
What is the most common energy in any beam
1/3 operation
70
what is the common saturation current
115 kVp
71
most projectile electrons have insufficient energy to ionize the target atoms, therefore
excitation occurs instead, thus the heat
72
to ionize the electron, the projectile electron must have
energy greater than the binding energy of the electron
73
what are the two types of production
characteristic and brems
74
which type of production runs around nucleus
brems
75
which type of production ionizes the target
characteristic
76
what percentage of x-rays produced are characteristic
10%
77
what percentage of x-rays produced are brems
90%
78
a photon that loses energy is what type of production
brems
79
describe characteristic production
1. when projectile electrons from cathode have sufficient energy to ionize an inner shell target electron at the anode
2. the ionization leaves a "hole" which is filled by an electron from a more distant shell
3. x-ray produced during this trasition has energy equal to the differetnce in binding energy between the shells involved
80
describe brems production
1. negative electron from the cathode comes under the influence of the positive nucleus of the target material at the anode
2. unlike charges attract
3. the electron leaves the target atom with reduced energy in new direction
81
what is a discrete spectrum represent
characteristic x-rays
82
the vertical line at specific energies is related to
electron binding energy of the target
83
the hights of the discrete spectrum relate to
quantity of photons at that energy-intensity-frequencey-number of photons at this energy
84
what changes the location of the discrete spectrum
atomic number
85
the lower the atomic number, the further______the discrete spectrum
left and vice versa
86
on a combined emission spectrum graph, amplitude is
quantity
87
energy is
position
88
the continuous spectrum represents
brems
89
the greatest number of photons (highest amplitude of curve is approximately
1/3 of maximum
90
area under curve represents
total number of x-rays
91
to move the discrete spectrum left or right, nedd new
target material
92
the discrete spectrum has the ______ energy all the time
same
93
change in kVp
shift of spectrum left and right and amplitude (gets more or less photons
94
shift of the continuous spectrum to the right means
greater effective energy when kVp in increased
95
changing mA changes
amplitude of curve of both discrete and continuous BUT position of neither.
96
long scale
lots of gray
97
compton interactions have what scale of contrast
long scale (lots of gray)
98
what is increase filtration
reduces amplitude of curve, more on left (low energies)
99
what happens to contrast when you significantly increase filtration?
fewer energis to work with so shorter scale of contrast.
100
when you increase mA or mAs
amplitude of curve changes up and down (quantity of x-rays up or down)
101
changes in atomic number of the target material changes
the amplitude of the entire curve
102
atomic number of target material change results in
amplitude (discrete and continuous) and shifting of the discrete spectrum (energy of characteristic x-rays produced changes)
103
full wave rectification to 3-phase
the amplitude increases and continuous spectrum shifts to the right. The amplitude of the discrete spectrum increases (but at same energy, so the location does not move)
104
if you increase kVp by 15% you
reduce mAs by 50% to maintain image density
105
Increaseing the kVp does not double the number of photons. it
increases the number of photons with sufficient energy to reach the film
106
if you decrease kVp by 15%, you
double your mAs to maintain image density.
107
for long scale contrast, kVp is ______, grid ratio is ______, film screen (system speed) is _______
increased, decreased, increased
108
shortest wavelength photons are associated with
maximum energy (controlled by kV operation)
109
wavelength min in angstroms =
12.4/kVp
110
quantity is effected by
mAs, mA, time, distance, voltage, waveform, filtration
111
quality is effected by
kVp
112
HVL
amount of absorbing material needed to cut beam in half
113
what is the minumum requirement for inherent filtration of a general purpose x-ray tube
.5 mm Al equivalent
114
x-ray quantity is proportional to
mAs; kVp^2
115
x-ray quantity is inversely proportional to
distance^2
116
increased filtration
removes soft photons from the beam; fewer photon energies means shorter scale of contrast
117
x-ray production is
at the target
118
x-ray interaction is
with matter, outside the tube
119
alpha and beta particles are interactions based on
electric charge on particle; mass of particle
120
x-ray and gamma photon interactions are based on
energy of photon; type of matter
121
types of photon interaction
1. transmitted through matter unchanged
2. change direction with no energy lossed
3. change direction and lose energy
4. deposit all energy in the matter
122
a photon interaction that changes direction with no energy loss is
classical scattering
123
a photon interaction that change direction and loses energy is
compton scattering
124
a photon interaction that deposits energy in the matter is
1. photoelectric effect
2. pair production
3. photodisitegration
125
pair production
only happens at extremely high energys and does not happen in diagnostic; 1.02 Mev to happen
126
photodisintegration
10MeVto happen
127
describe classical scattering (aka coherent or elastic)
1. low photon energy (less than 10 KeV) enters atom
2. atom excited by photon
3. releases photon of same keV and wavelength
4. new photon travels in different direction from original photon but usually forward (small scatter angle)
128
Describe photoelectric effect (total absorption)
1. photon ejects an inner shell electron
2. photon loses all energy
3. characteristic radiation emitted when hole filled
4. there is a cascade of photons until atom is neutral again
129
what are the results of the photoelectric effect
1. photon disappears
2. atom is ionized
3. characteristic photon is emitted
130
the liklihood of the photoelectric effect decreases as
energy increases
131
probablility of the photoelectric effect is about
z^3
132
describe compton scatterint (partial absorption)
1. photon changes direction with decreased energy
2. original atom is ionized
3. contributes to personnel/patient exposure; film fog (decreased contrast)
133
what is the probability of compton interactions happening
atomic number (Z) is independent
134
as energy increases both PE and Compton ______
decrease; PE more rapidly than compton
135
describe pair production (total absorption)
1. photon interacts with nuclear force field
2. uses 1.022 MeV to produce pair of electron like particles
3. photon ceases to exist; 2 photons of .511 MeV emitted
136
Describe photodisentegrtion (total absorption)
1. high photon energy (greater than 10 MeV)
2. photon absorbed by nucleus
3. nucleus excited
137
photon attenuation
removal of photons from primary beam; interactions are total or partial absorption
138
photon transmission
incident photons that do not interact
139
differential absorption
the difference between those x-rays absorbed photoelectrically in the patient and those transmitted to the image receptor.
140
What are 3 factors that influence differential absorption
1. beam energy
2. atomic number
3. mass density of matter
141
beam energy
* DA increases as E increases
* due to increase probability of PE at low E
* increase in patient dose
142
atomic number
* DA increases as Z increases
* probablility of PE increases with Z increases
* REMEMBER compton is not influenced by Z
143
Mass Density of Matter
* DA increases as density increases
* quantity of matter per unit volume
* increase matter/volume = increase interactions
* important since tissue densities cary more widely than Z
