M Part II Flashcards
1
Q
-special type of diode (anode & cathode)
-converts electrical energy to EM energy
A
X-ray Tube
2
Q
any tube with two electrodes
A
Diode
3
Q
-consists of two perpendicular sets of ceiling mounted rails
-most frequently used
-allows both longi and transverse travel of x-ray tube
A
Ceiling-Support System
4
Q
when x-ray tube is centered above examination table at the standard SID
A
Preferred Patient Pos’n
5
Q
-consists a single column with rollers at each end
-the x-ray tube slides up and down the column as the column relates
-variation: single floor support system
A
Floor-to-ceiling Support System
6
Q
-C-shaped and ceiling mounted system
-provides very flexible x-ray tube positioning
A
C-arm support system
7
Q
variations for C-arm support system
A
L-arm and U-arm
8
Q
lead -lined metal container into which the x-ray tube is fitted
A
Protective Housing
9
Q
3 Functions of Protective Housing
A
- Insulation (diala oil) - against electric shock
- Thermal cushion - to dissipate heat
- Mechanical support - for the tube and protect from damage caused by rough handling
10
Q
Properly designed protective housing reduces level of leakage radiation to less than [..]
A
100mR/hr at 1 m
11
Q
emission with intensity in all directions
A
Isotropic Emission
12
Q
a thin section of a glass/metal envelope that which useful beam emerges
size: 5 cm²
A
Window
13
Q
x-rays emitted through the window
A
useful beam/primary radiation
14
Q
x-ray that escape through protective housing
A
leakage radiation
15
Q
it contains x-ray tube and its components
A
glass/metal enclosure
16
Q
glass enclosure composition able to withstand tremendous heat
A
pyrex glass
17
Q
disadvantages of glass enclosure
A
more likely to fail
lesser tube life
18
Q
maintain constant electric potential b/w electrons of the tube current and enclosure
A
metal enclosure
19
Q
advantages of metal enclosure
A
less likely to fail
longer tube life
20
Q
negative side of x-ray tube
A
cathode
21
Q
2 primary parts of cathode
A
filament
focusing cup
22
Q
a coil of wire that emits electrons when it is heated
A
filament
23
Q
composition of filament
A
thoriated tungsten
24
Q
purpose of 1-2% thorium in filament
A
enhance efficiency of thermionic emission
prolongs tube life
25
Atomic no. [W]
74
26
Atomic no. [W]
74
27
melting point of W
3410 degrees celsius
28
purpose of W in filament
-provides higher thermionic emission
-does not vaporize easily
29
T/F If enclosure is vaporized, it can cause arcing and lead to tube failure
T
30
most common type of tube failure
tungsten vaporization with deposition
31
metal shroud that surrounds filament
focusing cup
32
effectiveness of focusing cup is determined by
-size and shape
-its charge
-filament size and shape
-pos'n of filament
33
serves as a grid (exposure switch) in grid controlled x-ray tube
focusing cup
34
The x-ray tube current is adjusted by controlling [..]
filament current
35
cloud of electron
space charge
36
a phenomenon of the space charge that makes it difficult for subsequent electrons to be emitted by filament bc of electrostatic repulsion
space charge effect
37
T/F Thermionic emission at low kVp and high mA can be space charge limited
T
38
filament current that has risen to its maximum value bc all available electrons have been used
saturation current
39
X-ray tube at saturation current emission is [..]
limited
40
a region of the anode target in which electrons interact to produce x-rays
-actual x-ray source
filament spot
41
filament spot sizes
small: [..]
large: [..]
shape: [..]
0.1-1mm
0.3-2mm
circular (preferred) / double banana
42
important for high resolution magnification radiography and mammography
round focal spot
43
used when small body parts are imaged and when better spatial resolution is required
assoc with small filament
small focal spot
44
limiting factor of small focal spot
the heating of target is concentrated onto smaller area
45
used when large body parts are imaged
assoc with large filament
large focal spot
46
positive side of XRT
conducts electricity, radiates heat and contains target
anode
47
two types of of anode
stationary
rotating
48
3 functions of anode
electrical conductor
mechanical support for the target
thermal dissipator
49
3 most common anode materials
copper
molybdenum
graphite
50
copper atomic no.
29
51
molybdenum atomic no.
42
52
difference if Mo and Graphite to tungsten
lower mass density
make anode lighter and easier to rotate
53
used when high tube current and power are not required
stationary anode
54
imaging systems using stationary anode
dental x-ray imaging system
portable x-ray imaging system
55
the area of the anode struck by the electrons from the cathode
target
56
composition of target
stationary - alloy embedded in the copper anode
rotating - entire rotating disk, alloying the W with Rhenium
57
purpose of rhenium in target
additional mechanical strength to withstand stresses (high-speed rotation)
the high effects of high repentive expansion and contraction
58
mammographic target composition
Mo 42
Rh 41
59
material of choice for target
Tungsten W
60
3 main reasons for using W for target
1. atomic no. : high Z results to high efficiency x-ray production and high energy x-ray
2. thermal conductivity: efficient material for dissipating heat
3. high melting point: 3410 deg C compared to Cu = 3000 deg C
61
it allows the electron beam to interact with a much larger target area
rotating anode
62
rpm of rotatong anode
3 600 - 10 000 rpm
63
T/F Higher tube currents and shorter exposure times are possible with the rotating anode
T
64
shaft between anode and rotor
anode stem
65
anode stem composition
molybdenum
66
characteristic of Mo as anode stem
poor heat conductor
67
used to turn the anode
Electromagnetic Induction Motor
68
2 Principal Parts of EM Induction motor
stator and rotor
69
located outside the enclosure
secondary coil windings/electromagnets
stator
70
located inside glass envelope
rotating part (shaft)
rotor
71
time it takes the rotor to rest after use
approx. 60s
coast time
72
-as the target angle decreases, the effective focal spot also decreases
-design incorporated into XRT target
-allows large area for heating while maintaining a small focal spot
Line Focus Principle
73
result of line focal principle
effective focal spot much smaller than the actual focal spot
74
line focal principle Limiting Factor
ability of the cone of x-ray produced to adequately cover the largest field size used
75
the area projected onto the patient and image receptor
effective focal spot/ effective target area
76
area on the anode target that is exposed to electrons from the tube current
actual focal spot
77
consequence of line focus principle
anode heel effect
78
describe ANODE HEEL EFFECT
absorption of x-rays in the heel of target
the smaller the anode angle, the larger the heel effect
79
results of ANODE HEEL EFFECT
1. smaller effective focal spot
2. radiation intensity on the cathode side (120%) of the x-ray field is greater than that on anode (75%)
80
important consequences of ANODE HEEL EFFECT
1. thicker part (cathode side), thinner part (cathode side)
2. larger effective focal spot size (cathode)
smaller effective focal spot size (anode)
81
the imaginary line generated by the center most x-ray in the bream
central ray
82
describe OFF FOCUS RADIATION/EXTRA FOCAL X-RAYS
x-rays produced in anode but not at focal spot
83
Results of OFF FOCUS RADIATION/EXTRA FOCAL X-RAYS
1. increases skin dose
2. reduces image contrast
3. exposure of tissue that was intended to be excluded by collimator
84
OFF FOCUS RADIATION/EXTRA FOCAL X-RAYS geometric solutions
1. designing a fixed diaphragm in the tube housing near the window of the XRT
2. Metal enclosure XRT
85
Factors affecting XRT life
increased:
-minimum radiographic factors mA, kVp and exposure time
-faster IR
86
Factors affecting XRT life
decreased:
excessive heat
87
3 ways of heat dissipation
radiation
conduction
convection
88
transfer of heat by emission of infrared radiation
radiation
89
transfer of energy from one area of an object to another
conduction
90
transfer of heat by the movement of a heated substance from one place to another
convection
91
causes of tube failure
1. a single excessive exposure
2. long exposure time causes excessive heating of the anode
3. vaporization of filament
92
a single excessive exposure
causes: [..]
pitting and cracking
93
long exposure time causes excessive heating of the anode
result: [..]
damage to the bearings on the rotor assembly
94
long exposure time causes excessive heating of the anode
causes: [..]
warping and rotational friction of the anode
95
vaporization of filament
causes: [..]
arcing
96
T/F The most frequent cause of abrupt tube failure is electron arcing from filament to enclosure due to vaporized tungsten
T
97
it guides and aids the RT in using x-ray tubes and acceptable exposure levels to maximize x-ray tube life
X-ray tube rating charts
98
-most important XRT rating charts
- it conveys which radiographic technique are safe and unsafe for XRT operation
Radiographic Rating Charts
99
used to detrmine the thermal capacity of an node and its heat dissipation characteristics
also used to determine the length of time required for complete cooling after any level of heat input
anode cooling chart
100
heat thermal energy
SI unit: [..]
British units: [..]
X-ray Applications: [..]
calories
joules
heat units (HU)
101
single phase
formula
HU = kVP x mA x s
102
three-phase 6-pulse
formula
HU = 1.35 x kVp x mA x s
103
three-phase 12-pulse
formula
HU = 1.45 x kVp x mA x s
104
used precisely in same way as anode cooling chart
-complete cooling: 1-2 hrs
housing cooling chart
