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Exposure I > Test One > Flashcards

Test One Flashcards

1
Q

Two benefits of the fact that the X-ray tube is a vacuum.

A
  1. Vacuum keeps electrons at a high speed, because if the there were air or gas molecules for the electrons to collide with, they would slow those electrons down.
  2. A vacuum prevents oxidation & burning out of the filament.
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2
Q

What constitutes the tube being called a “diode tube?”

A

Its two electrodes: the cathode (negative side) and the anode (positive side.).

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3
Q

What are the external structures of the X-ray tube?

A

The support structure
The protective housing
The glass or metal envelope

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4
Q

What are the internal structures of the X-ray tube?

A

The anode and the cathode.

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5
Q

Describe the ceiling support X-ray system.

A

Ceiling rails directly over radiographic table allow for longitudinal and transverse travel. The X-ray tube housing and column can be moved to change the SID and to center the tube to the IR (detent).

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6
Q

Describe the floor-to-ceiling support X-ray system.

A

The tube is attached to a single column (with rollers) which is attached to a floor-mounted rail.

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7
Q

Describe a C-Arm support X-ray system.

A

Tube located directly across from the IR, both attached to a C-arm. Used for special procedures such as cardiovascular, interventional suites and in the OR.

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8
Q

Most used SID’s:

A

40 inches for most procedures.

72 inches for chest and c-spine.

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9
Q

Benefits of metal tube housing.

A

Protects against accidental electrical shock, lead lining provides additional shielding, provides mechanical support for tube, houses a cooling fan to cool off tube and oil.

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10
Q

Characteristics of glass envelope:

A

Provides insulation from electrical shock, dissipates heat in tube by conducting to insulating oil which surrounds glass envelope, creates a vacuum to allow flow of electrons.

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11
Q

Characteristics of insulating oil:

A

Provides more insulation, helps dissipate heat away from tube.

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12
Q

What part of the housing is where the useful beam is emitted?

A

The housing window.

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13
Q

Leakage radiation:

A

X-rays that escape tube housing. must be reduced to less than 100mR/hr at 1 m

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14
Q

What are two things that help control leakage radiation?

A

Filters and collimators

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15
Q

What does “X-rays are emitted isotropically,” mean?

A

They are emitted in equal intensity in all directions.

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16
Q

The only X-rays we are interested in are:

A

the useful beam. (Leakage is not useful.)

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17
Q

What are the benefits of a metal envelope vs a glass envelope?

A

Tungsten deposits collect inside glass envelopes, decreasing insulating ability and lead to breakage of glass and tube failure.

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18
Q

The cathode consists of two parts:

A

A tungsten filament and a nickel/molybdenum focusing cup.

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19
Q

The filament is:

A

The source of electrons during X-ray production.

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20
Q

The focusing cup does what?

A

Focuses stream of electrons.

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21
Q

What are the benefits of a tungsten filament?

A

It has a high melting point and it does not vaporize easily.

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22
Q

What is the melting point of tungsten?

A

3410 degrees Celsius. (6170 degrees Fahrenheit.)

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23
Q

When outer shell electrons are boiled off and ejected from filament, we have:

A

Thermionic emission

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24
Q

Space charge:

A

electrons liberated from the filament during thermionic emission that form a cloud around the filament.

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25
Space charge effect:
The cloud of electrons not allowing more electrons to be boiled off the filament.
26
Saturation current:
The point where increasing kVp will not increase the tube current because all electrons are used up. At this point, the only way to increase tube current is to increase mA.
27
When the X-ray tube is operated at saturation current, it is known as:
emission limited. This is due to the fact that you cannot get anymore X-ray photons. The current is maxed out.
28
What are dual focus tubes?
The focusing cup simply has two filaments in it. One is a larger sized, for larger focal spots, the other is small for smaller focal spots. Only one filament is energized at a time.
29
Focusing cup:
A cup made of nickel and molybdenum with the filament embedded in the cup. Charged negatively, therefore keeping the negative cloud of electrons from spreading apart and it focuses the electrons to a small area on the anode.
30
Efficiency of focusing cup depends on:
Size, shape, charge, filament size and shape, position of filament in cup.
31
In a grid controlled tube, the focusing cup acts as a/n
exposure switch.
32
The two types of anodes are:
Rotating and stationary. Rotating being the more common type. Stationary used in dental X-ray.
33
An anode consists of:
A target, stators and a rotor.
34
The purpose of the target is:
To stop electrons in tube current to produce X-rays.
35
The target is made up of:
90% tungsten and 10% rhenium alloy. Rhenium adds mechanical strength to support high speed rotation and provides elasticity when focal track expands due to heat.
36
The target is embedded in:
molybdenum and graphite, which helps dissipate heat.
37
Why is the target angled?
To guide the X-rays out of the tube and through the window.
38
Why is the target made of tungsten?
Because of tungsten's high melting point (3410 C), high atomic number (74), its thermal conductivity for efficient X-ray production, and it can withstand high tube currents without cracking, pitting, or bubbling.
39
Materials with higher atomic numbers:
produce a higher amount of X-rays for the same tube current. (More efficient.)
40
Graphite bonding to back of rotating target helps:
To dissipate heat from the tube due to its high melting point.
41
Benefits of a rotating anode as opposed to a stationary:
Allows electron beam to interact with much larger target area, which allows a higher tube current which allows shorter exposure time.
42
Stator:
electric motor that turns rotor at very high speed.
43
Rotor:
Connected to target through anode stem - causes target to rotate rapidly during X-ray production.
44
How many RPM's does the rotator accelerate to while the filament is heating up?
3400 - 10,000 RPM's.
45
Two position switch:
First part starts the rotor, second part takes the exposure.
46
Once the exposure is taken, let go of the switch. Why?
This minimizes the amount of time the filament is heated and extends tube life.
47
How does the anode work as an electrical conductor?
It receives electrons emitted by the cathode and conducts them trough the tube to connecting cables and back to the high-voltage generator.
48
Two functions of the anode include:
Provides mechanical support for the target and acts as a thermal radiator.
49
Three ways heat is dissipated within the tube:
Convection - fan. Conduction - stem. Radiation - anode emits infrared energy.
50
How the anode itself helps to dissipate heat:
Heat goes from target to stem to rotor to glass or metal envelope to oil. (The fan also helps by blowing air over the tube.)
51
Focal spot:
Physical area of target bombarded by electrons during X-ray production.
52
Focal spot on stationary target:
Is a fixed area.
53
Focal spot on a rotating target:
area represented by a focal track. Because of this larger area being exposed, the rotating anode withstands higher heat loads.
54
Line focus principle
The relationship between the actual focal spot and the effective focal spot.
55
Actual focal spot:
where electrons in the tube current bombard the target (depends on the size of the filament.)
56
Effective focal spot:
Area as seen from directly below the tube - area projected onto patient and image receptor.
57
Benefit of large focal spot:
Can withstand heat produced by large exposures
58
Benefit of small focal spot.
Better quality images.
59
When the angle of the target is made smaller, the effective focal spot becomes:
smaller. This improves spatial resolution while maintaining heat capacity.
60
Anode heel effect
As X-rays are emitted from the anode, some are absorbed by the anode itself due to heel of target - more X-rays emitted toward the cathode.
61
Advantage of the anode heel effect:
Cathode side of tube placed over thicker body part resulting in more even density on radiograph. (Thoracic spine and foot.) The smaller the focal spot the larger the heel effect.
62
Extra-focal radiation
X-rays produced from outside the focal spot. This is undesirable as it extends focal spot size, reduces image contrast and increases patient dose.
63
Off focus radiation:
Most off focus radiation attenuated by tube housing & collimator located near window of glass envelope.
64
Causes of tube failure:
Excessive heat placed on the anode, high techniques applied to cold anode, elevated temps for prolonged periods, excessive heating of the filament, & tube arcing.
65
What happens when excessive heat is placed on the anode:
Surface melting and pitting of target. Results in reduced radiation output, and tungsten vaporizing and coating the inside of the glass envelope.
66
What happens when high techniques are applied to cold anode:
Temp increases too rapidly, the anode cracks and becomes unstable in rotation. Prevented by warming up the tube.
67
What happens when elevated temps for prolonged periods:
Excessive heating of the bearings, which increases rotational friction and an imbalance of the rotor assembly. Causes bearings to fail, glass envelope to crack and tube housing to fail.
68
What happens with excessive heating of the filament (or high mA for prolonged periods):
tungsten vaporizes and deposits in tube. Filament wire becomes thinner & breaks causing an open filament. (Caused by holding the exposure switch too long.)
69
Tube arcing:
Caused by tungsten that has vaporized and coated the inside of the tube. It disturbs the balance of the X-ray tube, causes changes in the tube current and tube failure. "Tube arcing" will show up on the computer console.
70
Ways to extend tube life:
Minimum factors of kVp and mAs Warm up the tube Don't hold down the rotor button without making an exposure. Lower tube currents Keeping the tube stationary while energized Not using tube if unusual noise from rotor.
71
Charts that help extend tube life:
Radiographic rating chart Anode cooling chart Housing cooling chart
72
Radiographic rating chart/Tube rating chart
Determines whether a particular exposure will be safe to use. Also determines what limits on kVp, mA & exposure time needed to make safe exposure.
73
Other names for radiographic rating chart:
Instantaneous load tube rating chart | Single-exposure rating chart
74
Anode cooling chart:
Shows the time required for a heated anode to cool down. x-axis time in minutes y-axis heat units
75
Heat unit on anode cooling chart:
Capacity of the anode and housing to store heat or the thermal capacity of the anode.
76
How to calculate heat units for the anode cooling chart on a single phase machine:
(kVp)(mA)(s)= HU
77
How to calculate heat units for the anode cooling chart on a three-phase, 6 pulse machine:
(1.35)(kVp)(mA)(s)= HU
78
How to calculate heat units for the anode cooling chart on a three-phase, 12-pulse & high frequency machine:
(1.4)(kVp)(mA)(s) = HU
79
Housing cooling chart
Similar to anode cooling chart, however it uses millions of heat units…the housing unit requires 1-2 hours to completely cool.
80
Atomic number is equal to?
The number of protons (Z)
81
Atomic mass number is equal to?
The number of nucleons (A)
82
Electron binding energies (levels) are:
K, L, M, etc...
83
The closer to the nucleus, the ______ the binding energy.
higher
84
The number of electrons is equal to:
the number of protons
85
What is special about an ionized atom?
It has an extra electron or is missing an electron.
86
What is the formula for the maximum number of electrons per shell?
2 n squared.
87
Electron kinetic energy is increased or decreased as kVp is increased?
increased.
88
What are electrons traveling from cathode to anode called?
Projectile electrons (tube current)
89
What does the interaction of projectile electrons and the target atoms result in?
heat and X-rays.
90
What are the four conditions required to produce an X-ray?
Separation of electrons Production of high speed electrons Focusing of electrons Stopping high speed electrons at target.
91
Explain the separation of electrons.
The filament heats up, electrons boil off the filament (thermionic emission) and the electrons form a space charge.
92
How are high speed electrons produced?
By raising the kVp which creates a strong electric field and causes electrons to travel at high speed from cathode to anode. The electron stream is expressed in milliamperes (mA).
93
How are the electrons focused?
The negatively charged focusing cup focuses them towards the positively charged focal spot on the anode.
94
Explain what happens when the electrons slam into the target?
When the electrons hit the target, their kinetic energy is transferred to the target, the electrons slow and almost stop

Exposure I (1 decks)

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