Lesson 4, 8-9, & 10-11 Test Flashcards Preview

RAD 352 - Special Imaging Modalities > Lesson 4, 8-9, & 10-11 Test > Flashcards

Flashcards in Lesson 4, 8-9, & 10-11 Test Deck (54)
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1

4 conditions necessary to produce x-rays

Source of free electrons (thermionic emission, cathode filaments)
Acceleration of electrons (kilovoltage)
Focusing of electrons (focusing cup)
Deceleration of electrons (anode/target)

2

The ejection of electrons from the surface of the filament wires due to increased heat, causing an electron cloud; "boiling off" of electrons
While current passes through the filament wire, heat (therm) causes the separation if electrons (ionic) which will be released from the wire (emission)

Thermionic emission
Space charge cloud

3

Negative side of the x-ray tube

Cathode

4

What is the function of the cathode?

To produce a thermionic cloud, conduct the high voltage to the cap between the cathode and anode, and focus the electron stream as it heads for the anode

5

3 things the cathode assembly consists of

Filament(s)
Focusing cup
Associated wiring

6

4 things the x-ray tube consists of

Cathode
Anode
Enclosed within an envelope
Encased in a protective housing

7

The entire cathode and anode assembly is housed within this glass or metal structure

Envelope

8

The entire cathode and anode assembly is housed within this metal structure

Protective housing

9

Small coil of wire set in the cathode assembly within the focusing cup; electron production happens here

Filament

10

What is the filament made of?

Thoriated tungsten; rhenium and molybdenum are also desirable materials

11

2 reasons tungsten is the material of choice for the filament

High melting point- permits the filament to operate at the high temperatures required of an x-ray tube
Difficult to vaporize- vaporization produces particles that deposit on other surfaces and reduce the vacuum within the tube

12

Positive side of the x-ray tube; serves as a target surface for high-voltage electrons from the filament, conducts the high voltage from the cathode back into the x-ray generator circuitry and serves as the primary thermal conductor

Anode

13

A shallow depression in the cathode assembly designed to house the filament; surrounds the two filament wires of the cathode
Holds the released electrons around the filament wires in a space charge cloud until the exposure switch is engaged; when the kilovoltage is applied, during the exposure, the electrons are focused into a narrow beam that is directed toward the anode because they repel each other
Because electrons all possess negative charges, their tendency is to diverge rather than travel in straight lines; this is provided with a low negative potential which focuses the electrons toward one another in a convergence pattern

Focusing cup

14

The parts within the cathode assembly that connect the elements of the assembly together

Wiring

15

A two-filament arrangement within the x-ray tube

Dual-focus

16

What is the function of the filament?

To provide sufficient resistance to the flow of electrons so that the heat produced will cause thermionic emission to occur
This process causes electrons to leave the surface of the filament wire and form a thermionic cloud
When the high voltage is released at exposure, the entire cloud is available to be driven toward the anode target where x-ray photons will be produced

17

Why are there two filament wires in the cathode?

They are different sizes because there are two different focal spots that handle different mA

18

2 focal spots and their associated mA

Large = high mA (greater than 100), more electrons flowing; large filament diameter
Small = 100 mA or below

19

What percentage of x-ray exposure is heat versus x-rays?

Heat = 99%
X-rays = 1%

20

Forces the electrons across the tube giving them kinetic energy

Kilovoltage (kVp)

21

The higher the kVp the _______ energy the accelerating electrons will have

More

22

2 reasons why you have to wait when you push the x-ray button

The anode needs to rotate
Filament wires need to be hot enough to boil off electrons

23

2 causes of tube failure by the cathode

Vaporization of tungsten (from both filament and anode)
Breaking of the filament itself

24

What is the focusing cup made of?

Nickel

25

As more and more electrons build up in the area of the filament, their negative charges begin to oppose the emission of additional electrons, limiting x-ray tubes to maximum mA ranges of 1,000-2,000

Space charge effect

26

As kVp increases, a greater percentage of the thermionically emitted electrons are driven toward the anode

Saturation current

27

What is the average diagnostic x-ra tube filament life?

6-9 hours (10,000-20,000 exposures)

28

When the x-ray machine is first turned on, a mild current is sent to the filament. The filament remains in this preheated mode until immediately prior to an exposure. When the switch labeled "rotor" is activated prior to an exposure, not only dose the rotor begin to turn by a higher current is sent to the filament to bring the thermionic cloud to the proper size for the mA selected. This increase in filament heating is what causes most of this of the filament. Tungsten (from both filament and anode) is also gradually deposited on the inner surface of a glass envelope

Vaporization

29

Positive side of the x-ray tube
Surface is where the high-speed electrons from the filament are suddenly stopped, resulting in the production of x-ray photons
To convert the electrons' kinetic energy into x-ray energy the accelerating electrons must be slowed down or stopped (deceleration)

Anode

30

3 functions of the anode

Serves as a target surface for the high-voltage electrons from the filament, thereby becoming the source of the x-ray photons
Conducts the high voltage from the cathode back into the x-ray generator circuitry
Serves as the primary thermal conductor

31

3 things the anode assembly consists of

Anode
Stator
Rotor

32

Induction-motor electromagnets that turn the anode

Stator

33

A hollow copper cylinder or cuff that is attached to the anode by a molybdenum shaft ad is affected by the electromagnetic field of the stator, causing it to turn
Located inside the stator and inside the envelope
The inside contains silver-plated steel ball bearings around a shaft that is anchored to the envelope; use silver plating as a high-temperature lubricant between the cuff and the anode shaft

Rotor

34

An anode assembly that turns during exposure

Rotating anode

35

What are rotating anode discs made of?

Molybdenum

36

What is the focal track material?

Rhenium-alloyed tungsten

37

3 reasons tungsten is the metal of choice for the source of x-ray photons

High atomic number
High melting point
Heat-conducting ability

38

Normal use of a rotating anode will eventually vaporize sufficient target focal tract material to roughen the target area; reduces the efficiency of the tube

Pitting

39

The portion of the anode where the high-voltage electron stream will impact (4 names); the precise point at which the x-ray photons are created
Area of the focal track that is impacted by the electron beam at one time
Rectangular area; have to focus on getting a good resolution because x-rays come from an area instead of a spot and cause the edges of the image to blur

Target
Focus
Focal point
Focal spot

40

The portion of the anode where the high-voltage electron stream will impact; the circular path that will be impacted by the electron beam
Focal spot/target/focus/focal point makes up this so the electrons aren't hitting the same spot on the anode

Focal track

41

2 focal spots on the anode

Actual
Effective

42

The physical area of the focal track that is impacted, in the tube

Actual focal spot

43

The area of the focal spot that is projected out of the tube toward the object being radiographed, what the patient sees

Effective focal spot

44

2 things size of the effective focal spot is determined by

Anode target angle
Actual focal spot size

45

What is the actual focal spot size controlled by?

Length of the filament

46

By angling the target, one makes the effective (focal spot) area of the target much smaller than the actual (focal spot) target area
Design incorporated into x-ray tube targets to allow a large area for heating while a small focal spot is maintained

Line-focus principle

47

What is the most common diagnostic radiography target angle?

12°

48

Why can the target angle be no smaller than 12°?

To cover a 14" x 17" field at 40"

49

Caused by the line-focus principle
Because of the geometry of an angled anode target, the radiation intensity is greater on the cathode side
Photons that are emitted toward the anode end are more likely to be absorbed by the target material itself than those that are emitted toward the cathode end
This can cause as many as 20% more photons at the cathode end of the tube and 25% fewer photons at the anode end; a total variation of approximately 45% exists parallel to the anode-cathode axis
The 45% variation is significant enough to cause a visible difference in exposure during radiographic examinations when large film sizes are used at short distances

Anode heel effect

50

Because the cathode end of the x-ray tube has a more intense beam, it should be positioned toward the ________ part of the body

Denser/thicker

51

Where is the stator located?

Outside the vacuum of the envelope; the electromagnetic effect that causes the rotor to turn can function through the envelope, permitting electrical isolation of the stator coils from the high voltage of the exposure

52

At what speed do high-speed rotating anodes operate?

10,000-12,000 revolutions per minute (rpm)

53

A structure where the primary x-ray beams exit the envelope, which allows less absorption or scatter of photons

Window

54

3 things the protective housing does

Controls leakage and scatter radiation
Isolates the high voltages
Provides a means to cool the tube