Chapter 5

Question 1

The x-ray tube consists of a _______ and an ______ enclosed within an _______, and then encased in a _______.

Answer 1

cathode; anode; envelope; protective housing

Question 2

An X-ray tubes life can be significantly extended by ________.

Answer 2

Proper care and handling by professional radiographers

Question 3

Is the cathode the positive or negative side of the x-ray tube?

Answer 3

Negative

Question 4

What is the function of the cathode?

Answer 4

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

Question 5

The cathode assembly consists of?

Answer 5

The filament or filaments, focusing cup, and associated wiring.

Question 6

What is a filament?

Answer 6

A small coil of thin tho rusted tungsten wire.

Question 7

How thick is the filament wire?

Answer 7

0.1-0.2 mm

Question 8

What are the dimensions of the filament?

Answer 8

1-2mm wide by 7-15mm long.

Question 9

_________ is the material of choice because of its high melting point.

Answer 9

Tungsten

Question 10

What is the melting point of tungsten?

Answer 10

3,370*C

Question 11

What are two desirable materials to use as a filament other than tungsten?

Answer 11

Rhenium & molybdenum

Question 12

Why is tungsten not easily vaporized a good material to used in a cathode?

Answer 12

Vaporization produces particles that deposit on other surfaces and reduce the vacuum witching the tube.

Question 13

What is another name for x-ray tubes that have dual filaments?

Answer 13

Dual-focus arrangement

Question 14

What is the function of the filament?

Answer 14

To provide sufficient resistance to the flow of electrons so that the heat produces will cause thermionic emission to occur.

Question 15

What temperature does tungsten need to reach to exhibit significant thermionic emission?

Answer 15

Above 2,200*C

Question 16

Older tubes are much more sensitive to ______ and a _____ that is jarred can break prematurely.

Answer 16

Rough handling; thin filament

Question 17

What is an average diagnostic x-ray tube filament life?

Answer 17

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

Question 18

What is the primary cause of premature turn failure?

Answer 18

The radiographer’s habit of holding the rotor switch prior to making exposures. Every second the rotor switch is depressed, life is removed from the filament.

Question 19

Routinely delayed exposures while the filament is enduring maximum current can shorten tube life by ___-___% (To 5,000-12,000 exposures)

Answer 19

50;60

Question 20

Single-phase generators are usually capable of initiating exposure within ____ milliseconds.

Answer 20

10 (0.01 second)

Question 21

High-frequency generators are usually capable of initiating exposure within ___ millisecond.

Answer 21

1 (0.001 second)

Question 22

Who developed the cathode using a hot filament?

Answer 22

William D. Coolidge

Question 23

When was the Coolidge tube developed?

Answer 23

1915

Question 24

What other development did Coolidge contribute to radiography other than the hot filament?

Answer 24

The focusing cup, the imbedded anode target, and various anode cooking devices.

Question 25

Why is the focusing cup?

Answer 25

A shallow depression in the cathode assembly designed to house the filament.

Question 26

What is the focusing cup made of?

Answer 26

Nickel

Question 27

What is the focusing cup’s purpose?

Answer 27

To narrow the thermionic cloud as it is driven toward the anode.

Question 28

Why do electrons have a tendency to diverge rather than travel in straight lines?

Answer 28

Because they possess a negative charge

Question 29

What is space charge effect?

Answer 29

As more electrons build up in the area of the filament, their negative charge begin to oppose emission of additional electrons. It also limits x-ray tubes to maximum mA range of 1,000-1,200.

Question 30

What is saturation current?

Answer 30

Affects the efficiency of the x-ray tube. As kVp increases, a grater percentage of the thermionically emitted electrons are driven toward the anode. The filament amperage curve flattens out when the kVp is driving the entire thermionic cloud toward the anode. Saturation current is achieved when there are no further thermionic electrons to be driven toward the anode.

Question 31

The anode is the positive or negative side of the x-ray tube?

Answer 31

Positive

Question 32

Why are the anode side of the x-ray tubes 3 functions?

Answer 32

-Serves as a target surface for the high voltage electrons from the filament(becoming the source of x-ray photons) -it conducts the high voltage form the cathode back into the x-ray generator circuitry -it serves as the primary thermal conductor

Question 33

The ________________ is where the high-speed electrons from the filament are suddenly stopped, resulting in the production of x-ray photons.

Answer 33

Anode target surface

Question 34

The anode assembly consists of?

Answer 34

The anode, static and rotor and serves as the path for the high-voltage flow during exposure.

Question 35

What are the two types of anodes?

Answer 35

Stationary anodes and rotating anodes

Question 36

When we’re rotating anodes developed?

Answer 36

1936

Question 37

How long is a given area of the target bombarded in modern rotating anodes?

Answer 37

7-50 microseconds

Question 38

The _______ the anode rotates, the better the heat dissipation.

Answer 38

Faster

Question 39

Stationary anodes comprise _______________ imbedded in a ___* angled end of a copper rod.

Answer 39

Rhenium-alloyed tungsten; 45

Question 40

Rotating anode disks range from ____ to ____ cm in diameter and comprise ____________.

Answer 40

5; 13; molybdenum

Question 41

Tungsten is the metal of choice for the source of x-ray photons for what three reasons?

Answer 41

- High atomic number(74) - High melting point - Heat-conducting ability

Question 42

During normal use, the focal track reaches a temperature between _________*C and _______*C

Answer 42

1,000; 2,000

Question 43

What does using an anode warm-up procedure designed to do?

Answer 43

To bring the anode heat form room temperature to near the range of operation.

Question 44

A tape measure attached to the side or bottom of a tube collimating may begin at 12 cm because?

Answer 44

The target is considered the point source of x-ray photons and it is from this point all tube-to-object and image-receptor distance are measured.

Question 45

Line-focus principle is used to

Answer 45

Reduce the effective area of the focal spot. This permits the vets resolution of detail while permitting as large an actual area as possible

Question 46

As the actual focal-spot size increases, the effective focal-spot size ____________.

Answer 46

Also Increases

Question 47

When the target angle is less than ____*, the effective focal spot is smaller than the actual focal spot.

Answer 47

45

Question 48

What is the most common diagnostic radiography target angle is?

Answer 48

12*

Question 49

When the target angle is decreased, ________ focal spots can be achieved.

Answer 49

Smaller

Question 50

Why is a disadvantage of extremely small target angles?

Answer 50

The geometry of the angle can limit the size of the primary beam field at short source-to-image receptor distances.

Question 51

To cover a 14”x17” field at 40”, a minimum of ____* target angle is required.

Answer 51

12

Question 52

Target angles of ______ than 14* have been shown to produce some degree of ______ as they age

Answer 52

Less; cutoff

Question 53

Why are x-ray focal spots rectangular?

Answer 53

Because the line-focus principle applies only in the direction is the angle.

Question 54

X-ray tube targets must be ________ to maintain the small effective focal-spot size in the horizontal direction, usually not over 2 mm.

Answer 54

Narrow

Question 55

The line-focus principle permits the actual focal spot’s vertical dimension to as much as 6 mm to gain __________________________________.

Answer 55

Maximum thermal conductivity

Question 56

The effective focal spot’s vertical dimension is the only one stated as the ______________.

Answer 56

Focal-spot size

Question 57

Who establishes standards for focal-spot size?

Answer 57

The National Electrical Manufacturers Association (NEMA)

Question 58

The ________ is the only part of the cathode or anode assemblies that is located _________ the vacuum of the envelope.

Answer 58

Outside

Question 59

Where is the rotor located?

Answer 59

Inside the stator and inside the envelope

Question 60

What is the rotor comprised of?

Answer 60

A hollow cylinder or cuff that is attached ti the anode disk by a molybdenum shaft.

Question 61

Common rotating anodes revolve at _____-______ revolutions per minute.

Answer 61

3,200-3,600

Question 62

A common cause of then failure is _______________ caused by long use at high temperatures.

Answer 62

Bad bearings

Question 63

What is the primary function of the envelope?

Answer 63

to maintain the vacuum between the cathode and anode

Question 64

What does the removal of air permit from the vacuum pump do?

Answer 64

permits electrons to flow from cathode to anode without encountering the gas atoms of air and greatly increases the efficiency of the tube's operation

Question 65

Modern x-ray tubes must be mounted inside what?

Answer 65

a protectives housing

Question 66

What does the housing control?

Answer 66

leakage and scatter radiation, isolates the high voltages, and provides a means to cool the tube

Question 67

When x-ray photons are produced in the anode, how are they emitted?

Answer 67

isotropically (in all directions)

Question 68

What is leakage radiation?

Answer 68

any photons that escape from the housing except from the port

Question 69

What must leakage radiation not exceed?

Answer 69

100 mR/hr at 1 meter

Question 70

Off-radiation may contribute as much as __-__ percent of the total beam.

Answer 70

25-30

Question 71

What are the three types of rating charts available to help radiographers avoid thermal damage to x-ray tubes?

Answer 71

radiographic tube rating charts, anode cooling charts, and housing cooling charts

Question 72

What do anode cooling charts permit?

Answer 72

the calculation of the time necessary for the anode to cool enough for additional exposures to be made

Question 73

What is the formula for calculating heat units?

Answer 73

kVp x mA x time x rectification constant