CHAPTER 5 TEST

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

X ray tube consists of what?

cathode and anode within an envelope and then encased in protective housing

the negative side of the x-ray tube

cathode

produces a thermionic cloud

cathode

conducts the high voltage to the gap between the anode and cathode

cathode

focuses the electron stream as it heads for the anode

cathode

Cathode assembly consists of

filament or filaments

focusing cup

associated wiring

a small coil of thin thoriated tungsten wire

filament

How big is the wire?

0.1-0.2 mm thick

How big is the coil?

1-2 mm wide by 7-15 mm long

What is the material of choice because of it's high melting point?

tungsten

What is the melting point of tungsten

3370 C

What else are desirable materials other than tungsten?

rhenium and molybdenum

A two filament arrangement within the x-ray tube

dual focus

When the high voltage is released at exposure what happens?

the entire cloud is available to be driven toward the anode target where x rays photons be produced

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

focusing cup

What is the focusing cup made off?

nickel and its purpose is to narrow the thermionic cloud as it is driven toward the anode

As more and more electrons build up around the filament, their negative charges begin to oppose the emission of additional electrons

space charge effect

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

saturation current

What are the anodes functions?

1) serves as a target surface for the high-voltage electrons from the filament

2) conducts the high voltage from the cathode back into the x-ray generator circuitry

3) serves as primary thermal conductor

1,2, and 3

What does Anode assembly consist of?

1) anode

2) stator

3) rotar

1, 2 and 3

What are the two types of anodes?

stationary and rotating

anode assembly that is immobile

stationary

an anode assembly that turns during exposure

rotating

The faster the anode rotates, the ___________ the heat dissipation

better

Stationary anodes are composed of

rhenium-alloyed tungsten imbedded in a 45o angled end of a copper rod.

Primary reasons for choosing tungsten for the x-ray source:

1) High atomic number

2) High melting point

3) Heat conducting ability.

1,2, and 3

What reduces the efficiency of the tube?

Pitting of the anode target focal track

How do you avoid pitting?

use an anode warm-up procedure whenever the machine is activated after a long period of time

What is the target?

the point source of x-ray photons, and it is from here that all tube-to-object and image receptor distances are measured

Actual focal spot

the physical area of the focal track that is impacted.

Effective focal spot

the area of the focal spot that is projected out of the tube toward the object being radiographed

Line-focus principle

used to reduce the effective area of the focal spot permitting the best resolution of detail while permitting as large an actual area as possible

The effective focal-spot size is controlled by

the size of the actual focal spot

Anode heel effect

due to the geometry of the angled anode target, the radiation intensity is greater on the cathode side

Stator

the only part of the cathode or anode assemblies that is located outside the vacuum of the envelope

The anode heel effect

the reason why each radiographic table has a standard natural "head" end

induction-motor electromagnets comprise what?

comprise the stator that turns the anode

Common rotating anodes revolve at

3,200-3,600 revolutions per minute (rpm)

High speed rotating anodes operate at

10,000-12,000 rpm

A common cause of tube failure is

bad bearings caused by long use at high temperatures

envelope is made of

Pyrex glass and is about 10" long, 6" in diameter at the center, and 2" in diameter at the ends

The window

The point where the primary x-ray beam exits the envelope

The primary function of the envelope is

to maintain the vacuum between the cathode and the anode

Leakage radiation

any photons that escape from the housing except at the port

Leakage radiation must not exceed

100 mR/hr at 1 meter.

Tube rating charts

provide a guide regarding the most common technical factor combinations that can be used without overloading the tube

Anode cooling chart

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

Housing cooling chart

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

Charts to help avoid thermal damage to the tube:

1) Radiographic tube rating charts

2) Anode cooling charts

3) Housing cooling charts

1, 2 and 3

When about 10% of the diameter of the filament has vaporized, what happens?

a filament becomes subject to breaking

The portion of the anode where the high-voltage electron stream will impact is called by different

names:

1) the target

2) the focus

3) the focal point

3) the focal spot

4) the focal track

1, 2, 3, and 4

When the target angle is less than 45 degrees, the effective focal spot is

smaller than the actual focal spot.

The most common diagnostic radiography target angle is

12 degrees

Tubes are available with angles ranging from

7 to 17 degrees

Diagnostic tubes are available with focal spots of

0.1mm to 3.0mm.

Stator

the only part of the cathode or anode assemblies that is located outside the vacuum of the envelope

Where is the rotor located?

inside the stator and inside the envelope

What is the rotor composed of?

a hollow copper cylinder or cuff that is attached to the anode disk by a molybdenum shaft

Metal envelopes are becoming more common because

they eliminate the problem of tungsten vaporization

The removal of the air 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

The housing does what?

1) controls leakage and scatter radiation

2) isolates the high voltages

3) provides a means to cool the tube

1,2, 3

What does the dielectric property of the oil do?

insulates the high-voltage components from the tube housing that you, the radiographer, touch

What will the tube housing absorb?

most of the off-focus radiation but enough can exit causing "ghosting" of structures adjacent to the edge of the primary beam

The ball bearings use

silver plating as a high-temperature lubricant because liquids would produce gas at high temperatures which would reduce the vacuum in the tube.

Some electrons produced in the thermionic cloud will

vaporize and deposit on the inside of the tube

When the electrons vaporize and deposit inside the tube, the deposits give the glass envelope what kind of look?

a mirrored appearance and can cause high-voltage arcing which can destroy the tube.

The deposits of vaporized tungsten can cause

increased filtration of the primary beam

What does the thermionic cloud deposits do to the inside of the tube?

reduces vacuum and makes the tube gassy

With a rotating anode target area, the term focal track is used to

represent the circular path that will be impacted by the electron beam

Rotating anode disks

range from 5-13 cm in diameter and are composed of molybdenum

The heated cathode was created by

William Coolidge for GE in 1915.

In the early days of radiography, what kind of cathode was used?

a cold cathode with an unheated filament