RAD Physics - Chapter 13: X-ray Tubes

thermionic emission

The process of heating the filament to burn off electrons is known as __________.

increases, increases

As the filament current increases, the temperature of the filament __________ and the rate of electron emission __________.

space charge

If no kilovoltage is applied across the tube, the electrons remain near the filament as a __________.

negatively, positively

When kilovoltage is applied between the filament and the target such that the cathode is __________ charged and the anode is __________ charged, electrons are driven towards the anode at a high speed

increases

As the kVp increases, the speed of the electrons __________.

tube current

The electron stream crossing the gap between the cathode and anode is the __________, measured in mA.

x-ray photons

If the speed of the electrons is great enough __________ are produced.

heat, x-ray photons

Ninety nine percent of all target interaction produce __________ and one percent produce __________.

window

The cathode and anode are enclosed in a thick Pyrex glass tube with a thinner __________, where x-rays exit the tube.

degassing

The tube contains as perfect a vacuum as possible, created by a process known as __________.

filament, wires, focusing cup

The cathode assembly consists of the __________, __________, and __________.

tungsten wire

The filament is a small coil of __________.

heat the filament, provide the potential difference needed to drive electrons to the target

The filament is mounted on two wires that support it and carry current. One wire carries the low voltage current used to __________ and the other wire carries the high voltage that is used to __________

focusing cup

A negatively charged concave metal __________ is located behind the filament.

focal spot

The purpose of the focusing cup is to confine the electrons to a narrow beam and focus them as the __________ on the tungsten target.

double focus tube

An x-ray tube with two filaments is known as a __________.

focal spots

The two filaments differ in size, producing two different __________ on the target.

three, filaments, high voltage current, small filament, large filament

The two filaments are connected with __________ wires. One wire is connected to both __________ and then each filament is connected to a separate wire. The wire that is common to both filaments carries __________ . Two wires are activated for each exposure, the common wire and either the one connected to
__________ or the one connected to __________.

increases

As the tube ages, the filament gradually evaporates as a result of heating, causing the wire to become thinner, which __________ electrical resistance.

heat

An increase in filament resistance will increase the __________ produced , which will in turn increase the rate of thermionic emission.

lower

As the tube ages, a progressively __________ filament current is required for a desired tube current.

wall of the glass tube

Some of the evaporated tungsten from the filament will be deposited on the __________ which may attract electrons and eventually puncture the tube.

window

If tungsten deposits occur on the __________, it will act as a filter and reduce x-ray output.

tungsten

Stationary anodes consist of a block of copper with a small button of __________.

target that the electrons strike, to conduct heat away from the tungsten

The tungsten serves as the __________, while the purpose of the copper is __________.

focal spot

The electron stream bombards a limited area on the target called the __________.

detail

Using the smallest practical focal spot produces images with the greatest __________.

increases, increases

As the size of the focal spot decreases, the concentration of heat in that area __________ and the risk of melting the target __________.

line focus principle, effective focal spot

The angle of the anode towards the image receptor results in the __________, in which the actual focal spot on the anode is larger than the __________.

decreases

As the target angle decreases, the effective focal spot size __________.

smaller, larger

The effective focal spot becomes _________ when projected towards the anode and __________ when projected away from the anode.

greater

The smaller the effective focal spot size, the __________ the image detail.

rotating

Diagnostic x-ray tubes use __________ anodes.

7.6 cm, 3600

Rotating anodes usually measure __________ in diameter and rotate at __________ rpm.

12.7 cm, 10,000

The rotating anodes of heavy duty models measure __________ in diameter and rotate at __________ rpm.

beveled

Rotating anodes display the line focus effect because they have a __________ edge.

target angle

The degree of beveling is called the __________.

short exposure rating

The ability of the target to withstand high exposures at very short exposure times is known as __________.

increases

As target angle decreases, short exposure rating __________.

outside the tube

The stator of the induction motor is located __________.

focal track

During rotation, the actual surface of the anode that is bombarded by electrons is constantly changing and is known as the __________ over which heat can be spread out.

higher

A rotating anode is able to withstand __________ mA than a stationary anode.

obtaining fine detail, higher mA at shorter exposure times

Rotating anode tubes usually have one filament to provide a .6 mm focal spot that is used for __________ and a second filament to provide a 1 mm focal spot that is used for ___________.

magnification

Special rotating anode tubes called fractional focus tubes have a .3 mm focal spot that is used in _______ radiography.

filament, anode

Overheating the __________ or __________ shortens tube life because of the effects of vaporized tungsten.

10%

A filament diameter reduction of ________ will cause the filament to break, ending tube life.

dissipate heat

Technical factor selection is limited by the ability of the anode to __________.

tube rating chart

The safe limits for exposure factors for a cold anode are provided by the manufacturers in the form of a _______.

less

Maximum tube rating is __________ with half wave rectification that with full wave rectification.

less

Maximum tube rating is __________ with smaller focal spots size than with larger focal spot size.

higher

A tube with a high speed anode rotation has a __________ tube rating that a conventional tube.

heat units (HU)

The thermal capacity of an anode is measured in __________.

size, cooling

The thermal capacity of the anode is dependent upon the __________ of the anode and the _______ method.

area of the focal track, anode mass

In general, an increase in __________ increases the short exposure rating, while an increase in the __________ increases thermal capacity.

1. provides an unobstructed path for the electrons

2. slows down the burnout of the filament

What are the two purposes of the vacuum within the x-ray tube?

It increases temperature and thus thermionic emission. This produces a sharp increase in tube current

How does an increase in filament current affect tube current?

stationary and rotating

What are the two types of anodes?

induction motor

What type of motor is used to rotate the anode?

12º

What is the most common target angle of a diagnostic x-ray machine?

Space charge effect

For a particular current value, the space charge has a definite size. The presence of a space charge hinders further emission of electrons from the filament because like charges repel. An equilibrium is reached between electron emission and return rate of electrons from space charge back to the filament.

Space charge compensator

It lowers filament current to compensate for an increase in kVp so that mA is constant when kVp changes. Higher kVp means that more electrons are driven towards the anode per second. This would increase tube current (mA). To keep this from happening, the ___ lowers filament current to create a smaller space charge when kVp is increased

1. using excessively high mA

2. prolonged heating from normal current loads

What are the 2 causes of filament burn out?

Filament booster circuit

It lowers the filament current to a lower standby mode until the exposure switch is closed, at which time it raises the filament current to the proper level. This extends tube life.

1. Keep filament boost time low (don’t rotor excessively)

2. Exposures should not exceed the tube rating chart

3. Perform manufacture suggested warm up exposures

4. Provide adequate cooling of the housing to avoid boiling the oil

5. Rapid sequence exposures must comply with anode cooling curve

List the rules that should be followed to extend tube life.

cathode

negative, tungsten filament (low voltage, heat up to thermionic emission)

anode

positive, tungsten target (site of x-ray production) (99% produce heat, 1% produce x-ray photons)

more, increases

As kVp increases it gives the electrons ___ energy and makes it faster (as kVp increases, energy + speed ___)

space charge

electrons burnt off cathode through thermionic emission (electrons stay hovering here until the big potential difference)

tube current

electrons traveling from cathode to anode (measured in mA)

glass envelope

a glass tube that is completely sealed 

glass envelope

this is also called a vacuum tube, bc there is no air

window

where glass is thinner (x-ray photons exit here), the glass is made of Pyrex

degassing

pyrex is baked and this is where air is expelled

1. unobstructed

2. burnout

2 reasons we need vacuum tube:

1. The lack of air gives us an ___ path for electrons moving from cathode to anode (not bumping into air molecules)

2. Slows down ___ of the filament

oil, insulation

Between glass tube and metal housing there is ___, and the this provides ___

1. filament

2. associated wires

3. focusing cup

what does cathode assembly consist of?

filament

a small coil of tungsten wire

wires

each filament is connected to 2 of these

Wire #2

which wire contains the low voltage current that is used to heat the small filament to thermionic emission

Wire #3 (common wire)

which wire has high voltage current that gives us the big potential difference across x-ray tube

Wire #1

which wire has low voltage current that is used to heat the large filament (connected to wire 3)

large

Large focal spot heats up the ___ filament (common and big wire)

small

Small focal spot heats up the ___ filament (common and small wire)

negatively, focusing cup

concave metal cup that completely surrounds the filament, it is ___ charged, job is to focus the electrons, what is this?

Double focus tube

this has 2 filament sizes (small and large)

ages

diameter of wire becomes thinner, as tube ___

more, more

The thinner the wire, the ___ it is going to heat up because there is ___ resistance, this takes less current to heat it up to thermionic emission

less current

to correct a thin wire, we need to give it ___

wall of tube, window

As the tungsten filament evaporates from the filament, it will condense or have more deposits on the ___ or ___

window

If we have tungsten deposits on the ___, we have less x-ray photons exiting the tube (acts as a filter and absorbs x-ray photons, so they don’t exit the tube)

wall of tube

If we have tungsten deposits on the ___, there is spark over (some of electrons are striking wall of tube and this can puncture the tube)

puncturing the tube

Most common cause of tube failure: ___

stationary and rotating

what are the 2 types of anodes?

rotating

In diagnostic x-ray we use the ___ anode

stationary

In dentistry they use the ___ anode

tungsten

area of the target where electrons strike is ___, but most of it is copper

1. high

2. heat

3. atomic #

3 reasons why tungsten is good:

1. Tungsten has a ___ melting point

2. Ability to conduct ___ or move the ___ around 

3. High ___ allows it to create x-ray photons that have high energy

Copper

___ also conducts heat well

focal spot

area of the target that the electrons strike

small

Small filament = ___ focal spot