Chapter 6 X-Ray Tube

Types of external structure supports:

-ceiling support -floor to ceiling support -floor-mount system -Fluoroscopic tube is mounted underneath the x-ray table. -C-arm, a C shaped support for portable fluoroscopy or routine radiography

Small focal spot:

0.5 - 1.0mm in size (300 mA)

Rotating anode provides approximately

1000 times more area for electron beam to interact than a stationary anode

1000 degrees F equals

538 degrees C

Average X-ray tube filament life expectancy is

6-9 hours = 10 to 20 thousand exposures

isotropically

Equal intensity in all directions

filaments will never be used at the same time. T OR F

True

stator and rotor

What are induction motors composed of?

positive side of x-ray tube

anode

most common anode materials for heat dissiaption

graphite, copper, molybdenum

what is the most common cause of tube failure?

vapor induced tube arcing

Functions of the anode

-electrical conduction -mechanical support for target -thermal conduction

Maximum leakage radiation allowed in the x-ray tube is

1 mGy a /h at 1 meter when operated at maximum conditions

Housings have heat capacities in range from

1 to 1.5 million heat units

Large focal spots

1.0 - 2.0mm in size (400 mA and up) if small is used it will be exceed in heat capacity

Heat units for three phase units: three phase six pulse =

1.35 x KvP x Ma x sec.

three phase twelve pulse =

1.41 x KvP x Ma x sec.

the anodes of high-capacity x-ray tubes rotate at

10,000 rpm

Leakage radiation limit

100 milliRoentgens per hour (mR/hr) at 1 meter (m)

Most common anode angle used is ________. Any smaller an angle can limit the primary beam field at shorter SID's.

12 degrees, the small the angle the better it will become

steel: melting point is

1370 degrees celcius

the glass or metal enclosure is relatively large, perhaps

30 to 50 cm long and 20 cm in diameter

Most rotating anodes revolve at ________ .

3400 rpm (revolutions per minute)

tungsten high melting point

3410 degrees C

how many heat units would result from a an exposure of 88 kVp at 300 mA and 2/10 sec using a single phase x-ray generator?

5,280 HU

when electrons have bombard the target __________% of their kinetic energy is converted to heat

99

one hear unit for a single phase unit equals

=1KvP x 1Ma x 1sec. Hu = KvP x Ma x sec.

Rotor

A rotating electromagnet inside the stator.

tube rating chart (3 types) #2

Anode cooling chart: can determine time required for heated anode to cool Thermal capacity of anode is measured in "heat units".

C-arm support system

Ceiling mounted and provide for very flexible x-ray tube positioning

capacitor discharge

Connecting a charged capacitor across a resistor and so enabling the charge to flow from one plate to another. The charge remaining decays exponentially.

line focus principle

Describes the relationship between the actual and effective focal spots in the x-ray tube. A smaller target angle produces a smaller effective focal spot.

Anode heel effect

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

floor to ceiling support system

Has a single column with rollers at each end, one attached to a ceiling-mounted rail and the other attached to a floor-mounted rail

tube rating chart (3 types) #3

Housing cooling chart: determines time required to cool tube housing.

tube rating chart (3 types) #1 -Most x-ray units have a built in safety feature that will not allow an unsafe exposure to be made.

Radiographic rating chart: provides a guide regarding the most common technical factor combinations that can be used without overloading the tube.

phenomena where thermionic emission is limited because negatively charged electrons make it difficult for subsequent electrons to be emitted by filament.

Space-charge effect, limits our ability to make a large technique

The target area of the anode stuck by electrons is made of:

Tungsten / rhenium alloy

most common cause of tube failure

Tungsten vaporization with deposition on the inside of the glass enclosure

grid biased x-ray tubes

Uses a negative potential difference at the focusing cup to terminate the exposure

in a line focus principle, when the target angle is made smaller, effective focal spot will be smaller.

X-ray tube target angles vary from between 50 - 200

cloud thermionic emission

a cloud of free electrons

Some housings have oil layer between tube and housing, this serves as

an electrical insulator and a thermal cushion. Heat has to be absorbed or it can destroy

grid based x-ray tubes are used in

angiography, capacitor discharge units and pulsed fluoroscopy

the x-ray tubs positive electrode is th

anode

negative consequence of the line focus principle is?

anode heel effect

Leakage radiation

any x-rays, other than the primary beam, that escape the tube housing, results in unnecessary exposure of the patient and technologist

focusing cup

area on the anode that directs the electrons to the focal spot during x-ray production. in order to do that we run a Negatively charged on focusing cup

Fractional focal spots used for magnification techniques in special procedure tubes can be as small

as 0.1mm

Mammography tubes use molybdenum or rhodium targets

because of their low atomic #

the stem of the anode is

between the anode and the rotor

produce two focal spots by using an anode that contains two different angles.

biangular targets

Large focal spot

can withstand heat produced by large exposures. Associated with a large filament. so used when larger body parts are imaged

the x-ray beam has a greater intensity on the _________ side of the x-ray tube

cathode

internal structure

cathode is the negative side of tube, contains filament and focusing cup.

movement of electrons is always from

cathode to anode

xray tube current is controlled by

changing the filament current

Anode construction

copper, graphite molybdenum.

stationary anode x-ray tubes are used in

dental imaging systems, some portable imaging systems, and other special-purpose units in which high current and power are not required

xrays produce alot of heat and heat

destroys xray equipment

tungsten has a thermal conductivity nearly equal to that of copper. Its therefore an efficient metal for

dissipating the heat produced

protective housing protects against

electric shock and serves as mechanical tube support

Anode is driven by an

electromagnetic induction motor

rotating anode allows

electron beam to interact with a much larger target area, so the heat of the anode is not confined in one spot

the focusing cup is negatively charged so that it

electrostatically confines the electron beam to a small area of the anode

x-ray tube is very heavy and requires a sturdy support structure to support its weight.

external structure

X-rays produced outside the target/focal spot. Also referred to as Off Focus

extrafocal radiation

the larger the effective focal spot the greater the spatial resolution. true or false

false it is small the local spot the greater the spatial resolution

usually made of thoriated tungsten

filament

The focusing cup is made of:

filament is embedded in metal focusing cup

The small focal spot will provide a _________

finer detail of image or superior spacial resolution. Associated with small filament

the area of the anode from which x-rays are emitted is known as the

focal spot

refers to an element in the tube that acts as a switch

grid

certain types of x-ray tubes designed to be turned off and on very rapidly

grid-controlled tubes

floor-to-ceiling support system

has a single column with rollers at each end, one attached to ceiling mounted rail and the ohter attached to a floor-mounted rail

Tungsten's high atomic # (74) results in

high efficiency x-ray production.

what is possible with rotating anode?

higher tube currents and shorter exposure times

Virtually all high capacity tubes now constructed use tubes enclosed

in metal. They are moving away from glass and pyrex.

an decrease in the degree of anode angulation equates to a/an __________ in the anode effect

increase

in order to increase the energy of X-ray photons produced at the tube, the kinetic energy of projectile electrons must be increased, this is accomplished by

increasing kVp

the heat capacity can be further improvised by

increasing the speed of anode rotation

The anode spins using an

induction motor

Stator

induction-motor electromagnets that turn the anode

Difference in __________ can vary as much as 45%.

intensity across useful beam

Effectiveness of focusing cup is determined by

it's size and shape, it's charge and filament size and shape, and the position of the filament in the focusing cup

small the angle the

larger the heel effect

The protective housing is made of what?

lead-lined cast steel

The protective housing controls leakage and scatter radiation through the tube window to radiation of

less than 1mGya/h at 1 m

anode heel effect is radiation intensity is greater at cathode side of x-ray beam. This is a by product of

line focus principle

Single excessive exposure can result in

localized surface melting and pitting of anode / target.

Tube failure can occur from

long exposure times

x-rays that are emitted through the heel of the target

lose intensity

the cathode has its own circuit of

low voltage and ?

Space charge effect limits tubes to

maximum Ma ranges from 1000 - 1200

the protective housing also provides __________________ for the x-ray tube and protects the tube from damaged caused by rough handling

mechanical support

As tungsten vaporizes in glass tube arcing can be a problem,

metal tubes maintain a constant electric potential have a longer life and fail less (metal enclosure tubes)

Dual focus tube

most x-ray tubes have two focal spots, small and large (focal track)

Biased focusing cup uses __________ to focus electron stream as it moves toward the anode

negative voltage

Cathode is _____ charged portion of the tube

negatively

anode is _______ charged portion of the tube

positively

What has been material of choice for tube construction?

pyrex

X-ray intensity

quantity of x-rays, how may photons you have

Tube heat must be dissipated through what?

radiation, conduction, convection

ir order to be used more efficiently by the x-ray tube, alternating current is changed to pulsating direct current by the

rectifiers

the stem of the anode is narrow to

reduce thermal conductivity. The stem is usually made of molybdenum bc it is a poor heat conductor

extrafocal radiation is not useful radiation, it

reduces image contrast and extends the size of the focal spot

filaments in a biangle x-ray tube have been placed "end to end" with the

small focus filament above the large filament

the phenomena that makes it difficult for subsequent electrons to be emitted by the filament because of electrostatic repulsion os referred to as

space charge effect

Line-focus principle simultaneously improves

spatial resolution of the x-ray beam as well as the heat capacity of the anode

two types of anodes

stationary and rotating

we produce electrons and we apply energy to them and move them through

the anode and cathode

in rotating anode tubes what is the target

the entire rotating disk

focusing cup must always maintain a greater negative charge than

the filament itself now matter what amperage we use. It must always be greater

in a Grid-controlled x-ray tube

the focusing cup is the grid and therefore the exposure switch

saturation current

the point in which a further increase in KvP will not result in a higher Ma because all the available electrons have been used. (you used your kvp so much that you used all your electrons)

thermionic emission

the release of electrons from the tungsten filament when the electrical current passes through it and heats the filament

Tube window is section of tube designed to let x-rays pass through. These x-rays going through the collimator and onto the body parts are called

the useful beam

the source of electrons within the x-ray tube is the result of which of the following processes

thermionic emission

general purpose x-ray tubes use the rotating anode bc

they must be capable producing high-intensity x-ray beams in a short time

What is the name of the tube failure where vaporized tungsten is collected on the envelope?

tube arcing

the mot common cause of x-ray tube failure is

tube arcing

Glass envelope

tube is evacuated and contains two electrodes (cathode/anode)

tube failure

tube life is extended by using appropriate radiographic factors of Ma, KvP and time.

this relationship between the filament current and the tube currently depends on the

tube voltage

in stationary anode tubes the target consists of

tungsten alloy embedded in the copper anode

most rotating anode x-ray tubes have

two filaments mounted in the cathode assembly "side by side", creating large and small focal spot sizes

xrays emitted through the window are called

useful beam

Target surface irregularities result in

variable and reduced radiation output

preferred detent position

when the x-ray tube is centered above the examination table at the standard SID

Protective housing

x-rays are emitted isotropically, with equal intensity in all directions.

tube failure can be caused by

• Excessive heat for long periods (thermal characteristics) • long exposure times • heating of neck, rotor are & bearings • bearing damage • cracked envelope • excessive MA • vaporized plating • open filament • oil overheating • patient danger