RADT 1390 test 2 review chapter 5

Protective housing

*Modern X-ray tubes must be mounted inside this* **it controls leakage and scatter radiation, isolates the high voltages and provides a means to cool the tube.

Filament

*Small coil of thin thoriated tungsten wire. *Set in the cathode assembly within the focusing cup. *Single or dual-focused

filament breakage

*major cause of tube failure *the filaments become increasingly thin as vaporization continues *when about 10% of the diameter has vaporized, a filament becomes subject to breaking

anode heel effect

*the use of the line-focus principle causes this. *Because of the geometry of the angled anode target, the radiation intensity is greater on the cathode side* **Electrons bombard the target, x-rays are produced and most are emitted at angles between 45 degrees and 90 degrees in the direction of the electron travel

high-voltage arcing

*vaporized tungsten collection on the envelope *this type immediately destroys the tube.

thermionic emission

-The filament is heated -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.

what is the function of the cathode?

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

anode functions

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

measurement of the filament wire

0.1-0.2 mm thick

measurement of the filament coil

1-2 mm wide by 7-15 mm long

Why is tungsten the metal of choice for the source of x-ray photons?

1. high atomic number 2. high melting point 3. heat-conducting ability

recommendations got extending tube life

1. warmup the anode following the manufacturer's recommendations 2. Do not hold the rotor switch unnecessarily. Double-press switches should be completely depressed in one motion. Dual Switches should have the exposure switch depressed first, followed by the rotor switch. 3. use lower-mA stations when possible 4. used a lower-speed rotor when possible 5. Do not make repeated exposures near tube loading limits 6. Do not rotate the tube housing rapidly from one position to another 7. Do not use a tube when you hear loud rotor bearings -(unless it has been checked by a qualified service person)

measurements of envelope

10" long, 6" in diameter at the center, and 2" in diameter at the ends

leakage radiation must not exceed?

100 mR/hr at 1 meter 100 milliRoentgens per hour at 1 meter

The most common diagnostic radiography target angle is?

12 degrees *this will fully cover a 14x17 IR at 40 SID

Molybdenum atomic #

42

tube filament life span

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

atomic # of tungsten

74 enhances the production of diagnostic-range photons

Tube failure

A very small percentage of the electrons are permanently vaporized from the filament and they contribute to reducing the vacuum, thus making the tube gassy. *vaporized tungsten is also gradually deposited on the inner surface of a glass envelope. --this causes old tubes to have a mirrored appearance and can eventually cause high-voltage arcing. *filament breakage

Rhenium

Atomic Number: 75 provides greater elasticity when the focal track expands rapidly due to the intense heat.

what are the main components of the X-ray tube?

Cathode Anode Envelope Tube Housing

The rotor

Located inside the stator and inside the envelope *it comprises a hollow copper cylinder or cuff that is attached to the anode disk by a Molybdenum shaft. *turns when the stator is energized*

rotating anode

Permit bombardant of a given area of the target for only 7-50 microseconds. The faster the anode rotates the better the heat dissipation. **Nearly all units designed for diagnostic radiography utilize this type of anode because of their greater efficiency.

as the actual focal-spot increases?

The effective focal-spot also increases.

Rotor button depressed=

activates stator=turns the rotor, spins anode

components of the anode assembly

anode, stator, and rotor

saturation current

another filament phenomenon that affects the efficiency of the X-ray tube. **as the kVp increases, a greater percentage of the thermionically emitted electrons are driven toward the anode.

Leakage radiation

any photons that escape from the housing except at the port

space charge effect

as more and more electrons build up in the area of the filament , their negative charges begin to oppose the emission of additional electrons. **limits the X-ray tubes to maximum mA ranges of 1,000-1,200

off-focus radiation may contribute?

as much as 25-30% of the total primary beam and is a special problem in digital radiographic units.

photons that are emitted towards 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 20% more photons at the cathode end of the tube & 25% fewer photons at the anode end. **a total variation of approximately 45% exists parallel ti the anode-cathode axis.

off focus radiation

can cause serious degradation of radiographic image quality. compromises photons (x-rays) that were not produced at the focal spot. Occurs when the high-voltage electrons striking the focal spot produce scattered electrons or photons **significantly lower energy than the primary beam itself

what happens when you fail to do the warm up procedure?

can cause the entire anode to crack if the molybdenum absorbs the heat too rapidly and exceeds its expansion capability.

cracked anode

can divert the electron and/or photon stream toward the envelope, which will crack the envelope permitting the implosion the vacuum which can suck the insulating oil into contact with the superheated anode assembly. ***causes vaporization of the oil in a violent explosion that can blow out the rubber expansion seals of the housing which causes hot oil to blow away from the tube housing. ***this is a severe hazard towards patients

graphite backed anodes

can double heat-loading capabilities without increasing bearing wear.

45% variation is significant enough to?

cause a visible difference in exposure during radiographic exams when large IR sizes are used at short distances.

In the X-ray tube, changing the angle of the target:

changes the effective focal spot

focal track heating capacity

circular path to interact with electron beam

inside the rotor

contains silver-plated steel ball bearings around a shaft that is anchored to the envelope.

high-voltage insulation and tube cooling

dielectric oil helps in insulation and tube cooling

Stress relieved anode

dissipates heat much more efficiently and does not require an elaborate tube-warm up procedure.

rotating anode heating capacity

dynamic=increased heat load

The anode heel affect is the reason why?

each radiographic table has a standard or natural head end.

Components of the cathode

filaments, focusing cup, and associated wiring

stationary anode

has become limited to low-power functions, such as those of dental units

what is the envelope made of?

heat resistant Pyrex glass or metal

Rating and cooling charts

help the radiographers avoid thermal damage to X-ray tubes. They are the most valuable b/c they provide a guide regarding the max technical factor combinations that can be used w/o overloading the tube. All charts plot milliamperage, kilovoltage, and time each filament of each tube has a unique tube rating chart

focusing cup

is a shallow depression in the cathode assembly designed to house the filament.

Anode

is the positive side of the X-ray tube and has three functions

anode target surface

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

envelope construction

it is constructed around both the cathode and anode assemblies and must be sealed tight to maintain a high vacuum.

what is pitting?

it reduces the efficiency of the tube; used to describe an older tubes focal tack.

Function of the focusing cup

its purpose is to narrow the thermionic cloud as it is driven toward the anode. *is provided with a low negative potential, because of its geometry, focuses the electrons toward one another in a convergence pattern.

A heat unit is calculated as

kVp x mA x time x rectification constant single phase =1 high frequency=1.40

dual filaments

known as dual-focus arrangement. 2 filament wires in focusing cup

protective housing is made out of what?

lead-lined cast steel a window to permit unrestricted exit for the useful photons from the envelope window.

biased focusing cup

maintains the cup at a more negative voltage than the filament *this causes the electron beam to be focused into a narrower stream as it heads toward the anode.

Tungsten

material of choice for the filament because of its high melting point and because it is difficult to vaporize. *the high melting point permits the filament to operate at the high temperatures required of an x-ray tube.

dual-focal spots

most X-ray tubes have this to include one for fine detail studies and the other for heavy tube loads.

two-step exposure switches

most tube manufactures recommend that these exposure switches be fully depressed in one motion. **all units have electronic interlocks that will not permit the exposure to occur until the rotor had brough the anode up to the proper speed**

what is the focusing cup composed of?

nickel

anode cooling charts

permit calculation of time necessary for anode to cool enough for additional exposures to be made **All cooling charts are calculated in terms of radiographic heat units

Gyroscopic effect

produced by the centrifugal force of the rotation of the rotor. this can cause trauma to the anode disk and bearing, causing destruction of the tube. *constant rough handling of this sort can cause undue wear on the rotor bearings

Vaporization

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

rotating anode disks

range from 5 to 13 cm in diameter and comprise molybdenum

Control of leakage radiation and scatter radiation

remaining photons are unwanted and the tube housing is designed to absorb them.

air fan

remove heat from the housing

stationary anodes are made of

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

Normal anode wear & tear will demonstrate what visually?

roughen or pit the target area

heat unit rectification constants

single phase = 1 high frequency = 1.40

What are 4 things we need to produce an x-ray?

source of electrons Target High Voltage vacuum

Anode warm up procedure

specified by the tube manufacturers and are designed to bring the anode heat from room temperature to near the range of operation. *This also permits the heat from the hot anode to serve as a vacuum pump to maintain a strong vacuum inside the envelope.

stationary anode heating capacity

static target areas= limited heat capacity

two types of anodes

stationary anodes rotating anodes

why is a strong vacuum important?

the anode warm up procedure permits the heat from the hot anode to serve as a vacuum pump, which keeps a strong vacuum. *this is why X-ray tubes should be warmed up regularly even when the unit is not being used for patients.

The cathode end should be positioned toward?

the denser (thicker) part of the body

another effect of stator or rotor failure?

the electron stream overheats the target area of the anode focal track. *when the temperature exceeds the melting point of tungsten, melting will occur. **melting tungsten that drips onto the envelope will Destroy the tube

The envelope

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

The tube anode is established at what end of the x-ray table?

the head of the table to utilize the anode heel effect to best advantage.

The Stator

the only part of the cathode or anode assemblies that is located outside the vacuum of the envelope. **works using electromagnetic induction motor

window

the point where the primary x-ray beam exits the envelope. **in some tubes this is simply a thinner section of the envelope to allow less absorption or scatter of the photons

anode target

the portion of the anode where the high-voltage electrons stream will impact. *precise point at which the x-ray photons are created **it is from this point that all tube-to-object and image-receptor distances are measured.

one primary main causes of premature tube failure?

the radiographers habit of holding the rotor switch prior to making exposures. *every second the rotor switch is depressed, life is removed from the filament. *delaying the exposures while the filament is enduring maximum current can shorten tube life by 50-60%

when switch labeled rotor is activated prior to exposure what happens?

the rotor begins to turn and also 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 the vaporization of the filament.

If the stator fails?

the rotor will cease to turn the anode, resulting in the immediate melting of a spot on the target because rotating anode targets are not designed to absorb the heat of a high-voltage exposure while stationary.

Effects of off focus radiation

the scattered electrons or photons have sufficient energy remaining that when they strike another object in the tube cathode assembly, vaporized metal on a glass envelope, they produce photons. These photons produced will exit through the tube window, housing port, and collimator causing ghosting of structures

the effective focal-spot size is controlled by?

the size of the actual focal spot (which is controlled by the length of the filament) and the anode target angle

what are other names for the anode target

the target, the focus, the focal point, the focal spot, or the focal track *most common is the target area

the envelope is commonly called?

the tube

anodes function is enhanced by:

the use of rhenium-alloyed tungsten as the target focal track material. -sometimes backed by graphite and molybdenum

Filament saturation current has been achieved when:

there are no further thermionic electrons to be driven toward the anode. **at this point an increase in kVp will not increase the tube mA.

primary function of the envelope?

to maintain the vacuum between the cathode and anode. The removal of the air permits electrons to flow from the cathode to anode without encountering the gas atoms of air and greatly increases the efficiency of the tubes operation.

function of a filament

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

3 types of rating charts and cooling curves

tube rating charts anode cooling charts housing cooling charts

ball bearings inside the rotor

use silver plating as a high temperature lubricant between the cuff and the anode shaft.

effective focal spot

used to describe the area of the focal spot that is projected out of the tube toward the object being radiographed *controlled by Actual focal spot & Target angle

actual focal spot

used to describe the physical area of the focal track that is impacted.

dielectric oil

used to fill the space between the envelope and the tube housing. insulates the high voltage components from the tube housing absorbs much of the heat that is produced by x-ray production. sometimes circulated through recirculation system (heat exchanger)

Line focus principal

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

focal track

used to represent the circular path that will be impacted by the electron beam.

This is accomplished by the line-focus principle

when the target angle is less than 45 degrees the effective focal spot is smaller than the actual focal spot.