Ch.9 Flashcards
What are the three essentials conditions for the production of x-rays?
- Must be a source of free electrons
- Must be a means of accelerating those electrons to extreme speeds
- Must be a means of decelerating the electrons.
The source of free electrons is a _____ wire heated sufficiently to produce thermionic emission.
Filament
What is a minimum filament temperature?
3700 F
The actual temperature of the filament above 3700, hence the rate of thermionic emission, is predetermined by what?
mA station selected
The ____ ____ constitutes electrons that are free to move across the X-ray tube to the anode.
Space cloud/electron cloud
Some electrons fall back into the filament but are replaced by other electrons jumping out such that a constant number of electrons hover within the cloud. This state of equilibrium is called what?
Space charge effect
The number of electrons is predetermined by setting the ____ station.
mA
In order to suddenly accelerate the electrons of the space charge in a direction toward the anode, extremely high voltage _______ force is applied to the same filament.
Electromotive (EMF)
Electrons can accelerate to more than one half the ______ in just one inch of travel before reaching the anode disc.
Speed of light
The anode disc, made of metals with very high atomic numbers, provides a means of precipitously _______ these projectile electrons as the smash into it.
Decelerating
By the law of conservation of energy, the energy lost by sudden slowing down of the electrons cannot disappear but must be converted into another form. It is converted into what forms?
Electromagnetic radiation waves, including infrared, visible light, ultraviolet waves and x-rays.
How much of the radiation emitted from an exposure is useful diagnostic X-ray?
.5%
The other 99.5% emitted from the X-ray tube and its housing is in what form of energy?
Heat including infrared radiation, and as visible light.
The X-ray tube is a diode tube meaning what?
That is has two charged electrodes.
In electrical circuits, a negatively charged electrode is called a ______.
Cathode
In an electrical circuit, a positively charged electrode is called an _____.
Anode
The focusing cup of the cathode is usually made up of what material?
Molybdenum or nickel
The filaments in the cathode are embedded in _____ carved into the focusing cup.
Pitts
What is a dual focus tube?
Provide two filaments to choose from with the larger one ranging from 1.5-2 times the length of the smaller one.
What is actually being selected when you choose a large or small focusing spot?
The large or small filament.
Because negatively charged electrons repel each other the beam of electrons traveling from the filament to the anode tend to spread out, therefore within its pit each filament is surrounded with negative charge. What effect does this create?
This has the effect of pushing the electrons back toward the middle of the beam as they leave the filament, narrowing and constricting the beam.
By the time the beam reaches the anode is becomes smaller than the ____ itself.
Filament
The area on the anode where the electrons strike is called what?
The focal spot.
Within its pit each filament is thereby surrounded with _____ charge. This has the effect of pushing the electrons back toward the _____ of the beam as they leave the filament, narrowing and constricting
Negative, middle
Typically the electron beam as it strikes the focal spot is about _____ the size of the filament from which the electron beam originated.
1/20th
What is the size of the small focal spot and the large focal spot on standard diagnostic tubes?
Small- .5-.6 mm
Large- 1.0-1.2mm
What are the smallest focal spots that can be engineered?
.1 mm
If the filament is embedded deeply enough into its ___, and the focusing cup is given a strong _____ charge, then the electrons can actually be withheld from leaving the vicinity of the filament by oblique repulsion.
Pits, negative
The focusing cup can act as a ______ for very short exposures.
Switch
When a rapid sequence of very short exposures is needed a negatively-charged wire mesh or grid can actually be placed in front of the filaments to hold back the electron beam until those precise times when its needed. These types of X-ray tubes are referred to as what?
Grid controlled tubes
Each filament is made from what?
Thorium impregnated tungsten.
Why is tungsten used in the X-ray tube?
Because of its high melting point of 6200 F and it is a good conductor of electricity.
What benefits the filament by adding a small percentage of thorium to it?
The efficiency of thermionic emission and long life of the filament are improved.
The filament has two wires going into it from above but only ____ wire leaving it at the bottom.
One
The ______ current has the sole purpose of heating the filament up to the temperature corresponding to the ___ station selected. This current flows into and back ____ of the filament accounting for one wire on each end of the filament.
Filament, mA, out
The second wire entering the filament carries the tubes ______.
High voltage current
What is the job of the high voltage current generated by the main circuit?
To kick the electrons boiled off in the space charge across the tube to the anode with an extra force.
What carries the energy of the high voltage current across the tube?
The electron beam
Why is a second wire not seen exiting the filament?
Because the anode is connected to the positive side of the high voltage circuit by a wire. As the electrons strike the anode, they feel the positive charge and move down the anode shank and out a wire on the anode end of the X-ray tube.
____ ____ between the cylinder and the shaft allow for smooth rotation of the rotor.
Ball bearings
The _____ track is a ring of tungsten-rhenium alloy embedded within the molybdenum disc near the outer perimeter of the anode disc.
Focal
Why is rhenium added to the focal track?
Because of its added mechanical stability for high speed rotation.
What is tungsten a atomic number? Rhenium?
74, 75
Why are tungsten and rhenium ideal metals for the stopping and catching of the high speed electrons?
Because their atoms are crowded with electrons.
What is electron density?
The number of electrons per cubic nanometer.
What happens to electrons passing through a tungsten or rhenium atoms space in regards to electron density?
The electron has a high probability of colliding with an orbital electron due to the high atomic number of electrons within the space of their atoms.
Any process which slows or stops the projectile electron can produce radiation due to the loss of what type of energy?
Kinetic energy
With the shaft that supports the anode disk made of molybdenum, it forces the heat from the focal track to be dissipated by radiation into what?
The glass housing and surrounding oil.
By spinning the anode disc the electrons do not strike the same spot which would cause what to happen to the material?
It would melt.
T/F In relation to the patient and the image receptor, the focal spot never changes its position.
True
T/F In relation to the spinning anode the focal spot is constantly moving.
True
A standard X-ray tube spins the anode at how many rpms? A high speed rotor?
3400rpm, 10,000rpm
What type of motor is used to rotate the anode?
Induction
What is the outer portion of the shank made of?
Copper
The outer portion of the shank acts as the ____ of the induction motor.
Rotor
The stator consists of a series of ____ which are fired in sequence so that the magnetic field around them rotates.
Coils
The induced electrical current ____ the cylinder. By magnetic repulsion the cylinder is then pushed by the circulating magnetic field of the stator causing the cylinder to rotate.
Magnetizes
After an exposure is taken one can hear the ____ sound from the X-ray tube as the rotor coasts to a stop.
Shushing
The bearings are so perfectly smooth that slowing down the rotor would take many minutes, but the induction motor is automatically run in ____ to create a braking effect.
Reverse
Each time the rotor switch is depressed at the console, the induction motor is energized and must bring the spinning anode to its full____ before the tube current is allowed to strike it.
Speed
The exposure ready light comes on when what two things happen?
The filament and the anode disc are prepared
When the small focal spot is selected on the X-ray machine it automatically locks out what?
High mA stations
Why are higher mA stations locked out if you choose the small focal spot?
Because of the limitations in the heat capacity of the anode
A large amount of current focused onto a small enough focal spot can ____ the anode material.
Melt
The advantage to using a small focal spot is that it provides for much better ____ of the image.
Sharpness
Most procedures requiring a maximum sharpness are for what?
Extremities
Extremities are usually exposed with the mA at ___or less.
200
The advertised focal spot it called what?
The nominal focal spot
Because electrons repel each other, as more electrons are packed into the space charge cloud produced around the filament, it tends to expand in size, this is an effect called?
Focal spot blooming
Blooming occurs at higher mA stations because more ____ ____ is occurring and there are more electrons in the space charge.
Thermionic emission
A swollen space charge around the filament also means a larger ___ ___ at the anode when the electron beam reaches it.
Focal spot
The actual shape of the focal spot is close to the letter ___ with the electrons distribution concentrated at the sidebars.
H
Why must the entire anode and cathode be placed in a vacuum?
Any other gases or air in the tube can impede the flow of projectile electrons from the filament to the anode target.
The glass of the X-ray tube is thinnest where?
The window where the effective focal spot is directed.
