Xray Machine Flashcards
Overall principle of an xray machine
Accelerate electrons towards a piece of metal in a vacuum
Electrons interact w metal = xrays
Function of glass tube in xray
creates vacuum for xray production
Cathode function in xray
generates electrons
anode function in xray
receives electrons and produces xrays
What is the power supply in xray machines
high/low voltage circuits
What are the components of a cathode
tungsten filament
focusing cup
Tungsten filament in cathode
connected to a low voltage circuit
Focusing cup in cathode
Negatively charged concave reflector
Functiono of tungsten filament in cathode
When heated - emits electron cloud
Source of electrons that will be sent to anode to produce xrays
Focusing cup in cathode function
Focusing cup electrostatically focuses the electron beam to the focal spot on anode
Electrons are repelled by the negatively charged focusing cup
Descibe anode in xray
Copper block
Tungsten focal spot = target
+ charged to attract electrons
Path from cathode to anode is the high voltage curcuit
What is the path from cathode to anode
high voltage circuit
Focal spot (anode)
Site of xray production
Small rectangular area in anode
+ charged
Attracts electron cloud from cathode
What 4 things should an ideal target (tungsten focal spot) have?
- High Z#
- High melting point
- High thermal conductivity
- Low vapor pressure at high temperature
Why is it advantageous to have a high Z# for an ideal target?
Promotes maximum conversion of electron energy to xray photon energy
Tungsten satisfies all requirements of an ideal target except what? How is this addressed?
Tungsten is a poor heat conductor
Solved by copper block and cooling oil to assist in heat dissipation
Why do we want a low vapor pressure at high temperature for an ideal xray target?
Can otherwise damage target
Focal spot should be _____ to assist in heat dissipation
large
Focal spot should be _____ to increase image sharpness
small
What are the opposing issues regarding focal spot size?
Should be large to assist in heat dissipation
Should be small to increase image sharpness
SOLVED BY LINE FOCUS PRINCIPLE
Line focus principle
Makes possible a large actual focal spot & “projected”/”effective” focal spot of small size
Placement of target at an angle
= better heat dissipation
= increased image sharpness
Copper block of anode
Good thermal conductor
Removes heat from tungsten target
Reduces risk of melting of target
Function of glass tube of xray machine
Vacuum tube
No gas to prevent collision of the electrons to the gas molecules which impair xray production
Function of cooling oil in xray
dissipates heat
Rotating anode
Tungsten target is a beveled disk that rotates when tube is operating
Electrons strike successive small areas of target = acheives obj of having small focal spot
Actual focal spot is the circumference of the disk = helps distribute heat over large areas
DISADVANTAGE - $$$$$
Most dental systems use what kind of anodes?
stationary
bc rotating is $$$$$
Purpose of low voltage circuit
Heat filament to cause thermionic emission
How does a low voltage circuit work
Variable resistor increases/decreases resistance in circuit and changes current in filament
Controls # electrons produced at filament
mA control of xray machine actually adjusts this current
Increase mA = increase electrons sent to anode
If you increase mA in a low voltage circuit, what is the result?
Increase electrons sent to anode
Milliampere current (mA)
Electrons from heated filament to the target WITHIN the xray tube
Controls # xray photons
Increase mA for a given exposure time = increase receptor exposure
Increase mA for a given exposure time, _____ receptor exposure
increase
High voltage circuit
Creates high potential difference between cathode/filament and anode/target
Gives high velocity to filament electrons
Voltage varies between 50 - 100 kVp
An increase in kVp results in a _____ in mean energy of polychomatic xray beam - ultimately influences ______
increase
image quality
as kVP increases, velocity of electrons (energy) ____
increases
As kVp increases, photon energy (quality, penetrability) ______
increases
An increase in photon energy (quality, penetrabiity) results in…
Fewer photons interact in pt
Radiographic contrast decreases
As kVp increases, # photons _____
increases
An increase in the # of photons results in…
Increase in receptor exposure
Autotransformer
Changes kVp
Located in control panel
Compensates for variations in incoming line voltage = VOLTAGE STABILIZER
Receptor exposure
Measure of total xray beam exposure to the sensor
Must be enough to properly penetrate/image pt with acceptable noise levels
Must not be too much to over expose the pt (unnecessary dose) or receptor (poor image quality)
The receptor exposure must be enough to…
properly penetrate/image pt with acceptable noise levels
the receptor exposure must not be too much to…
over expose pt - unnecessary dose
over expose receptor - poor image quality
How is receptor exposure controlled
kVp - quality, quantity
mA - quantity
Exposure time
Collimation, filtration, SID
kVP controls…
receptor exposure
beam quality - image contrast
beam quantity - exposure
For intraoral dental imaging, kVp is often set at…
60-70 kVp
mA controls…
receptor exposure
beam quantity - exposure
Reported on xray tube control head
How is exposure time measured
Seconds
Measured in pulses on older xray tubes
Related to 60 cycle alternating current (AC) incoming power supply
1 pulse - 1/60 sec
Newer machines report exposure time directly (ms)
