X-ray Tube Flashcards
what are the parts of a dental X-ray tube
Control Panel Extension Arm Tubehead PID (the tube) BAsic parts: tube head and control pannel
what does PID stand for
position indicating device
what does the tube head contain
X ray tube and Position indicating device
how is the tubehead suspended
from a flexible extension arm
why is the extension arm hollow
enables wires to pass through
what is found on the control panel of a dental radiographic machine
On/off switch
exposure botton with indicator light and audible signal
expose time, KvP and mA selection
where is the control panel mounded
Wall or portable control box
what does the control panel do
turs on machine and adjusts settings (Kilovolts, Milliampers, exposure time)
what does federal laq require the exposure button be connected to
Indicator light and audible signal
what happens when the exposure button is depressed
Indicator light and sound is activited
what type of switch is the exposure button
Dead man type(switch works only with continuous pressure)
how long do you hold the exposure button down
Until the sound is over and the light is off
what are the 4 conditions for the production of X-rays
- Separation of electrons from tungsten filament at cathode
- production of high speed electrons to ensure enought KE needed for the production of X-rays
- focusing of electrons on a small region on the anode
- sudden stopage of electrons creating x-ray photons
why do electrons repel each other
negative charge
where are electrons focuesed
On the focal spot of the anode
what does the anode do to the electrons
stops and slows electrons
what types of interactions form when an electron slows
Bremsstrahlung
Characteristic
how are electrons originally separated
By the formation of an electron cloud by heating of tungsten filament at low voltage
is the anode or cathode the source of electrons
Cathode
what is the cathode made of
tungsten
what voltage is used to heat the cathode
6-12 volts
what is the process of using heat to produce electrons
Thermionic emission
what provides the necessary current to the tungsten filament
Step down transform
why is tungsten used in the cathode
High Z number
what is the voltage difference between cathode and anode
1000 times higher than 110 volt wall outlet
65000-90000 volts
what provides the necssary voltage between the cathode and anode
step up transformed
how fast are electrons accelerated
.5 times the speed of light
what doe the High Voltage difference do to the electrons
Lead to high Kinetic energy and high speed
what is used to focus the electons
A Molybdenum focusing cup at the cathode
what is the target made of for the electron beam
Tungsten(anode)
copper stem
why is tungsten used as the Anode
High Atomic number
High melting point
Low VP
efficiency of X-ray production
99% of electron energy transformed as heat, 1% converted to X-rays
why is the tungsten target embeeded in a copper stem
Need a good thermal conductor to dissipate heat
what fills the X-ray tube
Nothing, it is a vaccume
why does a metal housing surround the glass X-ray tube
Protect tube from accidental damage
prevents overheating by giving a potential space to fill with oil
what determines the current of the X-ray tube
By the Low voltage side
High current X-ray tubes do what to Xrays
create lots of X-rays
High voltage difference does what X-rays
X-rays are higher energy
problem with placing metal in a vaccum
Metal must not vaporize(good cuz tungsten has low vapor pressure)
How “pure” is the beam of X-rays leaving the tube
Different wavelengths with Different enegy
what does filtration of X-rays do
preferentially removes low energy X-ray photons from beam, reducing dose to patient
what is inherent filtration
The glass covering and oil in the tube that stops low energy X-rays
What is total filtration
Aluminum and inherent Filtration
what is the Aluminum equivalent
The thickness of aluminum (in mm) which gives the same filtration as the inherent filtering material (glass and oil) of x-ray tubehead under same conditions
how much total filtration must be used if 50-70KVp
1.5mm total (inherent +AI)
How much total filtration must be used above 70KVp
2.5mm total(inherent +Al)
why is the tube placed in oil
Insulationa nad heat
What does filtration do to exposure time and skin dose
INcreases exposure time up to 50% to account for loss inintensity
reduces patient skin exposure as much as 80%
what do you do if you use an Xray machine below 60KVP
don’t use it, need to overexpose patient to produce an acceptable radiograph
what are the facotrs that control the X-ray beam
Tube Voltage Exposure time Tube current Filtration Collimation Distance
what does A high amount of voltage do number and energy of photons
More photons(higher quantity( and higher energy(higher quality)
what does KVp control in an electron
Control the speed (KE) of each electron) leading to higher energy photons and X-rays
what is the Rule of Thumb for KVP
an increase in 15% in KvP should be followed by a reduction in 1/2 mAs
what happens to the expsore timeif you increase by 15kVP to get the same density
1/2 exposure time
what happens to the exposure time if you decrease by 15kVP to get the same density
double exposure time
why does low KVP lead to less dense images
longer wavelengths and lower energy so less penetration
what does Increaseing exposure time do
INCresaes the amount of x rays produced
what does an increase in current ddo to the x-rays
Increases the quantiy of x-rays produced
what one factor determines the total number of x-ray photons produced in the beam
miliamp seconds
what should remain constant for a particular exposure
miliamp seconds(increase time, decrease miliamps
what does Collimation do
Minimizes the amount of radiation to patients and reduces scatter, protecteing patient from unnecessary radition
what is the most effective beam limiting device
Collimator
do all X-ray tubes have collimators
Yes, made of lead
what does Federal law require that a collimator should restrict a beam to
no more than 7cm(2.75 inches) at the skin surface of the patient when making an intraoral dental radiograph
what is the relationship between intesnity of radiation and distance
Inverse square(intensity decreases inversly as the square of the source film distance)
