Radiation Physics Flashcards
***for this lecture, know structure and function. Don’t just memorize
: )
part that surrounds the other parts, creates vacuum for x-ray production
glass tube
cathode ____ electrons, and has a ____ charge
generates, negative
anode ____ electrons and ______
receives, produces x-rays
Tungsten filament is connected to a _____ circuit
low-voltage
negatively charged concave reflector is a…
focusing cup
site of x-ray production, where is it? what is its charge?
focal spot
located in anode, positively charged
focal spot attracts _____
electron cloud from cathode
ideal target should have. Tungsten satisfies all except which?
- high Z#
- high melting points
- high thermal conductivity
- low vapor pressure
Tungsten does not satisfy thermal conductivity
how do we address Tungsten’s poor conductivity of heat?
copper block and cooling oils in glass tube
focal should be large to assist in _____
focal spot should be small to increase ____
what’s the solution?
heat dissipation, image sharpness
solution: line focus principle
line focus principle practicals
putting focal spot at an line
importance of glass tube vacuum
no gas to prevent collision of e- to the gas molecules (which impairs x-ray production)
what type of systems do dental x-ray systems use?
stationary (non-rotating)
what are the features of a rotating anode?
- beveled anode disk rotates while tube is operating
- e- strike successive small areas of target (small focal spot)
- distributes heat over larger areas d/t actual focal spot
two big concerns in anode/focal spot
heat dissipation, image sharpness
image sharpness in rotating anode: e- strike ________
successive small areas of target
heat dissipation in rotating anode: actual focal spot is the _______ which helps to distribute heat over ____
circumference of the disk, larger areas
low voltage circuit powers ____; high voltage powers ___
filament, cathode to anode
purpose of low voltage circuit
heat filament to cause thermionic emission
_____ increases or decreases resistance in circuit and changes the current in the filament
variable resistor
increase mA, _____ electrons sent to anode
increase
e- from heated filament to the target ___ the x-ray tube constitute mA current
within
increase mA for a given exposure time, ____ receptor exposure
increase
increase kVp, ______
increase mean energy of polychromatic x-ray beam
purpose of high voltage circuit
create high potential difference between cathode/filament and anode/target
increasing kVp will ultimately influence ____
image quality
as kVp increases, _______
- velocity of e- increases
- photon energy increases
- # of photons increases
as photon energy (quality, penetrability) increases…
- fewer photons interact in patient
- radiographic contrast decreases
of photons increases, ______
receptor exposure increases
item used to change kVp that compensates for variations in incoming line (voltage stabilizer)
autotransformer
measure of total x-ray beam exposure to the sensor/detector/recpetor
receptor exposure
how is receptor exposure controlled?
- kVp
- mA
- exposure time (seconds)
- other factors to be discussed later…..
kVp controls _______ and _____
beam quality (image contrast), beam quantity (exposure)
mA controls _____
beam quantity (exposure)
exposure time is measured in ____
pulses
1 pulse equals…
1/60th of a second
amount of radiation per second (mA)
exposure rate
duration of radiation production (seconds)
exposure time
overall measure of exposure (assuming kVp is fixed)
milliamp-seconds (mAs)
for constant receptor exposure, exposure time and mA are _____
inversely related
mAs is a product of _____
mA and exposure time (in sec)
for constant receptor exposure, mAs must be the ___
same
when exposure time is increased, mA _______
must be decreased and vice versa
if 10mA x 1 sec = 10mAs, then 5mA x ___ = 10mAs
which would be most ideal?
2 seconds
the lower time exposure (less time for exposure)
if sufficient cooling time is not allowed, damage to ____ can occur
target or filament
x-ray production is inefficient, as seen by…
1% x-ray production, 99% heat production
2 guidelines for cooling time
- duty cycle
2. tube rating (heat capacity)
cooling time applies when one is anticipiating a single, long exposure
tube rating
cooling time that applies when several minor exposures are made in a clinical setting
duty cycle
radiation that originates at focal spot, leaves tube through window and is useful in image formation
primary (usable beam)
radiation that originates at focal spot, leaves the tube through barriers around the tube
leakage radiation
radiation that originates in tissues that causes image noise
secondary/scatter radiation
what helps minimize leakage radiation?
glass, oil, metal enclosures
