physics of radiography Flashcards
properties of x-rays
invisible
painless
latent effect
cumulative
the tube head
the functional part of the x-ray machine, where x-rays are produced is called the tube head
genetic effects of radiation
when ionising radiation acts upon germ cells, it may cause damage to the genetic material, which can lead to genetically induced diseases
e.g. malformations
metabolic disorders
immune deficiencies
somatic effects of radiation
direct changes to tissues and can occur straight after exposure
e.g. burns, cataracts, hair loss, GIT upsets
carcinogenic effects
changes in the tissues can be malignant and develop into diseases such as leukaemia
What is an X-ray
x-rays have a shorter wavelength than visible light - this is associated with a higher frequency
x-rays and Gamma rays are both associated with damaging genetic and somatic effects
production of xrays
mains electrical current transformed into a high voltage current
converted into x-rays
thermionic emission
xrays are released out of the window area of tube head
leave machine from primary beam
primary beam is indicated by the light beam diaphragm
during thermionic emission - 1% x rays 99% Heat
How is an x-ray produced?
-Filament
-mA applied
- Electrons produced
- kV applied between cathode and anode
- Electrons accelerated across tube
- Electrons hit target
- Heat and X-rays produced
X rays ACP
Anode - Positive - typically made of tungsten
cathode - negative - typically made of tungsten
Panic - positive anode, negative cathode
X-ray Settings: kV and mAs
the intensity and quality of the beam are controlled by the millamperage mA, time(s) and the kilovoltage (kV)
KIlovoltage
if too high the degree of attenuation by different tissues will be less noticeable.
x-ray will appear very dark, with a loss of contrast
Factors affecting kV include
species
weight
tissue density
tissue atomic number
Milliamperage
number of electrons produced at the cathode
Greater mA = greater number of electrons produced and greater intensity of the x-ray beam
Higher mA - the blacker the film (overexposed)
Lower mA - the lighter the film (underexposed)
Time (s)
Longer the patient is exposed to the primary beam - more photons will be released from the x -ray machine and thus the more photon - tissue interactions there will be
longer the exposure time the blacker the film
exposure time is measured in seconds
Film Focal Distance
The greater the distance the more the beam spreads out.
rule governing this is inverse square law
usually kept constant between 75cm and 100cm
X Ray Movement
- pass straight through the patient to the film
- Be absorbed to differing extents by the tissues.
- be reflected off the animal/surrounding objects as scatter.
