2.1/2.2 X-RAYS I & II Flashcards
explain why X-rays can pass through tissues in the human body?
X-rays have short wavelengths that, unlike visible light, can pass right through tissues in the human body
explain why we can see bones clearer than tissues/ muscles on X-ray scans?
bones contain calcium, which is denser than most other tissues, so it stops X-ray penetration, which is why bones are clear while tissues/muscles form shadows on X-ray scans
when are X-rays produced?
when highly energetic electrons interact with matter
what does X-ray production allow for?
the conversion of some of the electrons’ kinetic energy into electromagnetic radiation
how much of electron energy is lost and how much is converted into heat (useful x-rays)?
99% is lost
1% is converted into useful x-rays
what does a device that produces X-rays in the diagnostic energy range contain?
- an electron source
- an evacuated path for electron acceleration
- a target electrode
[x-ray tube] - an external power source to provide a high voltage to accelerate the electrons
[X-ray generator]
what is the tube envelope (insert) made of? why?
- the tube envelope is made of glass
- this is in order to achieve vacuum inside the x-ray tube
why do we want to achieve a vacuum inside an x-ray tube?
- because light travels faster in vacuum
- there are no interactions between electrons & gases
where are the cathode and anode located?
in the tube insert (tube envelope)
what occurs at the filament current supply (cathode)?
where thermionic emission takes place
what is thermionic emission? how is it achieved?
- it is the emission of electrons from a heated metal (cathode)
[formation of an electron cloud] - thermionic emission is achieved by changing the current
what is the tube voltage supply connected to respectively?
- the negative output is connected to filament (cathode)
- the positive output is connected to the anode
what will happen if there is no vacuum inside the x-ray tube?
the electrons will collide with molecules of the air, and lose energy
describe electron flow inside an X-ray tube
- thermionic emission from the cathode releases the electrons, which are then accelerated towards the anode
- when electrons hit the anode, they decelerate as they collide with the anode metal (tungsten), losing their energy
- the energy lost by electrons is released as an x-ray
what charge does the anode have and why?
the anode is positively charged (unlike the cathode) in order to attract the electrons
what prevents the anode from melting during the energy release (as x-ray beams)?
- it continuously rotates, so the hot spot moves away from the electron beam
- it then cools down before it gets back in position in the beam of electrons
what does the x-ray tube insert contain?
(1) the electron source
(2) the target (within an evacuated glass or metal envelope)
what does the tube housing provide?
- protective radiation shielding
- cooling of the X-ray tube insert
what are X-ray beam filters?
a material placed in front of an X-ray source in order to reduce the intensity of particular wavelengths from its spectrum
(shape the X-ray energy spectrum)
what are the functions of X-ray beam filters?
(1) selectively filter out low energy X-rays
(2) reduce the patient radiation dose
(3) optimize image quality
what are X-ray filters made of?
aluminum / copper
why do low-energy photons need be removed/filtered out?
- X-rays are not mono-energetic, and have several energies
- low-energy photons do not have enough energy to exit the patient’s body; they will be absorbed and therefore increase patient radiation dose
what are collimators? what are some of its benefits?
- a device that defines the SIZE and SHAPE of the X-ray field
(1) decreases photons being absorbed into the patient’s body
(2) produces higher image quality
what does the X-ray generator do?
- supplies the voltage to accelerate the electrons
- permits control of the X-ray beam characteristics
X-ray beam filters, collimators, and an X-ray generator work together to create?
a beam of X-ray photons
what are the features of photons produced in a beam of X-ray photons?
(1) well defined intensity
(2) penetrability
(3) spatial distribution