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
what do we want to achieve when we produce radiographic images?
balance (1) patient radiation dose and (2) image quality
what are the properties of X-rays?
- invisible
- cannot be heard
- have no odor
- are not affected by electric or magnetic fields
what are X-rays commonly referred to? what are they able to do?
- commonly referred to as photons
- have the ability to ionize other substances (cause the atoms through which they pass to eject electrons from their electron shells)
what does the ability of X-rays to ionize other substances account for?
- their imaging properties
- their potential harmful effects
what are the different ways X-rays can be detected?
- by their ability to ionize other substances
- by causing fluorescence
- by giving rise to color changes in several substances
(barium platinocyanide) - producing changes which can be made visible in photographic film
what are the two different types of anodes?
rotating anode
- accounts for MOST X-ray equipment, used in all major diagnostic radiology equipment
- higher kV and mass
stationary anode
- used for low current examinations (ex: dental equipment)
what are the two components of an X-ray tube?
(1) the insert - which is evacuated and is where all the X-rays are produced
(2) the tube shield - which supports the insert and is responsible for electrical and radiation safety
what are the functions of the X-ray tube?
(1) provide a beam of X-rays from as near a point source as possible (focus)
(2) dissipate the heat produced effectively to prevent damage to the components (ex: anode melting)
(3) provide a consistent quality (kVp) and quantity (mAs) of radiation
(4) allow X-rays to emerge only from the port of the tube housing
(5) provide an electrically safe environment for the practitioner
(6) the tube is securely supported, but capable of easy movement into any position and then being maintained in that position
explain what is meant by quality and quantity of radiation
quality = energy of photons
quantity = number of photons
numerous materials are used in the construction of the X-ray tube components and tube shield; what are they? SOS
- tube housing: steel lined with lead (except port)
- port: plastic or beryllium
- insulation between housing and insert: mineral oil
- insert (tube envelope): metal/ceramics or borosilicate glass
- filament (cathode) assembly /focusing cup: nickel or stainless steel
- filament: tungsten
- anode disc: molybdenum alloy, graphite disc, tungsten (90%) and rhenium (10%) alloy to prevent crazing effect
- anode stem: molybdenum
- stator windings: copper
- additional filtration: aluminum and copper
why is tungsten most suitable for the construction of the filament and anode?
tungsten has a high atomic mass, therefore a high melting point, and can withstand high temperatures
(i.e thermionic emission)
what occurs at the port?
the exit of X-rays
what does the “+” sign on the tube housing indicate?
it indicates the location of the anode (where the production of X-rays takes place)
what is the focal track?
a region in the anode where the electron stream strikes and heats the anode
what is the insert responsible for?
maintaining a vacuum for X-ray production
why would the metal component within the insert need to be earthed?
so that there is no buildup of static
describe the placement of cathode and anode assemblies in the insert
- the cathode and anode assemblies are fixed within the envelope
- the envelope supports these two electrodes in correct alignment at the correct distance
what reduces the risk of damage to the insert during operation?
all seals and poles are carefully chosen to match the expansion coefficients (heat) of the different parts
what does the anode assembly consist of?
- anode disc and focal track
- anode stem
- rotor assembly
- HT connection for the positive side of the tube circuit
what does the rotating anode consist of?
a tungsten rhenium disc which is typically 90-150 mm in diameter
what does a larger size of the rotating disc affect?
- it increases tube rating
- increases thermal capacity
if the rotating disc was a composite disc, what would it be made of?
- a tungsten/rhenium focal track
- a graphite or molybdenum backing
what is the purpose of the anode disc having a bevelled edge?
- it forms the (1) anode angle and the (2) focal track
- this is done to decrease the size of the apparent focal spot
typical angles for rotating X-ray tubes are between ___ and ___
16˚ and 20˚
what does the angle of the anode determine?
it defines the size of the apparent focal spot
to obtain a radiograph of a smaller area, we use an anode with what angle?
a smaller angle
what is the purpose of the rotation of the anode?
- to dissipate heat over a larger area
- increases the tube current to increase the intensity of X-rays
what is the purpose of the molybdenum stem that is attached to the anode?
it connects the anode to the rotors
describe the anode (molybdenum) stem
- it has a small cross-section and is as long as possible (this is done to restrict the conduction of heat to the bearing assembly)
- the stem is connected to the copper rotor assembly
the rotors use induction to rotate the anode at speeds of ____ - ____ during exposures
3000 - 9000 r.p.m
the cathode has two filaments:
- fine filament
- broad filament
what is the function of the focusing cup?
it encloses the two filaments (fine and broad)
what does the cathode assembly consist of?
- the filaments (fine and broad)
- focusing cup, which is negatively charged
- electrical supply and connections:
- filament supply
- HT supply to the negative side of the X-ray tube
which filament do we use if we need to obtain a radiograph with a larger area?
the broad filament
what does a large actual focal spot and what does a small actual focal spot produce respectively?
- large effective focal spot
- small effective focal spot
what is the purpose of the cathode?
- to produce thermionic emission of electrons which can be focused on and attracted to the anode
how is thermionic emission achieved?
- by supplying a variable mA which heats the filament
- increasing the mA increases the temperature in the filament
- this increases the number of electrons in the electron cloud
what is the relationship between mA (current) and number of X-rays produced?
- mA is directly proportional to the number of X-rays produced
- CURRENT DEFINES NUMBER OF ELECTRONS
- NUMBER OF ELECTRONS DEFINES NUMBER OF PHOTONS (intensity)
how is the focusing of the electrons onto the focal track of the anode achieved?
- the cathode filament is encased in a nickel housing (focusing cup)
- the focusing cup has sharp edges which become negatively charged when the tube voltage is applied
- the sharp edge concentrates the negative charge
- this narrows the electron beam directed towards the anode (which focuses them onto the focal track)
