X-ray tube Flashcards
how are x-rays produced in the x-ray tube
by converting electrical energy into electromagnetic radiation
what are the two main steps when producing an x-ray
achieved by
1) accelerating electrons from an electrically negative cathode towards a positive ‘target’ anode
2) when electrons collide with the anode they are rapidly decelerated
what happens, and in what percentages, when the electrons are decelerated
99% of their kinetic energy is converted in heat…a major problem
1% of their kinetic energy is converted into x-rays photons, which we need for x-ray imaging
purpose of an x-ray tube assembly (4 points)
1) to produce a useful beam of x-ray photons from a very small source
2) to cope with the heat generated during the process of x-ray production
3) to allow control of the quality and quantity of the radiation
4) the radiation should only emerge from the window or port of the housing of the x-ray tube
an x-ray tube should be: (5 points)
1) electrically safe and provide protection from unwanted x-radiation
2) securely supported
3) capable of easy movement into any position and maintain that position
4) capable of precise angulation
5) easily cleaned and maintained
definition of radiolucent
describes a structure that allows x-ray photons to pass through it with relative ease
definition of radiopaque
a structure that offers more resistance to the passage of x-rays through it
what two features does the x ray tube housing have to protect it from damage
1) filled with oil
2) a relatively large exterior surface
helps to dissipate the heat created within the tube
what happens as the tube temperature increases
the oil expands. a flexible diaphragm within the housing moves in and out to allow the oil to safely expand and contract.
what happens if the oil expands too much in the x ray tube housing
the temperature within the housing is dangerously high. a microswitch is triggered and all further exposures are prevented until the environment within the housing cools
what is the x ray tube housed in
a protective steel, lead lined housing
what is the function of the lead lined housing
acts as a shield and absorbs stray X-radiation
what happens to the useful x ray beams
pass through a small radiolucent window/port in the tube housing
what does the light beam diaphragm do
allows adjustment of the field size (collimation)
what is collimation
allows the radiographer to adjust the size and shape of the x ray beam for each patient
how does the light beam diaphragm work
uses a mirror and lamp system to illuminate the area of the patient which will be irradiated by x-rays, but it does this before the x-ray exposure commences
what two things are achieved by limiting the area of the x ray beam on the patient
1) reduces the dose to the patient
2) reduces scattered radiation and so improves image quality
before an exposure is made, the filament is heated and it glows, producing electrons by…
thermionic emission
what happens to the anode and electrons during the x-ray exposure
the anode is made electrically very positive with respect to the cathode. The negatively charged electrons are attracted by the now positively charged anode
when is the filament (cathode) heated
during ‘prep’ mode before an actual x-ray exposure is made by an electric current (12 volts, 4 amps)
what controls the filament temperature
the tube current (mA) selector
what is thermionic emission
when the heat of the glowing filament transfers energy to some of the electrons within the filament. the electrons are able to free themselves and leave the wire at its surface
what does thermionic emission create
a ‘cloud’ of electrons that are held around the cathode in readiness to be attracted to the anode during the x ray exposure
how many coal points do most diagnostic x ray tubes have
two separate filaments of different sizes i.e. are dual focus.
each filament has its own focusing cup. the electrons can be produced from a large ‘broad’ focus or a small ‘fine’ focus
what is the benefit of having a small focal spot size
produces sharp images (very important in diagnostic radiography)
what is the problem with using a fine focus
potential overheating of the x-ray tube
the higher the x-ray exposure factors we use the…
more heat produced by the x-ray tube. Heat units produced in a given exposure= kV x mA x sec x 1.4
the larger the source of x-rays the…
greater the unsharpness or blurring of the image
when x-ray images are produced there is always a penumbra. what does this mean
that all images have inherent unsharpness and x-rays cannot be produced from a point source and cannot be focused like visible light
what 3 things cause penumbra of images
1) focal spot size
2) source to detector distance (SDD)
3) Object to detector distance (ODD)
what does the line focus principle aim to do (2 things)
1) having the x-rays generated from as large an area of the target as possible to cope with the heat produced
2) having the x-rays appear to come from as small an area as possible to produce a tolerable level of unsharpness in the image
what happens if the anode is stationary during exposure
all electrons from filament fired at the same area. leads to a dangerous temperature rise and it will melt the anode
what happens if the anode rotates during exposure
electrons from filament spread over a larger area. leads to lower temperature rise and will not melt the anode
2 benefits of the line focus principle in the design of the x-ray tube (rotating anode)
1) represents a way of meeting (to a certain extent) the conflicting requirements of having a small apparent area of the x-rays and a large actual area of origin
2) it enables adequate heat dissipation properties whilst achieving a small focus to produce small amounts of geometric unsharpness due to penumbra
what does the rotating anode assembly consist of
the anode disc, composed of tungsten/rhenium skin coated onto a molybdenum base. some anodes also have a carbon heat sink to further dissipate the heat produced.
what is the anode disc supplied with and what is its purpose
supplied with a very high (50 to 150 kV) positive (+) voltage during exposure.
its purpose is to attract electrons from the cathode, decelerating them rapidly and convert their kinetic energy into x-rays
main aim of the rotating anode assembly
has to be able to withstand high temperatures and be an efficient emitter of x-rays
why is tungsten used (3 points)
1) has a high atomic number (74)- good for x-ray production
2) a high melting point (3,370 degrees Celsius
3) a high thermal conductivity
why is rhenium used (3points)
1) increases thermal capacity and tensile strength
2) maintains a smoother surface- a rough surface would reabsorb a large proportion of the x rays produced
3) increases resistance to damage from overheating and continuous electron bombardment
what is the purpose of the anode stem
also made of molybdenum- it connects the anode disc mechanically to the rotor and electrically to the high voltage terminal
what is the purpose of the rotor & bearings
in conjunction with the stator windings, enables the anode to rotate freely at 3,000 rpm or 9,000 rpm
what is the purpose of the stator windings
surround the neck of the x-ray tube, but still within the steel housing. when supplied with an alternating voltage, they induce rotation of the rotor and hence the anode
