xray system design Flashcards
what is faradays law
states that a current will be induced in a conductor which is exposed to a changing magnetic field
what is flemings right hand rule
thumb = force
first finger = magnetic field
=middle finger = current
what is electromagnetic induction
production of an electromotive force (emf) across an electrical conductor in a changing magnetic field. Alternating …
1 cycle = 1 Hz = 1 rpm
how do you transform voltage? (between 2 circuits)
- coils of wire (provide AC current flowing thru primary circuit thats wrapped around a core)
- magnetic flux ( induced by flow of current)
- magnetic flux is induced into the coil of the secondary circuit and when the number of windings on secondary side is greater than the primary, voltage increases
what does SST stand for
solid state transformers
why are SST’s preferable
using small high power, high frequency semi-conductors they are more efficient and reliable for power transformation
they can also transform AC into DC
what is a generator cabinet
cabinet that sits in xray room supplying all electrical needs
converts power circuits into low or high voltages and amperage and its a high frequency generator
what is a watt
1 volt x 1 amp
what 6 things are in xray tube housing and what do they do
- steel outer casing to be robust
- lead shielding to protect from leakage
- output port for primary beam
- insulated high-voltage cable sockets
- oil which insulates against electrical shock and circulates to help dissipate heat
- expansion bellows which disable the exposure switch when tube gets too hot
what is shown on the exit port of the light beam diagphragm
DAP dose area product
measures radiation exiting tube (not radiation reaching patient)
cGyxcm^2
what is on the back of the LBD
tube suspension, which can rotate
whats on the sides of the LBD
locks
electromagnet
switches to engage motor
what is the envelpe of the xray tube made of
borosilicate glass (pyrex) with beryllium window
why is the envelope made of borosilicate glass
heat resistant
strong
electrical insulator
radiolucent
what 2 main things are found in the glass envelope
cathode with heated filament and focusing cup
anode on a spindle
why do most tubes have 2 filaments
to give large and small focal spot
2 foci provide choice of quick exposures or high resolution
what are the filaments made out of?
thoriated tungsten
what amps do the filaments carry?
3-4 amps
why does the cathode have 2 electrical circuts
1 for high voltage current to make X-rays
2 to heat the filament and make it hot
what does the focusing cup do
electrostatic ally confines electrons to keep them ready to be used
what is the focusing cup made of
nickel
how does the filament get hot
large CURRENT thru thin wire, thickness of wire and material cause resistance.
electrical resistance is limiting affect which gives off heat and when heated enough the wire will give electrons via thermionic emission
what is a gridded tube/ what is its use?
controls the electron flow electrostatic ally by having an electronic grid on the cathode, switching from the focussing cup
why might a focussing cup need additional electrostatic charge
when imaging using short and rapid exposures, need precision of switching and generating electrons
what are the 3 different types of anodes and give examples of what they are used in
stationary anode (metal or mobile applications)
rotating anode (standard tube)
heavy-duty anode, ceramic envelope (angiography or CT tube)
what are the 2 different types of anode targets used in mammography and why
molybdenum ( get k shell to match breast tissue)
rhodium (for higher k shell energies for thicker breasts)
why is the target at an angle
to reduce apparent focal spot size and decrease unsharpness
what is the cylinder of a stationary anode made of
copper
what are the 4 things that construct an anode
- disc (tolerate heat loading) ( molybdenum)
- track (tolerate head loading and characteristic radiation) (tungsten-rhenium alloy)
- head-radiant backing (removes heat) (graphite)
- thin spindle (reduce heat conduction to bearings) (molybdenum)
flemings left hand rule is used only in MOTORS
apply current to wire
place in external magnetic field
force perpendicular to current and magnet formed causing spinning
magnitude of EMF induced in coil is proportional to rate of change of magnetic field
SO
the faster you change the magnetic field, the faster it turns
What 3 things can make an xray classified as broken
- anode is pitted and xrays to consistent
- bearings seize
- glass cracks and hot oil goes everywhere
why might a xray tube break
-problems in manufacture
- overheating due to either large exposures or not long enough cooling time
what will the xray machine do when it is overheating
expansion due to heat connects the warning circuit
warning light on generator
beeping noise
system cuts out until it has cooled OR limits the amount of rays to complete procedure
what 2 things does heating affect
size (expansion or contractions)
transfer of heat (conduction, convection radiation)
what are the positive and negative things that heating affects in xray?
ADV:
- heats cathode filament
- anode expansion to expected size
DIS:
- anode over-loaded due to frequent or high mAs exposures
- LBD (light sticks on)
- bearings wear in rotor
what are 2 ways xray machines are cooled
internally - via oil
externally - via air or water (chiller)
what is the use of a collimator
to limit the beam
collimation is manual or motor-drive by automatic format collimation system e.g bucky
what 5 components make up the light beam diaphragm
- added filtration
- entrance shutters
- mirror
- light source (light field)
- lead shutters
What are the ADV and DIS of having a ceiling suspended tube
adv:
- useful in large room for trauma, orthopaedics etc
dis:
- may not go very low if ceiling is high
- only travels along length of track in each direction
- cant be used if ceiling cant take the weight
what are the ADV and DIS of having floor mounted tube
adv:
- cheaper
- can reach distance that a ceiling one would have difficulties reaching
dis:
- not as hygienic
what is the typical ration found in a moving grid within a bucky system
12:1
what dimensions are the detector that is permenantly within the wall stand and table?
43cm x 43cm
what type of grids are used in integrated flat detectors
stationary grids
how do digital tomography and ct differ
dt is simpler/ simple version of ct
