Lecture 3 Flashcards
bremsstrahlung radiation
incoming charged particle slows down as it interacts with the nucleus. Collisional energy loss is through heat and radiative energy loss forms the Xray
what is the intensity of the bremsstahlung determined by
target z. radiative energy loss/ total energy loss= EkZ/820000. Where EK is the kinetic energy
what happens with higher z
more efficient material- like tungsten or mollubenium or rhodium
what is a characteristic x-ray production
it is discrete energy levels
what is an electron cascade
fills the vacancy of the inner shells of electrons
when does emission of characteristic Xray happen
only happens when the incoming electron has kinetic energy that exceeds the binding energy of an orbital electron. They can also come from electron transitions as holes fill but for image formation purposes, the K characteristic X-rays because the others have energies too low for imaging
what is the electron source
cathode
what is the target
anode
what is the device to energize the electron to gain kinetic energy
generator
what are the components of an X-ray tube
cathode, anode, gnerator, envelope, rotator/stator, housing
cathode assembly
filament current heats a wire and thermionic emission of electrons. The filament current is 10mV and current goes up to 7000
what is a tungsten filament
helical wire coil, 5-20mmlong with 2-5 mm width
what do the electrons accelerate towards
anode
what does the focus cup do
it compresses the electron stream
what is dual focus
a second shorter filament is present and it can be selected during the exam
why is there not a point focal spot
it is impossible plus tube current limits
what is the max tube current dependent on
kV
what is the max focal spot size dependent on
KV and mA
why is tungsten good for X-ray tubes
relatively high bremmastralung output and high melting point alloyed with things to make it even higher
xray production is inefficient why
lots of heat and little X-ray production
what does heat do to the tube
it limits the tuber perforamtice
radiative energy loss/ total energy loss=
EkZ/820000
what is the X-ray tube envelope made of
glass or metal and air molecules would impede flow, so it is vacuumed and it cannot have air because it will oxidize. Envelope has a window or small region of glass so beam can go through
what is the tube rotor/stator
mechanism that allows the anode to rotate, it uses an induction motor. The anode is connected to rotor within envelope. Stator consists of cold of wire outside envelope. AC in stator induces rotation of rotor/anode within the tube
xray tube housing
provides strucutural support, insulation, and shielfing, there is an old bath around the envelope the carry away heat. Lead within the housing shields external environment from leakage radiation
what is the X-ray generator
it provides the power necessary for X-ray production, provides adjustable low voltage to heat tube filament and provides an electron source. Transforms low voltage AC to adjustable high voltage which is used to accelerate electrons. operator then an select X-ray energy quantitiy, and exposure time
rectification
tube only works one way
generator power
power (kW)= V(Kv)xI(A)
what is the generator power benchmarked for
.1 sec exposure
what is the max power of a generator
instanteanous limit or FS
what is tube heating
99% is converted to heat so heat unit is peak potentialxtubecurrent x time
what is the formula for energy
energy= potential x current x time
how do we balance heat and cooling in the X-ray tube
FS, anode, bearings, and tube housing have thermal limits and repeated exposures can damage the tube, and this must balance against energy input, and cooling proportional to the Temp to the fourth. The hotter the tube, the faster it cools
how do you calculate the effective focal spot length
sin (theta) times the true FS length
what happens with a large anode angle and small filament lenth
large field overage, but small effective focal spot- poor power loading
what happens with a large anode angle and long filatment
large coverage and large focal spot with good power loading
small anode angle and long filament lenth
small field overage small effective focal spot but good power loading
explain the heel effect
there is lower X-ray intensity under the anode
in terms of the X-ray spectrum, what is quntitiy
the number of X-rays or fluency. it is proportional to the area under the spectrum.
what is quality in terms of X-ray spectrum
describes the shape of the spectrum energy and canc be HVL
what is the probability of bremmstralung
2xpixr
what happens with the bremmstralung further from nucelus
higher probability and lower energy
who came up with xrays
roentgen
what is the inverse square law
exposure is proportional to (1/distance)squared
what is X-ray quantity proportional to
Zx KV2 x mA x time/distance2, so it decreases with increased filtration. and it increases with decreased voltage ripple.
what does X-ray quality change with
z, kVP (maximum energy_ filtration with increased percent of X-rays with increased filtration, waveform (increased percent of high energy rats decreased ripple
what creates a characteristic xray
it is the holes in the electron orbits
what creates a characteristic xray
it is the holes in the electron orbits
high Z emits more electrons because
they have so many electrons that it is hard to hold onto them
