Ch 5, 6, 8 Physics
incoming line current
60 Hz alternating current with a nominal root means square (rms) voltage of 200-240; supplied to buildings; usually supplied in a three phase power cycle also called the mains
binding energies for Tungsten
K = 69.5 keV L = 12.1 keV M = 2.8 keV N = 0.6 keV O = 0.08 keV
characteristic peak
Occurs when K-shell emissions reach their effective energy range of 69 keV.
power rating
P=IV single phase= 0.7 high frequency= 1.00 *REMEMBER TO CHANGE mA TO AMPS AND KVP TO V*
fractional focal spot
a very small focal spot; usually a fraction of a millimeter in size
characteristic cascade
The reaction of electrons dropping into the holes created during a characteristic interaction until there is only a hole in the outer shell
dual-focus
X-ray tubes that contain two filaments: one large and one small; only one filament is used at a time. 1. small focal spot (effective) - 0.1-0.5 mm -high resolution 2. large focal spot (effective) -1-1.5 mm -high heat units
only benefit of curved tabletop
decreases OID
heat unit formula
kvp x mA x time x rectification constant 1 phase= 1.00 high frequency= 1.4
what does deposits of tungsten cause on images?
mirrored appearance -can eventually cause arcing when sufficient current is attracted to a deposit during an exposure
arcing
occurs when sufficient current is attracted to a deposit during an exposure - immediately destroys the tube
pitting
occurs with normal use of rotating anode, will eventually vaporize sufficient target focal track material to roughen or pit target area - reduces efficiency of the tube
primary function of envelope
to maintain vacuum between the cathode and anode
function of filament
to provide sufficient resistance to the flow of electrons so that the heat produced will cause thermionic emission
mobile systems
tube suspension is based on the floor suspension system
effective focal spot
used to describe the area of the focal spot that is projected out of the tube toward the object being radiographed -small as possible to increase a sharper image -controlled by the size of the actual focal spot and anode target angle
actual focal spot
used to describe the physical area of the focal track that is impacted -you want this as long as possible -this is the incident electron beam -controlled by the size of the filament
focal track
used to represent the circular path that will be impacted by the electron beam
floor to ceiling suspension system
uses a pair of rails, one on ceiling and one on floor, for longitudinal positioning
Floor suspension system
uses a tube-support column mounted on the floor. the system most be carefully counterbalanced to avoid tipping
c-arm tube suspension system
utilizes a C-shaped arm to support the tube and image receptor. the tube and image receptor are fixed to opposite ends of the C-arm
when to use small focal spot
when high resolution and detail are desired
when to use large focal spot
when more technique is involved
other filaments that can be used
Rhenium (3,170C) Molybdenum ( 2,620C, typically in mammography)
window of X-ray tube
-thinner spot of glass -allows for primary x-ray beam to exit envelope
stationary anodes
-comprised of rhenium-alloyed tungsten -Imbedded in a 45 degree angled end of copper rod - typically for longer exposures, low mA -dental x-rays
ancillary devices for tilting tables
-footboard -shoulder supports -handgrips -compression band
disk of anode
- 5-13 cm in diameter - molybdenum - thermal conductivity - high melting point - anode warm up
bremsstrahlung interactions
- 80-90% of x-ray photons -occurs only when the incident electron interacts with the force field of the nucleus - nucleus is + -Incident electron is - - force field causes the incident electron to slow or brake then diverts the electron course - energy that is lost during the braking is emitted as an x-ray photon - the energy is exactly the difference between the entering and exiting kinetic energy of the electron -unpredictable and can range from the total value of the incident electron - if the incoming electrons are farther away from nucleus, less energy is lost because the attraction is less -happening at the anode
rotating anode
- much larger target area - the faster the anode rotates, the better the heat dissipation - greater efficiency -higher tube currents possible -shorter exposure times possible - used in most X-ray tubes
focusing cup
- shallow depression in cathode assembly designed to house the filament - made of nickel - negatively charged in order to condense the electron beam - purpose is to narrow the thermionic cloud as it is driven toward the anode (great picture on page 81 in textbook!)
filament life
- typically 6-9 hours - functional temp obtained with rotor initiation - do not rotor unnecessarily (heat will build up, decreasing tube life) - most exposure should be made with no "rotor first" time
Grid-Biased Tubes
-Precise control of thermionic cloud -Changes charge of focusing cup from negative to positive -used in angiography or capacitor discharge generators -desirable to quickly regulate the flow of electrons producing x-ray photons
exposure switch
-activates the rotating anode of the X-ray tube - two-step button (rotor and exposure switch) -If depressed halfway, it activates the anode rotation then depressed completely to initiate the exposure -must be attached in a way that the operator cannot be exposed
saturation current
-affects the efficiency of the X-ray tube -Increasing kVp increases the number of electrons that impact the anode - increasing kVp increases the tubes efficiency -reached when additional kVp will not increase the number of electrons
overhead suspension system
-allows controls of longitudinal and transverse positioning as well as vertical distance -motion is locked into place by solenoid
target
-area of anode struck by electrons from cathode -SID is measured from this point - tungsten alloy -vaporization of some atoms of tungsten -point where incident electron beam turns into xrays
X-ray tube consists of
-cathode -anode -envelope -protective housing
Envelope
-geissler tube ( made an attempt to create vacuum) -crookes tube -coolidge tube (complete air evacuation) - made of pyrex glass -maintains vacuum -can have metal (decrease potential for arcing, steel)
high frequency
-incoming power= almost 1 MH -3 to 4% voltage ripple -lower voltage %= high frequency -small compact, better efficiency -takes 10% of the time to get same kVp than 3 phase
rotor
-inside the envelope, inside the stator -made of hollow copper cylinder that attaches to the anode disk by molybdenum shaft -3,400 rpm of common -10,000 rpm of high speed anode -contains silver-plated steel ball bearings around a shaft -precisely balanced, low friction device
protective housing
-lead lined -Insulation from high voltage -mechanical support of anode and cathode assemblies -controls leakage, scatter radiation -Isolates high voltage -provides a mean to cool the tube
heat production
-less than 1% of the total kinetic energy of the incident electrons is converted to heat -is the kinetic energy of the incident electrons increase, so does the efficiency of photon production
changes in x-ray beam quality and quantity by factors that influence the emission spectrum
-mAs= increase in quantity, no change in quality -kVp=increase in quality and quantity - filtration= decrease in quantity, increase in quality -target Z= increase quantity and quality - voltage ripple= decrease quantity and quality
type of tabletops
-motor driven- movable along length -floating tops- can be moved along their length and width -fixed and tilting tabletops
cathode assembly
-negative side -filament (thoriated tungsten) -focusing cup -associated wiring
characteristic interaction
-occurs only when the incident electron interacts with an inner-shell electron -must have enough energy to knock an inner-shell electron from orbit -Ionizing an atom - incident election interacts with tungsten atom, knocks out K or L shell electron - electron in outer shell works hard to maintain spot, because it is a weaker binding energy - outer shell gives up more energy because it is working harder -predictable -only occurs when kVp is above 70
stator
-outside the envelope -series of electromagnets -each electromagnet is energized in sequence -turns the anode -kvp would destroy the stator's electromagnets - you rotor the stator which induces current
single phase generator
-permits the potential difference to drop to zero with every change in the direction of current flow -1 pulse for half rectified -1 wire coming in -2 pulse for full rectified -not very efficient -100% ripple `1
off-focus radiation
-photons produced away from the focal spot -electrons or photons have sufficient energy remaining that strike other objects within the tube, creating photons -causes ghosting structures -aperture diaphragm helps with off-focus -may contribute 25-30% of the total primary beam
anode assembly
-positive side -anode -stator -rotor
anode heel effect
-problem caused by the use of line-focus principle -because of the geometry of an angled anode target, radiation intensity if greater on the cathode side -45% exists parallel to the anode-cathode axis -cathode end should be positioned toward the denser part of the body -smaller angle= more narrow effected focal spot ie. higher anode heel effect -greater angle= less the heel absorbs the photon (more photons are available -more acute angle= higher anode heel affect which causes less anode photons
function of cathode
-produce a thermionic cloud - conduct the high voltage to the gap between cathode to anode -focus the electron stream as it heads to the anode
multiphase generator
-produced by the generator and is the common form in which power is supplied to users -never drops to zero - voltage ripple occurs with full wave rectification -13-25% voltage ripple with 6 pulse - 4 to 10% voltage ripple with 12 pulse -add more wires to maintain potential -number of pulses are the number of diodes
vaporization of filament
-produces particles that will deposit on other surfaces -reduce the vacuum within the tube -when 10% of diameter has vaporized, filament is more likely to break -causes arcing -Increases filtration of beam
purpose of anode
-serves as a larget surface for the high-voltage electrons from the filament - conducts high voltage from the cathode back into the x-ray generator circuitry - source of x-ray photons -serves as the primary thermal conductor
effectiveness of focusing cup
-size -shape -charge- biased - filament size - filament shape - position of the filament within the focusing cup
3 essential conditions for x-rays
-source of electrons - acceleration of electrons - deceleration of electrons (thermionic emission happens at filament, voltage causes the acceleration, deceleration happens at the target or anode)
electrical production of x-rays must have
-source of electrons -appropriate target material (tungsten) -high voltage -a vacuum
thermionic emission
-space charge- electron cloud -space- charge effect- Makes it difficult for subsequent electrons to be emitted by the filament -Vaporization of some electrons (gassy tube)
filament circuit
-supplies the filament of the X-ray tube with properly modified power -purpose is to create the appropriate thermionically emitted electron cloud at the filament -
factors affecting size and relative position of x-ray emission spectrum
-tube current= amplitude of spectrum (amplitude= quantity) -tube voltage= amplitude and position -filtration= amplitude and position - atomic # of target material= amplitude and position -voltage waveform= amplitude and positon
target interactions
-two types- bremsstrahlung and characteristic -Interaction will occur depends on the electron kinetic energy and the binding energy of the electron shells of the atom -tungsten and rhenium are the material used for effort to provide appropriate atomic number atoms and a max of similar electron shell binding energies
upright units
-upright IR holder or Bucky -may have a grid or may not
Benefits of tungsten
1. Increases efficiency of thermionic emission (2,200C) 2. high melting point (3370C) 3. highly ductile (can bend and shape) 4. not easily vaporized -small coil ( 0.1-0.2 mm thick, 1-2 mm wide, 7-15 mm long)
vacuum of tube
A sealed glass tube from which most of the air has been evacuated -10^-5 mm mercury is needed -removal of air permits electrons to flow from cathode to anode without encountering the gas atoms of air -greatly increases the efficiency of the tubes operation
SID relation to anode heel
SID decreases= anode heel increases inversely proportionate
anode angle relation to anode heel
anode angle decreases= anode heel effect increases
space charge effect
as more and more electrons build up in the area of the filament, their negative charges begin to oppose the emission of additional electron - limits the X-ray tube to a max mA of 1,000-1,200
field size relation to anode heel
directly proportionate -field size increases= anode heel increases
timer circuit
electronic timers are capable of accurate exposures as short as 0.001 second with only a 1-msec delay -milliampere-second timer- used in some capacitor discharge units to monitor the production of mA and time on the secondary side of high voltage step up -AEC-
main circuit
supplies the X-ray tube with properly modified power -purpose is to produce x-rays -modifies the incoming line power to produce x-rays by a sequence of devices -permit the radiographer to adjust amperage, voltage, and length of exposure, as well as incorporate appropriate circuity to increase the efficiency
line-focus Principle
used to reduce the effective area of the focal spot - permits the best resolution of detail while permitting as large an actual area as possible -less angle of actual focal spot= smaller effective focal spot -greater angle of actual focal spot= large effective focal spot -most common angle is 7 to 17 degrees -to cover 14 by 17, a min. of 12 degree target angle is required
