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