XrayFund

5/12/99

Beam angle vs FFD

• Need to get the correct bevel on the anode to have the correct focal position and beam coverage
• Could do most shots at 72" for everyone

X-ray production

• Characteristic radiation
• The radiation produced is characteristic for the anode material
• Depended on electron binding energies for the target material
• Projectile electron ionizes a K shell electron
• Electrons from higher orbits fill the "hole"
• The change in electron binding energies results in photons w/the keV value—69.5keV
• The K shell binding of electrons is different for different materials depending upon the nucleus
• The projectile energy of the electron from the cathode must have more energy than the energy binding the electron in the K shell
• When an electron from the K shell leaves, it pulls an electron from a higher orbit to fill the space, changing the electrical configuration of the molecule—X-ray
• at 70keV, 15% is characteristic radiation (actual x-ray)
• Bremsstrahlung radiation
• Below 70keV, all Bremsstrahlung radiation—not enough energy to create an x-ray
• at 70keV, 85% is Bremsstrahlung radiation
• Above 70keV, Bremsstrahlung radiation
• Braking (slowing, decelerating) radiation
• Projectile electron (negative charge) comes under influence of the nucleus (electric Coulomb force)
• Projectile electron changes direction, variably
• Change in direction causes bremsstrahlung x-ray photon production (conservation of momentum)
• Electrons always travel at the speed of light or not at all
• Change of direction = scatter radiation
• Three electrons enter at different distances form the nucleus
• Three photons exit at different directions, different wavelengths

Alternating Electrical Current

• Electricity from the wall is alternating current
• Sine wave (reproducing)
• Start electricity is 0, move up to 110V, moves to 0, moves down to –110V
• Need to boost energy to 1000's of V
• Negative portion of the wave is not useful as it is
• 1/60s is one cycle of the wave

Single Phase, Full Wave Rectified Waveform

• bring negative part of the cycle to the positive part make it useful
• this is done by the rectified waveform
• cut the exposure time in half
• would run it through a transformer to boost it up to 60,000V
• only in the top 30% of the curve is useful
• there is waiting time b/w reaching the top of the curve
• ripple factor—going form max to min—100%

Three phase electrical waveform

• add two more circuits
• as the first phase starting to decline, the next phase would kick in and so forth

• so the ripple factor is b/w 6% and 12%

• similar to what most hospitals have had in the recent past
• have to have a transformer hung on the electric pole outside

• do not want to buy a three phase machine
• came in during the 1950's

6 kHz Medium Frequency Electrical Waveform

• 1985 came about
• no extra transformers on electrical pole
• ripple phase at about 10%
• use single phase electricity

100 kHz High Frequency Electrical Waveform

• 1989
• a switch that turns on and off 100,000 times/sec
• ripple factor about 1%
• claimed dose reduction to pt's

• 20-40% less exposure to pt's that the 6kHz system

Ripple Factor

• single phase—100%
• three phase—10%
• medium frequency—10% (below 40keV)
• high frequency—1% (above 40keV)

X-ray Production

• x-ray emission spectrum, filtered beam kVp (keV) >70
• aluminum filter that is required by law—regulated by FDA
• line begins at 0
• very few Bremsstrahlung released upon people
• the peak largest number of x-rays
• the far right has the peak x-rays
• every thing under the curve is Bremsstrahlung
• the peak to penetrate the pt is Bremsstrahlung
• change in milliamperage, kVp >70
• the average energy of the two peaks is the same
• the only difference is the 400mA has twice as many photons as 200mA
• mA is directly proportional to x-ray photon production
• time is directly proportional to x-ray photon production
• tend to talk about mAs which is directly proportional to x-ray photon production

• change in tube potential
• characteristic is at the same point
• 90kVp is taller—more photons produced—more electrical force
• 90 hits the x-axis further out
• increase in the average energy of 90—a more penetrating beam
• has a darkening effect on the film

• effect of filtration, kVp >70
• reduces low energy radiation, creating less exposure to the pt
• cooper or aluminum as filters
• 2mm Al—thickness of filter
• 4mm Al—less photons, peak shifted to the right, made the beam stronger

mR per mAs

• use the same amount of ??? and

X-ray Properties

• Wilhelm Conrad Roentgen—discovered x-rays by accidents
• Essentially no mass
• No electrical charge—makes them hard to focus
• Travel at the speed of light or not at all
• Can penetrate most matter
• Travel in a straight line until they interact w/matter
• Capable of making certain chemical compounds fluoresce
• Capable of exposing photographic film—but normal photographic film is save to go through x-ray scanners like at airports
• Can change biologic matter by ionization
• Radiation is either electromagnetic or particulate—lots come from the sun
• Alpha and beta are particulate
• X and gamma are electromagnetic

• Part of the electromagnetic spectrum
• Average wavelength of 10^-9cm
• Average frequency of 10¹⁹hertz
• Average energy of 60,000 electrons volts

Interactions

• 5 basic interactions

1. classical scattering
2. Compton effect
3. photoelectric effect
4. pair production—will not be tested on
5. photodisintegration—will not be tested on

Classical Scatter

• aka coherent or Thompson scattering
• b/w low energy x-ray photon and an atom
• photon changes direction w/o loss of energy
• no ionization
• most scatter forward—towards the film
• may contribute to film fogging—layer of gray on top of anatomy—destroys detail
• do not worry about

Compton Effect

• x-rays are capable of scatter
• noble prize winner
• 1945, St. Louis, chancellor of Wash U
• b/w moderate energy x-ray photons and an outer shell electron—the one's we use
• photon changes direction , loses energy, ionizes atom creating a Compton electron—leaves at reduced energy and at a different directions, the loss of energy is what was needed to remove the electron
• scattered photon may ionize another atom—multiple ionizations
• may cause back-scatter (180° )—turn back towards the tube
• adds to film fog—measures more than 12cm, need a grid to help capture the scatter (80-90%)
• increases w/increasing kVp

Photoelectric Effect

• b/w moderate energy x-ray photons and an inner shell electron—like a K shell
• photon loses all of its energy (it can no longer go anywhere), ionizes the atom creating a photoelectron—produces the white areas of the film
• produces characteristic x-rays (secondary radiation) in the object being x-rayed—not very strong and can not exit the tissues
• affects smaller atoms like C, O, N, etc
• secondary x-rays behave like scattered x-rays

Differential Absorption

• Compton scattered x-rays contribute no useful information
• adds radiographic density, results in film fog
• structures that absorb a great amount of x-ray are radiopaque—cortex of bone, teeth fillings, etc
• structures that allow x-rays to pass through are radiolucent—calcium, metals
• the bigger the atom, the better the target for the x-ray—oxygen, carbon
• generally less than 5% of x-rays that hit the patient reach the film
• less than half of those interact w/the film to form an image
• therefore approx. 1% of the x-rays emitted from the tube make the image
• maximal differential absorption results from lower kVp values
• but as kVp is lowered pt dose increases
• at lower energies (kVp) the % of photoelectric interactions increases (relative to Compton)
• around 40keV have equal number of interactions
• above 40 have more Compton scatter 85%—15% photoelectric
• to image small differences in soft tissue (renal stone) use low kVp technique (for maximal differential absorption)—make white whiter and background darker—use less kVp
• you can "see" four things radiographically:

1. air—blackest
2. fat—lighter than air; dark gray; around muscle and such
3. water –lighter than fat; lighter gray; muscle, nerve, CSF
4. metal—bone; whitest