220 quiz 1

QUIZ on Wednesday 9/2

4 fundamental requirements for xrays

free electrons, target, very high voltage, vacuum

anode made of

tungsten

high frequency generators

most efficient for X-ray, least voltage ripple, requires the least kvp

rectification

changes alternating current to direct current

autotransformer

makes steady current, delivers precise voltage to xray circuit

rheostat

controls current to filament circuit with resistance

step down transformer

slows down voltage

isotropically

X-rays in all directions

99% of what happens at the anode

heat

what is the anode made of

graphite - it helps control the heat

tube housing

encloses and protects the vacuum tube

greater anode angle

wider, useful beam

smaller anode anglw

narrower useful beam

anode heel effect

put the cathode side on the thicker anatomy because it is stronger beam

line focus principle

relationship between actual and effective focal spot

off focus radiation

can increase skin dose and can reduce image contrast, not created in focal track

filament

thin coil of thoriated tungsten - 2mm diameter, 1-2 cm long

small focal spot

small detail

large focal spot

more technique can be used

focusing cup

keeps electrons tight and away from the filament

quality

average energy of the beam, kvp, penetrating ability

quantity

number of photons in the beam, mas or mGy/mAs

reciprocity

quality of image is constant if exposure time and intensity vary in reciprocal manner

intensity

mGy

mAs equation

ma x time (seconds)

3 interactions at the anode

heat, characteristic X-rays, bremsstrahlung radiation

heat

vibration/excitement turns into heat and produces xrays

bremsstrahlung radiation

- projectile electron loses its energy as it interacts with the nuclear field

- happens at any kvp and is the majority of the beam

characteristic interactions

- incident electron interacts with k shell electron

- minimum kvp of 70

- cascade effect

5 ways xrays interact with matter

coherent scatter, Compton scatter, photoelectric effect, pair production, photodisintegration

interactions important to us

Compton scatter, coherent scatter, photoelectric effect

photoelectric absorption

primary interaction that creates the image

Compton scatter

- patient/occupational dose

- ejects outer shell electron and causes ionization

- image fog

photoelectric effect

- ejects inner shell electron from the target atom and ionizes

- cascade effect

- similar to characteristic interactions at anode

- incident electrons need a slightly higher energy than the binding energy of inner shell electrons

attenuation

- difference between what hits the patient and what hits the detector

- reduction in radiation intensity from absorption and scattering as beam passes through the body