rad physics exam 1

what percentage of electron energy at the target becomes heat?

99%

which focal spot should be used for high mA techniques?

large focal spot

cathode

negatively charged electrode where electrons are produced

thermionic emission

process by which a heated filament releases electrons

off-focus radiation

electrons strike outside of the focal spot

anode

positively charged electrode with a target

anode heel effect

less intensity of x-ray beam on the anode side

isotope

protons same, neutrons different

ionization

removal of an electron

velocity of electromagnetic radiation

3×10^8 m/s

particulate radiation

alpha and beta particles emitted from radioactive decay and nuclear reactions

what electromagnetic radiation ionizes matter?

gamma rays and x-rays ionize matter

x-ray quantity

number of x-ray photons produced controlled by mAs

x-ray quality

average energy of photons of x-ray beam controlled by kVp

Planck’s equation

E=hf where h is Planck’s constant and f is frequency

equivalent vs effective dose

individual organs vs whole body

electric potential

electrical force that drives current through the circuit

electric current

flow of electric charge through the conductor

electric resistance

opposition to current

leakage radiation

radiation that escapes housing other than through the window

housing

lead-lined, oil-filled casing that absorbs, supports, and insulates

envelope

glass or metal shell that maintains vacuum

what is the limit on leakage radiation?

1 mGy per hour at 1 meter

why does there need to be a vacuum?

air molecules would collide with electrons in the tube

discovery of the x-ray

discovered by Wilhelm Rontgen in 1895 when experimenting with a cathode ray tube

2n²

maximum number of electrons in a shell

how is electromagnetic radiation produced?

when a charged particle changes velocity

what does exposure measure?

ionization in the air in coulomb/kg or R

what does absorbed dose measure?

quantity of radiation energy absorbed by tissue in Gy or rad

what does dose equivalent measure?

occupational exposure in Sv or rem

what does radioactivity measure?

number of atoms decaying per second in Bq or Ci

coulomb’s law

force is inversely proportional to the square of the distance

factors that impact resistance

temperature, material, cross-sectional area, length

what materials are ferromagnetic?

iron, cobalt, nickel

Faraday’s law

a current is induced when a conductor sees a changing magnetic field

remnant radiation

radiation that passes through the patient and reaches the IR

mutual induction

AC in a primary coil induces voltage in a second coil

self induction

AC cuts through turn on a single coil to induce voltage back into itself

step-up transformer

increase voltage to around 150 kV

step-down transformer

decreases voltage to around 12 V and current to 5-8 A

autotransformer

increases or decreases voltage via kVP selector to be later increased by the step-up transformer

rheostat

adjusts the amount of resistance (amount of electrons boiled off) in filament circuit via mAs selector

formula for step-up transformer

voltage in x secondary turns/primary turns = voltage out

ratio of turns on high voltage transformer

500-1000:1

where does the mAs timer sit and why?

in the secondary circuit because it reads the current of the x-ray tube

what helps reduce off-focus radiation?

collimation, metal envelope, diaphragm at the port

AEC

ends exposure when the IR receives the required radiation

why is thorium added to tungsten filaments?

to increase thermionic emission and extend tube life

what are three ways heat leaves the anode?

radiation during exposure, conduction between exposures, convection through the oil

what does oil in the housing provide?

insulation and heat removal

what happens when the rotor is prepped for too long?

filament tungsten is vaporized

why is a metal envelope better than glass?

it resists arcing from tungsten deposits

series vs parallel circuit

current is same throughout vs total resistance is less than smallest resistor

how do you know a technique is safe?

it plots below the curve of a rating chart

heat unit

kVp x mAs x s x c

space charge

electron cloud formed by focusing cup

space charge effect

the cloud’s negative charge makes it difficult for more electrons to leave the filament, capping at 1000 mA