rad physics 1 exam 1

mega/micro is 10 to the +-

6

nano/giga is 10 to the +-

9

electron volt

amount of ENERGY gained by particle of 1 charge when accelerated 1 volt

1.602×10^-19 J

1 eV

v= (when light behaves like a wave)

c/wavelength

100m

radio waves

10^-2

microwaves

10^-5

infared

5 × 10^-7

ultraviolet

10 ^ -9

xray gamma ray

E=

hv

plancks constant

6.63 x 10^-34

each shell or orbit can only hold how many electrons

2n squared

what results in more tightly bound inner electrons

higher Z

what holds electrons in orbitals

binding energy

excess energy from an excited electron dropping is

characteristic xrays

therapy energy range xrays

50 keV to 25 MeV

3 classes of therapeutic xrays

superficial, orthovoltage, megavoltage

xray machine components

tube, generator

what produces electrons at the cathode, and is heated by passing a current through it, emits via thermionic emission

filament

kvp

max voltage across tube

electron has an energy in keV numerically equal to

max kVp

small focal spots lead to

intense heating

line focus principle idea

apparent focal spot smaller than actual

line focus involves doing what to the target

tilting it

rotating anode

disc rotated 3000 to 9000 rpm

heel effect

gradient in intensity from anode to cathode

xrays that have to travel through more target material are

more likely to be attenuated

correcting heel effect

appropriate filter

megavoltage xrays are produced by a

linac

do therapy tubes have rotating anodes

no

therapy tubes designed to be run

continuously for longer

therapy tubes anode

hooded

mA (tube current) is determined by

amount of voltage and current supplied to filament

the technique

kvp, ma, sec

radiographic images are

negative

compound used in xray film

silver bromide

film is not very sensiitve to

xrays

a shorter exposure time reduces

image blurring

what reduces image contrast

scattering of xrays by the patient

xray grids

reduce amount of scattered radiation

what are the only two things an electron can interact with

atomic nuclei and atomic electrons

electron interacts with atomic nuclei

bremmstrahlung

electron interacts with atomic electron

characteristic xrays

in bremsstrahlung, the incident electron interacts in the

coulomb field of an atomic nucleus

in bremmstrahlung, the electron motion is (and also decelerated and emitting waves)

deflected

the probability that an electron emits bremsstrahlung is proportional to the

E and atomic number of target material

in characteristic xrays, the energy of the photon is equal to the

difference in binding energy of the orbital

most important electron transitions are in what shell

K

xray energy range bremsstrahlung

0 to max KE of incident

average energy of photons in bremsstrahlung spectrum is

1/3 max energy

increasing the tube current ONLY AFFECTS

increasing height of spectrum

increasing tube voltage shifts

spectrum to the right

tube voltage also affects

amplitude

does tube voltage affect the energy of characteristic xrays

no

bremsstrahlung more efficient in materials with (proton #)

higher Z

atomic number of target increases, energy of characteristic energy xrays

increase

self filtration (unavoidable)

absorption of low energy xrays by target and window

filters in diagnostic xrays made of

aluminum

increasing filtration reduces

intensity of beam

increasing filtration increases (known as beam hardening)

average energy of photon spectra

why use external filtation

absorbed by patient but not film

xray production effeciency

fraction of incident electron KE converted to photons

what increases efficiency of xray production

incident electron energy, Z

xray intensity can be decreased by

beam divergence, attenuation

assume that the radiation source dimensions are small compared to the distance from the source where the beam is used

point source approximation

narrow beam geometry only detects (is used to measure linear attenuation coefficient)

unattenuated photons

unit of linear attenuation coefficient

1/length

mean free path

average distance a photon travels before interacting

HVL is a measure of

beam quality

HVL reduces what by half

beam intensity

for a monoenergetic beam the second HVL is what to the first

equal

HVL due to a process called

beam hardening

HVL of xray tube depends on

kvp, tube, external filtration

HVL for superficial tubes

1-4 mm Al

orthovoltage tube HVL

4 mm Cu

HVLs used to describe

beam quality, thickness of shielding

larger HVL is for more

penetrating beam

mass attenuation coefficient, compared to linear, removes

dependence of material density

linear attenuation coefficient dependent on

energy, composition, density