Chapter 1 - Xrays Flashcards
define force
F= ma
causes bodies to deviate (push, pull)
what forces causes protons and electrons to attract each other?
electrostatic
what forces hold the nucleus together
strong force
what force is involved in beta decay
weak force
what is energy
ability to do work, Joule
define electron volt
kinetic energy gained by electron as it is accelerated across electric potential of 1 V
How much energy is required to eject outer shell electron vs inner shell electron
outer- several eV
inner- several keV
How much energy is reqauired to eject nuclear particles like alpha particles?
several MeV
Name of negative and positive regions of electrical circuit
cathode = negative
anode = positive
how much power do x-ray generators use compared to average household?
household = few kW
generator = 100 kW, about 30 households
how is energy utilization in making x-rays kept low despite a higher power use?
-exposure time is short (abdominal x-ray exposure time is about 100 ms)
define matter
made up of atoms that contain protons, neutrons, electrons
atomic number of
hydrogen
carbon
nitrogen
oxygen
calcium
hydrogen = 1
carbon = 6
nitrogen =7
oxygen = 8
calcium= 20
define atomic number vs mass number
atomic number = Z = number of protons in nucleus
mass number = A = total number of protons and neutrons in nucleus
name of innermost electron sjhell
K shell then L shell etc.
what is electron binding energy
energy required to completely remove any electron from an atom, increases with Z
K shell binding energy of oxygen
0.5 keV, Z = 8
k shell binding energy of calcium
4 keV, Z = 20
k shell binding energy of iodine
33 keV, Z = 53
k shell binding energy of barium
36 keV, Z = 56
k shell binding energy of tungsten
70 keV, Z = 74
k shell binding energy of lead
88 keV, Z = 82
what is electron density
number of electrons per volume
Einstein mass energy equation
E = mc^2
Define electromagnetic radiation
wave associated with oscillating electric and magnetic fields
-travels at speed of light
-characterized by wavelength, frequency, and velocity
Z and nominal density of lung
Z = 7.5, 0.3 g/cm3
Z and nominal density of fat
Z = 6.5, 0.9 g/cm3
Z and density of water
Z = 7.5, 1 g/cm3
Z and density of soft tissue
Z = 7.5, 1.04 g/cm3
Z and density of bone
Z = 12, 1.7 g/cm3
define wave velocity
product of wavelength and frequency
what is meant by electromagnetic radiation is quantized?
-exists in discrete quantities called photons
energy of EM wave
E = hf, E = hc/lambda
low energy and high energy EM waves- give examples
low E = radio waves
high E = gamma waves
difference between gamma rays and x-rays
x-rays- photons produced from electrons
gamma rays- photons produced from a nuclear process
define excitation
-atomic electron is raised to higher energy shell
define ionization
electron is ejected from neutral atom, leaving behind a position ion
what is ionizing radiation
EM radiation with sufficient energy to eject atomic electrons
examples of non-ionizing vs ionizing radiation
non-ionizing- radio, microwaves, visible light
ionizing- UV, x-ray, gamma ray
what happens when x-rays interact with tissue?
energy is transferred to electrons and then deposited locally
what is characteristic x-ray
-vacancy in k-shell is filled by electron from higher shell, emitting chracterisit x-ray
-occurs only for discrete energy levels
what is energy of characteristic x-ray
slightly less than k shell binding energy
what type of power supply is used for x-ray generators?
3-phase (ex 0, 120, and 240 degress)- together yield a “flatter composite)- less ripple than single phase
what does the transformer do?
increase or decrease voltage
what does rectifier do?
convert AC to DC
what is the max voltage in the ripple?
peak voltage
equation for power dissipated in x-ray tube
P = IV
typical x-ray transformer rating
100 kV
1000 mA
100 kW
components of x-ray tube
-evacuated glass tube
-negative filament (cathode)- typically heated
positive target (anode)- usually tungsten
-voltage is applied across the tube to accelerate electrons from cathode to anode. Electrons strike the anode and yield Bremstrahlung photons
define tube current
flow of electrons from cathode to anode, mA
how much energy is converted to x-rays in a x-ray tube?
1%, rest is heat. Anode stores the heat
define x-ray focal spot
target region producing the x-rays
advantage of large vs small focal spot
small - reduce blurring
large - tolerate high power loadings, reduce exposure times
what is bremstrahlung photons
x-rays produced when electrongs interact with nuclear electric fields. Electrons are decelerated by electric field and change their direction of travel. Energy lost appears as a photon.
what is max bremstrahlung energy?
-x-ray tube voltage
-brems. photons are range of energies
brems. production is proportional to what?
-Z and kV
what happens to L-shell characteristic xrays?
-low energy and are absorbed by x-ray glass tube
define x-ray spectruum
graph that shows number of x-ray photons at each energy
average photon energy of a spectrum
1/3 to 1/2 the peak kV
how much do characteristic x-rays contribute to the x-ray spectrum for radiography, fluorscopy, and CT?
~ 10 %
for what imaging modality are characterisitc x-rays a major contributor to the x-ray spectrum?
mammo
what are the 3 possible fates of x-rays incident on matter?
-scattering
-absorption
-penetration
define coherent scatter
-x-ray photons is scattered without energy loss
-< 5 % of photon interactions
In radiography, what fraction of incident photons are absorbed, scattered, and transmitted to form the image?
-2/3 absorbed
-1/3 scattered
< 1 % transmitted
How many ion pairs does each Compton or photoelectron produce?
-hundreds or thousands
describe photoelectric effect
-x-ray photons is absorbed by inner shell electron. That electron is ejected from atom (photoelectron). Outer shell electron fills the inner shell vacancy, with excess energy emitted as a characteristic x-ray.
What does PE effect depend on?
Z^3/E^3
the more tighly bound the electron is, the greater the chance of PE effect if energy of electron is above k edge
Where are PE interactions important?
mammo- spectra is mostly low energy photons and PE effect ~ Z^3/E^3
define compton scatter
-incident photons interact with outer shell, loosely bound electrons
-results in scattered photon (less energy than incident) and scattered electron
what does compton depend on?
-electron density/ energy
where are PE and compton effects equal for soft tissue and for bone?
soft tissue- 25 kV
bone - 40 kV
where are compton interactions most important?
chest/body radiogaphy
CT imaging
what do scattered photons that reach the detector do to the image?
degrade image quality
reduce contrast resolution
does scatter affect spatial resolution or image mottle?
No
what does scatter radiation from the patient do in fluoro procedures?
exposes the operator to radiation
definie linear attenuation coefficient
fraction of incident photons removed from a beam in traversing a unit distance (cm^-1)
-accounts for al interactions, including coherent
what does linear attenuation coefficient depend on?
-physical density, Z
-decreases with energy (exception is K-edge)
what is transmission of primary beam through average patient for skull rdiography?
1 %
what kV would you use with iodine contrast?
70 kV to get 35 keV photons and make use of 33 keV k-edge of iodine
what filters are typically used in mammo to transmit low energy photons and absorb higher energy photons with PE efect?
Mo- k edge 20 keV
rhodium - k edge 23 keV
silver- k edge 25 keV
what is kerma
kinetic energy released per unit mass
-J/kg (or Gy)
-obeys IS law
what is entrance Kair
measure of x-ray intensity incident on the patient undergoing an x-ray exam
-a few mGy
what is Kair at image receptor?
-measure of x-ray intensity that is used to generate an image
-a few micro Gy
what reduces Kair at image receptor?
patient attenuation
grid losses
distance from x-ray tubes
what is HVL
thickness of material that attenuates beam by 50 %
what is HVL of soft issue in mammo (30 kV) vs abdo radiography (80 kV) vs CT (120 kV)
1 cm vs 3 cm vs 4 cm
define beam quality
average x-ray beam energy
defined by HVL (thickness of aluminum)
beam quality in Al of mammo (30 kV) vs abdo radiography (80 kV) vs CT (120 kV)
0.5 vs 3 vs 6-10 mm Al
beam intensity vs tube current, exposure time
-directly proportional to exposure time and mA
what does changing mAs affect in the beam?
beam intensity (quantity) but not quality
if you need more photons what would you do?
-increase mA
-increase exposure time would increase photons but also motion blur
what does changing tube voltage impact?
-beam quality and quantity
-beam intensity is proportional to square of voltage for radiography, kV^2.6 for CT (not as simple for mammo)
-increasing kV increases average photon energy
rule of thumb for mAs to maintain Kair at image receptor when changing kV
-in radiography, increasing kV by 15% requires halving of the mAs to maintain Kair at receptor
what does increase in distance do to beam quality and quantity?
-decreases quantity, no change to quality
what does filtration do to quantity and quality?
decrease quantity, increase quality
what do low energy photons do to patient and image?
irradiate patient and add nothing to image
what are filters for?
absorb low energy photons which add nothing to image and add patient dose
what are most filters made from
3 mm Al
-chest radiography is performed at higher k V and may use Cu filtering
why does CT use very high filtration
reduce beam hardening artefacts
define beam hardening
preferential loss of low energy photons by an absorber
where does beam hardening not occur?
monochromatic x-ray beams
2nd HVL is always what than first HVL?
larger
when the linear attenuation coefficient is 0.1 cm^-1, the fraction of photons transmitted through 2 cm is most likely?
0.8
% of x-ray photon energy incident on a patient that is most likely absorbed within the patient
65%
tissue HVL for typical xray beam
3 cm
when the tissue linear attenuation coefficient is 0.1 cm^-1, the fraction of photons transmitted through 2 cm is?
2 cm absorbs about 0.2 so 0.8 is transmitted
radiation intensity Kair at image receptor for typical radiograph
3 uGy
exposure index = 300
