Test 1: Lecture 1 and 2 Flashcards
Xrays have short or long wavelength?
short
high energy
high radiation
the shorter the wavelength, the ___ the energy
higher
example: xrays short wavelength but high energy
X-rays are able to penetrate materials that ___ visible light
absorb or reflect
Xrays can produce ___and cause biological changes in tissue
ions
•X-rays are produced through conversion of ___ of accelerated electrons into ___
kinetic energy
electromagnetic radiation/energy
•Production, acceleration and deceleration of electrons takes place within the ___
X-ray tube
cathode
filament with negative charge
heating it results in electrons coming off and moving toward + anode which in turn reflects into Xrays
negative filament inside the Xray machine
cathode
what is the positive part of the Xray machine
anode
source of electrons in the xray machine
cathode
acceleration of electrons inside the xray machine is by
potential difference between +(anode) and - (cathode)
deceleration of electrons and conversion into Xrays is at the __
anode (+)
of the energy converted from kinetic energy into electromagnetic radiation what % are xrays?
less the 1%
99% is lost as heat
•Radiation emitted from the anode mostly by an interaction called ___(braking radiation)
Bremsstrahlung
nucleus has what charge and is made of ___
protons and neutrons
positive charge
electrons have __ charge
negative
outer shell electrons have __ energy then inner shell electrons
less
Bremsstrahlung
electron is deflected off course as it nears the nucleus, loss of energy is emitted as xrays
the closer to the nucleus the incident electron to the nucleus, the higher the energy of the emitted xray photon
___ is when electron is deflected off course as it nears the nucleus. Loss of energy is emitted as X-rays
bremsstrahlung/ braking radiation
label
cathode= negative filament
anode= positive
cathode is made of ___
tungsten filament that is heated and releases electrons
2 types of filaments (small and long)
mA
number of electrons flowing per second from the cathode to the anode
mAs
mA x exposure time= number of electrons flowing during the total exposure time from cathode to anode
how to get mAs
mA x exposure time
can get the same number with different mA or exposure times
want to use the smallest exposure time possible to reduce artifact from patient moving
500 mA x 0.02 seconds= 10 mAs
10 mA x 1 second= 10 mAs
what happens when you increase mAs
increases filament heating and tube current will increase number of electrons moving from cathode to anode= increase number of xrays made
changing mA will do what to the energy of the xrays made
max energy will be the same for different mA
xrays with higher mA will have more electrons or more xrays
increase quantity of xray, not quality of xray
(not stronger (not more penetrating), just more)
kVp
kilovolt peak
voltage difference between anode and cathode
high potential difference leads to acceleration of the electrons in a vacuum towards the anode
the bigger the kVp, the stronger/more penetrating the xray
what does it mean if 55 kVp on the control panel?
Maximum kinetic energy of the electrons: 55 keV (energy from traveling form cathode to anode)
Maximum photon energy: 55 keV (can not be bigger then the max kinetic energy
increasing kVp will do what?
will increase the energy/penetrating power of the xray produced
(peak energy and average energy will increase)
will also increase the number of electrons/xrays produced
increases quality and quantity of xray
change in ___ will increase the number of xrays as well as the penetrating power of the xray
kVp
why use tungsten for anode
- High atomic number (high conversion efficiency from electrons to X-rays)
- High melting point
- Relatively resistant to surface damage
prevents overheating by spinning
which filament size is good for fine detail
small
which filament size is good for high heat?
large
reduced detain but can use higher exposures
(large electron beam)
collimation
making xray beam smaller to decrease unnecessary exposure and improve picture
what does it mean when xray beam is polychromatic
has wide range of low to high power xrays
low xrays are not strong enough to get through patient and causes scatter and increases the dose of radiation to the patient
we use filters to reduce low energy xrays
filtration of xray is meant to ___
decrease low energy xrays
low energy not strong enough to make xray, just contributing to radiation
glass envelope around the xray tube is a form of ___
inherent filtration
what are some examples of inherent filtration
glass envelope, insulating oil, plastic tube window
what is an example of added filtration
aluminium (1.5-2 mm)
absorbs low energy xrays
Attenuation of the beam by the different tissues of the patient reduces its intensity and creates the image: ___
‘shadowgram’
film not exposed to xray are ___
white or light
(xrays where absorbed by material and did not make it to the table)
___ is decrease in intensity of an xray beam as it passes through matter
attenuation
___ is photon removed from a beam
absorption
___ is when a photon changes direction
scatter
two types of attenuation
absorption and scatter
___ is xray passes through matter without interaction
transmission
photoelectric effect
complete absorption of xray photon
xray photon hits inner shell electron, this becomes ionized. not strong enough to leave and gets absorbed
outer shell electron fills empty inner shell space→ create a low energy xray (characteristic radiation)
photoelectron and characteristic radiation: both absorbed, not strong enough to get out of cell
what effect is compete absorption?
photoelectric effect
photoelectron and characteristic radiation: both absorbed, not strong enough to get out of cell
photoelectric effect causes the atom to become ___
ionized
(loses an electron- this is where things go wrong and cause cancer from radiation)
what is the probability of the photoelectric effect
xray totally absorbed
Probability of occurrence: Z3/E3
Proportional to atomic number cubed (Z3)
Inversely proportional to X-ray energy cubed (E3)
In summary: PE reactions are most likely to occur with low energy photons and elements with high atomic numbers
when is PE more likely to occur?
Photoelectric effect (total absorption) reactions are most likely to occur with low energy photons** and elements with **high atomic numbers
___ magnifies differences in tissues composed of different elements such as bone and soft tissue (large difference in atomic number) – good contrast!
photoelectric effect
compton effect
scatter effect
- Incoming photon ejects free outer shell electron from the tissue atom. The photon is scattered
- The scattered photon has lower energy, but may produce more ionizations, fog the film and is a radiation safety hazard
- The ejected orbital electron is absorbed in the patient
in the ___ effect, photon will hit outer shell electron
compton/ scatter
what can happen to the scattered photon
ionize other atoms, create fog on the film and can exposure holder to radiation
what is the probability of compton effect
Increases with electron density, which depends on the tissue density and not the atomic number
Relative proportion of Compton vs PE interaction increases in favor of COMPTON as energy of X-ray beam increases
Compton Effect causes almost all of scatter reaching film
as energy increases what effect also increases in probability
compton / scatter
increases in ___ will increase the probability of the compton effect
electron density (tissue density)
which of kVp or mAs controls the energy of the xray beam produced in the tube?
kVp
higher kVp = more compton effect, less photoelectric
which one has more kVp
more kVp= more energy= more compton effect
•___increases with high atomic number materials and low energy X-rays: GIVES CONTRAST
Photoelectric Effect
___ increases with higher density tissues and high energy X-rays: REDUCES IMAGE QUALITY (DECREASES CONTRAST) AND EXPOSES PEOPLE AROUND PATIENT TO SCATTERED RADIATION!
•Compton Scatter
degree of xray attenuation is based on
- Effective atomic number (Z) of each tissue (higher Z = more PE effect)
- Density or specific gravity of tissue (higher density = more compton)
- Thickness of tissue
- Photon energy - kVp (higher kVp= more compton)
why is water and muscle a similar color on the xray
similar density and atomic number (Z)
what are the 5 basic radiopactities
___ radiation from sources other than the primary x-ray beam
scatter radiation
safety hazard
produces reduced detail and film fog
factors that influence the amount of scatter
tissue thickness
field size
kVp (higher = more energy= more scatter)
how to reduce scatter
collimation
low kVp
how to reduce the effect of scatter
grids (alternating lead and aluminum strips
used for tissue thickness greater than 10-12 cm
some primary beams absorbed → must increase mAs
the goal of ___ is to obtain maximal diagnostic information with minimal exposure of the patient, radiology personnel and the general public
radiation protection
ALARA
as low as reasonably achievable
use smallest exposure possible
damage to tissue from xrays is from:
- Direct interaction with DNA
- Indirect effect: ionization of water molecules leads to formation of free radicals, which can then alter cellular mechanisms directly or via damage to genetic material
what are the three most sensitive tissue to xrays?
- Actively dividing tissues such as bone marrow and epithelial cells of the gastrointestinal tract are more responsive to radiation.
- Gonadal cells: damage can be amplified (genetic damage)
- Fetal cells: the younger the fetus the greater the potential for damage (death, congenital malformations, growth defect)
stochastic effects
•NO threshold dose, BUT probability of damage increases with dose (cancer, genetic effects)
can happen 1st xray or 100th xray
main concern!
what is the main concern of xray radiation
stochastic effect
•NO threshold dose, BUT probability of damage increases with dose (cancer, genetic effects)
deterministic effect
- High radiation exposure
- Clinical signs appear after a threshold dose
- Severity increases with the dose (erythema, ulcerations, hematopoietic damage, cataract…)
- Mainly seen in radiation therapy and nuclear accidents
___ effect is seen after a threshold dose
deterministic
mainly seen in radiation therapy and nuclear accidents
methods of xray protection
- Indication! Is there a need for these radiographs?
- Good radiographic technique
•Time
•Sedation / anesthesia
•Distance: inverse square law
•Filtration
•Shielding
- Reduction of scatter: collimation!
- Personnel monitoring
inverse square law
•If the distance from the primary source is doubled, the intensity will decrease by a factor of four
true or false
protection such as shield works for direct radiation
false
protection only helps with reduction of scatter radiation
how to measure radiation exposure
dosimeter
amount of radiation allowed per year
- Whole body: 50 mSv (milli-Sievert) / YEAR
- Embryo/Fetus of a WORKER: 5 mSv over 9 months
natural background exposure in the US
3 mSv/year
The main type of interaction producing X‐rays in the tube is:
Heat production
Bremmstrahlung
Photoelectric effect
Reflection radiation
Acceleration radiation
bremmstrahlung
Compared to a 90 kVp setting, a 70 kVp setting (keeping mAs the same)…
Decreases intensity, peak energy and average energy of the X‐ray beam
What are the two main types of interaction of X‐rays with matter?
Photoelectric and Compton effect
You are standing 1 m away from the patient while someone is taking a radiograph. If you were standing 2 m away, your exposure to scatter radiations would be:
Divided by 4
The additional filtration at the outlet of the X‐ray tube:
Eliminates low energy photons that would increase exposure and not contribute to the image
