diagnostic imaging Flashcards
If an x-ray has a short wavelength, what would the frequency be?
What does a high frequency mean?
It would have a high frequency. This means very energetic electromagnetic waves. Therefore, x-rays are potentially harmful due to the short wavelength and high frequency, therefore high energy.
What are the 7 parts to the electromagnetic spectrum?
radio waves, microwaves, infra-red, visible light, x-ray, gamma-rays
When x-rays interact with matter, what 3 things could happen?
penetrate, absorb, scatter
what is the relationship between wavelength and energy?
energy is inversely proportional to wavelength. eg. short wavelength = high energy. and high frequency.
What is it that x-rays do which cause damage? What do they cause damage to?
They produce ions in material that they penetrate. This causes biological DNA damage.
What are the two types of damage that x-rays can cause?
They are ionizing - produce electron pairs in tissue.
Direct damage - destroys DNA
indirect damage - produce radicals by ionizing water.
Once cells are damaged - what could happen?
They are either repaired or may mutate. Mutation leads to genetic changes, somatic changes, teratogenic changes, death.
Which are the tissues most affected by x-rays? Give examples.
Tissues that divide regularly. For example, bone marrow, epithelial cells of GI tract, gonadal cells, embryonic cells (esp. at early stage.)
Name the two biological effects of x-rays? Explain each one.
- stochastic effect: probability of damage increases with dose - no known lower threshold.
- deterministic effects: damage only over a threshold dose, severity increases with dose.
What is the goal of radiation protection? name some of the principles.
Minimal exposure with maximal information gained. ALARA - as low as reasonably possible justification - medical indication high risk groups not involved dose monitoring dose limits controlled and regulated areas.
exposure units?
roentgen R
Limited effective dose to whole body for employee?
20mSv/year - Sv (sieverts) is the effective dose to different body parts.
What are the laws with regards to holding and restraining animals?
only permissable if there is a good clinical reason - small animals should be sedated and restrained with sandbags.
name some methods of protection from x-rays?
indication, time kept low, distance (inverse square law) shielding (clothing), collimation, personnel monitoring.
what is filtration of the X-ray tube?
built in metal plates to filter out low energy (harmful) radiation.
explain the inverse square law of distance in x-rays?
as the distance from the primary source is doubled, the intensity decreases by a factor of 4.
How could you increase distance from the X-ray tube? eg)horses?
use cassette holders, cords from the machine.
protective clothing only protects you from what kind of radiation?
scatter only - not the main direct beams.
what is collimation?
done from all four sides to reduce the area that is exposed to the x rays. Done using a built in collimator.
explain dosimeters?
analyse every 3 months. any professional should wear one. wear under protective clothing. film badge or TLD.
What happens within an x ray tube to produce x rays?
vacuum with an anode and a cathode inside. electrons fired from cathode to the anode. converted to EM radiation. they are produced, accelerated and decelerated. cathode made of tungsten. A lot of heat produced.
What is mAs?
number of electrons. longer exposure = more x rays. so milli amps per second used. + longer exposure time = motion artefacts.
what is kVp?
acceleration of electrons. faster they travel, more energy and more hit target.
anode made of tungsten… why?
high melting point and can be rotating to avoid heating up. also can be angled to get a smaller focal point from a larger one.
smaller focal point = less blurry.
what is it called when an image is more/less blurry?
penumbra
what is the advantage of a larger focal point?
reduced detail but can use higher exposures.
What happens if you increase the kVp? (energy electrons have)
increased velocity of electrons. increased number of x ray photons produced. increases the penetrating power and energy of electrons.
what happens if you increase the mAs? (number of electrons) longer exposure = more electrons
increase filament heating. increased number of x rays produced.
explain attenuation? 2 ways?
decreases intensity of an x ray beam as it passes through matter?
- absorption = photon removed
- transmission = photon changes direction
explain transmission?
x ray passes through matter without interaction.
What is the photoelectric affect?
complete absorption of x ray. = good image. atomic number dependant.
what is the Compton effect?
depends on density. - scatter. depends on kVp. not good image. results if kVp too high.
what do grids do?
they reduce scatter and produce a better contrast image. not a safety device.
