Flashcards in • FILTRATION• COLLIMATION• The INVERSE SQUARE LAW Deck (21):
what does the filter do for us? or what does not having a filter do to the patient?
No filter - longer wavelength radiation absorbed in first few millimeters of patient’s soft tissue; these photons do not participate in image production, but INCREASE PATIENT DOSE!
Most longer wavelength radiation is removed by aluminum filter…
Therefore, the primary purpose of filtration is to reduce patient dose!
what are the effects of filtration on the number of photons in x ray beam and the average energy of photons in resultant beam?
number of photons is reduced and the average energy of photons is somewhat higher in this filtered beam.
• removes (primarily) lower energy x rays
• therefore, average energy (quality) of beam increases
• decreases overall darkness of the final image slightly (because some high energy photons are removed from beam(assuming no other changes are made)
what is inherent filtration of xray and added filtration and what is the total filtration then?
INHERENT FILTRATION OF X RAY TUBE (glass envelope) is approx. 0.5 mm. Al equivalentADDED FILTRATION - usually Al disk
TOTAL FILTRATION = INHERENT FILTRATION + ADDED FILTRATION
what should the total filtration be for a machine operating at or above 70 kVp?
Total filtration must be 2.5 mm. Al equivalent for machines operating at or above 70 kVp.
This is something that we do not usually worry or think about – filtration is installed during manufacture of the machine, and the machine cannot be sold commercially unless it has the proper filtration…
what are collimators?
Collimators may have different designs, but all are intended to reduce the area of beam to a size close to that of the image receptor.
the beam can be rectangular or circular.
Collimation means controlling the area of the x-ray beam so it is as small as possible for our purposes.
The rectangular decrease the dose but you have to be more technical and accurate.
“Lead washer” collimator (arrow) in PID where it attaches to x ray tube head…
what is the specification of diameter of the collimator?
Beam diameter at end of cone must be no greater than 2-3/4 inches (7 cm.)
Again, this is something that is part of the manufacturing and inspection processes….
what shape of collimators do most dental xray machines use?
what does the collimator do then?
1. Reduces the volume of tissue irradiated (thus decreases patient dose).
2. Reduces the production of scatter radiation (thus improves image quality).
Therefore, whenever possible or practical, we try to collimate the x ray beam so that it is only as wide as we need it!!
how does scattering effect the final image?
Photon b is absorbed, photon C doesn’t interact. Photon C area will be dark. Part B should be quite white, but if A is scattered and ends up in the area that should be really white, and so it ends up grey and so there won’t be as much contrast as there could be.
what is the inverse square law?
The intensity of a beam of electromagnetic radiation is inversely proportional to the square of the distance from the source of radiation.
As with any other forms of electromagnetic radiation, the INTENSITY of the x ray beam decreases as distance from the x ray source increases
The intensity (i.e. number of photons per square unit) of the x ray beam at distance 2D is 1/4 what it is at distance D.
what is the significance of this in regards to image production? radiation protection?
Image production – further the image is away from the source the longer the exposure time to get a good image, this isn’t practical because the patient is more likely to move.
Radiation protection – further you are away, the less intense will be the radiation and the more safe you would be.
how does attenuation of xrays occur?
1. Absorption, or
There are some that go right through which is transmitted x-ray.
for attenuation of xrays what is coherent scatter?
Doesn’t remove it from it’s shell but pushes it to a slightly higher energy level and when it drops down to become stable again it emits a photon.
**In diagnostic radiology, this is relatively insignificant as far as scatter production is concerned.**
- no loss of energy is involved
- only very low energy photons are involved
what is the photoelectric effect?
Overcomes the binding energy of the electron, electron shoots off into space with a small amount of kinetic energy, this creates an ion. To regain electrical neutrality, one of the higher level electrons will move to fill that void, and then another one to fill that one, and so forth depending on the amount of electron shells in the species. This releases a photon each time an electron drops down.
Is inversely proportional to the third power of the photon energy
(so, the lower the photon energy, the more likely a photoelectric interaction will occur)
- predominant interaction with low-energy radiation (but higher than coherent scattering); probability of interaction is inversely proportional to the third power of the photon energy; the interaction is most probable when the energy of the photon is close to and slightly greater than the binding energy of the orbital electron
- 80% of photoelectric interactions in diagnostic radiology occur in the k-shell; the density of the electron cloud is greatest in this region
- the probability of interaction is directly proportional to the third power of the atomic number of the absorber; interactions with low atomic number elements mostly occur in k-shells, while interactions with high atomic number elements occur more frequently in higher shells as well as in the k-shell
- a photoelectric interaction results in total absorption of the incident energy by the matter; although some “scatter” is produced from formation of characteristic radiation photons, this is usually absorbed within the irradiated matter and does not exit it
what is the photoelectric probability?
Is directly proportional to the third power of the atomic number of the absorbing element
(so, the higher the atomic number of the absorbing material, the more likely a photoelectric interaction will occur)
So calcium and phosphorous in bone for example are metals and so they will create an effect more readily than soft tissue.
what is the compton scatter (compton effect)?
Outer shell electron doesn’t have very high binding energy and so it doesn’t take much to knock it out. This means that the scattered photon could still have some energy but the electron that is knocked out and has kinetic energy which can mean it is scattered radiation and can effect the final image.
Large amounts of scatter will reduce the radiographic contrast… (see lecture 6)
- probability of interaction dependent upon total number of electrons in absorbing matter, which is generally a function of its density; interaction is independent of atomic number
- probability is dependent on photon energy; this interaction usually occurs only with high energy photons interacting with free or loosely bound electrons (with an atom with a relatively high atomic number, only the outer shell electrons are free, while with low atomic number atoms e.g. those found in soft tissue, virtually all electrons could be considered to be free)
- some of the energy of the incident photon is absorbed, but most of it is usually results in scatter radiation
what happens to the recoil electrons produced during the photoelectric and compton interactions?
(1) May undergo collisional interactions producing ionization or excitation of other atoms or molecules.
(2) May undergo radiative interactions producing secondary radiation by Bremsstrahlung interactions (producing mostly heat).
NB - Photoelectric and Compton interactions cause ionizations that have potential biological effects. (possibility of cancer)
what's the summary for this?
In dental diagnostic imaging:
1. Coherent scatter is RELATIVELY INSIGNIFICANT.
2. The photoelectric effect is predominant with low energy radiation and high atomic number absorbers. It results in high contrast, but produces the greatest dose to the absorbing tissue.
3. The Compton effect is the MOST COMMON INTERACTION BETWEEN X RAYS AND BODY TISSUES. It is responsible for almost all of the scatter radiation produced.
4. Photoelectric effect - energy of incident photon totally absorbed;
Compton effect - some of energy of
incident photon absorbed, but most
results in scatter radiation
therefore factors affecting absorption of scatter of x-rays include:
Therefore, factors affecting absorption or scatter of x-rays include:
1. Energy of the incident x ray beam (higher energy beams will be more penetrating).
2. Thickness of the absorber (the greater the thickness of any matter, the greater the absorption).
3. The atomic number of the absorber (the greater the atomic number, the more photoelectric interactions will take place, and the greater will be the total absorption).
4. The density of free electrons in the absorber (absorbers with higher free electron densities will cause more scatter).
what is attenuation of an xray beam mean?
Attenuation of the x ray beam refers to a reduction in the number of photons in the beam by interaction with matter. This may occur by:
1. Absorption - The energy of the incident radiation is totally absorbed in the matter.
2. Scattering - Only a portion of the incident radiation is absorbed; the incident photon undergoes an interaction through which scatter radiation is formed.
In the energy range of diagnostic radiology, attenuation may occur by:
1. Coherent scattering
2. Photoelectric effect
3. Compton effect (Compton scattering)