Attenuation, HVL and Beam Quality Flashcards
For every 5cm, how much of the number of photons is reduced?
(not an exact amount)
By the same percentage for every 5cm exponentially
What does the test for the 1/2 value layer (QC) involve?
- A series of exposures is taken
- After each exposure, another layer of alumminum (or some attenuator) is added.
- Attenuator thickness and the dosimiter reading for each exposure is recorded and then graphed
- The result of the graph is an exponential curve, showing that as thickness increases, attenuation will increase by a certian amount
What is the purpose of a HVL test?
To find how thick an attenuator has to be to reduce the photon number by 50% (To meausure the quality/penatrating ability of the beam)
If our HVL test showed a lower number than 1.4mm, what would that indicate
Assuming that 1.4mm is ideal
That would mean that most absorption takes place with a small amount of material thickness (attenuator) showing that our average energy was too low.
At energy levels below 120 kV, what are HVLs measured in?
mm of Al
At energy levels of 120-400, what are HVLs measured in?
mm of Cu
Will the number of x rays emerging (being transmitted) from an attenuator reach zero?
No, there will always be at least one photon escaping
Why is the HVL semilogarithmic?
Because the HVL is representing thickness (which is linear) and the number of transmitted photons (which is exponential), which means that the graph is both when combines; i.e. semilogarithmic
What is tenth value layer?
The thickness of a material that will reduce the incident intenisty by a factor of 10% (90% attenuation, 10% transmittion)
What is the TVL used for?
Used for shielding calcualtions to determine the amount of attenuating material to protect people
What happens to the quality and HVL as a heterogenous beam penetrates though matter?
-Its quality/average E increases
-HVL increases
As a homogenous beam penetrates through matter, what happens to quality and HVL?
-Quality/average E remains the same
-HVL reamains the same
What is another way to say polychromatic?
Hetrogenous, polyenergetic
Would the u (linear attenuation coeffient) increase or decrease with a polychromatic beam? Why or why not?
It would decrease, because low E photons are attenuated more rapidly than the higher E photons, showing that a small amount of thickness causes attenuation.
Are diagnostic x ray beams monochromatic or polychromatic?
Polychromatic
What is beam hardening?
When the low energy photons are attenuated more rapidly than high energy photons (effective energy of the beam increasing)
What is the purpose of added filtration? Give examples
To remove low energy photons that increase patient dose. (Aluminum)
What are the 3 ways the beam is filtered?
- Added filtration
- Inherent filtration
- Patient
What is the minimum HVL for 70 kVP?
2.5 mm of Al
What is the minimum HVL for 100 kVP?
3.6 mm of Al
What is the minimum HVL for 120 kVp?
4.3 mm of Al
When photon energies are** just** below the absorption edge (k-edge) of a filter, what is the probability of attenuation?
-u is at minimum
-Transmittion at its maximum
When is attenuation seen in a filter?
- When energy is way below k edge
- Energy is at k-edge
- Energy just above k-edge
At higher E, attenuation decreases
What type of filter+target allows for the most useful x ray energies to be transmitted in mammography with minimal loss of intensity?
Mo filter, Mo target
What is the downside of having a filter and a target of the same material?
They will transmit thier own charecteristic radiation
