Attenuation, HVL and Beam Quality

Question 1

For every 5cm, how much of the number of photons is reduced?

Answer 1

By the same percentage for every 5cm exponentially

Question 2

What does the test for the 1/2 value layer (QC) involve?

Answer 2

1. A series of exposures is taken
2. After each exposure, another layer of alumminum (or some attenuator) is added.
3. Attenuator thickness and the dosimiter reading for each exposure is recorded and then graphed
4. The result of the graph is an exponential curve, showing that as thickness increases, attenuation will increase by a certian amount

Question 3

What is the purpose of a HVL test?

Answer 3

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)

Question 4

If our HVL test showed a lower number than 1.4mm, what would that indicate

Assuming that 1.4mm is ideal

Answer 4

That would mean that most absorption takes place with a small amount of material thickness (attenuator) showing that our average energy was too low.

Question 5

At energy levels below 120 kV, what are HVLs measured in?

Answer 5

mm of Al

Question 6

At energy levels of 120-400, what are HVLs measured in?

Answer 6

mm of Cu

Question 7

Will the number of x rays emerging (being transmitted) from an attenuator reach zero?

Answer 7

No, there will always be at least one photon escaping

Question 8

Why is the HVL semilogarithmic?

Answer 8

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

Question 9

What is tenth value layer?

Answer 9

The thickness of a material that will reduce the incident intenisty by a factor of 10% (90% attenuation, 10% transmittion)

Question 10

What is the TVL used for?

Answer 10

Used for shielding calcualtions to determine the amount of attenuating material to protect people

Question 11

What happens to the quality and HVL as a heterogenous beam penetrates though matter?

Answer 11

-Its quality/average E increases
-HVL increases

Question 12

As a homogenous beam penetrates through matter, what happens to quality and HVL?

Answer 12

-Quality/average E remains the same
-HVL reamains the same

Question 13

What is another way to say polychromatic?

Answer 13

Hetrogenous, polyenergetic

Question 14

Would the u (linear attenuation coeffient) increase or decrease with a polychromatic beam? Why or why not?

Answer 14

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.

Question 15

Are diagnostic x ray beams monochromatic or polychromatic?

Answer 15

Polychromatic

Question 16

What is beam hardening?

Answer 16

When the low energy photons are attenuated more rapidly than high energy photons (effective energy of the beam increasing)

Question 17

What is the purpose of added filtration? Give examples

Answer 17

To remove low energy photons that increase patient dose. (Aluminum)

Question 18

What are the 3 ways the beam is filtered?

Answer 18

1. Added filtration
2. Inherent filtration
3. Patient

Question 19

What is the minimum HVL for 70 kVP?

Answer 19

2.5 mm of Al

Question 20

What is the minimum HVL for 100 kVP?

Answer 20

3.6 mm of Al

Question 21

What is the minimum HVL for 120 kVp?

Answer 21

4.3 mm of Al

Question 22

When photon energies are** just** below the absorption edge (k-edge) of a filter, what is the probability of attenuation?

Answer 22

-u is at minimum
-Transmittion at its maximum

Question 23

When is attenuation seen in a filter?

Answer 23

1. When energy is way below k edge
2. Energy is at k-edge
3. Energy just above k-edge

At higher E, attenuation decreases

Question 24

What type of filter+target allows for the most useful x ray energies to be transmitted in mammography with minimal loss of intensity?

Answer 24

Mo filter, Mo target

Question 25

What is the downside of having a filter and a target of the same material?

Answer 25

They will transmit thier own charecteristic radiation

Question 26

Work on questions.

Answer 26

(Relating to HVL and attenuation)