Attenuation, HVL and Beam Quality Flashcards

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1
Q

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

A

By the same percentage for every 5cm exponentially

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2
Q

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

A
  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
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3
Q

What is the purpose of a HVL test?

A

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)

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4
Q

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

Assuming that 1.4mm is ideal

A

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

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5
Q

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

A

mm of Al

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6
Q

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

A

mm of Cu

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7
Q

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

A

No, there will always be at least one photon escaping

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8
Q

Why is the HVL semilogarithmic?

A

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

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9
Q

What is tenth value layer?

A

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

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10
Q

What is the TVL used for?

A

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

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11
Q

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

A

-Its quality/average E increases
-HVL increases

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12
Q

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

A

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

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13
Q

What is another way to say polychromatic?

A

Hetrogenous, polyenergetic

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14
Q

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

A

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.

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15
Q

Are diagnostic x ray beams monochromatic or polychromatic?

A

Polychromatic

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16
Q

What is beam hardening?

A

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

17
Q

What is the purpose of added filtration? Give examples

A

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

18
Q

What are the 3 ways the beam is filtered?

A
  1. Added filtration
  2. Inherent filtration
  3. Patient
19
Q

What is the minimum HVL for 70 kVP?

A

2.5 mm of Al

20
Q

What is the minimum HVL for 100 kVP?

A

3.6 mm of Al

21
Q

What is the minimum HVL for 120 kVp?

A

4.3 mm of Al

22
Q

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

A

-u is at minimum
-Transmittion at its maximum

23
Q

When is attenuation seen in a filter?

A
  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

24
Q

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

A

Mo filter, Mo target

25
Q

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

A

They will transmit thier own charecteristic radiation

26
Q

Work on questions.

A

(Relating to HVL and attenuation)