X-ray interactions Flashcards

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1
Q

What do kV and mA control during x-ray production?

A

-KV: controls the accelerating voltage between anode and cathode. increasing kV shifts spectrum to right, increased max/effective energy/increased total no of photons
-mA: controls the tube current (energy and quantity of electrons flowing between anode and cathode). increases quantity of photons

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2
Q

What effect does filtration have on the beam produced?

A

moves spectrum to right, increases minimum and effective energy, reduces no of photons.

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3
Q

Brief overview of x-ray production

A

-Tungsten filament within cathode is heated until incandescent –> allows thermionic emission.
-Electrons are focussed via the cup and attracted to the anode target (tungsten) while travelling through a vacuum.
-High frequency generators are used to maintain a near constant waveform (allows increase x-ray output and lower pt dose)
-Bremsstrahlung and characteristic radiation occurs to produce photons.

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4
Q

What is the focusing cup made up of? What kind of material is it

A

Molybdenum, negatively charged (repels electrons). Has very high melting temp and very poor thermionic emitter.

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5
Q

Anode: material, Z number, features

A

-Tungsten rhenium target (high Z number)
-Rhenium helps with longevity
-Molybdenum disk and stem for poor heat transfer. Can have silver lubricant/blackened copper rotor.

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6
Q

Whaat is the Tungsten K edge (to produce characteristic radiation)?

A

70kV

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7
Q

How is heat removed from the x-ray?

A

Removed by conduction through oil and radiation through glass vaccuum.

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8
Q

What are the types of x-rays produced at the anode?

A

Characteristic and Bremsstrahlung

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9
Q

What is Bremsstrahlung radiation? % of rays produced

A

-Braking of electron near nucleus released produces x-ray.
-80% of rays produced this way.

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10
Q

What is the effect on Bremsstrahlung radiation when you change the following features:
-kV
-mA
-Filtration

A

-Increasing kV: increases quantity, average energy and minimum energy
-Increasing mA: increases quantity of photons only
-Filtration: reduces quantity of photons, average energy and min energy.

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11
Q

How is atomic number linked to Bremsstrahlung radiadtion?

A

The higher the atomic number, the more protons in nucleus, the more radiation produced. (proportional)

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12
Q

What is characteristic radiation?

A

-Occurs at anode
-Incoming electron interacts with inner shell electron of target material and the electron is ejected.
-Creates a vacancy, which is filled by dropping down of an upper shell electron. This dropping down releases characteristic radiation.

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13
Q

When can characteristic radiation also be emitted (in tissues)

A

After photo-electric effect occurs, if the energy is higher than the binding energy of that shell.

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14
Q

Name 5 factors that affect the x-ray spectrum

A
  1. Filament current (mA) - affects quantity of electrons
  2. tube potential (kV): increases quality, quantity and max energy
  3. Filtration: filters lower energy electrons via photo-electric effect, decreases quantity if photons, increases average energy .
  4. Anode target: higher atomic Z, more Bremmstrahlung occurs (more quantity).
  5. Generator waveform: increases quality and quantity of electrons
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15
Q

Name the 3 types of interactions between x-rays and tissues

A
  1. Transmission
  2. Attenuation: absorption of x-ray (photoelectric effect)
  3. Scattering: deflection of photon (Compton, elastic and pair production)
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16
Q

Describe photo-electric effect (PEE). How does this contribute to patient dose?

A

-Photon hits inner shell electron and is completely absorbed + a photo-electron escapes (low energy <1 keV, will be attenuated in tissues and not contribute to image)
-Ejected electron creates a vacancy and a higher electron will come down and release characteristic radiation.
-End product of PEE: ion, low energy x-ray and photo-electron.

17
Q

What 3 factors affect the probabilities of PEE?

A
  1. higher atomic number (Z) (eg iodine): more PEE
  2. Higher tissue density (p): more PEE
  3. Higher energy (E): less PEE b/c more likely to pass straight through without interacting with the matter.

Probility = pZ^3 / E^3

18
Q

What is a K edge and why is it useful?

A

-Energy level at which the K-shell electron will be released (binding energy of the electron)
-Below this range: cannot release the electron and t/f no PEE occurs.
-At the edge: increase likelihood of the PEE to occur and can use this to our benefit at diagnostic ranges to increase contrast between iodine and surrounding tissues.

19
Q

What is Compton scatter mechanism?

A

-Incident x-ray interacts with outer shell electron –> photoelectron is released and the x-ray scatters (not all energy is given to photoelectron).
-The angle of theta will be proportional to energy of new photoelectron (larger scatter angle = more energy conferred to photoelectron) and the less energy the Compton scatter has.
*Increasing theta decreases Compton scatter energy

20
Q

What is the effect of atomic number, electron density, mass density on Compton scatter?

A

-Unaffected by atomic number
-Affected by density of the tissue and electron density.

21
Q

What contribution to image does the Compton effect provide? (to contrast/noise/image quality/patient dose)

A

-Contributed to patient dose
-Reduces image quality
-Decreases contrast and increases noise

22
Q

What is Elastic/Rayleigh’s scatter? When does it occur/does it contribute to patient dose?

A

-Xray interacts with all electrons in the atom –> all the electrons oscillate temporarily –> when they rest they release x-ray radiation at an angle.
-Occurs at lower diagnostic ranges, when photon energy is less than the binding energy. All energy is conserved.
-Doesn’t contribute to patient dose

23
Q

What is HVL? What is the effect with beam hardening?

A

-Thickness (distance) of tissue required to halve the number of photons in a beam
-Is inversely proportional to LAC: as LAC increases (removes a large proportion of x-rays quickly) - HVL decreases
-Increases with beam hardening.
(beam hardening only occurs if the beam is not mono-energetic)

24
Q

What is the LAC? Equation?

A

-Determines the amount of x-rays removed from a beam as it travels a set distance through tissue
-0.693/HVL

25
Q

What is the MAC? How is it related to LAC and density? What factors affect MAC?

A

MAC = LAC/density, t/f is independent of density.
-only Z and E affect MAC.
-Described the attenuation of a monochromatic beam per unit mass