X-Ray

Question 1

What are the properties of X-ray?

Answer 1

• similar to γ-rays, BUT: arise from e^- shell instead of nucleus
• wavelength 10 - 0.1nm
• frequency: 30 * 10¹⁵ - 10¹⁸ Hz
• E: 100 eV - 1 MeV
  
  • high E used for therapy
  • low E for diagnostics

Question 2

How does a X-ray tube work?

Draw it.

Answer 2

1. e^- produced by heating
2. U_anode accelerates e^- from cathode to anode
3. e^- strike anode
   → e^- decelerated = Bremsstrahlung
   → e^- released from inner shell of anode atoms = characteristic X-rays

Question 3

Why is the anode of an X-ray tube rotating?

Answer 3

to get rid of heat via convection

Question 4

Why is vacuum in the X-ray tube?

Answer 4

otherwise e^- wouldn’t be able to travel with E_kinmax

Question 5

How does a linear accelerator work?

Draw it.

Why is it used?

Answer 5

1. inside the electrodes: high alternating electric field
2. e- accelerated btw 2 electrodes (→ v_e^- increases → electrodes longer and longer to keep travelling time constant)
3. X-rays produced by deceleration (= Bremsstrahlung)

⇒ used for therapy because X-rays with higher E can be obtained

Question 6

How does a cyclotron work?

When is it used and why?

Answer 6

1. acceleration of e^- btw the “Ds” (source in the middle)
2. magn. field keeps e^- on circular path
   → as v increases → r increases
3. e^- reach exit slit → leave cyclotron

X-rays produced esp. used for production of positron radiating isotopes used in PET-scans

Question 7

What is a synchrotron?

When is it needed?

Answer 7

• huge cyclotron that produces very high E X-ray for research purposes
• synchronizes btw motion of particle and electric field (normal cyclotron wouldn’t be able to do so)

Question 8

What are the types of X-ray interactions with matter?

Answer 8

• diffraction
• absorption

Question 9

When does X-ray diffraction occur?

Give the formula.

Answer 9

• occurs when diffraction pattern in size of λ_X-ray = 1pm - 10nm
• constructive interference occurs only at Bragg-angle
  nλ = 2d sin (sign)
  
  • λ = wavelenght of monochr. X-ray
  • d = distance btw lattice pattern (e.g. atoms in a crystal)
  • n = degree of principle maximum
  • sign = angle of lattice plane

Question 10

What is X-ray diffraction used for?

Answer 10

if amplification occurs (d = x * λ):

1. “measured” by photographic film
2. electron density map rendered
3. atomic model reconstructed

Question 11

On which fundamental attenuation does X-ray absorption depend on?
Give some formulas.

On what do the attenuation coefficients depend on?

Answer 11

_exponential attenuation principle _

• J = J₀ * e^-μx
• μ = μ_m * η
• μ_m = τ_m + σ_m + κ_m
  
  • τ_m = photoeffect attenuation coefficient
  • σ_m = Compton attenuation coefficient
  • κ_m = pair production attenuation coefficient

⇒ depend on type of material and hf

Question 12

Explain briefly the photoeffect attenuation of X-radiation.

On which paramaters does the amount of attenuation depend on?

Why is it of special relevance in medical practice?

Answer 12

1. incoming X-ray photon of hf = 10 - 100 keV strikes atom
2. e^- released, E attenuated

absorption depends on: τ = c * Z^{3 *}λ³

• c = constant depend on type of material
• Z = atomic number
• λ = wavelength

⇒ main effect in diagnostic X-ray

Question 13

Explain briefly the Compton attenuation of X-radiation.

What is important to mention when we talk about Compton attenuation?

Answer 13

1. incident X-ray photon removes Compton e^- from outer shell
2. Compton photon of lower E (= larger λ) emitted

⇒ negligible, BUT main type of attenuation for absorbants with low Z_eff → σ_m ~ Z

e.g. H₂0, soft tissues

Question 14

1. Explain briefly the pair production attenuation of X-radiation.

Why is it of special relevance in medical practice?

Answer 14

1. incident X-ray photon of hf > 1022 keV absorbed near nucleus
2. e^- e⁺ pair created (hf = 2m_ec² + 2E_kin)

⇒ relevant in high E X-ray photons (therapy)

Question 15

Why do spikes appear in absorption spectrum of X-ray?

How are they called?

What are the best absorbers, on what does the overall absorption finally depend on?

What are good absorbers?

Answer 15

• absorption edges appear due to e- transition characteristic for each type of material (characteristic X-ray absorption)
• absorption depends on:
  
  • density (affects attenuation coefficient)
  • atomic number
• good absorbers:
  
  • high Z (e.g. Pb, bones = Ca, P)
  • blood: good absorber due to ions

Question 16

Why are filters applied in X-ray imaging and therapy?

What is the best filter material and why?

Answer 16

• most absorption in low E-ranges (= large λ)
  → hazard for patient, but no information carried that would be needed for imaging
• best radiation protection: Pb due to high Z

Question 17

What is the basic type of imaging in X-ray diagnostics?

Which areas are the brightest?

Answer 17

summation image

• all layers btw X-ray source + detector take part in imaging
• brightest areas = high density, high Z_eff

Question 18

How does an electronic X-ray image amplifier work?

Draw it.

Answer 18

1. radiation sent through body
2. hits 1st luminescent screen → light produced (attached to photocathode)
3. e^- released from photocathode at luminescating points
4. e^- accelerated (basically PMT)
5. e^- beam reaches 2nd luminescent screen → produces light

Question 19

Explain the different types of contrast agents used for X-ray imaging methods.

Give some examples when they are used.

Answer 19

• positive contrast materials: absorb better than environment (greater Z_eff) → darker
  
  • BaSO₄: used in gastroint. tract
  • iodine cont. comp: intravenously
• negative contrast materials: absorb less than environment (lower Z_eff) → brighter
  
  • air, 0₂, CO₂
• dual contrast materials: simultaneous application of positive and negative contrast materials

Question 20

Explain what DSA is.

Name an example when is it used.

Answer 20

DSA = digital subtraction angiography

picture with contrast - picture without contrast → difference btw them

e.g. used to check state of arteries

Question 21

How does traditional tomography work?

When is it used? Give some examples.

Answer 21

only center of circular path is sharp, everything above and below is blurred

• X-ray source travels on circular path around center (= object)
• detector travels antiparallel to X-ray source

used when not wanted areas should be hidden, e.g.

• pulmonology: ribs hidden
• dentral panoramic image

Question 22

How does a CAT-scan work?

What is the difference btw a “normal” CT and a spiral CT?

Answer 22

1. transillumination from many directions (180°) (parallel to sagittal axis) → sum of densities measured
2. computer calculates D of different elements
3. cross-sectional images of each dir. obtained (path)
4. pictures above each other → whole cross-section

spiral CT:

• patient moves also → 3D-images
• BUT: increased radiation hazard

Question 23

What is the Hounsfield-scale and why is it used?

Give the formula how the HU is calculated.

Name 3 important values.

Answer 23

instead of absolut attenuation coefficients, relative coefficients are used (in relation to H₂0)

HU = (μ-μ_water)/μ_water * 1000
→ HU_air = -1000 (lower end of the scale)
→ HUwater = 0
→ HU_bone = +3000 (upper ende of the scale)

Question 24

What is windowing and why was it introduced?

Answer 24

most soft tissue btw 0-80 HU, but human eye is only able to differentiate btw 20 greytones

(range -1000 - +3000 → 100HU/greytone)

Windowing:

greytones related to smaller part of HU-scale, windows can be set → greytones btw 0-60 HU → 3HU/greytone

• HU > window: white
• HU < window: black

⇒ different information content can be obtained from different windows from the same data set