X-Ray Flashcards
What are the properties of X-ray?
- 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
How does a X-ray tube work?
Draw it.
- e- produced by heating
- Uanode accelerates e- from cathode to anode
- e- strike anode
→ e- decelerated = Bremsstrahlung
→ e- released from inner shell of anode atoms = characteristic X-rays
Why is the anode of an X-ray tube rotating?
to get rid of heat via convection
Why is vacuum in the X-ray tube?
otherwise e- wouldn’t be able to travel with Ekinmax
How does a linear accelerator work?
Draw it.
Why is it used?
- inside the electrodes: high alternating electric field
- e- accelerated btw 2 electrodes (→ ve- increases → electrodes longer and longer to keep travelling time constant)
- X-rays produced by deceleration (= Bremsstrahlung)
⇒ used for therapy because X-rays with higher E can be obtained
How does a cyclotron work?
When is it used and why?
- acceleration of e- btw the “Ds” (source in the middle)
- magn. field keeps e- on circular path
→ as v increases → r increases - e- reach exit slit → leave cyclotron
X-rays produced esp. used for production of positron radiating isotopes used in PET-scans
What is a synchrotron?
When is it needed?
- 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)
What are the types of X-ray interactions with matter?
- diffraction
- absorption
When does X-ray diffraction occur?
Give the formula.
- occurs when diffraction pattern in size of λX-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
What is X-ray diffraction used for?
if amplification occurs (d = x * λ):
- “measured” by photographic film
- electron density map rendered
- atomic model reconstructed
On which fundamental attenuation does X-ray absorption depend on?
Give some formulas.
On what do the attenuation coefficients depend on?
_exponential attenuation principle _
- J = J0 * e-μx
- μ = μm * η
- μm = τm + σm + κm
- τm = photoeffect attenuation coefficient
- σm = Compton attenuation coefficient
- κm = pair production attenuation coefficient
⇒ depend on type of material and hf
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?
- incoming X-ray photon of hf = 10 - 100 keV strikes atom
- e- released, E attenuated
absorption depends on: τ = c * Z3 *λ3
- c = constant depend on type of material
- Z = atomic number
- λ = wavelength
⇒ main effect in diagnostic X-ray
Explain briefly the Compton attenuation of X-radiation.
What is important to mention when we talk about Compton attenuation?
- incident X-ray photon removes Compton e- from outer shell
- Compton photon of lower E (= larger λ) emitted
⇒ negligible, BUT main type of attenuation for absorbants with low Zeff → σm ~ Z
e.g. H20, soft tissues
- Explain briefly the pair production attenuation of X-radiation.
Why is it of special relevance in medical practice?
- incident X-ray photon of hf > 1022 keV absorbed near nucleus
- e- e+ pair created (hf = 2mec2 + 2Ekin)
⇒ relevant in high E X-ray photons (therapy)
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?
- 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
Why are filters applied in X-ray imaging and therapy?
What is the best filter material and why?
- 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
What is the basic type of imaging in X-ray diagnostics?
Which areas are the brightest?
summation image
- all layers btw X-ray source + detector take part in imaging
- brightest areas = high density, high Zeff
How does an electronic X-ray image amplifier work?
Draw it.
- radiation sent through body
- hits 1st luminescent screen → light produced (attached to photocathode)
- e- released from photocathode at luminescating points
- e- accelerated (basically PMT)
- e- beam reaches 2nd luminescent screen → produces light
Explain the different types of contrast agents used for X-ray imaging methods.
Give some examples when they are used.
-
positive contrast materials: absorb better than environment (greater Zeff) → darker
- BaSO4: used in gastroint. tract
- iodine cont. comp: intravenously
-
negative contrast materials: absorb less than environment (lower Zeff) → brighter
- air, 02, CO2
- dual contrast materials: simultaneous application of positive and negative contrast materials
Explain what DSA is.
Name an example when is it used.
DSA = digital subtraction angiography
picture with contrast - picture without contrast → difference btw them
e.g. used to check state of arteries
How does traditional tomography work?
When is it used? Give some examples.
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
How does a CAT-scan work?
What is the difference btw a “normal” CT and a spiral CT?
- transillumination from many directions (180°) (parallel to sagittal axis) → sum of densities measured
- computer calculates D of different elements
- cross-sectional images of each dir. obtained (path)
- pictures above each other → whole cross-section
spiral CT:
- patient moves also → 3D-images
- BUT: increased radiation hazard
What is the Hounsfield-scale and why is it used?
Give the formula how the HU is calculated.
Name 3 important values.
instead of absolut attenuation coefficients, relative coefficients are used (in relation to H20)
HU = (μ-μwater)/μwater * 1000
→ HUair = -1000 (lower end of the scale)
→ HUwater = 0
→ HUbone = +3000 (upper ende of the scale)
What is windowing and why was it introduced?
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
