Physics Applied to Radiology RADI R250 Fall 2003

1
Physics Applied to RadiologyRADI R250 -- Fall
2003

• Chapter 11

2
X-Ray Production

• Function of x-ray machine produce x rays
• Conditions needed inside the x-ray tube
• 1) accumulation of e- (to produce desired mA)
• thermionic emission at filament
• 2) give e- high speed (to produce electron
  energy KE)
• potential difference (kVp) between anode
  cathode
• 3) keep e- in a tight stream as they move toward
  anode
• electrostatic focusing cup
• 4) stop e- at anode (e- interact with atoms)
• converts KE to EM radiation

3
Electron Target Interactions

• e- (mA) accelerated across tube (kVp)
• transfer of KE from electrons to target
• projectile e- penetrates into target material
• one e- may interact with many atoms giving up
  some of its keV to each

anode ()
cathode (-)
çKE (keV)
e-
e-
e-
e-
e-
e-
e-
e-
e-
e-
e-
e-
e-
e-
e-
e-
e-
e-
e-
e-
e-
e-
e-
e-
e-
e-
e-
e-
e-
e-
e-
e-
e-
e-
e-
e-
e-
e-
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KE to EM Energy

• Kinetic energy of the projectile electrons
• kVp on tube gives e- KE
• KE ½ m v2
• not unusual for e- to have KE ½ c
• KE of e- is expressed as keV
• 80 kVp on tube will allow some e- to have a KE of
  a maximum of 80 keV
• interactions occur with orbital e- or nucleus

5
Types of Interactions

• 1) heat ( 95 to 99 of KE)

2) EM radiation ( 5 to 1 of KE)
a) characteristic x rays ( 10 of all x rays
produced)
b) bremsstrahlung x rays ( 90 of all x rays
produced)
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Heat 99 of KE transfer

• excitation of outer shell e- of target atoms
• 1) small amount of energy is transferred
• 2) many interactions per projectile e-
• 3) of e- (not KE) rapid heat build-up

7
Bremsstrahlung X-Rays

• general, white, breaking
• projectile e- comes near the nucleus
• nucleus attracts e-
• e- changes course must slow down
• e- gives up some of its KE as an x-ray photon

8
Brems Energy Level
70 keV

• KE loss by e- will vary
• éD path é E loss
• lost E photon energy
• Ex keVeB - keVeA
• photon Emax
• kVp on tube e-keV Ex
• all energies produced
• Smallest measurable to Emax

90keV

20 keV
45 keV
90keV

45 keV
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Minimum Wavelength

• Minimum ? maximum energy (keV)
• Since brems can be produced at any level below
  the maximum energy the minimum ? is the shortest
  ? in the beam
• It can be determined by formula when the keV is
  known

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Minimum ? Problem

• An x-ray machine is set to 76 kVp. Will there be
  photons of .024nm in the beam?

.024 nm is a longer ? than .016 ? and will not
be found in the beam
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Spectrum Graph for Brems X-Rays

• represents of photons (Y) vs. energy (X)
• maximum E to kVp on tube

• maximum of photons at 1/3 to 1/2 of the Emax

• does not includes photons lost by filtration
• Theoretical graph would start at maximum
  continually descend to Emax

12
Characteristic X-Ray Production

• 1. projectile e- enters atom at high KE
• 2. e-p ionizes atom
• KE of e-p ³ Eb of e-o
• excess energy shared by e-s as KE

• 3. "hole" filled by outer shell or free e-
• must give up energy to move into the hole

4. energy released characteristic x-ray photon
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Characteristic X-Rays (cont.)

• photon release by movement of e- in orbital
  shell
• E released when e- fills hole in orbital
  structure
• Ex D Ebe
• Ebe old- Ebe new
• (-2.8keV) - (-69.5keV) 66.7 keV

Eb
Ee
Ee
ionized
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Characteristic Photons

• have specific (discrete) energy levels
• Ex emitted depend on the target material
• unknown material can be identified by the photon
  energies emitted
• x-ray target materials
• tungsten (K shell Eb - 69 keV)
• general use and fluoro
• molybdenum (K shell Eb -20 keV)
• mammography

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Spectrum Graph Characteristic X-rays

• vertical lines grouped at Ebe of the orbital e-
• no x rays at E between lines

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Combined Emission Spectrum Graphs
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X-Ray Beam Characteristics

• QUALITY
• overall energy in the beam
• QUANTITY
• number of photons in the beam
• INTENSITY
• quantity and quality combined

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Combined Emission Spectrum Graphs

• D in quantity D in amplitude
• D in quality D in R to L placement

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D in kVp
of photons
25
50
75
100
photon energy (keV)

• é kVp é quality Brems peak to R
• highest Brems to R
• é kVp é quantity é amplitude
• characteristic pos. no effect, if produced

20
Intensity kVp

• If the intensity of an x-ray beam is 125 mR at 75
  kVp, what will it be at 85 kVp?
• I2 I1 kVp22 / kVp12
• 125 mR 852 / 752
• 160.555 mR
• 161 mR
• 75 to 85 kVp 13 increase
• 125 to 161 mR 29 increase

• Formula for kVp D

• Relationship
• direct
• Exponential
• I will increase at greater rate than kVp

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of photons
25
50
75
100
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D in mA, mAs, or t
of photons
25
50
75
100
photon energy (keV)

• é mAs quality Brems peak at E
• highest Brems at E
• é mAs é quantity é amplitude
• characteristic pos. no effect, if produced

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Intensity mA, mAs, t

• If the intensity of an x-ray beam is 140 mR at 25
  mAs, what will it be at 15 mAs?
• I2 I1 mAs2 / mAs1
• 140 mR 15 / 25
• 84 mR
• 25 to 15 mAs 40 reduction
• 140 to 84 mR 40 reduction

• Formula for mAs D

• Relationship
• Direct
• Linear
• proportional
• I will change at same rate as mAs

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Combined kVp / mAs ?

• A radiograph made using 75 kVp at 22 mAs was
  changed to 80 kVp at 11 mAs. If the original
  exposure is 68 mR, what will the new exposure be?
• ? in mAs
• I2 I1 mAs2 / mAs1 68 x 11 / 22 34 mR
• ? in kVp
• I2 I1 kVp22 / kVp12 34 x 802 / 752 39
  mR

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D in Target Material
of photons
25
50
75
100
photon energy (keV)

• é Z é quality Brems peak to R
• highest Brems at E
• é Z é quantity é amplitude
• characteristic pos. move to R with é Z

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D Voltage Waveform
of photons
25
50
75
100
photon energy (keV)

• é p/s é quality Brems peak to R
• highest Brems at E
• é p/s é quantity é amplitude
• characteristic pos. no effect, if produced

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D Rectification (1Ø)

• Full quality Brems peak at E
• highest Brems at E
• Full é quantity é amplitude
• characteristic pos. no effect, if produced

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D in Filtration
of photons
25
50
75
100
photon energy (keV)

• é filt. é quality Brems peak moves to R
• highest Brems at E
• é filt. êquantity ê amplitude
• characteristic pos. no effect, if produced