1. Medical Imaging
X-Rays I

2. Principle of X-ray
A source of radiation

3. Principle of X-ray A source of radiation A patient of non uniform
substance

4. Principle of X-ray A source of radiation A shadow
A patient of non uniform
substance

5. Principle of X-ray
A source of radiation

6. X-ray tube Working Principle: Accelerated charge causes EM radiation:
Cathode filament C is electrically heated (VC = ~10V / If = ~4 A) to boil off electrons
Electrons are accelerated toward the anode target (A) by applied high-voltage (Vtube = 40 – 150 kV);
Deceleration of electrons on target creates "Bremsstrahlung"
evacuated gas envelope
filament
VC, If + A - C - +

7. X-ray tube Cathode Filament (-) Tube (vacuum) Coil of tungsten wire
High resistance in coil ->temperature rise to > 2200oC
Thermionic emission of electrons
Tube (vacuum)
Typical: Vtube = 40 – 150 kVp, Itube = mA
evacuated gas envelope
filament
VC, If - + - - - - - - A - - - C
space charge
stops further
emission
kVp, Itube - +

8. X-ray tube Anode Tungsten (high atomic number Z=74)
Electrons striking the anode generate HEAT and X-Rays
In mammography ->Molybdenium (Z=42) and Rhodium (Z=45)
Stationary anode-> tungsten embedded in copper
Rotating anode (3000 to 10,000rpm) -> increase heat capacity, target area
evacuated gas envelope
filament
VC, If - + - - - - - - A - - - C
space charge
kVp, Itube - +

9. XRAY PRODUCTION

10. X-RAY production X-ray tube produces two forms of radiation
Bremsstrahlung radiation (white radiation)
Characteristic radiation

11. White radiation, Bremsstrahlung
(Brake)
Inelastic interaction with atoms nuclei
Loss of kinetic energy
Xray (E) = lost kinetic E
X-Ray
High kinetic energy
Forward radiation
Emission  Z2
electron
Coulombic interaction
(Atomic number)
# of protons

12. White radiation, Bremsstrahlung
-Smaller L produce larger X-ray
-Broad range of emitted wavelengths
X-Ray L

13. How many wavelength will be emitted by a beam of electrons underegoing “Bremsstrahlung ”

14. White radiation, Bremsstrahlung
-Smaller L produce larger X-ray
-Broad range of emitted wavelengths
X-Ray L
impact with nucleus
maximum energy

15. X-ray intensity -QUANTITY
Overall Bremsstrahlung intensity I :
90% of electrical energy supplied goes to heat, 10% to X-ray production
X-ray production increases with increasing voltage V

16. Bremsstrahlung spectrum
relative output
Theoretically, bremsstrahlung from a thick target creates a continuous spectrum from E = 0 to Emax
Actual spectrum deviates from ideal form due to
Absorption in window / gas envelope material and absorption in anode
Multienergetic electron beam
Peak voltage kVp

17. Characteristic radiation
relative output
Energy must be > binding energy
Discrete energy peaks due to electrons transitions
Ka transition L->K
Kb transition M,N,O->K
Peak voltage kVp

18. Characteristic radiation
Incident electron

19. Characteristic radiationl2
Incident electron
Occurs only at discrete levels
There is a possibility of forming Auger electrons

20. Characteristic radiation
In Tungsten characteristic X-ray are formed only if V>69.5 kV because K shell binding energy is 69.5 keV
Molybdenum K-shell can be obtained at V> 20kV
L shell radiation is also produced but it’s low energy and often
absorbed by glass enclosure

21. X-ray intensity -QUALITY
Effective photon energy produced
Effective = ability to penetrate the patient
Effective photon energy ~ 1/3 to ½ of energy produced
Higher energy better penetration
Beam filtration – beam hardening

22. Beam Hardening
Polyenergetic beam >monoenergetic beam

23. X-ray tube construction

24. Anode
Most of the energy deposited on the anode transfers
into heat

25. Reduction of anode heating
Made of Tungsten, high melting point high atomic number Z = 74
Kinetic energy of incident electrons
100keV electron
6 MeV electron

26. Anode the target angle, 7 to 20 (average 12)
Seffective = Sactual*sin() > Line focusing principle

27. Anode filament balance
General radiography

28. Heel effect - SID source to image distance
- Heel effect is smaller at smaller
SID
Reduction of intensity
on the anode side
SID
The reduction in intensity can be used to reduce patient exposure

29. Beam collimation Size and shape of the beam Lead shutters
Dose reduction

30. Reduction of anode heating
Anode angle of 7º…15º results in apparent or effective spot size Seffective much smaller than the actual focal spot of the electron beam (by factor ~10)
Rotation speed ~ 3000 rpm
Decreases surface area for heat dissipation from by a factor of

31. Limitations of anode angle
Restricting target coverage for given source-to-image distance (SID)
"Heel effect" causes inhomogeneous x-ray exposure

32. X-ray tube - space charge
Space charge cloud forms at low tube voltage
At low filament current a saturation voltage is achieved, rising tube voltage will not generate higher electron flow
At high filament current and low tube voltage, space charge limits tube current->space-charge limit

33. Space charge limited
At high filament current and low tube voltage, space charge limits tube current->space-charge limit

34. Generator Single phase Three phase Single phase input (220V, 50A)
Single pulse or double pulse->rectifier
Min exposure time 1/120 sec
Xray tube current non linear below 40kV
Three phase
Three phase wave, out of phase 120 deg
More efficient higher voltage
Better control on exposure

35. Rectifier
Protects cathode from anode thermionic emission

36. Rectifier
1 phase
3 phase

37. BREAK!

38. Principle of X-ray A source of radiation A patient of non uniform
substance

39. Attenuation N = Noe-mL N True for Loss of photons by monoenergetic
x-ray
Loss of photons by
scattering or absorption L1 L No N
m -> linear attenuation
coefficient L1

40. m linear attenuation coeff.
m = mr+ mph+ mc+ mp [cm-1]
rayleigh
photoelectric
Compton
pair

41. m linear attenuation coeff.
m = mr+ mph+ mc+ mp [cm-1]
depends on tissue
soft tissue, hard tissue, metals
m decreases when energy increase
soft tissue:
m = 0.35  0.16 cm-1 for E = 30  100keV
m depends on density of material
mwat > mice> mvapor

42. Mass attenuation coeff.

43. Mass attenuation coeff.
N = Noe- r(m/r)L
rL = mass thickness

44. Mass attenuation coeff.
N = Noe- r(m/r)L
rL = mass thickness I x

45. Poly-energetic beam
Mass attenuation coefficient and linear attenuation coefficient are for mono-energetic beam
Half-value layer is for quantifying poly-energetic beams

46. HVL half value layer
Thickness of material attenuating the beam of 50% - narrow beam geometry
HVL for soft tissue is 2.5  3.0 cm
at diagnostic energies

47. HVL half value layer Transmission of primary beam:
10% chest radiography
1% scull radiography
0.5% abdomen radiography
Mammography (low energy HVL 1 cm)

48. Mean free path 1/m Average distance traveled before interaction
MFP=1/m
HVL
mfp

49. Principle of X-ray A source of radiation A shadow
A patient of non uniform
substance