Medical Imaging

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Medical Imaging

• X-Rays I

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

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X-ray tube

• Cathode Filament (-)
• Coil of tungsten wire
• High resistance in coil -gttemperature rise to gt
  2200oC
• Thermionic emission of electrons
• Tube (vacuum)
• Typical Vtube 40 150 kVp, Itube 1-1000mA

evacuated gas envelope
filament
VC, If
-

-
-
-
-
-
A
-
-
-
-
C
space charge stops further emission
kVp, Itube
-

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X-ray tube

• Anode
• Tungsten (high atomic number Z74)
• Electrons striking the anode generate HEAT and
  X-Rays
• In mammography -gtMolybdenium (Z42) and Rhodium
  (Z45)
• Stationary anode-gt tungsten embedded in copper
• Rotating anode (3000 to 10,000rpm) -gt increase
  heat capacity, target area

evacuated gas envelope
filament
VC, If
-

-
-
-
-
-
A
-
-
-
-
C
space charge
kVp, Itube
-

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XRAY PRODUCTION
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X-RAY production

• X-ray tube produces two forms of radiation
• Bremsstrahlung radiation (white radiation)
• Characteristic radiation

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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
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White radiation, Bremsstrahlung
-Smaller L produce larger X-ray -Broad range of
emitted wavelengths
X-Ray
L
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How many wavelength will be emitted by a beam of
electrons underegoing Bremsstrahlung
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White radiation, Bremsstrahlung
-Smaller L produce larger X-ray -Broad range of
emitted wavelengths
X-Ray
L
impact with nucleus
maximum energy
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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

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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
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Characteristic radiation
relative output

• Energy must be gt binding energy
• Discrete energy peaks due to electrons
  transitions
• Ka transition L-gtK
• Kb transition M,N,O-gtK

Peak voltage kVp
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Characteristic radiation
Incident electron
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Characteristic radiation
l2
Incident electron
Occurs only at discrete levels There is a
possibility of forming Auger electrons
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Characteristic radiation

• In Tungsten characteristic X-ray are formed only
  if Vgt69.5 kV because K shell binding energy is
  69.5 keV
• Molybdenum K-shell can be obtained at Vgt 20kV
• L shell radiation is also produced but its low
  energy and often
• absorbed by glass enclosure

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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

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Beam Hardening
Polyenergetic beam -------------------------------
gtmonoenergetic beam
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X-ray tube construction
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Anode
Most of the energy deposited on the anode
transfers into heat
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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
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Anode

• the target angle, 7? to 20? (average 12?)
• Seffective Sactualsin(?) -----------gt Line
  focusing principle

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Anode filament balance
General radiography
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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
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Beam collimation

• Size and shape of the beam
• Lead shutters
• Dose reduction

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Reduction of anode heating

• Anode angle of 7º15º 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 18-35.

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Limitations of anode angle

• Restricting target coverage for given
  source-to-image distance (SID)
• "Heel effect" causes inhomogeneous x-ray exposure

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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-gtspace-charge
  limit

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Space charge limited

• At high filament current and low tube voltage,
  space charge limits tube current-gtspace-charge
  limit

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Generator

• Single phase
• Single phase input (220V, 50A)
• Single pulse or double pulse-gtrectifier
• 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

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Rectifier
Protects cathode from anode thermionic emission
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Rectifier
1 phase
3 phase
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• BREAK!

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Principle of X-ray
A source of radiation
A patient of non uniform substance
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Attenuation
N Noe-mL
N
True for monoenergetic x-ray
Loss of photons by scattering or absorption
L1
L
No
N
m -gt linear attenuation coefficient
L1
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m linear attenuation coeff.

• m mr mph mc mp cm-1
• ?rayleigh
• ?photoelectric
• ?Compton
• ?pair

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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 gt micegt mvapor

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Mass attenuation coeff.
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Mass attenuation coeff.
N Noe- r(m/r)L
rL mass thickness
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Mass attenuation coeff.
N Noe- r(m/r)L
rL mass thickness
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Poly-energetic beam

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

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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

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HVL half value layer

• Transmission of primary beam
• 10 chest radiography
• 1 scull radiography
• 0.5 abdomen radiography
• Mammography (low energy HVL 1 cm)

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Mean free path 1/m

• Average distance traveled before interaction

MFP1/m ?????HVL
mfp
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Principle of X-ray
A source of radiation
A shadow
A patient of non uniform substance