Medical Imaging
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Transcript of Medical Imaging
![Page 1: Medical Imaging](https://reader036.fdocuments.net/reader036/viewer/2022062321/568135fe550346895d9d70c6/html5/thumbnails/1.jpg)
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 uniformsubstance
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Principle of X-ray
A source of radiation
A patient of non uniformsubstance
A shadow
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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"
+-
CA
VC, If
+-
filamentevacuated gas envelope
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X-ray tube Cathode Filament (-)
Coil of tungsten wire High resistance in coil ->temperature rise to > 2200oC Thermionic emission of electrons
Tube (vacuum) Typical: Vtube = 40 – 150 kVp, Itube = 1-1000mA
+-
kVp, Itube
CA
VC, If
+-
filamentevacuated gas envelope
-
--
-
-
-- - -
space chargestops further emission
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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
+-
kVp, Itube
CA
VC, If
+-
filamentevacuated gas envelope
-
--
-
-
-- - -
space charge
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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
X-Ray
Coulombic interaction
-Inelastic interaction with atoms nuclei-Loss of kinetic energy-Xray (E) = lost kinetic E
-High kinetic energy-Forward radiation
-Emission Z2
(Atomic number)# of protons
(Brake)
electron
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White radiation, Bremsstrahlung
X-Ray
L
-Smaller L produce larger X-ray-Broad range of emitted wavelengths
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How many wavelength will be emitted by a beam of
electrons underegoing “Bremsstrahlung ”
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White radiation, Bremsstrahlung
X-Ray
L
-Smaller L produce larger X-ray-Broad range of emitted wavelengths
maximum energy
impact with nucleus
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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
€
I ∝Vtube2 Itube
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Bremsstrahlung spectrum 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
relative output
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Characteristic radiation Energy must be > binding energy
Discrete energy peaks due to electrons transitions
K transition L->K
K transition M,N,O->K
Peak voltage kVp
relative output
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Characteristic radiation
Incident electron
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Characteristic radiation
Incident electron
Occurs only at discrete levelsThere is a possibility of forming Auger electrons
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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 oftenabsorbed 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 ------------------------------->monoenergetic beam
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X-ray tube construction
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Anode
Most of the energy deposited on the anode transfersinto heat
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Reduction of anode heating Made of Tungsten, high melting
point high atomic number Z = 74
€
Radiative enegy loss
Collisional energy loss=
EkZ
820,000
€
100 ⋅74
820,000= 0.009 → 0.9% Xray production
99% Heat production
€
6,000 ⋅74
820,000≅ 0.54 → 54 % Xray production
46 % Heat production
100keV electron
6 MeV electron
Kinetic energy of incident electrons
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Anode the target angle, 7 to 20 (average 12)
Seffective = Sactual*sin() -----------> Line focusing principle
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Anode filament balance
General radiography
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Heel effect
Reduction of intensityon the anode side
SID
- SID source to image distance- Heel effect is smaller at smallerSID
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 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 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->space-charge limit
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Space charge limited At high filament current and low tube voltage, space charge limits tube
current->space-charge limit
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Generator Single 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
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RectifierProtects 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 uniformsubstance
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Attenuation N
No
L
L1
N
Loss of photons by scattering or absorption
N = Noe-L
L1
-> linear attenuationcoefficient
True for monoenergetic x-ray
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linear attenuation coeff.
= r+ ph+ c+ p [cm-1] rayleigh
photoelectric
Compton
pair
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linear attenuation coeff.
= r+ ph+ c+ p [cm-1] depends on tissue
soft tissue, hard tissue, metals decreases when energy increase
soft tissue: = 0.35 0.16 cm-1 for E = 30 100keV
depends on density of material wat > ice> vapor
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Mass attenuation coeff.
€
Mass Attenuation Coeff =Linear Attenuation Coeff
Density of Material=
μ
ρ
cm2
g
⎡
⎣ ⎢
⎤
⎦ ⎥
€
wat
ρ wat
= μ ice
ρ ice
=μ vap
ρ vap
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Mass attenuation coeff.N = Noe- (L
L = mass thickness
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Mass attenuation coeff.N = Noe- (L
L = mass thickness
I
x
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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
€
HVL=lnlinear
cm[ ]
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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/ Average distance traveled before interaction
mfp
MFP=1/HVL
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Principle of X-ray
A source of radiation
A patient of non uniformsubstance
A shadow