physics and electronics1-3 · 2013-03-11 · filament offers resistance > heat > electron...
Post on 03-Aug-2020
4 Views
Preview:
TRANSCRIPT
1
PHYSICS AND ELECTRONICS OF RADIOLOGY
Oral and Maxillofacial RadiologyOral and Maxillofacial RadiologyUniversity of FloridaUniversity of FloridaCollege of DentistryCollege of Dentistry
Oral and Maxillofacial RadiologyOral and Maxillofacial RadiologyUniversity of FloridaUniversity of FloridaCollege of DentistryCollege of Dentistry
PHYSICS AND ELECTRONICS OF RADIOLOGY
Terminology
Current: measured in milliAmperes – mAVoltage: measured in kiloVoltage – kV (note that we use voltage as peak value /max. voltage supplied –kVp)Time: measured in milliSeconds – sIntegrated exposure – product of mA and s: mAsDistances:Source to object distance (SOD)Source to image distance (SID)Object to image distance (OID)
High Voltage Lead
ElHeavyMetal
Third electrodeElectronsMetal
TargetFilament /
X-Rays
Window
ANODEANODECATHODE
is heated
Electron source - also called “filament”Filament of Tungsten: Two sizes in most tubes: small and large
Advantages of Tungsten (W):
Cathode: negative electrode
High atomic number (74)High melting point (3370 °C)Good absorber and dissipater of heatEasily available; economical.Tungsten target (ANODE) bonded to a large copper block for
better thermal distribution
Electron production:Filament is heated by allowing a current to flow through it.
Filament offers resistance > Heat > electron emission (boils off):
THERMIONIC EMISSION
FILAMENT CURRENT: Current used to HEAT FILAMENT- FC controls # of electrons boiled off
Positive electrode at a high potential difference.
Anode
Small tungsten plate (target) embedded in a large block of copper.
Major source of heat production
Stationary OR Rotating anodes
2
Stationary anode:
Anode:
Tungsten: 2-3 mm thick embedded in Cu; dimensions 1 x
1 cm.
Rotating Anode
Focal track
7 mm wide
Rotating Anode
Rotating anodes – more heat tolerant;
Lesser cooling time; less damage to anodeSpeed: 3600 – 10,000 rpm.
High Voltage Lead
ElHeavyMetal
Third electrodeElectronsMetal
TargetFilament /
X-RaysWindow
CATHODEis heated
Filament /CATHODE
is heated
Electrons accelerated from
CATHODE to ANODE by high
voltage (potential difference)
Electrons strike ANODE
suddenly decelerated
energy lost is converted to x-
X-ray Source
energy lost is converted to x-rays.
Glass Enclosure
Purpose: provides a vacuum.
Energy split-up:
99% converted to heat; only 1% to x-rays
To prevent binding of moving parts, disruption of target
surface: roughening, pitting, cracking etc.
Dry lubricants such as graphite provide for lubrication
Methods of heat dissipation………….
Heat dissipated by the following mechanisms:
Radiation through vacuum;
Convection through surrounding oil and tube housing;
Conduction through solid tube parts
3
Overview of generator assembly
TimerTimer
Control PanelControl Panel kVpkVp
mAmA
GeneratorGenerator Low VoltageTransformerLow VoltageTransformer
Transformer Assembly
Transformer Assembly
High VoltageTransformerHigh VoltageTransformerRectifiersRectifiers
X-ray ProductionSource ComponentsX-ray ProductionSource Components Machine Schematic
TransformersTransformers
• Step-up transformer: increases voltage between • Step-up transformer: increases voltage between
electrodes
• Step-down transformer: reduces voltage – used for
heating the filament, for instance
• Change in voltage achieved at expense of current.
electrodes
• Step-down transformer: reduces voltage – used for
heating the filament, for instance
• Change in voltage achieved at expense of current.
Power Supply
• Most units are AC powered.• Most units are AC powered.
• Need rectifiers (reverses –ve current flow)
• Without rectifiers, production efficiency suffers due to
change in direction of current
• DC is more production-efficient but expensive
• Need rectifiers (reverses –ve current flow)
• Without rectifiers, production efficiency suffers due to
change in direction of current
• DC is more production-efficient but expensive
4
Types of power supplyTypes of power supply
Alternating current (AC): electrons flow in both directions; no constancy of current / voltage;
Ch di ti f tl d i l
Direct Current (DC): electrons flow in one direction only; current & voltage
remain constant; most efficient for x-ray production; expensive
Direct Current (DC): electrons flow in one direction only; current & voltage
remain constant; most efficient for x-ray production; expensive
Changes direction frequently – expressed in cycles
Relatively inefficient x-ray production
RectificationRectification
• Half wave rectification: suppression of negative
half of AC cycle only
• Half wave rectification: suppression of negative
half of AC cycle only
Full wave rectification
Full wave rectification
• Negative half of AC cycle is suppressed and reversed
– used for x-ray production, therefore.
• Voltage varies from 0 to peak value; no x-ray
production at 0 volts!
• Negative half of AC cycle is suppressed and reversed
– used for x-ray production, therefore.
• Voltage varies from 0 to peak value; no x-ray
production at 0 volts!
NEGATIVE WAVE
Peak Voltage: maximum energy x-rays produced
POSITIVE WAVE
Alternating CurrentAlternating Current
HALF-WAVE RECTIFICATION
FULL-WAVE RECTIFICATION
Single-phase vs.three-phase power source
Single-phase vs.three-phase power source
Three phase: 3 single-phase currents, out of phase with one thanother.
Voltage never drops to 0; stays closer to peak value
Advantages More x-ray photons produced; more mean energy of the beam (quality and quantity)
Exposure time can be reduced – less patient dose
Single versus three-phase powerMore x-ray photons
Higher energy
Exposure time reduced
PHASE 1
PHASE 3
PHASE 2
PHASE 1, 2 & 3
5
X-ray productionX-ray productionBremsstrahlung radiationBremsstrahlung radiation
Electron hitting target atom may be:• Completely stopped >> max. energy x-ray
OR• Deflected >> lower energy x-ray photon
If you plot the energy of all photons in the resulting x-ray beam, a continuous spectrum results, with energies of x-ray photons ranging from near-zero to a maximum.
Maximum energy will be equal to kVp. Energy
Intensity
BremsstrahlungBremsstrahlung
X ray ProductionCharacteristic Radiation
X ray ProductionCharacteristic Radiation
Characteristic
Energy
Intensity
BremsstrahlungBremsstrahlung
Physical Factors
AffectingAffecting
X-Ray Beam
Machine ControlsMachine Controls
6
o Tube current (mA)
o Time
o Tube potential (kVp)
o Tube current (mA)
o Time
o Tube potential (kVp)
Factors affecting beam quality:Factors affecting beam quality:
o Distance
o Intensity
o Target
o Filtration
o Collimation
o Distance
o Intensity
o Target
o Filtration
o Collimation
Tube current (mA)
Time
Tube potential (kVp)
Distance
Tube current (mA)
Time
Tube potential (kVp)
Distance
Factors affecting beam quality:Factors affecting beam quality:
Intensity
Target
Filtration
Collimation
Intensity
Target
Filtration
CollimationMORE X-RAYS
MORE CURRENT
Effect of current on
x-ray production
mA Controls – 10 mA or 15 mAmA Controls – 10 mA or 15 mA
Machine Controls
mA control
Machine ControlsIntegrated exposure control
Tube current (mA)
TimeTube potential (kVp)
Tube current (mA)
TimeTube potential (kVp) Eff t f ti
Factors affecting beam quality:Factors affecting beam quality:
Tube potential (kVp)
Distance
Intensity
Target
Filtration
Collimation
Tube potential (kVp)
Distance
Intensity
Target
Filtration
Collimation
INCREASED TIME
MORE X-RAYS
Effect of time on x-ray production
Timer dial
Integrated exposure control
Increasing time will increase number of photonsproduced >> darker image
Tube current (mA)
Time
Tube current (mA)
Time
Tube potential (kVp)
Distance
Intensity
Target
Filtration
Collimation
Tube potential (kVp)
Distance
Intensity
Target
Filtration
Collimation
7
Tube current (mA)
Time
Tube potential (kVp)
Tube current (mA)
Time
Tube potential (kVp)
Inverse Square Law:Intensity of the beam varies inversely as the square of the distance from the source
Distance
Intensity
Target
Filtration
Collimation
Distance
Intensity
Target
Filtration
Collimation
I1 / I2 = (D2)2 / (D1) 2I1 = Initial beam IntensityI2 = Beam intensity at a new location.D2 = distance of the new location from the sourceD1 = original distance from the source when intensity was I1
Problem: If the intensity of the beam at 2m from an x-ray source is 'x', what will the intensity be at a distance of 1m?
Ans.:
Inverse Square LawIntensity of the beam varies inversely as (distance)2 from the source
Inverse Square LawIntensity of the beam varies inversely as (distance)2 from the source
x/ I2 = 12 / 22 = 1/4.
I2 = 4x
Distance : 1m
Distance : 4m
Density = 1/16
Density = 1/4Density = 1
Distanc: 2m
Tube current
(mA)
Time
Tube potential
Tube current
(mA)
Time
Tube potential
Factors affecting beam quality:Factors affecting beam quality:
(kVp)
Distance
Intensity
TargetFiltration
Collimation
(kVp)
Distance
Intensity
TargetFiltration
Collimation
Tube current
(mA)
Time
Tube potential
Tube current
(mA)
Time
Tube potential
Factors affecting beam quality:Factors affecting beam quality:
(kVp)
Distance
Intensity
TargetFiltration
Collimation
(kVp)
Distance
Intensity
TargetFiltration
Collimation
Tube current
(mA)
Time
Tube potential
(kVp)
Tube current
(mA)
Time
Tube potential
(kVp)
Filtration removes most of the low energy photons
(long wavelength) that only add to the patient's
skin exposure(kVp)
Distance
Intensity
Target
Filtration
Collimation
(kVp)
Distance
Intensity
Target
Filtration
Collimation
skin exposure.
• Reduces patient dose
• Improves image contrast
Mean (average) energy of the beam goes up.
FiltrationEffect on X ray Beam
8
Types:
• Inherent (x-ray tube and its housing)
• Added filtration (Aluminum)
• The patient
Filtration:Filtration:
Federal regulation stipulates the thickness of filter material for the
different voltages used:
Below 50 kVp 0.5 mm Al equivalent
50-70 kVp 1.5 mm Al equivalent
Above 70 kVp 2.5 mm Al equivalent
Tube current (mA)
Time
Tube potential
(kVp)
Tube current (mA)
Time
Tube potential
(kVp)
• Is the process of shaping a beam to make it no bigger
than the size of the receptor.
• Collimators/beam limiting devices used.
The three different categories of beam limiting devices
are:
Distance
Intensity
Target
Filtration
Collimation
Distance
Intensity
Target
Filtration
Collimation
o Aperture diaphragms
o Cones and cylinders
o Collimators
Advantages of beam limiting d iAdvantages of beam limiting d idevices:
• Less patient exposure• Less operator exposure• Less scatter generation and so improved contrast..
devices:• Less patient exposure• Less operator exposure• Less scatter generation and so improved contrast..
• Intensity of x-ray decreases but mean
energy of beam goes up
Beam Hardening:Beam Hardening:
4 cm of
100 kVp1000 photons
40 keV (mean)
• Most of the low energy photons get
absorbed
4 cm of tissue
200 photons60 keV (mean)
Radiographic UnitsRadiographic UnitsExposure: MEASURE OF IONIZATION PRODUCED IN AIR BY X-/GAMMA RAYS
Absorbed Dose: ENERGY IMPARTED TO MATTER BY IONIZING RADIATION PER UNIT
MASS OF IRRADIATED MATERIAL.
Dose Equivalent: ABSORBED DOSE x QUALITY FACTOR
Radiographic Units:g p
SI Traditional Conversion
Exposure: C/kg Roentgen (R) 1R=2.58x10-4 C/kg.
Absorbed dose Gy (Gray) RAD 1 Gy=100 rad
Effective dose Sv (Sievert) REM 1 Sv=100 rem
Radioactivity Bq Ci (Curie) 1 Ci=3.7x1010 Bq
9
Occupational Exposure:Occupational Exposure:
MPD: Max. Permissible Dose Equivalent:MPD: Max. Permissible Dose Equivalent:
Max. dose that a person or specified parts thereof shall be
allowed to receive in a stated period of time.
Whole body MPD: 20 mSv per year.
Based on ALARA PRINCIPLE (As Low As Reasonably Achievable)
Max. dose that a person or specified parts thereof shall be
allowed to receive in a stated period of time.
Whole body MPD: 20 mSv per year.
Based on ALARA PRINCIPLE (As Low As Reasonably Achievable)
AttenuationAttenuation
Reduction in the intensity of x-ray beam as it traverses matter by
either absorption or deflection of photons from the beam
• Measured using the half value layer (HVL): defined as the
thickness of any material required to reduce the intensity of the
beam in half.
Half value layer: a measure of attenuationHalf value layer: a measure of attenuation
Factors affecting attenuation:Energy of radiation, atomic number (Z) of material through which it passes, tissue density, electron density etc.
Differential attenuationDifferential attenuation
Different body parts attenuate the x-ray beam to
diff d di h i d idifferent extents depending on their density,
thickness, energy of the beam, beam size etc.
Image contrast.
Scatter Radiation: reduces contrast……Scatter Radiation: reduces contrast……
Aim: To reduce the intensity of scatter radiation:Field size: Collimation.Kilovoltage: Sufficient kVp only.Part thickness: too much thickness of tissue generates tooPart thickness: too much thickness of tissue generates too
much scatter.
Scatter (secondary) radiation is also a health hazard!
Types of radiation
Primary radiation: Produced at focal spot and exiting tubehead through glass window – used to expose radiograph. MAIN BEAM.Secondary radiation: Radiation produced by alteration of direction of collimated primary beam by other objects as patient’s face.Stray (Leakage) radiation: Radiation produced at the focal spot and exiting the tubehead at points other than the glass window.
10
Interactions with matter
Interactions of x-rays with matterInteractions of x-rays with matter
In the diagnostic range: 3 types of interactions canoccur depending on the nature of the beam.
Coherent scatter, Photoelectric absorption and Compton scatter.
• X-ray interactions with matter: probability increases with atomicnumber and thickness of absorbing material.
Coherent scatter
K
ML
Incoming x-ray photon is deflected by an outer orbital electron.
Res ltant photon has a change in direction itho t loss of
Coherent ScatteringCoherent Scattering
Resultant photon has a change in direction without loss of energy.
Only interaction where no energy loss occurs.8% of all interactions
X-ray photon strikes an electron near the nucleus with enough energy to knock it off its orbit.Photon is totally absorbed and its energy transferred to the ejected /recoil electron.
Photoelectric EffectPhotoelectric Effect
Recoil electronRecoil electron
ee
K
ML
ee
Most interactions take place in the K-shell: electron density is highest here
• Characteristic x-ray formation: produced when a higher orbital electron falls
Photoelectric EffectPhotoelectric Effect
y p g
into the void in K orbit.
• Approximately 30% of all interactions.
• Most important interaction in diagnostic radiology
11
Compton ScatterCompton Scatter
Distinguish this from Coherent scatter
Caused when an x-ray photon scatters off an electron, ejecting it
from orbit
• Recoil electron
Compton ScatterCompton Scatter
• Scattered photon: may exit tissue or interact with more material
Compton scattering decreases contrast
• 62% of all interactions
top related