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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

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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

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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

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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

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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

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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


Distance


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



Distance

Intensity

Target

Filtration

Collimation


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


Distance

Intensity

Target

Filtration

Collimation


Distance

Intensity

Target

Filtration

Collimation

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Tube current (mA)

Time


Tube current (mA)

Time


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


(kVp)

Distance

Intensity

TargetFiltration

Collimation

(kVp)

Distance

Intensity

TargetFiltration

Collimation

Tube current

(mA)

Time

Tube potential

Tube current

(mA)

Time

Tube potential


(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

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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

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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.

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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

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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

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