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hat is a Photon?,Under the photon theory of light a photon

( )is a discrete bundle or quantum of( ) .electromagnetic or light energy Photons, ,are always in motion and in a vacuum have a

,constant speed of light to all observers at(the vacuum speed of light more commonly

) = .just called the speed of light of c 2 998x 108 / .m s

In physics, a quantum ( :plural quanta) is theminimum unit of any physical entity involved in an.interaction

A photon, , ,for example is a single quantum of light"and may thus be referred to as a light quantum".

http://en.wikipedia.org/wiki/Physicshttp://en.wikipedia.org/wiki/Physicshttp://en.wikipedia.org/wiki/Photonhttp://en.wikipedia.org/wiki/Photonhttp://en.wikipedia.org/wiki/Light_quantumhttp://en.wikipedia.org/wiki/Light_quantumhttp://en.wikipedia.org/wiki/Light_quantumhttp://en.wikipedia.org/wiki/Light_quantumhttp://en.wikipedia.org/wiki/Photonhttp://en.wikipedia.org/wiki/Physics

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Fun Photon Facts,The photon is an elementary particle despite the fact that it

. ,has no mass It cannot decay on its own although the energy of( )the photon can transfer or be created upon interaction with

.other particles Photons are electrically neutral and are one of,the rare particles that are identical to their antiparticle the

antiphoton

Basic Properties of Photons, . . .According to the photon theory of light photons

, = .move at a constant velocity c 2 9979 x 108 / ( . . "m s i e the speed of"),light in free space

.have zero mass and rest energy

,carry energy and momentum which are also related to the frequency nu=and wavelength lamdba of the electromagnetic wave by E h nu and p

= / .h lambda

/ / .can be destroyed created when radiation is absorbed emitted - ( . . )an have particle like interactions i e collisions with,lectrons and other particles such as in the Compton.ffect

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:Exposure The amount of ionizations-produced in air by x ray or gamma. ,photons The unit is Roentgen the SI unit

.is the coulomb per kilogram

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:Activity A measure of theintensity of radioactivity in asample of material quantified by

the number of radioactivedisintegrations occurring in a

given quantity of material per. ,unit time Unit is the curie the

.SI unit is Becquerel

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( ):Absorbed Dose D The energy impartedto matter by ionizing radiation per.unit mass of irradiated material The

absorbed dose is expressed in unit

; ( ).rad the SI unit is the gray Gy

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( ):Dose equivalent H The product of( : )absorbed dose in rads SI Gray in, ( )tissue a quality factor Q and other

( ).modifying factors N Dose equivalent

is expressed in the unit rem( )ADIATION EQUIVALENT MAN ,the SI unitis the Sievert.

=Dose Equivalent rads x Q x N

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a

Exposure Roentgen (R) Coulomb/kg 1 R= 2.54 x10-4 coul/kg

Activity Curie (Ci) Becquerel (Bq)1 Ci = 3.7 x

10

10

Bq1Bq=2.7 x 10-11

AbsorbedDose (D)

Doseequivalent(H)

Sievert (Sv) =Gray x QF

1 Sv = 100Rem

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matter

nnihilationphotonIncidentphotons

Secondaryphotons

Secondaryelectrons

cattered photonompton effectluorescence photon( )haracteristic radiation

ecoilelectron

lectron pair> .1 02 MeV

Photoelectron( hotoelectric)ffect

on interactingphotons

(simplified)representation

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:Photoelectric Effect Occurs between

tightly bound inner shell electron and- .incident x ray photon The inner shell

electron filled by outer shell electronand excess energy is emitted as

.characteristic radiation Thephotoelectric effect occurs when photon istotally absorbed by the inner shell

.electron and a photoelectron is emitted

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PHOTOELECTRIC

EFFECT

Incident photon

Photoelectron

Characteristic

x-ray

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:Compton Scatter In Compton scatterincident photon interact with loosely

bound outer shell electron resulting

.is scattered photon This is a causeof most scattered radiation in.diagnostic radiology

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COMPTON SCATTERING

Incident photon

Compton Electron

Scattered Photon

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:Coherent scatter - a termsometimes used for Rayleigh;scattering Incident photon changes

.direction without losing energy

imilar with that of Bremsstrahlung-ray production

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Rayleigh Scattering

Incident photon

Scatteredphoton

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:Pair Production High energy photoninteract with the nucleus of an

.atom The photon disappear andenergy is converted in to an

.electron and positron

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PAIR PRODUCTION

Incident photon

e-

e+

electron

positron

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:Photodisintegration When a highenergy photon is absorbed by anucleus resulting in immediate

.disintegration of the nucleus

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-Diagnostic x rays are produced when

electrons with high energies of 20.to 150 Kev are stopped in matter-X rays are produced by two

:different process known as

.1 Bremsstrahlung. -2 Characteristic x ray production

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( )Bremsstrahlung Breaking or General-X rays are produced when incident

electrons interact with nuclearelectric fields which slow them down

.and change their direction Some of the

-kinetic energy is emitted as x ray

. -photon Bremsstrahlung x ray produce.continous spectrum of radiation

-Bremsstrahlung x ray production

increases with the accelerating

( ) ( )voltage KV and atomic number z of.anode

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electron of the anode target are ejected by

.the incident electron The inner shell vacancyare filled by outer shell electrons and theenergy difference is emitted as characteristic

.radiationExcess energy may also be emitted as Auger

.electron-K shell electron is emitted only if incident

-electron have energies greater than K shell.binding energy

= =For tungsten 70 kv Molybdenum 20 kv- -L shell electron also normally accompanies K. - -shell radiation L shell characteristic x rays

have very low energies and are absorbed by the-glass of the x ray tube.

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X Ray tube components

Cathode: heated filament which is thesource of the electron beam directedtowards the anode

tungsten filament

Anode (stationary or rotating): impactedby electrons, emits X Rays

Metal tube housing surrounding glass (ormetal) X Ray tube (electrons are

traveling in vacuum) Shielding material (protection againstscattered radiation)

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Parts of fixed Anode x-ray Tube

High Tension Generator

Cathode

Anode

The Glass Envelope and VacuumThe Tube shield

Cooling mechanism

Filtration mechanism

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High Tension Generator

The high tension voltage is applied to anode of x-raytubegives the kinetic energy for the electron to leavecathode and bombard anode.

For diagnostic radiology 40-120 KVP is used.

This high voltage are provided by step up transformeror high tension generator.

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The cathode

Cathode is negative pole of the x-ray tube. It is a metal structuresupporting the filament, which on heating emits electrons.

Focusing Cup: Where the filament is located in the cathodeFilament made up of tungsten wire which tolerates high temperatureup to 3370*c

Has high resistance so as to produce amount of heat needed to boilthe electron

Shaped in helical or spiral winding to increase surface area foremitting electrons.

Its size is small so as to produce electron beam covering small area.

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Cathode structure

Cathode includes filament(s) andassociated circuitry tungsten material : preferred because of

its high melting point (3370C)

slow filament evaporation

no arcing (spark)

minimum deposit of W on glass envelope

To reduce evaporation the emissiontemperature of the cathode is reached justbefore the exposure

in stand-by, temperature is kept at 1500C so that 2700C emissiontemperature can be reached within a

second

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Modern tubes have two filamentsa long one : higher current/lower resolutiona short one : lower current/higher

resolution

Coulomb interaction makes the electronbeam divergent on the travel to theanode

focal spot increased low r m r solut on o l s t on o

l trons s ru l !

Cathode structure

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The Anode

A piece of tungsten in a form of plate of 2mm thick ,rectangular or circular in shape larger than the focal area is

embedded on a thick copper rod.

This is essentially a metal plate to receive the electronwhich bombard it.It is so designed the that bombarded electron gives great

amount of heat(99%) and small amount of x-ray(1%)Anode has relatively larger surface area so produced heatcan be dissipated and tube damage can be prevented.

Tungsten and copper is used for this purpose but why?

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Tungsten As a target:-Highmelting point, high atomic number (efficient

to produce x-ray), good conductor of heat, can be shaped andmade smooth as required.As a filament:-High thermionic emission, can be convertedinto wire, high melting point and does not vaporize easily.

Copper-Good conductor of heat so dissipates heat to outside to outsideof tube and also serves as electrode of positive pole (anode)where high KV current is connected.

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-H o w to p ro d u ce fa ste r a n d p o w erfu l x ra y?

T h e la rg e r th e d iffe re n ce b e tw e e n in th e( )ch a rg e p o te n tia l d iffe re n ce b e tw e e n a n o d e

a n d ca th o d e th e fa ste r th e e le ctro n.a cce le ra te to w a rd s th e a n o d e

,T h e fa ste r th e e le ctro n g o e s th e h a rd e r th e y-co llid e w ith th e a n o d e a n d m o re p o w erfu l x

.ray are produced

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-W h a t is th e e ffe ct o f kV P a n d m A s on x ra y

p ro d u ctio n ?

m A s In cre a sin g th e m A s it in cre a se th e- .q u a n tity o f th e x ra y p ro d u ctio n

kV p In cre a se in kV P it in cre a se s th e q u a lity- .o f th e x ra y p ro d u ctio n

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X Ray tube characteristics

Anode mechanical constraints : , , ,

Focal spot : surface of anode impacted by

electrons Anode angle

Disk and annular track diameter (rotationfrequency 3,000 10,000 / )

Thickness ( ) Anode thermal constraints

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Focusing of the electron beam

The electron spreading away from the filament are brought togetherby means of an electric field which exist between anode and cathode.

The filament sits in a slot in the cathode.

The filament and slot are carefully designed in shape, size andposition so that the emitted electron can only leave the filament

through the slot.

By this electron leaving the filament comes together in a beam so asto cover small area on the anode, also called FOCAL SPOT of x-raytube.

Focal Spot:The exact area of the anode where the electron hit theanode.

oca spo s ze an mag ng

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oca spo s ze an mag nggeometry

Focal spot Large image unsharpened Improving sharpness small focal spot size

For mammography focal spot size 0.4

Small focal spot size ( )

Large focal spot allows high output (shorter

exposure time) Balance depends on organ movement (fast moving

organs may require larger focus)

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Anode angle

The Line-Focus principle :

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Anode heel effect

Anode angle (from 7 to 20) induces avariation of the X Ray output in theplane comprising the anode-cathodeaxis

Absorption by anode of X photons withlow emission angle

The magnitude of influence of the heeleffect on the image depends on factors

such as : anode angle size of film

focus to film distance

Anode aging increases heel effect

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The heel effect is not always a negative factor

It can be used to compensate for differentattenuation through parts of the body

For example:

thoracic spine (thicker part of the patient towardsthe cathode side of the tube)

mammography

Anode heel effect

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Conditions necessary for theproduction of x-rays

There must be a High Voltage (potentialdifference)

There must be Fast Moving Electrons.

There must be a Target.

It must be in Vacuum.

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% W h a t is 1 5 ru le for kV P ?%If th e kV P is in cre a se d b y 1 5 th e

.d e n sity is d o u b le d

W h a t is sa n te s s R u le o r e q u a tio n ?

= )kV P 2 x th icken d d o f th e b od y p art in cm

+40

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Production ofx-ray

anodecathode

PHOTOELECTRIC

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PHOTOELECTRIC

EFFECT

Incident photon

Photoelectron

Characteristic x-ray

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The gas envelope and vacuum

The electron that leave the cathode have unimpeded passage tothe anode.

If there is no vacuum it collide with the gas with in the tube asa result of which they loose the energy before colliding with

the anode.

The end result would be production of less intense and lesspenetrating x-ray .

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The tube shield

It will not allow any x-ray to escape out of the x-ray tube expectthrough the glass window which is provided to allow only the

primary beam from focal spot.

Metallic casearound the x-ray tube that provides protectionagainst radiation risk and electrical risk. Theoretically it should

be ray proof and shock proof.

Ray proof is impossible according to physic of x-ray

absorption but significantly reduces amount of radiationcoming through the absorber within he safety limit.

Cooling Mechanism of x ray tube

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Cooling Mechanism of x-ray tube

Amount of heat production=KV x mA x Seconds

The greater the factor are used the more heat are produced.

If the dissipation of heat were not to take place simultaneouslywith heat production, the melting point of tungsten(3360*c)will

be soon reached.

Heat dissipation occurs by-1.Conduction-through the solid part of anode-copper

2.Convection-through the oil surrounding the tube(glass andcopper block transmit the heat to oil in which tube is immersed.

3.Radiation-occur through the vacuum of the tube to glassenvelope.

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Filtration in the x-ray tube

X-ray beam comprises many wavelength in it.A filter acts by

absorbing preferably the useless but harmful longerwavelengths in the beam.There by only shorter wavelength x-ray leaves the tube whichwould cast sharp radiographic image.

This useful beam still has to pass through:1.Thin window in the glass envelope2.Oil within the shield3.Lead lined plastic cover aperture call the portal.(these are also called inherent filtration of x-ray tube.)

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When x-ray strikes the body part ,secondary radiation areproduced which have longer wavelength than the primary beam.

These secondary radiations are scattered in all direction andproduce a veil of fog on the diagnostic x-ray film.Scattered radiation affect the image of part at a distance from thefilm by fogging.

Small parts like hands and feet shows negligible scattering butpelvic and trunk shows maximum scattering.

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Control of scattered radiationSome radiographic accessories are used to cut down the secondaryradiation fog:1.Shutters/diaphragms:-The shutter diaphragm have lighting

arrangement above them, incorporated inside the tube shield whichshows the field size (after operating the shutters) on the patients body.2.Cones:Cones of different sizes and shapes are available to restrict fieldof radiography to the obsolete required size.eg-mastoid cone , PNS cone

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. :3 G rid T h e se a re th e d e vice s w h ich co n sist o f( )se rie s o f a lte rn a te strip e s o f le a d o r tu n g ste n a n d

( ). -w o o d o r sim ila r ra d io lu ce n t m a te ria l T h e x ra yp e n e tra te s g rid a n d fa ll o n th e film to p ro d u ce

. -im a g e T h e sca tte re d x ra y a re cu t o ff b y le a d.strip e s

- : (Ty p ica lly 2 5 tim e s B u cky fa cto r o r g rid ra tio T h ere la tio n o f th e h e ig h t o f th e le a d strip s to th e w id th.o f th e n on op a q u e m ate ria lb etw e en th em

: , : , : )C om m on g rid ratio s a re 2 8 2 1 2 a n d 2 1 6

:G rid s a re m a in ly tw o ty p e s. : ( )1 S ta tin o n a ry g rid s LY S H O LM G rid. : . - .2 M o v in g G rid a Po tte r b u cky g rid b O scilla tin g g rid.c C ro ss g rid

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A Scattered and primary x-ray photons reachingthe same point Pon film. B Scattered photon isremoved by antiscatter grid, while primary

photon gets through.

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X-ray Film Cassette. Diagram demonstrating asheet of x-ray film between two fluorescentscreens within a light-proof cassette.

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:Emulsion Film consist of an approximately 10micrometer thick emulsion supported by mylar

.base which is 150 micrometer thickThe emulsion contains silver halide grains

which can be sensitized by radiation or

.light to hold a latent image( )Several light photon approximately 4 must.be absorbed to sensitize each grain

P i f Fil

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Processing of Films1.Proper mixing of the chemical solutionaccording to manufacture instruction.

2.Unloading of the exposed film from thecassette, writing patient ID with copying penciland then mounting it on to proper hanger.

3.Developing Process4.Rinsing process5.Fixing process6.Washing Process

7.Drying process

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:Developing Development of radiographic film-is a chemical process which reduces x ray

-exposed silver bromide of the x ray film into

,plain metallic silver in a finely divided.form to bring out the latent image

Sensitized grains are reduced in the.developer by the addition of electron

A developed grain results in a speck of.silver that appears black on the film Grainswith no latent image are also developed but

at a much slower rate

Contains 4 main chemical mixed with

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distilled water. / :1 Developing Reducing agent 2 main chemicals. -a Hydroquinone slowly build up back tones

and contrast. b Elon quickly builds up gray tones.2 Preservative agent contains sodium

sulfite which protects the rapid oxidation

of the developing agent. : ,3 Activator Contains sodium carbonate also

called alkalizer which provides necessary.alkaline medium

. :4 Restrainer Contain potassium bromide

restrain the developing agent fromdeveloping the unexposed silver halide.crytals

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Oth 2 h i l

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:Other 2 chemicals. :1 Wetting agent A form of detergent

which reduces surface tension andhelps the film to dry faster

. :2 Cutting agent A combination ofpotassium ferrocyanide and fixer

which can be used in an emergencyto lighten film that have been

accidentally overexposed or

.overdeveloped

What is difference between Spatial resolution and

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What is difference between Spatial resolution andcontrast resolution?

Spatial resolutionis a measure of the ability of animaging technique to demonstrate that two nearby

, , .objects are indeed separate objects It is measured" ,"in line pairs per millimeter referring to theability of a modality to demonstrate that very small

pairs of lines are indeed separatelines and not a. - ,single line Of the digital cross sectional modalities

.CT has the highest spatial resolution-Contrast resolution The density difference between the.two adjacent area on the radiograph

Refers to the ability of an imaging modality torender different objects or tissues as different

. -shades of gray Of the digital cross sectional

, .modalities MRI has the highest contrast resolution

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Normally 1-3 mSv/year

In areas of high background, 3-13

mSv/yearLD 50/60 = 4 Gy (The LD50/60 isthat dose at which 50%of the

exposed population will die within60 days)

O ti l di l d bli

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Occupational, medical and publicexposures

Occupational exposure All exposures of workers incurred:

in the course of their work, with the exception ofexposures excluded from the Standards

exposures from practices or sources exempted by the

Standards

O ti l di l d bli

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Occupational, medical and publicexposures

Medical exposure: Exposure incurred by patients

as part of their own medical or dental diagnosis ortreatment;

by persons, other than those occupationally exposed,knowingly while voluntarily helping in the support andcomfort of patients;

by volunteers in a programme of biomedical researchinvolving their exposure

O ti l di l d bli

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Occupational, medical and publicexposures

Public exposure: Exposure incurred by:

members of the public from radiation sources,

excluding any occupational or medical exposure and

the normal local natural background radiation but including exposure from authorized sources and

practices and from intervention situations.

D li it ( ti l

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Dose limits (occupationalexposure)

The occupational exposure of any worker shouldbe controlled so that the following limits be notexceeded:

500 mSvThe hands and feet

500 mSvThe skin

150 mSvThe lens of the eye

Annual equivalent dose in:

20 mSv per year, averaged overdefined periods of 5 years

50 mSv in any single yearEffective dose

Occupational dose limitApplication

PUBLIC O ti i ti d

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PUBLIC - Optimization underConstraints

DOSE LIMITS effective dose of 1 mSv in a year

in special circumstances, effective dose of 5 mSvin a single year, provided that the average over

five consecutive years in less than 1mSv peryear

equivalent dose to lens of the eye 15 mSv in ayear

equivalent dose to skin of 50 mSv in a year.

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Dose limits (public)

50 mSvThe skin

15 mSvThe lens of the eyeAnnual equivalent dose in:

1 mSv in a year(*)Effective dose

Public dose limitApplication

(*) In special circumstances, an effective dose of up to 5 mSvin a single year provided that the average dose over fiveconsecutive years does not exceed 1 mSv per year.

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Dose Limits (ICRP 60) OccupationalPublicEffective dose 20 mSv/yr averaged* 1 mSv in a

yr

over 5 yrs.

Annual equivalent

dose to Lens of eye 150 mSv 15 mSv

Skin 500 mSv 50 mSv

Hands & Feet 500 mSv

* with further provision that dose in any single yr > 30mSv (AERB) and =50 mSv (ICRP)

N.B.: M.P.D. 1931 = 500 mSv, 1947=150 mSv, 1977=50mSv&

in 1990=20 mSv

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We live with

1-3 mSv

Can kill

4000 mSv

Where to stop, where is the safe point?What are the effects of radiation?

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DETERMINISTICSomaticClinically attributablein the exposedindividual

CELL DEATH

STOCHASTICsomatic & hereditaryepidemiologicallyattributable in largepopulations

ANTENATALsomatic andhereditary expressedin the foetus, in the liveborn or descendants

BOTH

TYPEOF

EFFECTS

CELL TRANSFORMATION

o og ca e ec s o on z ngradiation

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Deterministic e.g. Lens opacities,

skin injuries,

infertility,epilation(hairremoval), etc

Stochastic

Cancer, geneticeffects.

Deterministic effects

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Deterministic(Threshold/non-stochastic)

Existence of a dosethreshold value (belowthis dose, the effect is notobservable)

Severity of the effectincreases with dose

A large number of cells areinvolved

adiation injury from an industrial sou

Deterministic effects

Threshold Doses for DeterministicEffects

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Cataracts of the lens of the eye 2-10 Gy

Permanent sterility

males 3.5-6 Gy

females 2.5-6 GyTemporary sterility

males 0.15 Gy

females 0.6 Gy

dose

everity ofeffect

threshold

Effects

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Muscle

Bones

Nervous

system

Skin

Mesodermorgans (liver,

heart, lungs)

Bone Marrow

Spleen

Thymus

Lymphaticnodes

Eye lensLymphocytes(exception to the RS laws)

Low RSMedium RSHigh RS

Factors affecting the

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G1

S

G2

M

G0

LET

LET%survivorcells

MM

Physical LET (linear energy transfer): RS Dose rate: RS

Chemical Increase RS: OXYGEN, cytotoxic drugs. Decrease RS: SULFURE (cys, cysteamine)

Biological Cycle status:

RS: G2, M

RS: S Repair of damage (sub-lethal damage

may be repaired e.g. fractionated dose)

Effects of antenatal exposure

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Effects of antenatal exposure

Lethal effects can be induced by relatively

small doses (such as 0.1 Gy) before orimmediately after implantation of the embryointo the uterine wall. They may also beinduced after higher doses during all the

stages during intra-uterine development.

Time

%

Pre-implantation Organogenesis Foetus

Lethality

0.1 Gy

Effects of antenatal exposure

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Effects of antenatal exposure

Mental retardation:

ICRP establishes that mental retardation can beinduced by radiation (Intelligence Quotient score< 100).

It occurs during the most RS period: 8-25 week of

pregnancy.

Risks of antenatal exposure related to mentalretardation are:

Severe mental retardation

with a risk factor of

0.1/Sv

Severe mental retardationwith a risk factor of

0.4/Sv

15-25 week8-15 week

Exam.

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number of chest x-rays

0 50 100 150 200

, , & ( )rm head ankle foot 1& ( )ead Neck 3( )ead CT 10 ( )horacic Spine 18, ( )ammography Cystography 20( )elvis 24, , & ( )bdomen Hip Upper lower femur 28( )a Swallow 30 ( )bsteric abdomen 34- ( )umbo sacral area 43( )holangiography 52 ( )umber Myelography 60 ( )ower abdomen CT male 72( )pper Abdomen CT 73( )a Meal 76- , - ( )ngio head Angio peripheral 80( )rography 87- ( )ngio abdominal 120( )hest CT 136. . ( )ower Abd CT fem 142( )a enema 154 . ( )ymphan 180

mSv

.050.15

0.49

0.92

1.0

1.22

1.4

1.5

1.72.15

2.59

3.0

3.61

3.67

3.8

4.0

4.366.0

6.8

7.13

7.69

9.0

Exam.(as multiple of chest x-ray)

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Typical effective doses from

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Typical effective doses fromdiagnostic medical exposures

: . ,Fro m R e fe rra lC rite ria Fo r Im a g in g C E.2 0 0 0

Diagnostic

procedure

Typical effective

dose (mSv)

Equiv. no. of

chest x-rays

Approx. equiv. period of

Chest (single PAfilm)

0.02 1 3 days

Skull 0.07 3.5 11 days

Thoracic spine 0.7 35 4 months

Lumbar spine 1.3 65 7 months

Typical effective doses from

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Typical effective doses fromdiagnostic medical exposures

Diagnostic

procedure

Typical effective

dose (mSv)

Equiv. no. of

chest x-rays

Approx. equiv. period of

Hip 0.3 15 7 weeks

Pelvis 0.7 35 4 months

Abdomen 1.0 50 6 months

IVU 2.5 125 14 months

: . ,Fro m R e fe rra lC rite ria Fo r Im a g in g C E.2 0 0 0

Typical effective doses from

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Typical effective doses fromdiagnostic medical exposures

Diagnostic

procedure

Typical effective

dose (mSv)

Equiv. no. of

chest x-rays

Approx. equiv. period of

Barium swallow 1.5 75 6 months

Barium meal 3 150 16 months

Barium followthrough

3 150 16 months

Barium enema 7 350 3.2 years

: . ,Fro m R e fe rra lC rite ria Fo r Im a g in g C E.2 0 0 0

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How to measure doses

Absolutemethods

Relativemethods

Calorimetry

Chemical(Fricke dosimeter)

Ionometry(ionization chamber)

Photography

Scintillation

TL

Ionometry

They needto know a

characteristic

parameter