rpdir-p18 qc for ct web

7/26/2019 Rpdir-p18 Qc for Ct Web

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IAEA

18: Optimization of protection in CT scanner 2

Contents

Quality control tests

CT accuracy, uniformity, linearity. an noise,

!o" an #i$# contrast resolution

%-axis sensiti&ity

'li$nment, ta(le top tra&el accuracy

)antry tilt measurement

*osimetry


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IAEAInternational Atomic Energy Agency

Part !"# O$timi%ation o& $rotection in CTscanner

Topic 1: Quality Control

IAEA Training Material on Radiation Protection in Diagnostic and Interentional Radiology


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IAEA

18: Optimization of protection in CT scanner +

P'ysical $arameters o& CT image

ma$e uality

ay (e expresse in terms of p#ysical parameters suc#as uniformity, linearity, noise, spatial resolution, lo"

contrast resolution

ma$e uality epens on t#e tec#nical c#aracteristics oft#e CT scanner, t#e exposure factors use an ima$e

&ie"in$ conitions.

Quality may (e assesse (y uantitati&e measurementusin$ test p#antoms, an (y t#e appearance of artifacts.

easurements s#oul (e conucte re$ularly


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IAEA

18: Optimization of protection in CT scanner /

Scanner $er&ormance# tec'nical

$arameters (I)

Test P#antoms: Test p#antoms of a stanarize #uman s#ape

or test o(0ects of a particular s#ape, size an

structure, are use for t#e purposes ofcali(ration an e&aluation of t#e performancesof CT scanners

P#antoms s#oul allo" for e&aluation of CT

num(er uniformity noise spatial resolutionslice t#icness ose positionin$ of ta(le top


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IAEA

18: Optimization of protection in CT scanner 3


$arameters (II)CT 4um(er 'ccuracy

CT num(er epens on tu(e &olta$e, filtration, o(0ect t#icness

CT num(er of "ater is (y efinition eual to 5

easure CT num(er s#oul (e 6 7 + 9 in t#e central O

CT 4um(er !inearity T#e linear relations#ip (et"een t#e calculate CT num(er an t#e linear

attenuation coefficient of eac# element of t#e o(0ect

*e&iations from linearity s#oul (e 6 7 / 9

CT 4um(er 9niformity T#e CT num(er of eac# pixel in t#e ima$e of an #omo$eneous o(0ect

s#oul (e t#e same o&er t#e ima$e area

T#e ifference in t#e CT num(er (et"een a perip#eral an a centralre$ion of an #omo$eneous test o(0ect s#oul (e 6 8 9

*ifferences are lar$ely ue to (eam #arenin$


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IAEA 18: Optimization of protection in CT scanner ;

4oise T#e local statistical fluctuation


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IAEA 18: Optimization of protection in CT scanner 8


$arameters (IV)

@patial esolution

T#e #i$# contrast resolutionetermines t#e minimumsize of etails &isualize in t#e plane of t#e slice "it# a

contrast A15>.

t is affecte (y:

t#e reconstruction al$orit#m

t#e etector "it#

t#e effecti&e slice t#icness

t#e o(0ect to etector istance

t#e x-ray tu(e focal spot size

t#e matrix size.


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IAEA 18: Optimization of protection in CT scanner B


$arameters (V)

@patial esolution

T#e lo" contrast resolutionetermines t#e sizeof etail t#at can (e &isi(ly reprouce "#en

t#ere is only a small ifference in ensity relati&eto t#e surrounin$ area

!o" contrast resolution is limite (y noise.

T#e perception t#res#ol in relation to contrast anetail size can (e etermine (y creatin$ contrast-

etail cur&e.


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$arameters (VI)

@lice T#icness

T#e slice t#icness is etermine int#e center of t#e fiel of &ie" as t#e

istance (et"een t#e t"o points on

t#e sensiti&ity profile alon$ t#e axis

of rotation at "#ic# response #as

fallen to /5>.

T#e use of post-patient collimationto reuce t#e "it# of t#e ima$e

slice leas to &ery si$nificant

increasesin t#e patient ose

Positionin$ of couc#

T#e accuracy of positionin$ of t#epatient couc# is e&aluate (y

mo&in$ t#e loae couc# a efine

istance an mo&in$ it (ac to t#e

start position


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Minim*m re+*irements# CT scanner (I)

ma$e noiseT#e @tanar *e&iation of CT num(ers in t#e central /55mm2O for a "ater or tissue eui&alent p#antom s#oulnot e&iate more t#an 25> from t#e (aseline.

CT num(er &alues

T#e e&iation in t#e CT num(er &alues for "ater or tissueeui&alent material an materials of ifferent ensitiess#oul 67 25 9 or />.

CT num(er uniformity

T#e @* of t#e CT num(er o&er a /55 mm2

re$ion ofinterest for "ater or tissue eui&alent material at t#e centeran at t#e perip#ery of p#antom s#oul &ary (y less t#an1./> of t#e (aseline


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Minim*m re+*irements# CT scanner

(II)

Compute tomo$rap#y ose inex

from t#e (aseline.

rraiate slice t#icness

T#e D of t#e ose profile s#oul not iffer more t#an 7 25> from(aseline.

i$# contrast resolution from (aseline.

!o" contrast resolution

Polystyrene pins of 5.E/ cm iameter inserte in a uniform (oy "ater

p#antom s#oul (e &isi(le in t#e ima$e.


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Topic 2: 4oise



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IAEA 18: Optimization of protection in CT scanner 1+

Imaging $er&ormance (Noise)

4oise is assesse usin$ cylinricalp#antoms, "#ic# are eit#er fille "it# "ateror mae of a tissue eui&alent material

Once an axial ima$e of t#e p#antom #as

(een acuire, noise is measures as t#e

stanar e&iation in CT num(er in a re$ionof interest


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IAEA 18: Optimization of protection in CT scanner 1/

e$ion ofe$ion ofinterestinterest


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4oise &alues are $i&en in manufacturersFspecifications are uote for a specific p#antom


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IAEA 18: Optimization of protection in CT scanner 1;


Gaseline noise &alues s#oul (e o(tainefor se&eralscan protocols t#at "ill (e useclinically, usin$ t#e routine QC noise

p#antom

To ensure t#at noise fi$ures are (ot#

accurate an representati&e, it is essential tofin t#e mean &alue from se&eral scans


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Topic E: CT uniformity an linearity



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IAEA 18: Optimization of protection in CT scanner 1B

CT n*m,er *ni&ormity

CT num(er uniformity can (e assesse at t#esame time as measurin$ noise, (y placin$ fouraitional O


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'xial ima$e of'xial ima$e of

a #omo$enousa #omo$enous

p#antomp#antom



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CT num(er linearity is assesse usin$ ap#antom containin$ inserts of a num(er ofifferent materials


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Topic +: !o" an #i$# contrast resolution



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IAEA 18: Optimization of protection in CT scanner 2+

Lo- contrast resol*tion

!o" contrast resolution


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IAEA 18: Optimization of protection in CT scanner 2/

TypicalTypicalima$e of t#eima$e of t#e

Catp#an !CCatp#an !C

insertinsert

Lo- contrast resol*tion


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S$atial resol*tion ('ig' contrast)

T#ere exist t"o (roa cate$ories of measurin$tec#niues : analysis of t#e point sprea function, usually (y

calculation of t#e moulation transfer function


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IAEA 18: Optimization of protection in CT scanner 2;

S$atial resol*tion ('ig' contrast)

T#e num(er of linepairs per cm 0ust&isi(le in t#e ima$e isapproximately

eui&alent to t#e 2>&alue of t#e T

T#is result can t#en (ecompare "it# t#e 2>

T, as uote in t#emanufacturerFsspecifications


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Topic /: %-axis sensiti&ity



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IAEA 18: Optimization of protection in CT scanner 2B

./Sensitiity (Imaged slice -idt')

'xial measurements

P#antoms use for axial measurements may containt#in metal plates, "ires or arrays of air #oles, incline atan an$le to t#e ima$e plane

anufacturers s#oul (e a(le to supply an appropriatep#antom or, alternati&ely, t#e Catp#an contains an insertsuita(le for t#is test

4ote:to o(tain meanin$ful measurements, t#e t#icnessof t#e plates, "ires or #oles cannot (e $reater t#an t#e

nominal slice "it# concerne. T#is may createpro(lems for t#e su(-millimetre slice "it#s offere onmulti-slice scanners.


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IAEA 18: Optimization of protection in CT scanner E5


'xial measurements

P#antoms manufacturers may uote t#e tolerancefor eac# nominal slice "it# settin$ in t#eirspecification ocumentation

%-sensiti&ity measurements in axial moe can (euse to c#ec t#at ima$e slice "it#s are "it#int#e tolerances $i&en

T#is can also (e use in con0unction "it# irraiateslice "it# measurements to assess t#e accuracy ofpost patient collimation an to calculate t#e$eometric efficiency for t#e scanner


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Plan &ie" of a test o(0ect use to measure ima$e

slice "it#s for axial scans



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Topic 3: 'li$nment



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IAEA 18: Optimization of protection in CT scanner EE

Alignment o& indicating lig'ts -it'

scan0 coronal and sagittal $lanes

@e&eral met#os can(e use to perform

t#ese tests

T#e tec#niuesescri(e #ere are

strai$#tfor"ar to

implement an reuire

minimal testeuipment.


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IAEA 18: Optimization of protection in CT scanner E+

Agreement ,et-een internal and

e1ternal scan $lane lig'ts

9se an en&elope-"rappe film


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IAEA 18: Optimization of protection in CT scanner E/

Co/incidence o& internal scan $lane

lig'ts and scan $lane

Pin prics are mae in a piece of t#erapy&erification film


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or multi-slice scanners, pin prics


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IAEA 18: Optimization of protection in CT scanner E;

%

M

Pin $ric2s made in

&ilm at $osition o&

scan $lane lig't

3/ray ,eam

e1$os*re




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Coronal and Sagital $lane lig'ts

' lon$, t#in o(0ect, "it# a #i$# CT num(er relati&e toair, suc# as t#e NleaF in a pencil or a strai$#tenepaper clip, can (e use as a marer to perform t#istest

T#e marer is supporte a(o&e t#e patient ta(le anali$ne, usin$ t#e inicatin$ li$#ts, so t#at it ispositione at t#e isocentre, parallel to t#e z-axis anperpenicular to t#e scan plane

f inicatin$ li$#ts are accurately ali$ne to t#e coronal

an sa$ital planes, t#e marer s#oul appear as a otat exactly x 5, y 5 on t#e axial ima$e.


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Topic ;: Ta(le tra&el accuracy



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IAEA 18: Optimization of protection in CT scanner +5

Ta,le trael acc*racy

' ruler or tape measure place alon$sie t#e ta(le,can (e use to c#ec t#at t#e amount of ta(le

mo&ement inicate on t#e $antry a$rees "it# t#e

actual istance mo&e.

' loa of approximately ;5- 85 $ s#oul (eplace on t#e ta(le in orer to simulate t#e "ei$#t

of a patient.

T#e test s#oul (e performe t"ice: (y ri&in$ t#eta(le top (ot# a"ay from an to"ars t#e $antry.


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Assessment o& distance indicator acc*racy

Ta,le trael acc*racy

A i l i i


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A1ial scan incrementation acc*racy

erification of incrementation accuracy (et"eensuccessi&e axial slices can (e ac#ie&e (y placin$

t#erapy &erification film on t#e couc#


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IAEA 18: Optimization of protection in CT scanner +E

Ta,le trael acc*racy &or 'elical scans

n #elical scannin$, it is not sufficient to use a simplemec#anical test (ecause t#e istance ima$e epens oncouc# spee an scanner soft"are

One met#o of assessin$ istance accuracy is to use an

acrylic test o(0ect containin$ t"o small raio-opauemarers, separate (y a fixe istance


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IAEA 18: Optimization of protection in CT scanner ++

Co*c' trael acc*racy &or 'elical

scans


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Topic 8: )antry tilt measurement


G t Tilt


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Gantry Tilt

T#e accuracy of isplaye $antry tilt can (eassesse (y supportin$ t#erapy &erification

film at t#e $antry en of t#e patient ta(le

T#e film must (e #el &ertically


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IAEA 18: Optimization of protection in CT scanner +;

Gantry Tilt

T#ree axial exposures are mae usin$ t#e samefilm:

one for t#e maximum superior $antry tilt,

one for t#e maximum inferior $antry tilt

one at 5 $antry tilt

T#e t#ree scan planes s#oul t#en (e &isi(le ont#e e&elope film

T#e an$les Jan -(et"een scan planes atmaximum tilt relati&e to t#at at 5 tilt s#oul eual

tilt an$les isplaye on t#e $antry.

A t & & t tilt


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Assessment o& acc*racy o& gantry tilt


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Topic B: *osimetry


D i t CTDI i i


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IAEA 18: Optimization of protection in CT scanner /5

Dosimetry / CTDI in air

T#e Compute Tomo$rap#y *ose nex


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Ioni%ationc'am,er

Ta,le


Dosimetry CTDI in air


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t is important to measure t#e CT* in airuner t#e follo"in$ conitions D#en

commissionin$ a CT scanner:

or all (eam s#apin$ filters or all nominal slice "it#s

or all clinical settin$s

or a ran$e of scan times or a ran$e of m' settin$s



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IAEA 18: Optimization of protection in CT scanner /+


Dosimetry CTDI in air ('elical)


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IAEA 18: Optimization of protection in CT scanner //

'xial slice positions'xial slice positions

elical scan


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Dosimetry / CTDI in Acrylic P'antoms

T#e (oy p#antom is place on t#e patient ta(lean t#e #ea p#antom is supporte in t#e #ea

rest

P#antoms are ali$ne an centre at t#e scanisocentre

T#e ion c#am(er is inserte into eit#er t#e centralor one of t#e perip#eral #oles of t#e p#antom


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IAEA 18: Optimization of protection in CT scanner /;

Dosimetry / CTDI in Acrylic P'antoms

*ose measurements at t#e center are use tocalculate t#e central CT*

Perip#eral CT* is measure in at least four

positions aroun t#e p#antom, so as to ac#ie&e atrue a&era$e

4ote:f $antry rotation is initiate from ifferent

an$ular positions for successi&e scans it may (e

necessary to tae a num(er of measurements ateac# position in orer to $et a representati&e mean

ose

Dosimetry / CTDI in Pers$e1


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4ead $'antom4ead $'antom5ody $'antom5ody $'antom (or ann*l*s(or ann*l*s

to &it oer 'ead $'antom)to &it oer 'ead $'antom)

Insert to $l*g 'olesInsert to $l*g 'oles


P'antoms



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IAEA 18: Optimization of protection in CT scanner /B

Central an perip#eral CT*Fs are use tocalculate "ei$#te CT*, CT*":

CT*"

s can (e compare a$ainst ia$nosticreference le&els for stanar scan

examinations


P'antoms

)( CTDI32+CTDI31C1=CTDI p100,c100,wn

E1am$le o& CTDI Val*es &or some CT


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E1am$le o& CTDI Val*es &or some CT

*nitsMODEL 6V 7AD S nCDTI (mGy8mAs)

Air -eig'ted4ead 5ody

Siemens4I9

!:: ;


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Dose Pro&iles (Irradiated slice -idt')

easurement of irraiate slice "it#s, for allnominal slice "it#settin$s, pro&ies a irect testof pre-patient (eam collimationan allo"s$eometric efficiencies to (e calculate for t#e

scanner )eometric efficiency



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irst met#o : expose t#erapy &erification film, supporte inair at t#e isocentre, at eac# of t#e slice "it# settin$s

Once e&elope, optical ensity profiles may (e plotteusin$ a i$itizer or t#e "it# measure usin$ a ruler

To pro&ie an accurate ose profile, a cali(ration cur&e can(e applie to con&ert optical ensity profiles into ose

profiles, from "#ic# irraiate slice "it#s


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IAEA 18: Optimization of protection in CT scanner 3E

Meas*rement o&Meas*rement o&irradiated sliceirradiated slice

-idt's &or a range-idt's &or a range

o& nominal sliceo& nominal slice

-idt' settings-idt' settings


9*ality control $rogram


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9*ality control $rogram

E1am$le o& a +*ality control $rogram &or a CT *nit#

Gaseline 7 25>1 yGi$# contrast resolution

A 7 25> or 1 mm1 yGma$e slice t#icness

A 7 25> or 1 mm1 yGrraiate slice t#icness

Gaseline 7 25>1 y'CT*

A 7 1./>1 yGCT num(er uniformity

Dater: 7 15 9

Ot#er materials 7 25 9

Rmore scansKparameters

*KD

1 IR

'

G

CT num(er &alues

Gaseline 7 25>

Rmore scansKparameters

*KD

1 yR

'

G

ma$e noise

emeial le&elPriorityreuency!e&el ofexpertise

P#ysical parameter

rpdir-p18 qc for ct web

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