radiaation protection in diagnostic and interventional imaging p18_qc_for_ct_web - copy

8/13/2019 Radiaation Protection in Diagnostic and Interventional Imaging P18_QC_for_CT_WEB - Copy

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IAEAInternational Atomic Energy Agency

RADIATION PROTECTION INDIAGNOSTIC ANDINTERVENTIONAL RADIOLOGY

Part 18: Optimization of protection in CT scannerPractical exercise - Quality Control

IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology


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18: Optimization of protection in CT scanner 2

Contents / Overview

Quality control test description on: CT accuracy, uniformity, linearity and noise, Low and high contrast resolution Z-axis sensitivity Alignment, Couch travel accuracy Gantry tilt measurement Dosimetry


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Part 18: Optimization of protection in CTscanner

Topic 1: Quality Control



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Physical parameters of CT image

Image quality May be expressed in terms of physical parameters such

as uniformity, linearity, noise, spatial resolution, lowcontrast resolution

It depends on the technological characteristics of the CTscanner, the exposure factors used and image viewingconditions.

Quality may be assessed by quantitative measurementusing test phantoms, and by the appearance of artifacts.

Measurements should be conducted regularly


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Scanner performance: technicalparameters (I)

Test Phantoms: Test phantoms of a standardized human shape

or test objects of a particular shape, size and

structure, are used for the purposes ofcalibration and evaluation of the performancesof CT scanners

They should allow for the parameters to be

checked: CT number; uniformity; noise; spatialresolution; slice thickness; dose; positioning ofcouch


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Scanner performance: technicalparameters (II)CT Number Accuracy

CT number depends on tube voltage, filtration, object thickness CT number of water is by definition equal to 0 Measured CT number should be < 4 HU in the central ROI

CT Number Linearity

It concerns the linear relationship between the calculated CTnumber and the linear attenuation coefficient of each element of theobject

Deviations from linearity should be < 5 HUCT Number Uniformity

It relates to the fact that a CT number of each pixel in the image ofan homogeneous object should be the same over various regions The difference in the CT number between a peripheral and a central

region of an homogeneous test object should be < 8HU Differences are largely due to beam hardening phenomenon


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The noise Is the local statistical fluctuation (standard deviation) ofCT numbers of an homogeneous Region Of Interest(ROI)

It strongly affects the low contrast resolution Noise is above all dependent on the radiation dose

Image noise should be measured over an area of about10% of the cross-sectional area of the test object.

The medical problem : to obtain an image with anacceptable level of noise while keeping the patient doseas low as reasonably achievable

Scanner performance: technicalparameters (III)

d o s e N o i s e 1 =


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Scanner performance: technicalparameters (IV)

Spatial Resolution The high contrast resolution determines the minimum

size of detail visualized in the plane of the slice with acontrast >10%.

It is affected by: the reconstruction algorithm the detector width the effective slice thickness

the object to detector distance the X-ray tube focal spot size the matrix size.


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Scanner performance: technicalparameters (V)

Spatial Resolution The low contrast resolution determines the size

of detail that can be visibly reproduced when

there is only a small difference in density relativeto the surrounding area

Low contrast resolution is considerably limited bynoise.

The perception threshold in relation to contrast anddetail size can be determined, for example, by meansof a contrast-detail curve.


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Scanner performance: technicalparameters (VI)

Slice Thickness The slice thickness is determined in

the center of the field of view as thedistance between the two points onthe sensitivity profile along the axisof rotation at which response has

fallen to 50%. The use of post-patient collimation

to reduce the width of the imageslice leads to very significantincreases in the patient dose

Positioning of couch The accuracy of positioning of the

patient couch is evaluated bymoving the loaded couch a defineddistance and subsequently moving itback to the start position


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Minimum requirements: CT scanner(I)

Image noiseThe Standard Deviation of CT numbers in the central 500mm2 ROI for a water or tissue equivalent phantom shouldnot deviate more than 20% from the baseline.

CT number valuesThe deviation in the CT number values for water or tissueequivalent material and materials of different densitiesshould


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Minimum requirements: CT scanner(II)

Computed tomography dose index (CTDI)The CTDI for a single slice for each available beam shaping filter andfor each available slice thickness should not deviate more than 20%from the baseline.

Irradiated slice thickness

The FWHM of the dose profile should not differ more than 20% frombaseline.

High contrast resolution (spatial resolution)The FWHM of the point spread function of a pin, or the edge responsefunction of an edge should not differ more than 20% from baseline.

Low contrast resolutionPolystyrene pins of 0.35 cm diameter inserted in a uniform body waterphantom should be visible in the image.


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Topic 2: Noise



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Imaging performance (Noise)

Noise is generally assessed using cylindricalphantoms, which are either filled with wateror made of a tissue equivalent material

Once an axial image of the phantom hasbeen acquired, noise is obtained from thestandard deviation in CT number in a regionof interest (ROI) placed centrally within theimage


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Region ofinterest(ROI)



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Noise figures given in manufacturersspecifications are quoted for a specific phantom(e.g. manufacturers QA phantom) and forspecified scan parameters

These conditions must be matched exactly for thepurposes of the acceptance test

Manufacturers often quote noise at a particularsurface dose

If this is the case, dose for axial scans can bemeasured by taping an ion chamber to the surfaceof the phantom


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Baseline noise values should be obtained forseveral scan protocols that will be usedclinically , using the routine QC noise

phantom

To ensure that noise figures are bothaccurate and representative, it is essential tofind the mean value from several scans (10scans.)


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Topic 3: CT uniformity and linearity



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CT number uniformity

CT number uniformity can be assessed at thesame time as measuring noise, by placing fouradditional ROI (N, E, S and W) at positions nearthe edge of the image of a uniform phantom

Mean CT number is then noted for these fourregions, as well as the central one

The deviation from the central value should becalculated

It can be valuable to check CT number uniformityfor large fields of view


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Axial imageof anhomogenousphantom



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CT number linearity is assessed using aphantom containing inserts of a number ofdifferent materials (materials should cover a

wide range of CT numbers) One example of a suitable phantom to useat acceptance is the Catphan (The PhantomLaboratory, Salem, NY), which contains fourinserts with CT numbers ranging fromapproximately -1000HU to +1000HU


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Topic 4: Low and high contrast resolution



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Low contrast resolution

Low contrast resolution (or low contrastdetectability) is often quoted in specificationdocumentation, as the smallest visible object at agiven contrast for a given dose

Since this measurement relates directly to imagingperformance, it is an important parameter to verifyat acceptance

At least 20 images of the low contrast insert (LCR)should be acquired and then viewed by at least 3observers under optimal viewing conditions, so asto obtain an average


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Typical imageof theCatphan LCRinsert

Low contrast resolution


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Spatial resolution (high contrast)

There exist two broad categories of measuringtechniques : those involving analysis of the point spread function,

usually by calculation of the modulation transfer function(MTF)

those involving either objective analysis or visualassessment of images of a resolution bar phantom.

The resolution is quoted as the spatial frequency (inline pairs / cm) at which the modulation falls to 50%,

10% or 2% MTF. These figures are often given for more than onereconstruction algorithm, e.g. for standard and high-resolution scans.


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Spatial resolution (high contrast)

The number of linepairs per cm justvisible in the image isapproximatelyequivalent to the 2%value of the MTF

This result can then becompared with the 2%MTF, if this is quoted inthe manufacturersspecification


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Topic 5: Z-axis sensitivity



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Z-Sensitivity (Imaged slice width)

Axial mode measurements Phantoms used for axial measurements may

contain thin metal plates, wires or arrays of airholes, inclined at an angle to the image plane

Manufacturers should be able to supply anappropriate phantom or, alternatively, the Catphancontains an insert suitable for this test

Note: to obtain meaningful measurements, the

thickness of the plates, wires or holes cannot begreater than the nominal slice width concerned.There may be problems for the sub-millimetre slicewidths offered on multi-slice scanners.


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Axial mode measurements Phantoms Manufacturers may quote the tolerance

for each nominal slice width setting in theirspecification documentation

Z-sensitivity measurements in axial mode can beused to check that imaged slice widths are withinthe tolerances given

They can also be used in conjunction withirradiated slice width measurements to assess theaccuracy of post patient collimation and tocalculate the geometric efficiency for the scanner


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Plan view of a test object used to measure imagedslice widths for axial scans



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Topic 6: Alignment


Alignment of indicating lights with


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Alignment of indicating lights withscan, coronal and sagittal planes

Several methods canbe used to performthese tests

The techniquesdescribed here arestraightforward toimplement and require

little specialist testequipment.

Agreement between internal and


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Agreement between internal andexternal scan plane lights

Use the envelope-wrapped film recommended forthat measurement, however, a piece of paper orcard can also be used

The wrapped film is placed flat on the table andilluminated by the external scan plane light

The position of the light is marked on the filmenvelope and the table is moved automatically tothe scan plane

If the distance between the internal and externallights is correct, the internal light should nowcoincide with the mark on the film envelope.

Co incidence of internal scan plane


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Co-incidence of internal scan planelights and scan plane

Pin pricks are made in a piece of therapyverification film (or similar) along the line of theinternal scan plane light, and the film is exposed toa narrow axial scan and developed

Coincidence between the pin pricks and the x-raybeam exposure indicates alignment between theinternal lights and the scan plane


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Co incidence of internal scan plane


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Z

X

Pin pricks made infilm at position ofscan plane light

X-ray beamexposure

Co-incidence of internal scan planelights and scan plane


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Coronal and Sagital plane lights

A long, thin object, with a high CT number relativeto air, such as the lead in a pencil or astraightened paper clip, can be used as a markerto perform this test

The marker is supported above the patient tableand aligned, using the indicating lights, so that it ispositioned at the isocentre, parallel to the z-axisand perpendicular to the scan plane

If indicating lights are accurately aligned to thecoronal and sagital planes, the marker shouldappear as a dot at exactly x = 0, y = 0 on the axialimage.


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Topic 7: Couch travel accuracy



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Couch travel accuracy

A ruler or tape measure placed alongside thetable, can be used to check that the degree ofcouch movement indicated on the gantry agreeswith the actual distance moved.

A load of approximately 70- 80 kg should beplaced on the table in order to simulate the weightof a patient.

The test should be performed twice : by driving thetable top both away from and towards the gantry.


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Assessment of distance indicator accuracy

Couch travel accuracy


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Axial scan incrementation accuracy

Verification of incrementation accuracy betweensuccessive axial slices can be achieved by placingenvelope-wrapped film on the couch (in theisocentre plane) and exposing it to an axial scansequence

Narrow slices separated by a couch incrementgreater than 1 slice width can be used, and thedistance between the lines on the film measured

Couch travel accuracy for helical


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Couch travel accuracy for helicalscans

In helical scanning, it is not sufficient to use a simplemechanical test because the distance imaged depends oncouch speed and scanner software

One method of assessing imaged distance accuracy is touse a Perspex test object containing two small radio-opaque markers, separated by a fixed distance (ex:20 cm)

The test object is scanned in Scan Projection Radiography(SPR) mode and a helical run is planned to start at the firstmarker and to end at a distance x from the first marker

If couch travel is accurate during the helical scan, themarkers should be clearly seen on the first and final imagesof the series.

Couch travel accuracy for helical


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Couch travel accuracy for helicalscans


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Topic 8: Gantry tilt measurement



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Gantry Tilt

Three axial exposures are made using the samefilm: one for the maximum superior gantry tilt,

one for the maximum inferior gantry tilt

one at 0 gantry tilt The three scan planes should then be visible on

the developed film

The angles + and - between scan planes atmaximum tilt relative to that at 0 tilt should equaltilt angles displayed on the gantry.

f f il


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Assessment of accuracy of gantry tilt


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Topic 9: Dosimetry


D i CTDI i i


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Dosimetry - CTDI in air

The Computed Tomography Dose Index(CTDI) in air can be measured using a 10cmpencil ionization chamber, bisected by the

scan plane at the isocentre, supported fromthe patient table

The ion chamber can be supported using aretort stand and clamp, if a dedicated holderis not available

D i CTDI i i


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Ionizationchamber

Table


D i CTDI i i


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When commissioning a CT scanner, it isvaluable to measure the CTDI in air underthe following conditions:

For all beam shaping filters For all nominal slice widths For all clinical kV settings

For a range of scan times For a range of mA settings

D i t CTDI i i


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An additional test : comparison of dose from ahelical scan with that from an axial scan

A helical CTDI value cannot be measured directly,as CTDI is a single slice measurement

To obtain a relative dose for helical scanning, theentire length of the chamber can be scanned firstly in axial mode (contiguous slices) and secondly inhelical mode, at pitch 1

The ratio of the 2 doses provides a correctionfactor, which can then be used to convert axialCTDIs into helical CTDIs

D i t CTDI i i (h li l)


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Axial slice positions

Helical scan (pitch 1)

Dosimetry - CTDI in air (helical)

Dosimetry - CTDI in Perspex


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Dosimetry CTDI in PerspexPhantoms

The body phantom placed on the patient table andthe head phantom is supported in the head rest

Phantoms are aligned centred at the scanisocentre

The ion chamber is inserted into either the centralor one of the peripheral cavities of the phantom (allother cavities being filled with Perspex rods)



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Dosimetry CTDI in PerspexPhantoms

Dose measurements at the center are used tocalculate the central CTDI

Peripheral CTDI is measured in at least four

positions around the phantom, so as to achieve atrue averageNote: if gantry rotation is initiated from different

angular positions for successive scans it may be

necessary to take a number of measurements ateach position in order to get a representative meandose



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Head phantom Body phantom(or annulus

to fit over head phantom)

Insert to plug holes

Dos et y C D e spePhantoms



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Central and peripheral CTDIs are used tocalculate weighted CTDI, CTDIw:

CTDIws can be compared against diagnosticreference levels for standard scanexaminations

y pPhantoms

) ( CTDI 3 2 + CTDI 3 1 C 1 = CTDI p 100, c 100, w n

Example of CTDI Values for some CT


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punits

MODEL KV FAD S nCDTI (mGy/mAs)

Air weightedHead BodySiemensHIQ

133 700 10 0.195 0.161 (0.83) 0.093 (0.48)

Siemens

Plus S

120 700 10 0.128 0.110 (0.86) 0.062 (0.48)

SiemensPlus S

137 700 8 0.161 0.082 (0.51)

GE Pace 120 525 10 0.344 0.200 (0.58) 0.094 (0.27)

PhilipsLX

120 606 10 0.200 0.160 (0.80) 0.081 (0.41)

FAD(mm): Focus to Axis distanceS (mm): Slice thickness

Dose Profiles (Irradiated slice width)


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Measurement of irradiated slice widths, for allnominal slice width settings, provides a direct testof pre-patient beam collimation functionality andallows geometric efficiencies to be calculated for

the scanner Geometric efficiency (GE) is defined as:

GE = width slice irradiated x 100 %width slice imaged

It is recommended that GE value is displayed onthe console if it is less than 70%



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First method : expose envelope-wrapped x-ray film,supported in air at the isocentre, at each of the slice widthsettings

Once developed, optical density profiles may be plotted

using a scanning microdensitometer or the width measuredusing a ruler

To provide an accurate dose profile, a calibration curve canbe applied to convert optical density profiles into dose

profiles, from which irradiated slice widths (FWHM of doseprofiles) may be derived Alternative technique: measure dose profiles using TLDs.



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Measurement ofirradiated slicewidths for a range

of nominal slicewidth settings


Quality control program


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Quality control program

Example of a quality control program for a CT unit:

Baseline 20%H1 yBHigh contrast resolution

> 20% or 1 mmM1 yBImage slice thickness

> 20% or 1 mmH1 yBIrradiated slice thickness

Baseline 20%H1 y ACTDI

> 1.5%H1 yBCT number uniformity

Water: 10 HUOther materials 20 HU*more scans/parameters

H

H

D/W

1 Y*

A

B

CT number values

Baseline 20%*more scans/parameters

HH

D/W1 y*

AB

Image noise

Remedial levelPriorityFrequencyLevel ofexpertise

Physical parameter

radiaation protection in diagnostic and interventional imaging p18_qc_for_ct_web - copy

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