CT Image QualityCT Image Quality

Dr M A OghabianDr M A Oghabian

Medical Physics Department,Medical Physics Department,

Tehran University of Medical Tehran University of Medical SciencesSciences

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SinogarmSinogarm

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Reconstruction FiltersReconstruction Filters

CT number=1000×(pixel-water)/water

CT NumberCT Number

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Window width & window levelWindow width & window level

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Quality criteria for CT imagesQuality criteria for CT images

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Image QualityImage Quality

1) Spatial Resolution1) Spatial Resolution 2) Density Resolution2) Density Resolution 3) Noise3) Noise 4) Artifact4) Artifact

Partial volume effectPartial volume effect Beam HardeningBeam Hardening Star ArtifactStar Artifact Slice profileSlice profile

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Spatial Resolution (high contrast)Spatial Resolution (high contrast)

Spatial Resolution is defined as: The number of line pairs per cm just visible in image

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The minimum size of detail visualized in the The minimum size of detail visualized in the image with a contrast >10%. image with a contrast >10%. It is affected by:It is affected by: Reconstruction algorithmReconstruction algorithmDDetector widthetector width Slice thicknessSlice thicknessObject to detector distanceObject to detector distanceX-ray tube X-ray tube focal spotfocal spot size sizeMatrix size.Matrix size.

Spatial Resolution Spatial Resolution high contrast resolutionhigh contrast resolution

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Spatial ResolutionSpatial Resolution Low contrast resolutionLow contrast resolution

The size of detail The size of detail that can be that can be visualized when visualized when there is only a there is only a small difference in small difference in densitydensity

It limited by It limited by noise. noise.

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Slice Thickness and Pixel sizeSlice Thickness and Pixel size

Thicker slice or bigger pixel size causes Thicker slice or bigger pixel size causes worse Spatial Resolution, but better worse Spatial Resolution, but better SNRSNR

Thicker Slice improve Density resolutionThicker Slice improve Density resolution

Higher mAs and more patient dose Higher mAs and more patient dose leads to better SNRleads to better SNR

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NoiseNoise

Noise is obtained Noise is obtained

from the standard from the standard deviation in CT deviation in CT number in a number in a region of interest region of interest (ROI) placed (ROI) placed within the imagewithin the image

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It affects the low contrast resolutionIt affects the low contrast resolution Noise dependents on the radiation doseNoise dependents on the radiation dose The medical problem is: to obtain an image The medical problem is: to obtain an image

with an acceptable level of noise while keeping with an acceptable level of noise while keeping the patient dose as low as reasonably the patient dose as low as reasonably achievableachievable

doseNoise 1

NoiseNoise

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Scatter!!Scatter!!

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Typical Scatter SurveyTypical Scatter Survey

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CT Number AccuracyCT Number Accuracy

Measured CT number should be < Measured CT number should be < ±± 4 HU in 4 HU in the central the central ROI ROI

CT number of water is by definition equal to 0 CT number of water is by definition equal to 0 CT number depends on tube voltage, filtration, CT number depends on tube voltage, filtration,

object thicknessobject thickness

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CT number uniformityCT number uniformity

•The difference in the CT number between a peripheral and a The difference in the CT number between a peripheral and a central region should be < 8HUcentral region should be < 8HU•Differences are largely due to Differences are largely due to beam hardening beam hardening phenomenonphenomenon

CT number of CT number of each pixel in each pixel in the image of the image of an an homogeneous homogeneous object should object should be the same be the same over various over various regionsregions

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CT number linearityCT number linearityLinear relationship between the calculated CT number and Linear relationship between the calculated CT number and the the linear attenuation coefficientlinear attenuation coefficient of each element of the of each element of the object. Deviations from linearity should be < ± 5 HUobject. Deviations from linearity should be < ± 5 HU

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ArtifactsArtifacts

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Partial Volume ArtifactPartial Volume Artifact

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Star ArtifactStar Artifact

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Ring ArtifactRing Artifact

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The slice sensitivity profile (SSP)The slice sensitivity profile (SSP)

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Z-Sensitivity Z-Sensitivity (Imaged slice width)(Imaged slice width)

Plan view of a test object used to measure imaged slice widths for axial scans, to assess the accuracy of post patient collimation, and to calculate the geometric efficiency for the scanner

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Alignment of indicating lights with Alignment of indicating lights with scan, coronal and sagittal planesscan, coronal and sagittal planes

Several methods can be used to perform these tests

The wrapped film is placed flat The wrapped film is placed flat on the table and illuminated on the table and illuminated by the external scan plane by the external scan plane lightlight

The position of the light is The position of the light is

marked on the film envelope marked on the film envelope and the table is moved and the table is moved automatically to the scan automatically to the scan planeplane

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Couch travel accuracyCouch travel accuracy

To assess the distance indicator accuracy, a ruler or tape a ruler or tape measure placed alongside the table, to check that the measure placed alongside the table, to check that the degree of couch movement indicated on the gantry degree of couch movement indicated on the gantry agrees with the actual distance moved. agrees with the actual distance moved.

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Axial scan incrementation Axial scan incrementation accuracyaccuracy

VVerification of incrementation accuracy erification of incrementation accuracy between successive axial slices can be between successive axial slices can be achieved by placing envelope-wrapped film achieved by placing envelope-wrapped film on the couch (in the isocentre plane) and on the couch (in the isocentre plane) and exposing it to an axial scan sequenceexposing it to an axial scan sequence

Narrow slices separated by a couch Narrow slices separated by a couch

increment greater than 1 slice width can be increment greater than 1 slice width can be used, and the distance between the lines on used, and the distance between the lines on the film measuredthe film measured

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Couch travel accuracy for Couch travel accuracy for helical scanshelical scans In helical scanning, to assess imaged distance In helical scanning, to assess imaged distance

accuracy, a Perspex test object containing two small accuracy, a Perspex test object containing two small radio-opaque markers, separated by a fixed distance radio-opaque markers, separated by a fixed distance (ex:20 cm) is used.(ex:20 cm) is used.

A helical run is planned to start at the first marker and A helical run is planned to start at the first marker and

to end at a distance x from the first markerto end at a distance x from the first marker If couch travel is accurate during the helical scan, the If couch travel is accurate during the helical scan, the

markers should be clearly seen on the first and final markers should be clearly seen on the first and final images of the series.images of the series.

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Couch travel accuracy for Couch travel accuracy for helical scanshelical scans

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Assessment of accuracy of gantry tiltAssessment of accuracy of gantry tilt

A film must be held vertically, so that it is parallel to the sagital plane Three axial exposures with 0, max superior and max inferior gantry Three axial exposures with 0, max superior and max inferior gantry

tilt are made using the same film:tilt are made using the same film: The three scan planes should then be visible on the developed film The three scan planes should then be visible on the developed film

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Dosimetry - CTDI in air (helical)Dosimetry - CTDI in air (helical)

Axial slice positionsAxial slice positions

Helical scan (pitch 1)Helical scan (pitch 1)

The Computed Tomography Dose Index (CTDI) in air The Computed Tomography Dose Index (CTDI) in air can be measured using a 10cm pencil ionization can be measured using a 10cm pencil ionization chamber, bisected by the scan plane at the isocentre.chamber, bisected by the scan plane at the isocentre.

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Dosimetry - CTDI in Perspex Dosimetry - CTDI in Perspex PhantomsPhantoms

Head phantomHead phantom Body phantom Body phantom (or annulus (or annulus to fit over haed phantom)to fit over haed phantom)

Insert to plug holesInsert to plug holes

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Dosimetry - CTDI in Perspex Dosimetry - CTDI in Perspex PhantomsPhantoms Central and peripheral CTDI’s are used to calculate

weighted CTDI, CTDIw:

CTDIws can be compared against diagnostic reference levels for standard scan examinations

)( CTDI3

2 + CTDI

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C

1 = CTDI p100,c100,wn