CTSeeram Chapter 5:Data Acquisition in CT

Data Collection BasicsX-ray source & detector must be in & stay in alignmentBeam moves (scans) around patientmany transmission measurementsPatientX-Ray beams

Data Collection BasicsPre-patient beamcollimated to pass only through slice of interestshaped by special bow tie filter for uniformityPatientFilter

Data Collection Basics (cont)Beam attenuated by patientTransmitted photons detected by scannerDetected photon intensity converted to electrical signal (analog)Electrical signal converted to digital valueA to D converterDigital value sent to reconstruction computer

CT RayThat part of beam falling onto a single detectorRay

Each CT Rayattenuated by patientprojected onto one detectordetector produces electrical signalproduces single data sample

CT View# of simultaneously collected rays

Scan Requires Many Data Samples# Data Samples = [# data samples per view] X [# views]# Data Samples = [# detectors] X [# data samples per detector]

Acquisition GeometriesPencil Beam Fan Beam Spiral Multislice

Pencil Beam GeometryTube-detector assembly translates left to rightEntire assembly rotates 1o1o1st Generation CT

Fan Beam Geometry2nd Generation3nd Generation4th Generation

Comparing Long vs. Short GeometryScanFOVScanFOVSmaller fan angleLonger source-detector distanceLower beam intensityLower patient doseMore image noiseLess image blurRequires larger gantryLong Geometry

Spiral GeometryX-ray tube rotates continuously around patientPatient continuously transported through gantryNo physical wiring between gantry & x-ray tubeRequires Slip Ring technology

Whats a Slip Ring?

Slip RingsElectrical connections made by stationary brushes pressing against rotating circular conductorSimilar to electric motor / generator design

X-Ray Generator Configurations with Slip Ring TechnologyProblem:Supply high voltage to a continually rotating x-ray tube?Options#1Stationary Generator & Transformer#2Stationary GeneratorTransformer & x-ray tube rotate in gantry#3Transformer, generator & tube rotate in gantry

X-Ray GeneratorHigh Voltage TransformerX-Ray TubeIncoming AC PowerPrimary VoltageSecondary Voltage

Option #1: Stationary High Voltage Transformerhigh voltage must pass through slip ringsHV TransformerGenerator

X-Ray GeneratorHigh Voltage TransformerX-Ray TubeIncoming AC PowerPrimary VoltageSecondary Voltage

Option #2: Rotating High Voltage Transformerlow voltage must pass through slip ringsGenerator

X-Ray GeneratorHigh Voltage TransformerX-Ray TubeIncoming AC PowerPrimary VoltageSecondary Voltage

Rotating Generatorlow line voltage must pass through slip rings

Spiral CT AdvantagesFaster scan timesminimal interscan delaysno need to stop / reverse direction of rotationSlip rings solve problem of cabling to rotating equipmentContinuous acquisition protocols possible

X-Ray System ComponentsX-Ray GeneratorX-Ray TubeBeam FilterCollimators

X-Ray Generator3 phase originally usedMost vendors now use high frequency generatorsrelatively smallsmall enough to rotate with x-ray tubecan fit inside gantry

X-Ray Tube

X-Ray TubeMust provide sufficient intensity of transmitted radiation to detectorsRadiation incident on detector depends uponbeam intensity from tubepatient attenuationbeams energy spectrumpatientthicknessatomic #density

Maximizing X-Ray Tube Heat Capacityrotating anodehigh rotational speedsmall target anglelarge anode diameterfocal spot size appropriate to geometrydistancesdetector size

Special Considerations for Slip Ring Scannerscontinuous scanning meansHeat added to tube fasterNo cooling between slicesNeed more heat capacityfaster cooling

Why not use a Radioactive Source instead of an X-Ray Tube?High intensity requiredX-ray tubes produce higher intensities than sourcesSingle energy spectrum desiredProduced by radioactive sourceX-ray tubes produce spectrum of energiesCoping with x-ray tube energy spectrumheavy beam filtering (see next slide)reconstruction algorithm corrects for beam hardening

PatientCT Beam FiltrationHardens beampreferentially removes low-energy radiationRemoves greater fraction of low-energy photons than high energy photonsreduces patient exposureAttempts to produce uniform intensity & beam hardening across beam cross sectionFilter

CT Beam CollimationPre-collimatorsbetween tube & patientTubeDetectorPost-collimatorsbetween patient & detector

Pre-CollimationConstrains size of beamReduces production of scatterMay have several stages or sets of jaws

Pre-collimator

Post-CollimationReduces scatter radiation reaching detectorHelped define slice (beam) thickness for some scannersPost-collimator

CT Detector Technology:Desirable CharacteristicsHigh efficiencyQuick response timeHigh dynamic rangeStability

CT Detector EfficiencyAbility to absorb & convert x-ray photons to electrical signals

Efficiency Components

Capture efficiencyfraction of beam incident on active detectorAbsorption efficiencyfraction of photons incident on the detector which are absorbedConversion efficiencyfraction of absorbed energy which produce signal

Overall Detector EfficiencyOverall detector efficiency = capture efficiency X absorption efficiency X conversion efficiency

Capture EfficiencyFraction of beam incident on active detector

Absorption EfficiencyDepends upon detectorsatomic #densitysizethicknessDepends on beam spectrum

capture efficiency X absorption efficiency X conversion efficiencyFraction of photons incident on the detector which are absorbed

Conversion EfficiencyAbility to convert x-ray energy to lightGE Gemstone Detector made of garnet

Conversion EfficiencySiemens UltraFastCeramic (UFC) CT DetectorProprietaryFast afterglow decay

Ability to convert x-ray energy to lightUFC PlateUFC Material

Response TimeMinimum time after detection of 1st event until detector can detect 2nd eventIf time between events < response time, 2nd event may not be detectedShorter response time better

StabilityConsistency of detector signal over timeShort termLong termThe less stable, the more frequently calibration required

Dynamic RangeRatio of largest to smallest signal which can be faithfully detected Ability to faithfully detect large range of intensities Typical dynamic range: 1,000,000:1much better than film

Detector Types: Gas IonizationX-rays converted directly to electrical signalIonizationChamber-++-Filled with Air

CT Ionization DetectorsMany detectors (chambers) usedadjacent walls shared between chambersTechniques to increase efficiencyIncrease chamber thicknessx-rays encounter longer path lengthPressurize air (xenon)more gas molecules encountered per unit path lengthX-Raysthickness

Older Style Scintillation DetectorsX-rays fall on crystal materialCrystal glowsLight flash directed toward photomultiplier (PM) tubeLight directed through light pipe or conduitPM tube converts light to electrical signalsignal proportional to light intensityPMElectricalSignal

Detector Types: ScintillationX-ray energy converted to lightLight converted to electrical signal

Photomultiplier TubeElectricalSignalScintillationCrystal

Photomultiplier TubesLight incident on Photocathode of PM tubePhotocathode releases electrons

X-RaysLightScintillationCrystalPMTubePhotocathode-+Dynodes

Photomultiplier TubesElectrons attracted to series of dynodeseach dynode slightly more positive than last oneX-RaysLightScintillationCrystalPMTubePhotocathode-+++++Dynodes

Solid State DetectorsCrystal converts incident x-rays to lightPhotodiode semiconductor current proportional to lightX-RaysLightPhotodiodeSemiconductorElectricalSignal

PhotodiodeMade of two types of materialsp-typen-typeLens focuses light from crystal onto junction of p & n type materialspnLensJunctionX-RaysLight

PhotodiodeLight controls resistance of junctionSemiconductor current proportional to light falling on junctionpnLensJunctionX-RaysLight

Solid State DetectorsOutput electrical signal amplifiedFast response timeLarge dynamic rangeAlmost 100% conversion & photon capture efficiencyScintillation materialscadmium tungstatehigh-purity ceramic material

Pre-AmplifierAnalog to DigitalConverterLogarithmic AmplifierFromDetectorToComputerCompresses dynamic range; Converts transmission intensity into attenuation dataIncreases signal strength for later processing

Logarithmslogarithms are exponentslog10x is exponent to which 10 is raised to get xlog10100 =2 because 102=100Log10x = ? means 10? = x?

LogarithmsUsing logarithms the difference between 10,000 and 100,000 is the same as the difference between 10 and 100

Compression3 = log 100011010010002 =log 1001 = log 100 = log 10Hard to distinguish between 1 & 10 hereDifference between 1 & 10 the same as between 100 & 1000Logarithms stretch low end of scale; compress high end

Logarithmic Amplifieraccepts widely varying inputtakes logarithm of inputamplifies logarithmlogarithm output dynamic range now appropriate for A/D conversion100,00010,0001,000100101543210InputLogarithm

Improving Quality & DetectionGeometrySmaller detectorsSmaller focal spotLarger focus-detector distanceSmaller patient-detector distanceThinner slicesless patient variation over slice thickness distance

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