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CT in Radiation CT in Radiation OncologyOncology

Rachel Liu, Ph.D.Rachel Liu, Ph.D.

ContentsContents

1.1. What is multiWhat is multi--detector CT?detector CT?

2.2. How is CT used in Radiation How is CT used in Radiation Oncology?Oncology?

1.1. What is multiWhat is multi--detector detector CT?CT?

AcknowledgementAcknowledgementBorrowed some slides from Dr. Dianna Borrowed some slides from Dr. Dianna Cody, Dept of Imaging Physics, MDACCCody, Dept of Imaging Physics, MDACC

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Axial Platforms: Axial Platforms: Step and shoot, rewind coil between slicesStep and shoot, rewind coil between slices

Helical or Spiral CTHelical or Spiral CT•• SlipSlip--ring gantry allows continuous rotationring gantry allows continuous rotation

–– Reduction of interscan delaysReduction of interscan delays•• Constant table motion during scanningConstant table motion during scanning•• Special reconstruction methods developedSpecial reconstruction methods developed

SlipSlip--ringring

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SpiralSpiral Pitch Pitch DefinitionDefinition

•• Image and beam width are same Image and beam width are same for conventional axial CTfor conventional axial CT

•• Pitch = table travel per rotation Pitch = table travel per rotation ÷÷ beam widthbeam width

•• Typical pitch values are 0.5 to Typical pitch values are 0.5 to 2.02.0

PitchPitch

For Pitch < 1

Spirals overlap

Dose is increased (at same mAs)

For Pitch > 1

Spirals are stretched

Dose is decreased (at same mAs)

For Pitch =1

Similar to Axial

Image ExamplesImage Examples

•• Pitch affects image quality:Pitch affects image quality:GE GE LightSpeed LightSpeed RTRT

Pitch=0.75 Pitch=1.5

4×1.25

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Helical InterpolationHelical Interpolation

Collect data (black dots)

Interpolate to estimate image between collected data

Advantage: Can reconstruct slices at any position with any interval. Increased resolution in axial (z)-direction.

MultiMulti--Channel CTChannel CT

•• Acquisition of multiple images per scanAcquisition of multiple images per scan•• Faster volume acquisition timesFaster volume acquisition times•• Better bolus tracking and thin slices for Better bolus tracking and thin slices for

3D3D

MultiMulti--Channel CT detectorsChannel CT detectors

Table direction (z-axis)

64 x 0.625 mmz

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Detectors Detectors –– GE GE LightSpeedLightSpeed RTRT

4-channel system: 16 @ 1.25mm, total 20mm

4×1.25mm4×2.5mm

4×3.75mm

4×5mm

16 detectors

MCCT MCCT DetectorsDetectors

z z

z

chan

nel

chan

nel

chan

nel

chan

nel

THIRD Gen.

Detector Configuration Affects Detector Configuration Affects Image Quality: GE LS RTImage Quality: GE LS RT

Pitch=0.75 Pitch=1.5

Detector configuration

4×1.25

4×2.5

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Image Examples: GE LS RTImage Examples: GE LS RT

Pitch=0.75 Pitch=1.5

Detector configuration

4×1.25

4×2.5

MultiMulti--Channel CTChannel CT

•• Helical nonHelical non--planar dataplanar data•• Data from multiple channelsData from multiple channels

1 2 3 41st rotation 2nd rotation

direct data

complementary data

Longitudinal direction

00

1800

3600

1 2 3 4

Consequences of MCCTConsequences of MCCT

•• Breath hold feasibleBreath hold feasible•• However, high scan speed may However, high scan speed may

not be desirable in not be desirable in Rad OncRad Onc–– Problem with synchrony of table Problem with synchrony of table

motion and diaphragm motion motion and diaphragm motion giving rise to artifactsgiving rise to artifacts

–– That leads to 4D or average CTThat leads to 4D or average CT

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Control ConsoleControl Console

•• Scan protocolsScan protocols•• Data managementData management•• Image reconstructionImage reconstruction•• Image analysis toolsImage analysis tools•• Network/PACS Network/PACS

connectivityconnectivity•• Archive mediaArchive media

CT CT –– Covers OffCovers OffX-ray Tube HousingHV Transformer

HV Transformer

Detector array

GeneratorGenerator•• kVpkVp

•• mAmA

•• Power rating 30 Power rating 30 –– 60 kW60 kW

kilovolts-peak id voltage across tube(relates to max energy of x-ray beam)

current across tube(relates to number x-ray photons produced)

kW = kV • mA / 1000@ 120kVp, 60kW gen has max 500mApractical limit is less than max available

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Spatial ResolutionSpatial Resolution

•• Display Field of View (DFOV)Display Field of View (DFOV)•• Reconstruction filter (kernel)Reconstruction filter (kernel)•• XX--ray tube focal spot sizeray tube focal spot size•• Image thickness (blurs edges of Image thickness (blurs edges of

objects)objects)•• Pitch (blurs edges of objects)Pitch (blurs edges of objects)•• Patient motionPatient motion•• Image zoomImage zoom

Std Recon Edge Recon

Effects of Recon Filters on Effects of Recon Filters on Resolution & NoiseResolution & Noise

Contrast ResolutionContrast Resolution

•• Effective Effective mAsmAsmAmA * time / pitch* time / pitch

•• Image thicknessImage thickness•• Patient sizePatient size•• Reconstruction Reconstruction

filterfilter•• Viewing conditionsViewing conditions

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ACR Phantom ACR Phantom -- Low Contrast SectionLow Contrast Section

120 kVp, 1600 mAs 120 kVp, 192 mAs

Partial Volume Averaging

Viewing Conditions Viewing Conditions --ContrastContrast

•• DistanceDistance•• Ambient (room) lightingAmbient (room) lighting

–– Cannot see the stars in the daytimeCannot see the stars in the daytime•• Monitor brightnessMonitor brightness•• ReflectionsReflections•• Viewing angle (flat screens)Viewing angle (flat screens)•• [Age of eyeballs[Age of eyeballs……]]

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Image MatrixImage Matrix

Pixels and Image MatricesPixels and Image Matrices

133133124124112112126126202202

86868282103103131131207207

73739696127127158158200200

103103146146200200220220222222

Pixel Values (HU)

CT Number: CT Number: Hounsfield Hounsfield Unit (HU)Unit (HU)

•• Pixel bitPixel bit--depth of 2depth of 21212 = 4096 values= 4096 values•• Contrast scaleContrast scale

HU = HU = 1000*1000*((µµmm –– µµwaterwater) / ) / µµwaterwater–– CT number for water = 0 HU CT number for water = 0 HU –– CT number range CT number range ––1024 to +3072 HU1024 to +3072 HU

•• CT number affected by CT number affected by kVpkVp

•• CT number in Pinnacle Treatment CT number in Pinnacle Treatment Planning System is Planning System is HU+1000HU+1000, 12, 12--bit bit unsigned integer unsigned integer (detail in part 2)(detail in part 2)

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Resolution Related with DFOVResolution Related with DFOV

512 pixels

512 pixels

25 cm DFOV

Pixel = ~ 0.5mm

512 pixels

512 pixels

50 cm DFOV

Pixel = ~ 1mm

Image DisplayImage Display

•• The human eye resolves 256 The human eye resolves 256 shades of grayshades of gray

•• Display monitors have about 2Display monitors have about 288

gray levelsgray levels•• Digital CT data has 4096 possible Digital CT data has 4096 possible

valuesvalues

•• WW/WL to select desired CT WW/WL to select desired CT numbers for display with 256 numbers for display with 256 shades of grayshades of gray

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Typical CT Typical CT Numbers (HU)Numbers (HU)

•• AirAir•• LungLung•• FatFat•• WaterWater•• BrainBrain•• Soft TissueSoft Tissue•• BoneBone•• MetalMetal

-1000~ -700

~ - 120 to ~ -800 +/- 5~ 40

~ 40 to ~ 100200 to > 600

> 1000

2. CT Application in 2. CT Application in Radiation OncologyRadiation Oncology

Different from CT in Diagnostic Different from CT in Diagnostic ImagingImaging

CT simulation control area: CT simulation control area: More complicatedMore complicated

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DifferencesDifferences

•• Patient groupPatient group•• Imaging purposeImaging purpose•• Bore sizeBore size•• CouchCouch•• Laser alignmentLaser alignment•• SoftwareSoftware•• QA procedureQA procedure•• Clinical protocols/proceduresClinical protocols/procedures……

Patient Group Patient Group (in a cancer center)(in a cancer center)•• Diagnostic CT: screen or diagnosticDiagnostic CT: screen or diagnostic

–– General public suspected to have cancersGeneral public suspected to have cancers–– Previously cured cancer patients for follow upPreviously cured cancer patients for follow up–– Diagnosed cancer patients for metastases, Diagnosed cancer patients for metastases,

staging or treatment evaluation staging or treatment evaluation Large group: ~23,000 patients seen by MDACCLarge group: ~23,000 patients seen by MDACC

•• Therapy CT in Radiation OncologyTherapy CT in Radiation Oncology–– Diagnosed cancer patients who will receive Diagnosed cancer patients who will receive

radiation treatmentradiation treatment–– For treatment planning purposesFor treatment planning purposesSmall group: ~6,500 evaluated, 5,000 treated, ~1/4Small group: ~6,500 evaluated, 5,000 treated, ~1/4

Imaging Purpose: Imaging Purpose: Diagnostic CTDiagnostic CT

•• Requires high image qualityRequires high image quality–– See lesions, diagnosisSee lesions, diagnosis

•• Requires Requires low dose low dose –– To reduce stochastic risk: To reduce stochastic risk:

radiation generated cancer or radiation generated cancer or carcinogenesiscarcinogenesis

•• Gold Standard in DI about Gold Standard in DI about radiation: ALARAradiation: ALARA–– As Low As Reasonably AchievableAs Low As Reasonably Achievable

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Imaging Purpose: Imaging Purpose: Therapy CT (1)Therapy CT (1)

•• Treatment planning and Treatment planning and simulationsimulation–– Accuracy and Accuracy and

reproducibility of patient reproducibility of patient positioningpositioning -- Extremely Extremely important!!!important!!!

–– Imaging position must Imaging position must represent treatment represent treatment positionposition

A Linac Treatment Machine

Imaging Purpose: Imaging Purpose: Therapy Therapy CT (2)CT (2)

•• Treatment planning and Treatment planning and simulationsimulation–– Image entire outline of Image entire outline of

the patients the patients -- No No truncation is allowedtruncation is allowed

–– Spatial accuracy Spatial accuracy extremely importantextremely important

Patient imaged 3 times to get rid of truncation!!!

Display Field of View (DFOV)Display Field of View (DFOV)

•• Image Matrix: 512 Image Matrix: 512 ×× 512 pixels512 pixels

•• Diagnostic: Optimized DFOVDiagnostic: Optimized DFOV–– Head, 16 Head, 16 -- 25cm, 25 most common25cm, 25 most common–– Body, 25 Body, 25 -- 50 cm, 36 most common50 cm, 36 most common

•• Therapy: Large DFOVTherapy: Large DFOV–– Head, 50 cm Head, 50 cm (35cm for Stereotactic (35cm for Stereotactic RadiosurgeryRadiosurgery

only)only)•• Partial shoulder scan is includedPartial shoulder scan is included

–– Body, 50 Body, 50 -- 65 cm (Extended DFOV)65 cm (Extended DFOV)

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Bore sizeBore size

Scan field of view

(SFOV)

DFOV is limited by SFOV, hence bore size

Large BoreLarge Bore

Diagnostic CTDiagnostic CT•• Regular bore Regular bore

size GE 16size GE 16--channelchannel– Bore Opening: 70 cm– SAD: 54 cm– SDD: 95 cm– SFOV: 50 cm

Therapy CTTherapy CT•• Regular bore Regular bore •• Large boreLarge bore

–– Philips: Philips: • Bore Opening: 85 cm• FOV: 60 cm

–– GE 4GE 4--channel channel RT:RT:

• Bore Opening: 80 cm• SAD: 61 cm• SDD: 106 cm• Field of View: 65 cm

Why need large bore CT?Why need large bore CT?

•• To enclose treatment accessories in the To enclose treatment accessories in the large bore when neededlarge bore when needed

Patient immobilization foam for a thoracic stereotactic patient

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Flat Couch: Therapy CTFlat Couch: Therapy CT

Couch Sag Couch Sag ––Serious problemSerious problemfor Therapyfor Therapy

•• When couch moves into the gantry, When couch moves into the gantry, couch sags due to patient weightcouch sags due to patient weight

Displacement

Courtesy of Karl Prado, PhD

DisplacementDisplacement

Not a problem for DiagnosticNot a problem for Diagnostic

Serious concern for Therapy (2 mm Serious concern for Therapy (2 mm positioning accuracy required)positioning accuracy required)

Depending on manufacture and Depending on manufacture and specific scanners, sag may vary in 1specific scanners, sag may vary in 1--6 6 mmmm

Correction method is still under Correction method is still under investigationinvestigation

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Complicated Immobilization Complicated Immobilization AccessoriesAccessories

Complicated Laser Complicated Laser SystemSystem

Ceiling Ceiling laserlaser

Wall laserWall laser

Bump protector

Laser control panel

Patient ThroughputPatient Throughput

•• Diagnostic CTDiagnostic CT–– 20 minutes/patient20 minutes/patient–– Contrast generally usedContrast generally used

•• Therapy CT Therapy CT –– Generally ~1 hour/patient Generally ~1 hour/patient

-- positioning takes most timepositioning takes most time-- mark treatment centermark treatment center-- make customized immobilization make customized immobilization

devicedevice-- tattoo the patienttattoo the patient

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Clinical Protocol: Clinical Protocol: kVpkVp

•• Therapy CT Therapy CT –– Fixed Fixed kVpkVp for all for all patientspatients–– HounsfieldHounsfield Units (Units (HUsHUs) converted to ) converted to

electron densityelectron density to calculate patient to calculate patient dose in dose in MeV MeV treatment beam using treatment beam using keVkeVCT dataCT data

–– HU varies with HU varies with kVpkVp–– Only one calibration curve is stored in Only one calibration curve is stored in

TPS, usually 120kVp is chosenTPS, usually 120kVp is chosen

Electron Density v.s. HU Calibration CurveElectron Density v.s. HU Calibration Curve

HU+1000

Why electron density is Why electron density is needed?needed?

•• MeVMeV beam attenuation is related with beam attenuation is related with tissue electron densitytissue electron density

•• KeV KeV beam attenuation (HU) is related beam attenuation (HU) is related with electron density and Z atomic with electron density and Z atomic number number

•• A conversion from HU to electron A conversion from HU to electron density is needed for treatment dose density is needed for treatment dose calculation forcalculation for MeVMeV beamsbeams

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Clinical Protocol: Scan timeClinical Protocol: Scan time

•• Therapy CT: Therapy CT: Free breathingFree breathing–– Free breathingFree breathing to simulate treatment to simulate treatment

conditioncondition–– Special CT applications, especially Special CT applications, especially

for lung patientsfor lung patients•• Average CTAverage CT

–– Slow scan to get averaged tumor position Slow scan to get averaged tumor position •• 4D CT4D CT

–– Cine mode, multiple scans in the same Cine mode, multiple scans in the same position, sort images according to position, sort images according to breathing phasebreathing phase

–– Used in gated treatmentUsed in gated treatment

Example of Average CTExample of Average CT1 32

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4

11 1312109

876

Courtesy of Tinsu Pan, PhDAverage CT

Example of 4D CTExample of 4D CTX-ray on

off

1st table position(0 - 2cm)

2nd table position(2 - 4 cm)

3rd table position(4 - 6 cm)

EE 50% EE 50%

EI 0% EI 0%

0 10 20 30 40 50 60 70 80 90%0 10 20 30 40 50 60 70 80 90%

Respiratory singal

CT images

4D-CT images

Courtesy of Tinsu Pan, PhD

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SummarySummary

•• Diagnostic Imaging CT Diagnostic Imaging CT Image QualityImage Quality

•• Therapy CTTherapy CTPosition Position

Image QualityImage Quality