Norbert J. Pelc, Sc.D.Norbert J. Pelc, Sc.D.

Departments of Radiologyand BioengineeringDepartments of Radiologyand Bioengineering

Stanford UniversityStanford University

Dual Energy CT:Dual Energy CT:PhysicsPrincip lesPhysicsPrinciples

DisclosuresDisclosures

Uri Shreter andRobert Senzig, GE Healthcare

ThomasFlohr and BernhardSchmidt, SiemensMedical Solutions

Researchsupport: NIH grant EB006837, GEHealthcare,theLucasfoundation

This lectureincludes off-labeluseof CT scanners

MotivationMotivation

“ Two pictures are takenof the sameslice,one at 100kV and theother at 140kV... areasof high atomicnumberscanbeenhanced... Testscarriedout to datehaveshownthatiodine (Z=53) canbereadilydifferentiatedfrom calcium (Z=20)”.

G.N. Hounsfield, BJR46, 1016-22,1973.

• Materialspecificity• Improvedtissuecharacterization

OUTLINEOUTLINE

• Physical principlesof multi-energy x-raymeasurements

• Dual energy CTprocessing

obtaining dual-energy measurements

scanning

• Strengthsandlimitations

unknown amountsofunknown amounts oftwo known materialstwo known materials

energyE1

I1

mb (g/cm2)

I01

water

bone10 10020 50 200

corticalbonewater

photonenergy(keV)

mas

sat

tenu

atio

nco

effi

cien

t(cm

2 /g)

0.1

1.0

10.0

100.0

I1 = I01e-(µ/ρ)w1mw + (µ/ρ)b1mb

I2 = I02e-(µ/ρ)w2mw + (µ/ρ)b2mb

I2

E2I02

E2E1

solve for mw and/or mb

unknown amountsofunknown amountsoftwo known materialstwo known materials

energyE1

I1

I01

water

bone10 10020 50 200

corticalbonewater

0.1

1.0

10.0

100.0

I2

E2I02

E2E1

mb = A{ ln(I01/I1) - (µw1/µw2)(ln(I02/I2)}

subtractwater

scale for lostbone signal

makes the water contributionat E2 match that at E1

••

photonenergy(keV)

mas

sat

tenu

atio

nco

effi

cien

t(cm

2 /g)

mb (g/cm2)

dual energy projection methodsdual energy projection methods

DEXA

Singleenergy “Boneminic tissue” imageBone image

DualEnergyRadiography

• 2 energies 2 materials

material analysiswithmaterial analysis withabsorptiometryabsorptiometry

• canwe generalizethis? N energiesfor Nmaterials?

• limitation: two stronginteraction mechanismsCompton scatteringandphotoelectric absorption

Eachhas~ sameenergydependencefor all elements

Basis material decompositionBasis material decomposition

0.1

1

10

100

1000

0 20 40 60 80 100 120 140

OCaCuCa'

Cu

.61*O + .04*Cu

Ca

O

Basismaterial decompositionBasis material decomposition

M gramsof Ca.04 M gramsof Cu

.61 M gramsof O

I

I0 I0

I

=

Indistinguishableat anyx-rayenergyabove their K-edge

(K-edgemethodsare not coveredhere)

• Barringa K-edge:µ(E) = a*Compton(E)+ b*Photoelectric(E)

2 fundamentalparameters characterize material behavior

electrondensity, effectiveatomicnumber

basismaterial decompositionbasismaterial decomposition basismaterial decompositionbasis material decomposition• Barringa K-edge:

µ(E) = a*Compton(E)+ b*Photoelectric(E)

2 fundamentalparameters determinematerial behavior

electrondensity, effectiveatomicnumber

• anymaterialcanbemodeled asa weightedsumof twoothermaterials

µ(E) = α* µi(E) + β* µj(E)

basis material decomposition

commonbases:aluminumandplastic

• in anyprojection measurement,we canonly isolatetwomaterials

OUTLINEOUTLINE

• Physical principles of multi-energyx-raymeasurements

• Dual energy CTprocessing

obtainingdual-energy measurements

scanning

• Strengthsandlimitations

• material selective imagesbasismaterialimagesmaterialspecificor cancelled images“ virtual non-contrast” scansweightedsubtractionsof thetwo energiesprocessingamplifiesnoise

• monoenergetic imagesweightedsumsof thetwo energiescanbehigh SNR,depending on the weighting

DualDual--energy processingenergy processing

DualDual--energy processingenergy processing

• reconstructimagesin thenormal manner,and combineHU imageseasyto implement

• combineprojectiondataprior toreconstructionsomewhat moredifficult

requiresaligned projections

enables“ exact” beamhardening correction

nonlinearcombination

PrePre--reconstr uction processingreconstruction processing

material selective images

monoenergetic images

adaptedfrom Lehmannet al: MedPhys8, 659-67, 1981.

calibration

low energyprojections

high energyprojections

“ aluminum”projections

“ plastic”projections

“ aluminum”image

“ plastic”image

linearcombination

finalimage(s)

CT recon

canincorporateaccuratebeamhardeningcorrection

basismaterialimages

Monochromatic CT from projection-based recon

Water Aluminum Water AluminumMono-

chromaticImage based Projection based

Projection based MD reduces beam hardening

80kVp 140kVp

Potential for beam hardening streak-free images

Courtesyof Uri Shreter, GE Healthcare

Potential impact on Cardiac IQ +Perfusion

Polychromatic

Courtesyof Uri Shreter, GE Healthcare

Heart Chamber Phantom, 8.3%

L = 0, W = 350 HU

80kVp 140kVp

MonochromaticIodineWater

Monochromatic CT from projection-based recon*

Material separation

Monochromatic CT – keV

tuned

Natively eliminates beam

hardening CT # shifts

Courtesyof R. Senzig, GE Healthcare

nonlinearcombination

PrePre--reconstr uction processingreconstruction processing

material selective images(noisy)

monoenergetic images(canbelow noise)

adaptedfrom Lehmannet al: Med Phys 8, 659-67, 1981.

calibration

low energyprojections

high energyprojections

“ aluminum”projections

“ plastic”projections

“ aluminum”image

“ plastic”image

linearcombination

finalimage(s)

CT recon

higher,negativelycorrelatednoise

Two known materials in aTwo known materials in a voxelvoxel55 keV 80 keV

iodineCNR=7.9 iodineCNR=3.8

water image

SNR=37

iodineimage

SNR=3.4

optimal combination(“ mixed” image)

iodineCNR=10

waterSNR=67 waterSNR=71“water” contrast

iodinecontrast

Dual energy CTDual energy CT

HUlow

HUhigh

line of identity: “water-like” materials

-1000

-1000

effectiveZ > water

effectiveZ < water

HUlow/HUhigh

(and relatedmetrics)dependon effective Z

Principle of Dual Energy CT – Image Based Evaluat ion

Each material is characterized by its „Dual Energy Index“

x80 and x140 are the Hounsfield numbers at 80 kV and 140 kV, resp.

Dual energy CT can measure chemical compo si tion!

Material DEI

Bone 0.1148

Liver 0.0011

Lung -0.0021

Sof t Tissue -0.0052

Skin -0.0064

Protein s -0.0087

Fat -0.0194

Gall fluid -0.0200

Courtesyof B. Krauss,B. Schmidt,andTh. Flohr, SiemensMedical Solutions

atomic number and density asatomic number and density asmaterial parametersmaterial parameters

• tissuespecific ity? applicationsbeinginvestigated

kidneystonecharacterization

fat or iron in theliver

plaquecharacterization

• imagesegmentationboneor plaqueremoval

identifying ligamentsor tendons

Three known materialsThr eeknown materialsdual energy CTdual energy CT

water

iodine

HUlow

water+iodine

bone water+bone

HUhigh

identity

Kelczet al: Med Phys6, 418-25,1979.

boneand iodinein water

Cancalculatebothiodine andbone.Requiresknownmixing propertiesand consistentwater density

mb = A{ HUlow - (Μ1,I/ Μ2,I) HUhigh}

Three known materialsThree known materialsdual energy CTdual energy CT

Reliableseparationrequires large(R1 - R2)2

where R = µhigh/µlow, dependsonmaterialand energies

Works bestfor onehigh Z andonelower Z material, andverydifferentx-rayenergies

iodine

calcium

Kelczet al: Med Phys6, 418-25,1979.

Spectral separationSpectral separation

• verycritical for SNRefficiency,separationrobustness,etc.

• implementationsdifferent kVp and/or filt ration

layereddetector

photoncountingwith energyanalysis

DualDual kVpkVp, dual filt rati on, dual filt rati on

135 kVp1.5 mm bronze

85 kVp0.1 mm erbium

• switchedfi ltrationimprovesseparation

• differentmAs helpsapportiondose

Lehmannet al: Med Phys 8, 659-67,1981.

Carmi R, NavehG, andAltman A: IEEENSSM03-367,1876-78,2005

Layered detectorLayereddetector• simultaneous dualenergysensing• relatively poor spectral separation

Energy discriminating, photonEnergy discriminating, photoncounting detectorscounting detectors

broadspectrumx-ray source

pulseshaping

pulseheight

analysis

two(or more)

energybins

High enoughcountrateis diffi cult to

achieve

Spectral separationSpectral separation

• verycritical for SNRefficiency, separationrobustness,etc.

• implementationsphotoncounting with K-edgefilterphotoncounting with energyanalysisdifferent kVp andfiltrationdifferent kVplayereddetector

betterspectral

separationanddose

efficiency

Dual energy implementationsDual energy implementations

• Sequential scansat different kVpmotion sensitivity > 50%Trot

highermotion sensitivity in helical mode

• Two sourcesat 90º on thesamegantry

SiemensDefinition80 kVp & 140kVp

Dual Source Challenge: Inconsistent scans

Coincident

Moving PhantomSimulation

Dual Source system

Moving Objec ts

Does not see movement

Courtesyof R. Senzig, GE Healthcare

Dual energy implementationsDual energy implementations

• Sequential scansat different kVpmotion sensitivity > 50%Trot

• Two sourcesat 90º on thesamegantrysomemotionsensitivity (~ 25%Trot)

• Switching kVp within a singlescan1, 2

technically challenging with rapidgantryrotation

1. Lehmann etal: Med Phys 8, 659-67,1981.2. Kalenderetal, Med Phys13, 334, 1986.

Courtesyof Uri Shreter, GE Healthcare

RapidRapid kVpkVp switchingswitchingDual energy CTDual energy CT Dual energy implementationsDual energy implementations

• Sequential scansat different kVpmotion sensitivity > 50%Trot

• Two sourcesat 90º on thesamegantrysomemotionsensitivity (~ 25%Trot ?)

• Switching kVp within a singlescan

• Energydiscriminatingdetectorslayereddetector,photoncounting

betterimmunity

tomotion

Strengths and limita tionsStrengthsand limitations

• perfect beamhardeningcorrection(pre-recon)effectivemonoenergeticimages

• extrapolationto high energiesmore accurate RTPandN/M attenuation correction

• somematerial specificity (e.g.,effectiveZ, DEI)mayprovidediseasespecificityimproved imagesegmentation

Strengths and limita tionsStrengths and limita tions

• virtual non-contrast imageperfectlyregisteredandsimultaneouslyacquiredBewareof noise propagation. Separate optimizedscans

probablyhavelower total dosefor sameIQ

• isolatecontrastmedia from calcified plaquedif ficult, especially for smallamountsof either

• molecularimaging?I don’ t think so

• “ tomochemistry”only with K-edgemethodsandrelativelylargeconcentrations

Thank You