Dual Energy CT (DECT) Dual Energy CT (DECT)

––

Technology Approaches and Technology Approaches and

Scan ModesScan Modes

MarcMarc KachelrieKachelrießß

Institute ofInstitute of MedicalMedical PhysicsPhysics (IMP) (IMP)

FriedrichFriedrich--AlexanderAlexander--UniversityUniversityErlangenErlangen--NNüürnbergrnberg

SCCT 2010

www.imp.uniwww.imp.uni--erlangen.deerlangen.de

DisclosuresDisclosures

• I have the following financial relationships to disclose

– Consultant to CT Imaging GmbH

– Managing director of RayConStruct GmbH

– Grant supports from AiF, DFG, Intel, Siemens, Varian, Ziehm

• I will discuss the following off-label use in my presentation

– Exact Material Decomposition from Inconsistent Rays

Standard CT image

Calcium density image Soft tissue density image

Kalender WA et al. Radiology 164:419-423, 1987

1980ies: The First Clinical DECT 1980ies: The First Clinical DECT Product ImplementationProduct Implementation

DECT (ImageDECT (Image––based)based)

C/W=0/500 HU

2500 HU

1700 HU

E0

High energyspectrum 140 kV

Low energy spectrum 80 kV

80 keV 140 keV

wj

µ–images

Aluminum densityWater density 70 keV image

DECT ApplicationsDECT Applications• Selective display of body substances with high atomic number:

Quantification of calcium, iron or iodine concentrations, bone mineral density etc.

• Separate displays of bones and soft tissue: Material-selective projection radiography of the chest, the skeleton etc., and image segmentation

• Distinction between iodine and calcium: Differentiation between contrast medium in blood and calcified plaque or bone for CT angiography

• Selective display of contrast media and other injected tracers:Concentration measurement of injected substances such as iodine or gadolinium

• Exact quantification of contrast media: Perfusion measurement

• CT numbers for hypothetical monoenergetic sources: Attenuation correction for PET/CT (at 511 keV) and for SPECT/CT

• Electron density: Planning of radiation therapy with protons, electrons or high-energy x-rays

• …

Dual Energy whole body CTA: 100/140 Sn kV @ 0.6mm

Courtesy of Friedrich-Alexander University Erlangen-Nuremberg - Institute of Medical Physics / Erlangen, Germany

Single DECT

Scan

DE bone removal

Virtual non-contrastand Iodine image

Examples(Slide Courtesy of Siemens Healthcare)

Technology ApproachesTechnology Approaches• Multiple scans at different spectra

• Dual source CT

• Fast tube voltage switching

• Slow tube voltage modulation

• Dual layer detectors (sandwich detectors)

• Split detector (different prefiltration)

• Photon counting detectors (two or more energy bins)

DemandsDemands

• Simultaneous acquisition to avoid motion artifacts

• Independent tube current curves for both spectra– Select tube currents

– Select anatomy-dependent tube current curves1

• Free and application-dependent choice of spectra– Select prefiltration

– Select tube voltages

• Achieve good angular sampling

• Avoid scatter and cone-beam artifacts

• Acquire consistent rays– Each ray should be measured twice

– or reconstruction should correctly handle inconsistencies2

1 Stenner, Kachelrieß. Dual energy exposure control (DEEC) for computed tomography. Med. Phys. 35(11):5054-5060, November 2008. 2 Maaß, Meyer, Kachelrieß. Exact dual energy material decomposition from inconsistent rays (MDIR). Med. Phys 37:under consideration, 2010.

U0= 120 kV, U1= 80 kV, U2= 140 kV, (C=0 HU / W=200 HU)

Optimal TubeOptimal Tube CurrentCurrent ModulationModulation

Stenner, Kachelrieß. Dual energy exposure control (DEEC) for computed tomography. Med. Phys. 35(11):5054-5060, November 2008.

80 kV / 140 kV80 kV / 140 kV

80 kV / 140 kV + Prefilter80 kV / 140 kV + Prefilter

100 kV / 140 kV + Prefilter100 kV / 140 kV + Prefilter

ImageImage––based DECTbased DECT

C/W=0/500 HU

2500 HU

1700 HU

µ–images

Aluminum densityWater density 70 keV image

E0

High energyspectrum 140 kV

Low energy spectrum 80 kV

80 keV 140 keV

wj

Aluminum densityWater density

RawdataRawdata––based DECTbased DECT

70 keV image

E0

High energyspectrum 140 kV

Low energy spectrum 80 kV

80 keV 140 keV

wjq1

q2

pAl =DAl(q1,q2)

pH2O=DH2O(q1,q2)

Consistent RawdataConsistent Rawdata

• two subsequent circle scans + no object motion

• special detectors (sandwich, energy resolving)

140 kV 80 kV

Almost Consistent RawdataAlmost Consistent Rawdata

• dual source circle scan(source misalignment)

• tube voltage switching

140 kV

80 kV

Inconsistent RawdataInconsistent Rawdata

• spiral scans(subsequent or dual source)

• two orthogonal circular source trajectories

140 kV

80 kV

MDIR MethodMDIR MethodMaterial Decomposition with Inconsistent RaysMaterial Decomposition with Inconsistent Rays

Init image–based

Estimate error

New estimate

� conventional image–based DECT

� use current material images toestimate the error that would appearif this was the true object

� correct for that error

� repeat with a more accurate material image estimation

Maaß, Meyer, Kachelrieß. Exact dual energy material decomposition from inconsistent rays (MDIR). Med. Phys. 37:under consideration, 2010.

start image

current error estimate

measurementsimulation

current imageestimate

f (k+1) = BP(q) + f (k) – BP(FP(f (k)))

MDIR Update Formula MDIR Update Formula –– First IterationFirst Iteration

SimulationSimulation

• Forbild head phantom– inconsistent rays

(two orthogonal circles)

– 80 kV and 140 kV

– 3 iterations

MDIR ResultsMDIR Results

70 k

eV

µ–

ima

ge

(C/W

=50

/50 H

U)

Bo

ne

de

nsit

y(C

/W=

0%

/10

%)

Wa

ter

den

sit

y(C

/W=

10

5%

/10

%)

Phantom Image–based MDIR

MDIR –Phantom

MDIR –Image–based

Clinical Phantom MeasurementClinical Phantom Measurement

– Siemens Somatom Definition Flash DSCT scanner

– 20 cm diameter PE disc with HA400 inserts

– 80 kV and 140 kV

– 1 iteration

HA400

80 kV 140 kV

Measurement ResultsMeasurement Results

Bone material density(C/W=100%/10%)

Soft material density(C/W=100%/10%)

Bone material density(C/W=0%/10%)

70 keV µ–image (C/W=0/50 HU)

Imag

e-b

ased

MD

IR

Somatom Definition Flash operating at 80 kV and 140 kVSiemens Healthcare, Forchheim, Germany

y

x

140 kV

80 kV

Thank You!

Please also visit:

Low-Dose Phase-Correlated Micro-CT of the Mouse Heart, breakout room 4, Saturday morning, 11:00-11:15

Single and Dual Source Temporal Resolution Improvement, poster session XII, Saturday afternoon, 13:00-13:45

The Near Future in Cardiac CT Image Reconstruction, main room, Sunday morning, 10:30-10:45

Please also visit:

Low-Dose Phase-Correlated Micro-CT of the Mouse Heart, breakout room 4, Saturday morning, 11:00-11:15

Single and Dual Source Temporal Resolution Improvement, poster session XII, Saturday afternoon, 13:00-13:45

The Near Future in Cardiac CT Image Reconstruction, main room, Sunday morning, 10:30-10:45