Quantifying Cardiac Deformation by strain (-rate) imaging

Hans TorpNTNU, Norway

Hans Torp Department of Circulation and Medical Imaging

Norwegian University of Science and Technology Norway

Quantifying Cardiac Deformation

by strain (-rate) imaging

• Describe deformation by strain and strain rateDescribe deformation by strain and strain rate

• Ultrasound methods for strain rateUltrasound methods for strain rate– speckle tracking versus Doppler methodsspeckle tracking versus Doppler methods

– Clutter noise and thermal noiseClutter noise and thermal noise

– angle dependencyangle dependency

• Frame rate issuesFrame rate issues

• Visualization of strain and strain-rateVisualization of strain and strain-rate

Hans TorpNTNU, Norway

Velocity gradient – strain rateVelocity gradient – strain rate

V2 – V1L

= Rate of deformation= Strain Rate

V1

V2

Velocity gradient:

L

(Myocardial) velocity gradient is an instantaneous property

Integrated velocity gradientIntegrated velocity gradientversus strainversus strain

”Growth function” Strain = exp{ IVG } - 1

Integrated velocity gradientIntegrated velocity gradientversus strainversus strain

Envelope RF

What is the best velocity What is the best velocity estimator?estimator?

s1 s2Autocorrelation method is optimal(Maximum likelihood estimator)

velocity ~ angle(R)

RF signals IQ signals

x1 x2

R=x1*x2

What is the best velocity What is the best velocity estimator?estimator?

R=x1*x2 v=c/4piT angle(R)

Estimation error is minimumEstimation error is minimumwhen correlation is maximumwhen correlation is maximum

Hans TorpNTNU, Norway

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5

x 106

0

0.5

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1.5

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abs(R)

angl

e(R

))

Correlation magnitude

Vel

ocit

y es

tim

ate

Estimate of velocity gradient Estimate of velocity gradient (strain rate)(strain rate)

0 0.005 0.01 0.0150.01

0.015

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depth range [mm]

Ve

loci

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m/s

ec]

Linear regression

Weighted Linear regression(Maximunlikelihood)

Simulation experimentSimulation experimentStrain rate estimatorsStrain rate estimators

10 20 30 40 50

0

0.5

1

1.5

Simulation no

Strain rate

Linear regression

Weighted Linear regression(Maximunlikelihood)

Clutter noise

Clutter noiseClutter noise

•bias towards zero for velocity measurements

•increased variance for strain rate

•Clutter filter helps when tissue velocity is high•limited effect in apical region

•Second harmonic (octave) imaging reduces clutterindependent of tissue velocity

Fundamental and second Fundamental and second harmonic signal separated by harmonic signal separated by

filterfilter

0 1 2 3 4 5x 106

-20

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80

20 40 60 80 100

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Fundamental Signal from septumNoise from LV cavity

2. harmonic-500 0 5000

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250

-10 0 100

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250

Hans TorpNTNU, Norway

Second Harmonic TDISecond Harmonic TDI

Fundamental, f=1.67MHz

Second harmonic, f=3.33MHz

• Fundamental and Fundamental and second harmonic second harmonic calculated from the calculated from the same data setsame data set

• No significant noise No significant noise differencedifference

• SSecond harmonic econd harmonic TDI TDI gives more aliasing.gives more aliasing.

Second Harmonic SRISecond Harmonic SRI

Fundamental, f=1.67MHz

Second harmonic, f=3.33MHz

• Fundamental and Fundamental and second harmonic second harmonic calculated from the calculated from the same data setsame data set

• Significant noise Significant noise reduction when using reduction when using the second harmonic the second harmonic frequency bandfrequency band

• Aliasing is not a Aliasing is not a problem due to small problem due to small velocity differencesvelocity differences

Frame rate issues in tissue velocity Frame rate issues in tissue velocity and strain rate imaging and strain rate imaging

Packet Packet acquisitionacquisition

30 - 80 frames/sec30 - 80 frames/sec

Packet acquisitionPacket acquisition

tissue interleavingtissue interleaving

100 - 150 100 - 150 frames/secframes/sec

Continuous Continuous acquisition tissue acquisition tissue interleavinginterleaving

250 - 350 frames/sec250 - 350 frames/sec

- TVI aliasing- TVI aliasing

+ Offline spectral + Offline spectral DopplerDoppler

(Work in progress)(Work in progress)

Image sector: 70 deg.Parallell beams : 2

Packet acquisition - continuous acquisitionPacket acquisition - continuous acquisition

Myocardial velocity and strain rateMyocardial velocity and strain ratewith 300 frames/secwith 300 frames/sec

Velocity

v1

v2

Strain rate

TimeHans TarpNTNU, Norway

Lateral movementLateral movement

Angle corrected strainAngle corrected strain

Summary 1Summary 1

• Strain rate from Tissue Doppler is possible for motion Strain rate from Tissue Doppler is possible for motion along the ultrasound beam with high temporal along the ultrasound beam with high temporal resolutionresolution

• Weighted linear regression gives minimum estimation Weighted linear regression gives minimum estimation errorerror

• Second harmonic Tissue Doppler reduce clutter noise Second harmonic Tissue Doppler reduce clutter noise artefacts in strain rate imagingartefacts in strain rate imaging

Summary 2Summary 2• Integrated strain is improved by Integrated strain is improved by

tracking material pointstracking material points

• 2D speckle-tracking gives angle-2D speckle-tracking gives angle-independent strain, with reduced independent strain, with reduced temporal resolution temporal resolution

• A combination of high frame rate A combination of high frame rate tissue Doppler and lower frame rate tissue Doppler and lower frame rate speckle tracking is probably the best speckle tracking is probably the best solution for strain imagingsolution for strain imaging

• 3D reconstruction of strain (-rate) 3D reconstruction of strain (-rate) covering the left ventricle can be covering the left ventricle can be obtained from 3 standard apical viewsobtained from 3 standard apical views