Quantifying Cardiac Deformation by strain (-rate) imaging Hans Torp NTNU, 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 methods –Clutter noise and thermal noise –angle dependency Frame rate issuesFrame rate issues Visualization of strain and strain-rateVisualization of strain and strain-rate Hans Torp NTNU, Norway

Velocity gradient – strain rate V2 – V1 L = Rate of deformation = Strain Rate V1 V2 Velocity gradient: L (Myocardial) velocity gradient is an instantaneous property

Integrated velocity gradient versus strain ”Growth function” Strain = exp{ IVG } - 1

Integrated velocity gradient versus strain

Envelope RF

What is the best velocity estimator? s1s1 s2s2 Autocorrelation method is optimal (Maximum likelihood estimator) velocity ~ angle(R) RF signals IQ signals x1x1 x2x2 R=x1*  x2

What is the best velocity estimator? R=x1*  x2 v=c/4piT angle(R)

Estimation error is minimum when correlation is maximum Hans Torp NTNU, Norway Correlation magnitude Velocity estimate

Estimate of velocity gradient (strain rate) depth range [mm] Velocity [m/sec] Linear regression Weighted Linear regression (Maximun likelihood)

Simulation experiment Strain rate estimators Simulation no Strain rate Linear regression Weighted Linear regression (Maximun likelihood)

Clutter 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 clutter independent of tissue velocity

Fundamental and second harmonic signal separated by filter FundamentalSignal from septum Noise from LV cavity 2. harmonic Hans Torp NTNU, Norway

Second Harmonic TDI Fundamental, f=1.67MHz Second harmonic, f=3.33MHz Fundamental and second harmonic calculated from the same data setFundamental and second harmonic calculated from the same data set No significant noise differenceNo significant noise difference Second harmonic TDI gives more aliasing.Second harmonic TDI gives more aliasing.

Second Harmonic SRI Fundamental, f=1.67MHz Second harmonic, f=3.33MHz Fundamental and second harmonic calculated from the same data setFundamental and second harmonic calculated from the same data set Significant noise reduction when using the second harmonic frequency bandSignificant noise reduction when using the second harmonic frequency band Aliasing is not a problem due to small velocity differencesAliasing is not a problem due to small velocity differences

Frame rate issues in tissue velocity and strain rate imaging Packet acquisition frames/sec Packet acquisition tissue interleaving frames/sec Continuous acquisition tissue interleaving frames/sec - TVI aliasing - TVI aliasing + Offline spectral Doppler (Work in progress) Image sector: 70 deg. Parallell beams :2

Packet acquisition - continuous acquisition

Myocardial velocity and strain rate with 300 frames/sec Velocity v1v1 v2v2 Strain rate Time Hans Tarp NTNU, Norway

Lateral movement

Angle corrected strain

Summary 1 Strain rate from Tissue Doppler is possible for motion along the ultrasound beam with high temporal resolutionStrain rate from Tissue Doppler is possible for motion along the ultrasound beam with high temporal resolution Weighted linear regression gives minimum estimation errorWeighted linear regression gives minimum estimation error Second harmonic Tissue Doppler reduce clutter noise artefacts in strain rate imagingSecond harmonic Tissue Doppler reduce clutter noise artefacts in strain rate imaging

Summary 2 Integrated strain is improved by tracking material pointsIntegrated strain is improved by tracking material points 2D speckle-tracking gives angle- independent strain, with reduced temporal resolution2D speckle-tracking gives angle- independent strain, with reduced temporal resolution A combination of high frame rate tissue Doppler and lower frame rate speckle tracking is probably the best solution for strain imagingA combination of high frame rate tissue Doppler and lower frame rate speckle tracking is probably the best solution for strain imaging 3D reconstruction of strain (-rate) covering the left ventricle can be obtained from 3 standard apical views3D reconstruction of strain (-rate) covering the left ventricle can be obtained from 3 standard apical views