Session: P3B MEDICAL IMAGING Chair: N. de Jong Erasmus Medical Centre P3B-1

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1 using ultrasound data to test the performance of this algorithm and compare it to currently accepted delay estimators implementing a variety of sub-sample interpolation methods. Simulation results show that this algorithm significantly outperforms other algorithms in terms of jitter and bias over a broad range of conditions. For a 5 MHz, 50% bandwidth signal sampled at 40 MHz with a sub-sample delay of 0.3 samples, the bias of the proposed method is over 300 times smaller than that for the normalized correlation with grid slope interpolation, and over 80 times better than that for normalized correlation implementing cosine fitting interpolation. Computational costs associated with this algorithm are comparable to those of other methods. Session: P3B MEDICAL IMAGING Chair: N. de Jong Erasmus Medical Centre P3B-1 A VIRTUAL ENVIRONMENT FOR THE EVALUATION, VALIDATION AND OPTIMIZATION OF STRAIN AND STRAIN RATE IMAGING J. D HOOGE* 1, S. I. RABBEN 2,P.CLAUS 1,F.IRGENS 3, J. THOEN 1,F. VAN DE WERF 1, and P. SUETENS 1, 1 Catholic University, Leuven, Belgium, 2 Rikshospitalet University Hospital, Oslo, Norway, 3 NTNU, Trondheim, Norway. Corresponding jan.dhooge@uz.kuleuven.ac.be Ultrasound based strain (S) and strain rate (SR) imaging are new tools for the assessment of regional myocardial function. Although their clinical use has already been shown, the origin of some typical image artifacts is still not fully understood. The aim of this study was thus to create an environment that allows to study these artifacts in a controllable manner. Methods: The left ventricle was approximated as a thick-walled deforming ellipsoid. Equatorial wall thickness, global long and short axes motion over a typical cardiac cycle were used as input to the model. To simulate torsion (T), the material points were rotated about the long axis of the model with a base-to-apex gradient. Point scatterers were positioned at random within the left ventricle and their motion as a function of time was determined by the model. 3D models were generated every 20 micros. They were used as input for a simulation environment for ultrasonic imaging that is based on the impulse response method. As a first application, the influence of cardiac T on the quality of the SR estimates was investigated. Hereto, tissue Doppler data (TDD) were simulated for T ranging from 0-20 degrees. An ultrasound system sampling at 40MHz having a 3.5MHz 64-element array transducer was simulated. TDD were calculated by autocorrelation for realistic acquisition parameters and subsequently analysed using the free software Speqle. The estimated SR and S curves together with 398

2 the position at which they were calculated were extracted. Based on these positions the theoretical S/SR curves along the ultrasound beam could be extracted from the kinematic model for comparison. Results: The mean relative error in the estimated velocity and SR were 4% and 21%. The mean error in peak systolic and early and late diastolic SR increased with cardiac T (0.13 vs 0.21; p<0.05). Visually, the SR curves in the mid-wall segments were good irrespective of T while worsening occurred in apical segments where out-of-plane motion is maximal. Conclusion: A kinematic model was combined with an ultrasound simulator to create a virtual environment to study the influence of physiologic, acquisition and post-processing parameters on the quality of the S/SR estimates. In a first study the influence of T on SR estimates was studied. This work was supported by the Fund for Scientific Research - Flanders (FWO-Vlaanderen) P3B-2 CLINICAL DEMONSTRATION OF FUNCTIONAL WAVE FRONT OF INTRAMYOCARDIAL ISCHEMIC REGION IN PATIENTS WITH CORONARY STENOSIS Y. KOIWA* 1,B.H.ONG 1,M.SUTOH 1, K. IWABUCHI 1, Y. KAGAYA 1,J. WATANABE 1,K.SHIRATO 1,H.HASEGAWA 2, and H. KANAI 2, 1 Graduate School of Medicine, Tohoku University, 2 Graduate School of Engineering, Tohoku University. Corresponding koiwa@int1.med.tohoku.ac.jp In the experimental studies, the deterioration of the transmural myocardial contractility has been reported to occur heterogeneously under the coronary hypoperfusion. That is, the subendocardium is the first layer to be vulnerable to the ischemia. In the longer period of ischemia, the ischemic zone progresses heterogeneously from the endocardium to the epicardium (the wave front phenomenon). At present, however, there is still a serious deficiency of clinical information concerning the transmyocardial functional heterogeneity of such wave front across the ventricular wall in patients with coronary ischemia. Our novel transcutaneous ultrasonic-based method, the Phased Tracking Method (PTM) (IEEE Trans. UFFC,1997;44:752), is unique in its higher spatial and temporal resolutions (accuracy: 0.5 µm in 375 µm-thickness layer) as well as its potential to detect the contractility of each myocardial layer across the wall. In this clinical study, we examined whether PTM can be of use to describe the functional wave front in the ventricular wall. Methods: PTM was applied to four patients with single coronary stenosis ( 75%) to evaluate the transmural contractility (% systolic thickening: %tkn) at each layer with 750µm thickness across the basal septum both at pre- and 5 days after the coronary intervention (Rf: 9 khz, resolution: 0.5 µm/s). Results: In patients, a decrease in %tkn especially at the subendocardial side was quantitatively detected in the region related to a significantly narrowed coronary artery (%tkn; 95±24 vs 130±36% of 4 normal subjects). The transmural profile of %tkn across the wall during systole became dull and/or unclear compared to the normal subjects. After the 399

3 successful coronary reperfusion, a larger recovery in magnitude of %tkn as well as the increase in the number of layers showing 130% in tkn were observed at subendocardial side, eventually resulting in the normal profile of %tkn. Other intramural layers showed no significant improvement in systolic function. Conclusion: PTM can be a bedside diagnostic modality to evaluate the functional deterioration and recovery of ischemic endocardium, describing the sequential change in functional wave front in the wall during the clinical course including the percutaneous coronary intervention. P3B-3 INFLUENCE OF ACOUSTIC INTENSITY ON THE SECOND-HARMONIC BEAM PROFILE T. VARSLOT*, T. JOHANSEN, R. HANSEN, B. ANGELSEN, and H. TORP, Norwegian University of Science and Technology, Trondheim, Norway. Corresponding varslot@math.ntnu.no A method for fast numerical simulation of nonlinear wave propagation based on a quasilinear approximation, has previously been presented. The aim of the current study was to validate this model by comparison to a conventional nonlinear simulation model, and to experimental measurements. Experimental verifications were performed with hydrophone measurements in a water tank. An annular array probe (Vingmed Sound APAT 3.25) with a diameter of 14.7 mm and 78 mm ROC, designed for cardiac imaging, was used for the experiments. Simulations were performed for the same experimental setup. The KZK equation is a good model for the nonlinear nature of sound propagation through water. As the comparison was done for an approximate f-number of 5.2, the limited accuracy of the diffraction description at low f-numbers was not relevant. The reference simulation used here was a solution of the KZK equation for a forward propagating pulse using an operator-splitting approach. Second harmonic beam profiles were produced by filtering the hydrophone signal at 6 MHz, with a 1 MHz bandpass filter. Beam profiles from the two simulation methods and the hydrophone measurements were compared at depths 4 cm, 7 cm, and 10 cm, for pulses with a mechanical index (MI) ranging from 0.1 to 1.8. The results showed almost perfect match between the two simulation models for MI less than 0.5. For higher MI values, the KZK method showed a gradual increase in the side lobe level, whereas the main lobe width and shape was preserved. These effects were also demonstrated by the hydrophone measurements; however the side lobe level for low MI values was not accurately measureable, due to thermal noise. 400

4 P3B-4 INVESTIGATION OF SCATTERER DEPENDENCY IN MEDICAL ULTRASOUND ABERRATION CORRECTION S.-E. MAASOEY, T. VARSLOT, and B. ANGELSEN, Norwegian University of Science and Technology. Corresponding Investigation of how different types of aberration affect the estimation of arrival time and amplitude fluctuations (ATAF) from a random speckle signal in ultrasound medical imaging, has been performed. The ATAF were used in a time delay, and a time delay and amplitude aberration correction filter. Such a correction filter, which is the most commonly tested in aberration correction techniques, inherently assumes scatterer independent aberration. 2D-simulations were performed to investigate three different aberration situations: scatterer independent aberration (SIA), scatterer dependent aberration (SDA), and corrected scatterer dependent aberration (CSDA). The SDA was simulated using two different, 2 cm thick human body wall models (HBWM), representing situations of weak and strong aberration respectively. For the CSDA, the same HBWM were used, but the transmitted beam was corrected with a time delay and amplitude aberration correction filter estimated from a point source in the focus point of the array. The SIA was simulated by concentrating all the aberration of the two HBWM into a plane on the transmitting/receiving array. ATAF were estimated with a correlation method based on statistical averaging of the cross-spectrum of a set of twenty statistically independent speckle signals. Estimates from the point source simulations were used as a reference for evaluation of the results. Results show that for SIA and CSDA, estimates for the arrival time fluctuations were very close to the point source estimates for both the weak and strong HBWM. Used in a time delay correction filter they produced equivalent transmitted beam profiles as correction with the point source estimates. The estimate of amplitude fluctuations in the SIA situation was approximately equal to the point source estimates. In the CSDA situation the estimates were somewhat poorer. This did not influence the aberration corrected beam profiles significantly. For SDA the arrival time fluctuations deviated from the point source estimates. With the strong HBWM, the estimate was very poor giving almost no correction of the beam profile. The amplitude estimates were only able to detect amplitude variations of about 4 db for both HBWM, while variations of up to 18 db existed for the strong HBWM. The work presented was supported by the Medicine and Health program of the Research Council of Norway 401

5 P3B-5 SIMULATIONS OF THE NONLINEAR ACOUSTIC PRESSURE FIELD WITHOUT USING THE PARABOLIC APPROXIMATION K. HUIJSSEN* 2, A. BOUAKAZ 1 3,M.VERWEIJ 2, and N. DEJONG 1 3, 1 Erasmus Medical Center, Rotterdam The Netherlands, 2 Delft University of Technology, Delft, The Netherlands, 3 ICIN Utrecht The Netherlands. Corresponding a.bouakaz@erasmusmc.nl Aim: The popular KZK equation has been widely used to model the nonlinear acoustic wave field. However, since it is based on a parabolic approximation, it has various limitations, at close distances from the transducer and at large radial extents from the beam axis or in case of phased arrays for steering angles larger than 16 degrees. The purpose of this paper is to model the nonlinear acoustic wave field without using the parabolic approximation. Method: The model is based on the full nonlinear wave equation known as the Westervelt equation. This equation is solved numerically using an explicit finite difference scheme with fourth order centered space differences and second order centered or backward time differences. To check the validity of our model, the numerical results for the linear propagation case were compared to the analytical results obtained using FIELD program. Moreover, the nonlinear beam profiles were compared to the numerical results of the KZK equation. In addition, our numerical predictions were compared to hydrophone measurements performed with a single element, unfocused transducer with an effective radius of 6.9 mm and a center frequency of 1 MHz. We used a pulsed excitation of 6 cycles with pressure levels at the source of 39, 219 and 495 kpa. The axial beam profiles and the lateral beam profiles at several distances were measured. Results: For a low drive level, our numerical predictions showed a good agreement with the analytic solution as well as with the measured data for all fundamental frequency beam profiles. At higher excitations, the measurements and the simulations gave a significant second harmonic component at 15 db below the fundamental level. The numerical data reproduced all the features at the fundamental and harmonic frequency beam profiles. For regions where the KZK equation is known to be less accurate, our model performed much better. The sidelobes created at larger observation angles (>16degrees) and the nearfield variations were accurately reproduced. Conclusions: The performance of our model is good since it predicts accurately the harmonic field at any position with respect to the transducer. It can be used for the whole scanning sector, which is required for harmonic beam profile simulations for phased arrays. 402