| abstract |
An ultrasound system and method for calculation and display of tissuendeformation parameters are disclosed. An ultrasound acquisition techniquenthat allows a high frame rate in tissue velocity imaging or strain rate imagingnis employed. With this acquisition technique the same ultrasound pulses arenused for the tissue image and the Doppler based image. A sliding windowntechnique (160, 161, 162, 163 and 164) is used for processing. The tissuendeformation parameter strain is also determined by an accumulation of strainnrate estimates for consecutive frames over an interval. The interval may be antriggered interval generated by, for example, an R-wave in an ECG trace.nThe strain calculation (150) may be improved by moving the sample volumenfrom which the strain rate is accumulated from frame-to-frame according tonthe relative displacement of the tissue within the original sample volume. Thenrelative displacement of the tissue is determined by the instantaneous tissuenvelocity of the sample volume. An estimation of strain rate (150) based uponna spatial derivative of tissue velocity is improved by adaptively varying thenspatial offset, dr . The spatial offset, dr , can be maximized to cover the entirentissue segment (e.g., heart wall width) while still keeping both of the samplenvolumes at each end of the offset within the tissue segment. This may benaccomplished by determining whether various parameters (e.g., grayscalenvalue, absolute power estimate, magnitude of the auto-correlation functionnwith unity temporal lag and/or magnitude of strain correlation) of the samplenvolumes within in the spatial offset are above a given threshold. Strain ratenmay be estimated (150) using a generalized strain rate estimator that is basednon a weighted sum of two-sample strain rate estimators with different spatialnoffsets. The weights are proportional to the magnitude of the strain ratencorrelation estimate for each spatial offset, and thus reduce the effect ofnnoisy, i.e. poorly correlated, samples. An improved signal correlationnestimator that uses a spatial lag in addition to the usual temporal lag isndisclosed. The spatial lag is found from the tissue velocity. The improvednsignal correlation estimator can be utilized both in the estimation of strain ratenand tissue velocity. Tissue velocity may be estimated in a manner that nreduces aliasing while maintaining spatial resolution. Three copies of anreceived ultrasound signal are bandpass filtered at three center frequencies.nThe middle of the three center frequencies is centered at the secondnharmonic of the ultrasound signal. A reference tissue velocity is estimatednfrom the two signals filtered at the outside center frequencies. The referencentissue velocity is used to choose a tissue velocity from a number of tissuenvelocities estimated from the signal centered at the second harmonic. Anmethod to estimate (150) the strain rate in any direction, not necessarily alongnthe ultrasound beam (144), based on tissue velocity data from a small regionnof interest around a sample volume is disclosed. Quantitative tissuendeformation parameters, such as tissue velocity, tissue velocity integrals,nstrain rate and/or strain, may be presented (152) as functions of time and/ornspatial position for applications such as stress echo. For example, strain ratenor strain values for three different stress levels may be plotted together withnrespect to time over a cardiac cycle. Parameters which are derived fromnstrain rate or strain velocity, such as peak systolic wall thickening percentage,nmay be plotted with respect to various stress levels. |