Patent EP-1079240-A2 (EP)
Inventor: TORP HANS GARMANN
IPC: A61B5/352
Filed: 2000-08-09
Abstract: An ultrasound system and method for calculation and display of tissue
deformation parameters are disclosed. An ultrasound acquisition technique
that allows a high frame rate in tissue velocity imaging or strain rate imaging
is employed. With this acquisition technique the same ultrasound pulses are
used for the tissue image and the Doppler based image. A sliding window
technique (160, 161, 162, 163 and 164) is used for processing. The tissue
deformation parameter strain is also determined by an accumulation of strain
rate estimates for consecutive frames over an interval. The interval may be a
triggered interval generated by, for example, an R-wave in an ECG trace.
The strain calculation (150) may be improved by moving the sample volume
from which the strain rate is accumulated from frame-to-frame according to
the relative displacement of the tissue within the original sample volume. The
relative displacement of the tissue is determined by the instantaneous tissue
velocity of the sample volume. An estimation of strain rate (150) based upon
a spatial derivative of tissue velocity is improved by adaptively varying the
spatial offset,  dr . The spatial offset,  dr , can be maximized to cover the entire
tissue segment (e.g., heart wall width) while still keeping both of the sample
volumes at each end of the offset within the tissue segment. This may be
accomplished by determining whether various parameters (e.g., grayscale
value, absolute power estimate, magnitude of the auto-correlation function
with unity temporal lag and/or magnitude of strain correlation) of the sample
volumes within in the spatial offset are above a given threshold. Strain rate
may be estimated (150) using a generalized strain rate estimator that is based
on a weighted sum of two-sample strain rate estimators with different spatial
offsets. The weights are proportional to the magnitude of the strain rate
correlation estimate for each spatial offset, and thus reduce the effect of
noisy, i.e. poorly correlated, samples. An improved signal correlation
estimator that uses a spatial lag in addition to the usual temporal lag is
disclosed. The spatial lag is found from the tissue velocity. The improved
signal correlation estimator can be utilized both in the estimation of strain rate
and tissue velocity. Tissue velocity may be estimated in a manner that 
reduces aliasing while maintaining spatial resolution. Three copies of a
received ultrasound signal are bandpass filtered at three center frequencies.
The middle of the three center frequencies is centered at the second
harmonic of the ultrasound signal. A reference tissue velocity is estimated
from the two signals filtered at the outside center frequencies. The reference
tissue velocity is used to choose a tissue velocity from a number of tissue
velocities estimated from the signal centered at the second harmonic. A
method to estimate (150) the strain rate in any direction, not necessarily along
the ultrasound beam (144), based on tissue velocity data from a small region
of interest around a sample volume is disclosed. Quantitative tissue
deformation parameters, such as tissue velocity, tissue velocity integrals,
strain rate and/or strain, may be presented (152) as functions of time and/or
spatial position for applications such as stress echo. For example, strain rate
or strain values for three different stress levels may be plotted together with
respect to time over a cardiac cycle. Parameters which are derived from
strain rate or strain velocity, such as peak systolic wall thickening percentage,
may be plotted with respect to various stress levels.