An Approach to Multibeam Covariance Matrices for Adaptive Beamforming in Ultrasonography

Abstract—Medical ultrasound imaging systems are often
based on transmitting, and recording the backscatter from, a
series of focused broadband beams with overlapping coverage
areas. When applying adaptive beamforming, a separate array
covariance matrix for each image sample is usually formed.
The data used to estimate any one of these covariance matrices
is often limited to the recorded backscatter from a single
transmitted beam, or that of some adjacent beams through
additional focusing at reception. We propose to form, for each
radial distance, a single covariance matrix covering all of the
beams. The covariance matrix is estimated by combining the
array samples after a sequenced time delay and phase shift.
The time delay is identical to that performed in conventional
delay-and-sum beamforming. The performance of the proposed
approach in conjunction with the Capon beamformer
is studied on both simulated data of scenes consisting of point
targets and recorded ultrasound phantom data from a specially
adapted commercial scanner. The results show that the
proposed approach is more capable of resolving point targets
and gives better defined cyst-like structures in speckle images
compared with the conventional delay-and-sum approach. Furthermore,
it shows both an increased robustness to noise and
an increased ability to resolve point-like targets compared with
the more traditional per-beam Capon beamformer.