  Ultrasound imaging is extensively employed in clinical settings due to its
non-ionizing nature and real-time capabilities. The beamformer represents a
crucial component of an ultrasound machine, playing a significant role in
shaping the ultimate quality of the reconstructed image. Therefore, Transmit
Beam Pattern (TBP) optimization is an important task in medical ultrasound, but
state-of-the-art TBP optimization has well-known drawbacks like non-uniform
beam width over depth, presence of significant side lobes, and quick energy
drop out after the focal depth. To overcome these limitations, we developed a
novel optimization approach for TBP by focusing the analysis on its narrowband
approximation, particularly suited for Acoustic Radiation Force Impulse (ARFI)
elastography, and considering transmit delays as free variables instead of
linked to a specific focal depth. We formulate the problem as a non linear
Least Squares problem to minimize the difference between the TBP corresponding
to a set of delays and the desired one, modeled as a 2D rectangular shape
elongated in the direction of the beam axis. In order to quantitatively
evaluate the results, we define three quality metrics based on main lobe width,
side lobe level, and central line power. Results obtained by our synthetic
software simulation show that the main lobe width is considerably more intense
and uniform over the whole depth range with respect to classical focalized Beam
Patterns, and our optimized delay profile results in a combination of standard
delay profiles at different focal depths. The application of the proposed
method to ARFI elastography shows improvements in the concentration of the
ultrasound energy along a desired axis.
