Abstract
Second harmonic imaging is currently adopted as standard in commercial echographic systems. A new imaging
technique, coined as superharmonic imaging (SHI), combines the 3rd till the 5th harmonics, arising during
nonlinear sound propagation. It could further enhance resolution and quality of echographic images. To meet
the bandwidth requirement for SHI a dedicated phased array has been developed: a low frequency subarray,
intended for transmission, interleaved with a high frequency subarray, used in reception. As the bandwidth of the
elements is limited, the spectral gaps in between the harmonics cause multiple reflection artifacts. Recently, we
introduce a dual-pulse frequency compounding (DPFC) method to suppress those artifacts at price of a reduced
frame rate. In this study we investigate the feasibility of performing the frequency compounding protocol within
a single transmission.
The traditional DPFC method constructs each trace in a post-processing stage by summing echoes from two
emitted pulses, the second slightly frequency-shifted compared to the first. In the newly proposed method, the
transmit aperture is divided into two parts: the first half is used to send a pulse at the lower center frequency,
while the other half simultaneously transmits at the higher center frequency. The suitability of the protocol
for medical imaging applications in terms of the steering capabilities was performed in a simulation study using
the FIELD II toolkit. Moreover, an experimental study was performed to deduce the optimal parametric set
for implementation of the clinical imaging protocol. The latter was subsequently used to obtain the images of
a tissue mimicking phantom containing strongly reflecting wires. For in-vitro acquisitions the SHI probe with
interleaved phased array (44 odd elements at 1MHz and 44 even elements at 3.7MHz elements, optimized for
echocardiography) was connected to a fully programmable ultrasound system.
The results of the Field II simulations demonstrated that the angle between the main and grating lobe
amounted to 90°. The difference in the fundamental pressure level between those lobes was equal to -26.8 dB.
Those results suggest that the superharmonic content in the grating lobe was acceptably low. A considerable
improvement in the axial resolution of the SHI component (0.73 mm) at -6 dB in comparison with the 3rd
harmonic (2.23 mm) was observed. A similar comparison in terms of the lateral resolution slightly favored the
superharmonic component by 0.2 mm. Additionally, the images of the tissue mimicking phantom exhibited an
absence of the multiple reflection artifacts in the focal and post-focal regions.
The new method is equally effective in eliminating the ripple artifacts associated with SHI as the dual pulse
technique, while the full frame rate is maintained.
