An ultrasound vector Doppler imaging is useful for detecting flow components normal to the ultrasound beam
direction. However, the conventional vector Doppler imaging method suffers from the bias of the time interval between
samples caused by the mismatch between transmit and receive directions during demodulation. In this paper, a new
directional demodulation method, in which demodulation is performed with a modified sample interval depending on the
receive beam steered angle to reduce the bias occurred in a conventional ultrasound vector Doppler imaging is presented.
To evaluate the performance of the proposed directional demodulation method, the pre-beamformed radio-frequency
(RF) data from in-vitro experiments were obtained using a commercial ultrasound system and a Doppler string phantom.
The true flow velocity of the phantom was 0.3 m/s. The center frequency of 5 MHz and the pulse repetition frequency of
4 kHz were used for the experiments. Also, a 32-element sub-aperture on a 128-element 7.2-MHz linear array probe
were used for emission and reception while changing the flow direction from -45 degrees to 0 degree by a step of 5
degrees. The proposed directional demodulation method successfully visualizes all flow directions. In addition, it lowers
a bias on flow estimation compared to the conventional method (i.e., 0.0255±0.0516 m/s vs. 0.0248±0.0469 m/s of error
of velocity, 2.4862±3.8911 degrees vs. 2.4857±3.5115 degrees of error of direction, respectively). These results indicate
that the proposed directional demodulation method can enhances the accuracy in flow estimation for vector Doppler
imaging.
KEYWORDS: Ultrasonography, Signal to noise ratio, Phased arrays, Image quality, Tissues, In vivo imaging, Functional imaging, Transducers, In vitro testing, Image enhancement
In medical ultrasound imaging, a multi-beamforming (MBF) method is used for supporting high frame rate imaging
or functional imaging where multiple scanlines are reconstructed from a single excitation event. For efficient MBF, a
time-sharing technique (i.e., MBF-TS) can be applied. However, the MBF-TS could degrade image quality due to the
decreased beamforming frequency. In this paper, the multi-access register-based MBF (MBF-MAR) method running on
the post-fractional filtering (PFF) architecture is presented. In PFF-MBF-MAR, instead of lowering beamforming
frequency, a multi-access register at each channel is utilized for generating multiple scanlines simultaneously. To
evaluate the performance of the proposed PFF-MBF-MAR method, the phantom experiment was conducted where 64-
channel pre-beamformed radio-frequency (RF) data were captured from a tissue mimicking phantom by using a modified
commercial ultrasound system (SONOLINE G40, Siemens Inc., USA) using a 3-MHz phased array probe. From the
phantom experiment, the PFF-MBF-MAR method showed 4.7 dB and 0.6 improvements in the signal-to-noise ratio
(SNR) and the contrast-to-noise ratio (CNR), respectively, compared to the PFF-MBF-TS method, while slightly
increasing the hardware complexity (<5.2%). The similar results were achieved with the in vivo thyroid data. These
results indicate that the proposed PFF-MBF-MAR method can be used for high frame rate imaging or functional imaging
without sacrificing image quality while slightly increasing the hardware complexity.
In such applications as fast 3D imaging with 2D arrays and point-of-care imaging with an ultra portable devices, periodic
sparse arrays(PSA) can be efficiently used to increase the effective aperture size with less number of active elements
than the conventional method. Generally, PSA can be represented as sub-arrays distributed uniformly in P -element
intervals, each with L consecutive elements, where L < P. Since the continuous wave beam pattern in the far-field is
given by Fourier transform of aperture function, the beam pattern of PSA is a multiplication of beam patterns of the
upsampled dense array by the ratio of P and L -elements sub-array. A recent method to design a PSA pair provides
analytically the values of P and L for transmit and receive arrays to eliminate the dominant grating lobes, which occur
at the same position on both transmit and receive. In this work, we present a method to design a PSA pair with improved
performance by further suppressing the residual grating lobes of PSA. It can be accomplished by properly shading
amplitude of the transmit and receive sparse arrays. This shading window function is also obtained by signal analysis of
aperture functions. The beam patterns of various PSA pairs based on the proposed design method are evaluated through
computer simulations. The simulation results show that the residual grating lobes are reduced by about 10dB more in all
cases. Consequently, our method can be used to improve the performance of beam pattern or enhance the periodicity of
sparse array.
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