This paper further investigates the use of coded excitation for blood flow estimation
in medical ultrasound. Traditional autocorrelation estimators use narrow-band excitation
signals to provide sufficient signal-to-noise-ratio (SNR) and velocity estimation performance. In this
paper, broadband coded signals are used to increase SNR, followed by sub-band processing.
broadband signal, is filtered using a set of narrow-band filters.
Estimating the velocity in each of the bands and averaging the results
yields better performance compared to what would be possible when transmitting a narrow-band
pulse directly. Also, the spatial resolution of the narrow-band pulse would be too poor for
brightness-mode (B-mode) imaging and additional transmissions would be required to update the
B-mode image. In the described approach, there is no need for additional transmissions, because
the excitation signal is broadband and has good spatial resolution after pulse compression.
Two different coding schemes are used in this paper, Barker codes and Golay codes. The
performance of the codes for velocity estimation is compared to a conventional approach
transmitting a narrow-band pulse. The study was carried out using an experimental ultrasound scanner
and a commercial linear array 7 MHz transducer. A circulating flow rig was scanned with a beam-to-flow angle
of 60°. The flow in the rig was laminar and had a parabolic flow-profile with
a peak velocity of 0.09 m/s. The mean relative standard deviation of the reference
method using an eight cycle excitation pulse at 7 MHz was 0.544% compared to the peak
velocity in the rig. Two Barker codes were tested with a length of 5 and 13 bits, respectively.
The corresponding mean relative standard deviations were 0.367% and 0.310%, respectively.
For the Golay coded experiment, two 8 bit codes were used, and the mean relative
standard deviation was 0.335%.
A new Plane wave fast color flow imaging method (PWM) has been investigated, and performance evaluation of the PWM based on experimental measurements has been made. The results show that it is possible to obtain a CFM image using only 8 echo-pulse emissions for beam to flow angles between 45o. and 75o. Compared to the conventional ultrasound imaging the frame rate is ~ 30-60 times higher. The bias, Best of the velocity profile estimate, based on 8 pulse-echo emissions, is between 3.3 % and 6.1 % for beam to flow angles between 45o. and 75o, and the standard deviation, σest of the velocity profile estimate is around 2 % for beam to flow angles between 45o. and 75o. relative to the peak velocity, when the flow angle is known in advance. A study is performed to investigate how different parameters influence the blood velocity estimation. The results confirmed expectations for beam to flow angles between 45o. and 75o. The parameter study shows that the PWM using Directional velocity estimation gives the best results using spatial sampling interval ≤λ/10, correlation range ≥10λ, and number of directional signals ≥6. It is hereby shown that, by carefully choosing the set of parameters, PWM is feasible for fast CFM imaging with an acceptable bias and standard deviation.
In conventional ultrasound color flow mode imaging, a large number (~500) of pulses have to be emitted in order to form a complete velocity map. This lowers the frame-rate and temporal resolution. A method for color flow imaging in which a few (~10) pulses have to be emitted to form a complete velocity image is presented. The method is based on using a plane wave excitation with temporal encoding to compensate for the decreased SNR, resulting from the lack of focusing. The temporal encoding is done with a linear frequency modulated signal. To decrease lateral sidelobes, a Tukey window is used as apodization on the transmitting aperture. The data are
beamformed along the direction of the flow, and the velocity is found by 1-D cross correlation of these data. First the method is evaluated in simulations using the Field II program. Secondly, the method is evaluated using the experimental scanner RASMUS and a 7 MHz linear array transducer, which scans a circulating flowrig. The velocity of the blood mimicking fluid in the flowrig is constant and parabolic, and the center of the scanned area is situated at a depth of 40 mm. A CFM image of the blood flow in the flowrig is estimated from two pulse emissions. At the axial center line of the CFM image, the velocity is estimated over the vessel with a mean relative standard deviation of 2.64% and a mean relative bias of 6.91%.
At an axial line 5 mm to the right of the center of the CFM image, the velocity is estimated over the vessel with a relative
standard deviation of 0.84% and a relative bias of 5.74%. Finally the method is tested on the common carotid artery of a healthy 33-year-old male.
In this paper a method for spatio-temporal encoding is presented for synthetic transmit aperture ultrasound imaging (STA). The purpose is to excite several transmitters at the same time in order to transmit more acoustic energy in every single transmission. When increasing the transmitted acoustic energy, the signal to noise ratio will increase. However, to focus the data properly using the STA approach, the transmitters have to be separated from each other. This is done by dividing the available spectrum into several subbands with a small overlap. Separating different transmitters can be done by bandpass filtering. Therefore, the separation can be done instantaneously without the need for further transmissions, unlike spatial encoding relying on Hadamard or Golay coding schemes, where several transmissions have to be made before the decoding can be done. Motion artifacts from the decoding can, thus, be avoided. To further increase the transmitted energy, the excitation waveforms are designed as linear frequency modulated (FM) signals. This makes it possible to maintain the full excitation amplitude during most of the transmission. The design of the separation filters will also be discussed. The method was tested using the experimental ultrasound scanner RASMUS and evaluated using a reference setup with a linear FM excitation waveform and STA beamforming. The point spread function (PSF) was measured on a wire phantom in water. A wire phantom with an attenuating medium was also measured, where the proposed method achieved approximately 2 cm improvement in penetration depth. The signal to noise ratio was also measured, where the gain was approx. 7 dB in comparison to the reference.
In conventional synthetic transmit aperture imaging (STA) the image is built up from a number of low resolution images each originating from consecutive single element firings to yield a high resolution image. This may result in motion artifacts making flow imaging problematic. This paper describes a method in which all transmitting centers can be excited at the same time and separated at the receiver. Hereby the benefits from traditional STA can be utilized and a high fframe rate can be maintained and the images are not influenced by motion artifacts. The different centers are excited using mutually orthogonal codes. The total signal at the receiver is then a linear combination of the transmitted signals convolved with the corresponding pulse-echo impulse response. The pulse-echo impulse responses for the different elements are modeled as FIR channels and estimated using a maximum likelihood technique. The method was verified using Field II. A 7 MHz transducer was simulated with 128 receiving elements and 64 transmitting elements divided into subapertures so that 4 virtual transmission centers were formed.
The point spread function was measured and the axial resolution was 0.2312 mm (-3dB) and 0.3083 mm (-6dB), lateral resolution 0.5301 mm (-3dB) and 0.7068 mm (-6dB) and maximum lateral sidelobe level less than 44 dB. Conventional STA is given as a reference with the same setup excited with a single cycle sinusoid at 7 MHz with axial resolution 0.2312 mm (-3dB) and 0.3083 mm (-6dB), lateral resolution 0.5301 mm (-3dB) and 0.7068 mm (-6dB) and maximum lateral sidelobe
level less than 44 dB.