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.
Conventional ultrasound scanners are restricted to display the blood velocity component in the ultrasound beam direction. By introducing a laterally oscillating field, signals are created from which the transverse velocity component can be estimated. This paper presents velocity and volume flow estimates obtained from flow phantom and in-vivo measurements at 90° relative to the ultrasound beam axis. The flow phantom experiment setup consists of a SMI140 flow phantom connected to a CompuFlow 1000 programmable flow pump, which generates a flow similarly to that in the femoral artery. A B-K medical 8804 linear array transducer with 128 elements and a center frequency of 7 MHz is emitting 8 cycle ultrasound pulses with a pulse repetition frequency of 7 kHz in a direction perpendicular to the flow direction in the phantom. The transducer is connected to the experimental ultrasound scanner RASMUS, and 1.4 seconds of data is acquired. Using 2 parallel receive beamformers a transverse oscillation is introduced with an oscillation period 1.2 mm. The velocity estimation is performed using an extended autocorrelation algorithm. The volume flow can be estimated with a relative standard deviation of 13.0% and a relative mean bias of 3.4%. The in-vivo experiment is performed on the common carotid artery of a healthy 25 year old male. The same transducer and setup is used as in the flow phantom experiment, and the data is acquired using the RASMUS scanner. The peak velocity of the carotid flow is estimated to 1.2 m/s and the volume flow to 290 ml/min. This is within normal physiological range.