Naohiko Tanaka

Estimation of Doppler shift frequency using selected phase information for high frame rate color flow mapping

PurposeWe describe a new approach to processing signals used to estimate the Doppler shift frequency in high frame-rate color flow mapping with fewer pulse transmissions. When an ultrasound pulse is transmitted to a large number of scatterers, the echoes from the scatterers overlap and interfere with one another. This interference causes the phase of the received echo signal to fluctuate, thus disturbing the estimated shift in Doppler frequency. The technique proposed here eliminates this disturbed phase information, leaving the remaining information for use in estimating the shift in Doppler frequency. The instantaneous frequency of the echo signal can serve as an index of the influence of interference.MethodsTo test this technique in vivo we used radio-frequency echo signals from the carotid artery for simulation and evaluated the error of the estimated Doppler shift frequency in several cases.ConclusionPerformance was enhanced when the number of pulses transmitted was limited and this technique was used.

Experimental verification of color flow imaging based on wideband Doppler method

The purpose of this study is to eliminate the aliasing in color flow imaging. The wideband Doppler method is applied to generate a color flow image, and the validity of the method is experimentally confirmed. The single beam experiment is carried out to confirm the velocity estimation based on the wideband Doppler method. The echo data for the conventional pulsed Doppler method and the wideband Doppler method are obtained using a flow model, and the estimated velocity for each method is compared. The color flow images for each method are also generated using several types of flow model. The generated images are compared, and the characteristics of the imaging based on the wideband Doppler method are discussed. The high velocity beyond the Nyquist limit is successfully estimated by the wideband Doppler method, and the availability in low velocity estimation is also confirmed. The aliasing in color flow images is eliminated, and the generated images show the significance of the elimination of the aliasing in the flow imaging. The aliasing in color flow imaging can be eliminated by the wideband Doppler method. This technique is useful for the exact understanding of blood flow dynamics.

Alias‐free estimation of blood velocity using wideband pulse transmission

Blood velocity information that is obtained from a pulsed Doppler system is applied to clinical diagnoses of the circulatory system. However, the conventional pulsed Doppler technique has a fundamental limitation on Doppler velocities: blood velocity is measured erroneously when it is greater than the Nyquist limit. This phenomenon is called aliasing. This study is aimed at expanding the range of measurable blood velocities while avoiding aliasing. A new technique proposed here is based on wideband pulse transmission and phase correction. A series of short pulses with a wide frequency band is transmitted in this technique. Received echoes are Fourier transformed and converted to the cross spectrum. The phase of the cross spectrum is proportional to the frequency and the blood velocity. When the blood velocity is greater than the Nyquist limit, a part of the phase of the cross spectrum is larger than ±π and is wrapped in ±π. The wrapped phase data are corrected using wideband echo information to avoid alia...

Estimation of the Doppler shift frequency using selected phase information for high‐frame rate blood flow imaging

A new signal processing approach is presented for the estimation of the Doppler shift frequency in high‐frame rate blood flow imaging with fewer pulse transmission. When an ultrasound pulse is transmitted to a large number of scatterers, the echoes from the scatterers overlap and interfere with one another. This interference causes a fluctuation in the phase of the received echo signal, and the estimation of the Doppler shift frequency will be disturbed by this phase fluctuation. A new technique proposed here makes it possible to eliminate the disturbed phase information by the interference, and the remaining information is applied to the estimation of the Doppler shift frequency. The instantaneous frequency of the echo signal can be used as an index of the influence of interference. To test this technique, radio‐frequency echo signal from the carotid artery is used for the simulation, and the error of the estimated Doppler shift frequency is evaluated for several cases. The result shows that better perfo...