Improving image contrast and accuracy in velocity estimation by convolution filters for intracardiac blood flow imaging.

In this study, the point spread function (PSF) of an ultrasound imaging system was estimated and used as a reference signal in a filtering method for improvement of image quality. The PSF of the imaging system was estimated from measured echo signals from an imaging target. Convolution filters (including deconvolution) were used for improvement of image contrast and spatial resolution. Furthermore, the accuracy in estimation of velocity vectors was evaluated for investigation of the impact of the proposed filters on velocity estimation. In the phantom experiment, contrast of the B-mode image was improved from 76.4 dB to 81.1 dB and 77.8 dB using the convolution and deconvolution filters, respectively. Also, the two-dimensional (2D) velocity distribution in the phantom was estimated by the block matching method, and the bias error (BE) in the estimated lateral velocity was reduced from -19.7% to 2.16% and 2.29% using the convolution and the deconvolution filters, respectively.

Measurement of flow velocity vectors in carotid artery using plane wave imaging with repeated transmit sequence.

PURPOSE: Doppler-based methods are widely used for blood flow imaging in clinical settings. However, they inherently estimate the velocity component only in the axial direction. Therefore, various studies of angle-independent methods have been conducted. The multi-angle Doppler method is one such angle-independent method, in which the velocity vector is estimated using axial velocities obtained from multiple directions by steering an ultrasonic beam. Recently, plane wave imaging, which realizes a very high frame rate of several thousand frames per second, was applied to the multi-angle Doppler method. However, the maximum detectable velocity, i.e., the aliasing limit, was reduced depending on the number of steering angles. In the present study, the feasibility of a specific transmit sequence, namely, the repeated transmit sequence, was examined using the plane-wave multi-angle Doppler method. METHOD: In the repeated transmit sequence, plane waves were emitted to the same direction twice, after which the steering angle was changed. By repeating the same procedure, a pair of beamformed radio-frequency (RF) signals could be obtained under each beam steering angle. By applying the autocorrelation method to each pair of RF signals, the time interval between the RF signals could be kept as the pulse repetition interval (PRI). The feasibility of such a transmit sequence was examined by numerical simulation and in vivo measurement of a human carotid artery. RESULTS: The simulation results showed that the maximum steering angles of over 10 degrees were not feasible with the linear array used in the present study. The feasible maximum steering angle would depend on the element pitch of the probe relative to the ultrasonic wavelength. By limiting the maximum steering angles to 5 and 10 degrees, bias errors were 9.2% and 11.3%, respectively, and root mean squared errors were 21.5% and 16.9%, respectively. Also, flow velocity vectors in a human carotid artery could be visualized with the proposed method. CONCLUSION: The multi-angle Doppler method was implemented in plane wave imaging with the repeated transmit sequence, and the proposed method was shown to be feasible through numerical simulation and in vivo measurement of a carotid artery.

Investigation of feasibility of singular value decomposition clutter filter in plane wave imaging with packet transmission sequence.

PURPOSE: Assessment of blood flow is an important function in diagnostic ultrasound imaging. Color flow imaging is one such method widely used in the clinical setting. Since autocorrelation suffers from aliasing, the time interval between successive transmissions of ultrasonic pulses should be as short as possible. For this purpose, a specific transmit-receive sequence, namely, packet transmission, is widely used in color flow imaging. Also, plane wave imaging recently introduced to ultrasound imaging significantly contributes to improvement of the temporal resolution. Furthermore, a singular value decomposition (SVD) clutter filter reportedly outperforms a conventional clutter filter. In the present study, the feasibility of the SVD clutter filter in plane wave imaging with the packet transmission sequence was investigated. METHOD: In the present study, the packet transmission sequence was implemented in plane wave imaging by sending plane waves multiple times in the same direction before changing the steering angle. In the first strategy, like conventional color flow imaging with line-by-line acquisition using a focused transmit beam, a clutter filter was applied to ultrasonic radio-frequency (RF) signals in each packet. In the second strategy, the number of transmissions per packet was set at two, and a clutter filter was applied to RF signals obtained from the first or second transmission in different packets. RESULTS: The in vivo experimental results on a human carotid artery showed that the second strategy with an SVD filter realized significantly better performance than the first strategy with a polynomial regression filter used as a conventional filter. CONCLUSION: An SVD clutter filter was feasible in plane wave imaging with the packet transmission sequence, and the performance was improved by limiting the number of transmissions per packet to two.