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Photoacoustic (PA) imaging is an emerging imaging technology combining optical imaging with ultrasound. There has been a strong demand for PA flow imaging of contrast agents such as blood or nano-particles for observing the disease process or drug delivery. In conventional PA flow imaging, however, the temporal resolution is limited by the acoustic time-of-flight (TOF), which is the highest possible frame rate, and the sensitivity is determined by the signal-to-noise ratio (SNR). We propose coded excitation using periodic pseudorandom sequences to achieve PA flow imaging with high sensitivity and ultra-high temporal resolution beyond that of acoustic TOF. A periodically and unipolarly implemented m-sequence was chosen as the code for coded excitation. The frame-rate, which was limited by the acoustic TOF, can be maximized up to the pulse repetition frequency since the start point of decoding can be set in any code in the periodic sequence. Therefore, velocities can be acquired at shorter intervals, and higher temporal resolution is achieved. Moreover, the SNR can be improved by compressing coded signals. We demonstrate the feasibility through experiments. Coding significantly improved the SNR, so that the estimated velocity agreed well with the true designated velocity. In addition, we confirmed a shorter interval of temporal velocity change from the coded results. This indicates that slight flow variations, which could not be observed without coding, can be detected. [ABSTRACT FROM PUBLISHER]

Doppler flow estimation suffers from the weak backscattering from the red blood cells in the vessel. Thus, a novel method is proposed in this study for Doppler detection utilizing Golay-coded excitation to improve signal-to-noise ratio (SNR). Unlike traditional Golay decoding which is limited to low-velocity flow to avoid motion artifact, the proposed decoding is applicable to both high flow velocity and low flow velocity. The analyses show that the main lobe component of Golay code can produce the correct Doppler frequency but the range side lobe counterpart erroneously shifts by half of the pulse-repetition-frequency (PRF). Thus, an optimized decoding filter whose low-pass cut-off frequency is PRF/4 in the slow-time is developed. The PRF/4 filter can retain the main lobe component with Doppler frequency within ±PRF/4 while eliminating the side lobe component outside ±PRF/4. The experimental results demonstrate that the range side lobe level (RSLL) is effectively suppressed by the PRF/4 filter compared to the conventional and the modified methods. In the case of high flow velocity, the color-flow image (CFI) with the PRF/4 filter exhibits accurate estimation of the flow region while the conventional and the modified methods suffer from noticeable side lobe artifacts. In condition, the performance of Golay excitation improves in Doppler detection by using the PRF/4 filter for decoding. [ABSTRACT FROM PUBLISHER]

The frequency bandwidth of CMUTs (capacitive micromachined ultrasonic transducers) is known as relatively broader than that of other ultrasonic transducers. To utilize the wide bandwidth characteristic of the CMUT arrays, in this paper, we report on coded excitation techniques in the CMUT array. Through simulations, STMF (Simultaneous Transmit Multiple-zone Focusing) based ultrasound imaging techniques using orthogonally frequency-divided chirp signals are investigated. In the simulations, the frequency divided sub-band chirps that have orthogonal property are designed within the frequency bandwidth of the CMUT arrays, and simultaneously fired on multiple ranges, in which each signal is focused at a different range, in one transmission event. This paper also presents ultrasound images through a modulation and demodulation process of orthogonal sub-band coded signals. Experiments on the chirp-coded excitation in CMUT arrays are reported in this paper as a feasibility study of the FDMA (frequency division multiple access) like STMF method. In the experiment, mixed two orthogonal chirp signals are simultaneously fired with the CMUT arrays and the received signals are successfully separated into two compressed signals. [ABSTRACT FROM PUBLISHER]

Shear wave elasticity imaging using acoustic radiation force consists of exciting a shear wave by a “push pulse” and tracking the shear wave by ultrafast imaging. The push pulse is radiated at several hundred micro seconds, which is about 100 times higher than the pulse used in imaging. We propose a new pushing-and-tracking sequence, coded push pulse excitation, which can decrease energy density of the push pulse with respect to time by shortening the burst duration or reducing the amplitude without degrading the signal-to-noise ratio (SNR) of the observed shear wave propagation. Phantom experiments successfully demonstrated the validity of the proposed scheme implemented with Golay codes. [ABSTRACT FROM PUBLISHER]