Coded excitation waveform engineering for high frame rate synthetic aperture ultrasound imaging

Rail fracture monitoring based on ultrasonic-guided wave technology with multivariate coded excitation

Steel lay the basis of railroad train traffic. Rail fracture is the most serious injury to the rail, which should be monitored in time. The conventional mono-pulse exciting ultrasonic guide wave (UGW) has low energy, the conventional Barker code has limited coding sequence length and the orthogonal complementary Golay code has the problem of low monitoring efficiency. This study proposes multivariate coded excitation (MCE) to excite UGW to monitor rail fracture, and the feasibility of coding and decoding the method is theoretically derived and verified. Ideally, the MCE generated based on the 3-bit Barker code (B3: 1,1, -1) and 4-bit orthogonal complementary Golay code (GA4: 1,1,1, -1; GB4: 1,1, -1,1) is calculated to have a main-lobe power level (MPL) gain of 8.1020 dB, which is significantly higher than the MPL of Barker and Golay codes. For the proposed method, finite element modeling simulation and experimental study are carried out respectively. Analyze and process the data, and calculate the gain of the difference in amplitude (DIA) between the amplitude of echo caused by rail fracture and the amplitude in the healthy rail. The gain of the DIA of the echoes in the MCE is above 50 dB (experimental data, the value of simulation data is 5 dB) under different degrees of rail fracture, while the gain of the DIA of the echoes caused by the other three excitation methods is below 40 dB (experimental data, the value of simulation data is 1 dB). Simulation and experimental results show that the MCE makes up for the shortcomings of the conventional Barker code and Golay code, improves the excitation energy of the monitoring system, and the high gain of the DIA of the echoes is more conducive to the identification of rail fracture damage.

Synthetic Aperture Ultrasound Imaging through Adaptive Integrated Transmitting-Receiving Beamformer

Synthetic aperture (SA) technique is very attractive for ultrafast ultrasound imaging, as the entire medium can be insonified by a single emission. It also permits applying the dynamic focusing as well as adaptive beamforming both in transmission and reception, which results in an enhanced image. In this paper, we firstly show that the problem of designing the transmit and receive beamformers in SA structure can be formulated as a problem of designing a one-way beamformer on a virtual array with a lateral response equal to that of the two-way beamformer on SA. It is also demonstrated that the length of the virtual aperture is increased to the sum of the transmit aperture length and the receive one, which can result in an enhanced resolution. Moreover, a better estimation of the covariance matrix can be obtained which can be utilized for applying adaptive minimum variance (MV) beamforming method on the virtual array, and consequently the resolution and contrast properties would be enhanced. The performance of the new method is compared with other existing MV-based methods and is quantified by some metrics such as the full width at half maximum (FWHM) and generalized contrast to noise ratio (GCNR). Our validations on simulations and experimental data have shown that the new method is capable of obtaining higher GCNR values while retaining or decreasing FWHM values almost all the time. Moreover, for the same subarray length for estimating the covariance matrices, the computational burden of the new method is significantly lower than those of the existing rival methods.

Efficient synthetic transmit aperture ultrasound based on tensor completion

One of the most important methods in medical ultrasound imaging is the synthetic transmit aperture (STA). Despite the image quality improvement in the STA, this method suffers from several limitations, including a limited data acquisition rate and an increase in the overall time to form a single frame. Tensor completion (TC) is a powerful technique that uses rank minimization to recover missing information from a low-rank tensor. This paper provides a novel random synthetic transmit aperture (RSTA) method based on using only a randomly selected part (a fraction) of the linear array elements in the transmit mode to increase the data acquisition rate and then applying the tensor completion (TC) to improve the image quality. By the proposed method, as it is not necessary to transmit all elements sequentially, the data acquisition rate is improved and the overall time for creating an image is also significantly reduced. We investigated the proposed idea by using several simulated and experimental phantoms. Results showed that the proposed method could increase the data acquisition rate up to three times with the image quality difference of less than 6% compared to the original STA method.

Research on Golay-coded excitation in real-time imaging of high frequency ultrasound biomicroscopy

High frequency ultrasonic imaging provides clinicians with high-resolution diagnostic images and more accurate measurement results. The technique is now widely used in ophthalmology, dermatology, and small animal imaging. However, since ultrasonic attenuation in tissue increases rapidly with increasing frequency, the depth of detection of high frequency ultrasound in tissue is limited to a few millimeters. In this paper, a novel method of using Golay-coded excitation as a replacement for conventional single-pulse excitation in high frequency ultrasound biomicroscopy was proposed, and real-time imaging was realized. While maintaining the transmission voltage and image resolution unchanged, the detection depth can be effectively improved. The ultrasonic transmission frequency is 30 MHz and the transmission voltage is ± 60 V p-p. In this study, 4-bit, 8-bit, and 16-bit coding sequences and decoding compression were used. To verify the effectiveness of the coding sequence in real-time imaging of ultrasound biomicroscopy, we designed a 10-μm diameter line target echo experiment, an ultrasound phantom experiment, and an in vitro porcine eye experiment. The experimental results show that the code/decode method of signal processing can not only maintain a resolution consistent with that of single-pulse transmission, but can also improve the detection depth and signal-to-noise ratio.

A High-Resolution Ultrasonic Ranging System Using Laser Sensing and a Cross-Correlation Method

Ultrasound has been proven to be a valid tool for ranging, especially in water. In this paper, we design a high-resolution ultrasonic ranging system that uses a thin laser beam as an ultrasonic sensor. The laser sensing provides a noncontact method for ultrasound detection based on acousto-optic diffraction. Unlike conventional methods, the ultrasound transmitted from the transducer is recorded as the reference signal when it first passes through the laser. It can be used to improve the accuracy and resolution of the time-of-flight (TOF) by a cross-correlation method. Transducers with a central frequency of 1 MHz and diameters of 20 mm and 28 mm are used in the experiment. Five targets and a test piece are used to evaluate the ranging performance. The sound velocity is measured by the sound velocity profiler (SVP). The repeatability error of TOF is less than 4 ns, and the theoretical resolution of TOF is 0.4 ns. The results show a measurement resolution within one-tenth of the wavelength of ultrasound and an accuracy better than 0.3 mm for targets at a distance up to 0.8 m. The proposed system has potential applications in underwater ranging and thickness detection.

Diverging wave compounding with spatio-temporal encoding using orthogonal Golay pairs for high frame rate imaging

Golay coded excitation for diverging wave compounding (DWC) has been demonstrated to increase the signal-to-noise ratio (SNR) and contrast for high frame rate cardiac imaging. However, the complementary codes need to be transmitted in two consecutive firings for decoding, which reduces the frame rate by 2 folds. This paper proposes an orthogonal Golay pairs coded (OGPs-coded) DWC sequence to overcome this problem, which implements spatio-temporal encoding for DWC. Two diverging waves (DWs) at different steering angles coded by an orthogonal Golay pair are transmitted simultaneously, thus compensating the frame rate reduction caused by transmissions of complementary codes. The two DWs can be separated based on the orthogonality of Golay pairs. To test the feasibility of the proposed sequence, we performed simulations of point targets and tissue phantoms in both static and moving states. Compared with non-coded DWC at the same frame rate, OGPs-coded DWC obtains comparable resolution, SNR gains of 7.5-10 dB and contrast gains of 3-5 dB. The OGPs-coded DWC sequence was also tested experimentally on a tissue-mimicking phantom. Compared with non-coded DWC, OGPs-coded DWC achieves improvements in the SNR (3-6 dB) and contrast (1-2 dB). Preliminary in vivo results show brighter myocardium and larger penetration depth with the proposed method. The proposed OGPs-coded DWC sequence has potential for high frame rate and high quality cardiac imaging.