Shigeaki Okumura

Remote heartbeat monitoring from human soles using 60-GHz ultra-wideband radar

Measurement of heartbeats is essential in cardiovascular magnetic resonance imaging because the measurement must be synchronized with the phase of cardiac cycles. Many existing studies on radar-based heartbeat monitoring have focused on echoes from the torso only, and such monitoring cannot be applied to subjects in magnetic resonance scanners because only the head and soles can be seen from the outside. In this study, we demonstrate the feasibility of the remote monitoring of heartbeats from the subject’s soles using a 60-GHz ultra-wideband radar. The heartbeat intervals measured using the radar are quantitatively compared with those measured using conventional electrocardiography.

Ultrawideband Radar Imaging Using Adaptive Array and Doppler Separation

Ultrawideband Doppler radar interferometry is known as an effective method that enables high-resolution imaging when using a simple antenna array. The technique, however, suffers from image artifacts when multiple moving targets with the similar Doppler velocities are present in the same range bin. To resolve this problem, we combine the Doppler interferometry technique with the Capon methods. Through numerical simulations and experiments, we show the remarkable performance improvement achieved by the proposed method.

Stabilization techniques for high resolution ultrasound imaging using beamspace Capon method

Several adaptive beamforming imagers have been proposed to improve the spatial resolution of medical ultrasound imaging. These imagers employ spatial averaging technique and diagonal loading technique to suppress correlated interferences and to acquire robustness, respectively. In this study, we propose the technique that selects the optimum size for spatial averaging with respect to the measurement depth. We employ a small diagonal loading factor of -40 dB to avoid the deterioration in image quality. In a simulation study, the proposed method succeeded to improve the image quality compared to a conventional beamspace Capon method. The sidelobe level and the -6 dB beam width in the proposed method were -26 dB and 0.27 mm, respectively, where those in the conventional method were -16 dB and 0.40 mm, respectively. The proposed method succeeded to depict the higher resolution image than that of the conventional method.

High-contrast and low-computational complexity medical ultrasound imaging using beamspace capon method

Several adaptive beamforming techniques have been proposed to improve the quality of medical ultrasound images. The beamspace (BS) Capon method is one common method used to depict high-resolution images with low computational complexity. However, the complexity is not low enough for real-time imaging in clinical situations because the conventional BS Capon method employs a time-delay process and a transition process from elementspace signal processing to BS signal processing at all points of interest. Thus, we propose a technique that replaces the time-delay process using a steering vector. In addition, the Capon method employs a spatial averaging (SA) technique to stabilize the estimation in intensity. However, when the averaging size is not adequate, the estimated intensity might be smaller than that given by the delay-and-sum (DAS) method. Because most medical diagnoses are presented based on the estimation of intensity acquired by the DAS beamformer, accurate estimation of intensity is also required. Therefore, we propose a compensation technique that uses both small and large sizes for SA. In an experiment, the -6 dB beam width, sidelobe level, and estimation error in the intensity of the proposed method were 0.17 mm, -27 dB, and 0.92 dB, respectively, where those of the conventional BS Capon method were 0.29 mm, -22 dB, and 8.1 dB. The complexity of the proposed method is one-fourteenth that of the conventional method. Compared with conventional methods, the proposed method succeeded in depicting a higher-contrast image with accurate intensity estimation and lower computational complexity.

Numerical simulation and machine learning based analysis of the ultrasonic waveform propagating in bone tissue

Since bone has a complex structure, it is difficult to analytically understand the behavior of the ultrasound although it is useful for bone quality diagnosis. Our group, therefore, had been working on simulating ultrasound propagation inside bone models. In this paper, the results of the neural network based bone analysis using waveforms derived by the 3-D elastic FDTD (finite-difference time domain) simulation will be presented as well as its basis and some representative results of the FDTD method applied for the bone assessment. Since the FDTD simulation only requires the 3-D geometry of the model and the acoustic parameters (density, speed of longitudinal wave and shear wave) of the media, it has been very useful for evaluating each effect of a certain acoustic parameter or the bone geometry such as bone density, respectively, in addition to the purpose of visually understanding the wave behavior inside the model including the propagation path. Moreover, thanks to the recent powerful computational resource, it is now realized to prepare a huge number of waveforms for machine learning by using FDTD method. As a result, it was shown that the neural network can estimate the bone density better than the traditional method. (Grant: JSPS KAKENHI 16K01431.)

High-resolution wavenumber estimation in ultrasonic guided waves using adaptive array signal processing for bone quality assessment

Quantitative ultrasound is a modality that is used to evaluate bone quality. It is considered that the analysis of ultrasound guided wave propagating along cortical bone may be useful for the assessment of cortical bone quality. Because the frequency-dependent wavenumbers reflect the elastic parameters of the medium, high-resolution estimation of the wavenumbers at each frequency is important. We report an adaptive array signal processing method with a technique to estimate the numbers of propagation modes at each frequency using information theoretic criteria and the diagonal loading technique. The proposed method estimates the optimal diagonal loading value required for guided wave estimation. We investigate the effectiveness of the proposed method via simple numerical simulations and experiments using a copper plate and a bone-mimicking plate, where the center frequency of the transmit wave was 1.0 MHz. An experimental study of 4 mm thick copper and bone-mimicking plates showed that the proposed method estimated the wavenumbers accurately with estimation errors of less than 4% and a calculation time of less than 0.5 s when using a laptop computer.Quantitative ultrasound is a modality that is used to evaluate bone quality. It is considered that the analysis of ultrasound guided wave propagating along cortical bone may be useful for the assessment of cortical bone quality. Because the frequency-dependent wavenumbers reflect the elastic parameters of the medium, high-resolution estimation of the wavenumbers at each frequency is important. We report an adaptive array signal processing method with a technique to estimate the numbers of propagation modes at each frequency using information theoretic criteria and the diagonal loading technique. The proposed method estimates the optimal diagonal loading value required for guided wave estimation. We investigate the effectiveness of the proposed method via simple numerical simulations and experiments using a copper plate and a bone-mimicking plate, where the center frequency of the transmit wave was 1.0 MHz. An experimental study of 4 mm thick copper and bone-mimicking plates showed that the proposed method...

Noncontact respiration monitoring of multiple closely positioned patients using ultra-wideband array radar with adaptive beamforming technique

Contactless respiration monitoring using Doppler radar is an important technology for healthcare applications. The radar measures small displacements of the body surface. In this study, we propose a new algorithm to separate multiple targets placed closely together at the same range but at different lateral positions using ultra-wideband array radar and the Capon method. The Capon method, which is an adaptive beamforming - technique, assumes that arrival echoes are not correlated. However, echoes from the human body should be correlated and this reduces system performance. We improve the performance - by introducing two different diagonal loading factor values for direction of arrival estimation and weight vector calculation. In an experimental study, the proposed method separates the echoes from two patients spaced at a lateral distance of approximately 70 mm. The estimated displacement error when using the proposed method is less than 0.13 mm, while that of the conventional method is 2.7 mm.

Accurate Characterization of Ultrasonic Guided Waves Propagating in Cortical Bone Acquired by a Single Transmitter–Receiver Pair Using Adaptive Signal Processing

The characterization of Lamb waves propagating in cortical bone is important for diagnosing bone quality. Using fewer transmitters and receivers is desirable for low-cost diagnosis. Thus, in this study, we propose a method based on an adaptive beamforming technique to characterize Lamb waves using a single transmitter-receiver pair. We begin by estimating the longitudinal and shear wave velocities as these are the primary determinants of the Lamb wave transfer function. The frequency domain interferometry (FDI) with Capon method, which is one of the adaptive signal processing methods, accurately estimates the position of the wave having the same frequency components as the reference wave. The high-frequency components of the zeroth-order modes of the Lamb wave are almost non-dispersive. Therefore, we use the transmitted wave as the reference and apply the Capon method to the high-frequency components where the phase velocity is converge to the Rayleigh wave velocity. Because the Rayleigh wave velocity is determined by the longitudinal and shear wave velocities, we can estimate candidates of the velocities by using estimated Rayleigh wave velocity. Finally, we calculate the transfer function using a least squares method (LSM) applied to the estimated velocities. We conduct 2-D numerical simulations using a semi-analytical finite element method. The center frequency of the transmitted wave is 1.0 MHz, the thickness of cortical bone is 5.0 mm, signal-to-noise ratio is 20 dB, longitudinal and shear wave velocities are 4430 and 2120 m/s, respectively, and the distance from the transmitter to the receiver is 21 mm. We use eight candidates in the fitting process using LSM. The predicted longitudinal and shear wave velocities were 4458 and 2116 m/s, respectively, and the residue normalized with respect to the received signal intensity was −16 dB. The proposed method accurately predicts the waveforms and longitudinal and shear wave velocities.

A fast signal processing technique for characterizing Lamb wave propagation in viscoelastic cortical long bones using one transmitter and two receivers

Axial transmission technique is useful in assessing bone quality. Most conventional techniques require multiple transmitters and receivers. Decreasing the number of elements and computational complexity are desired for low-cost diagnosis. We propose a technique that characterizes the Lamb wave using one transmitter and two receivers. Because the transfer function of the Lamb wave is mainly determined by the longitudinal wave velocity (LWV), shear wave velocity (SWV), and bone thickness, these parameters may be determined in a fitting procedure using the transfer function. The number of combinations of these parameters is enormous, and we thus select the candidates for these parameters using spatial domain interferometry. The method estimates the phase velocity of zero-th order anti-symmetric mode. We applied the proposed method to numerical simulation data generated by the semi-analytical finite element method. We employed a viscoelastic plate with LWV, SWV, and thickness of 2120 m/s, 4430 m/s, and 1.0 mm...

Computational complexity reduction techniques for real-time and high-resolution medical ultrasound imaging using the beam-space Capon method

The beam-space (BS) Capon method is an adaptive beamforming technique that reduces computational complexity. However, the complexity is not low enough for real-time imaging. Reducing the number of time-delay and transformation processes from element-space to BS signal processing is required. We propose a technique that replaces the time-delay processes by the multiplication of steering vectors and covariance matrices. In addition, we propose a compensation technique for estimating the intensity accurately. In an experimental study using a 2.0 MHz transmission frequency on a 15 × 10.4 mm2 region of interest, the first side-lobe level, the −6 dB beam width, the intensitys estimation error, and the calculation time of the conventional method were −15 dB, 0.70 mm, 3.2 dB, and 656 ms. Those of the proposed method were −17 dB, 0.36 mm, 1.6 dB, and 81 ms, respectively. Using our method on three CPUs achieves imaging of 37 frames/s.