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Purpose: In the receive beamforming of an ultrasonography system, a B-mode image is reconstructed by assuming an average speed of sound (SoS) as a constant value. In our previous studies, we proposed a method for estimating the average SoS based on the coherence factor (CF) and the reciprocal of phase variances of element signals in delay-and-sum (DAS) beamforming. In this paper, we investigate the accuracy of estimation of the average SoS for compound imaging., Methods: For this purpose, two numerical simulations were performed with k-Wave software. Also, the estimation methods based on the CF and the reciprocal were applied to in vivo data from the common carotid artery, and B-mode images were reconstructed using the estimated average SoS., Results: In the first numerical simulation using an inhomogeneous phantom, the relationship between the accuracy and the transmission angles for the estimation was investigated, and the root mean squared errors (RMSEs) of estimates obtained based on the CF and the reciprocal of the phase variance were 1.25 ± 0.09, and 0.765 ± 0.17% at the transmission sequence of steering angles of (- 10°, - 5°, 0°, 5°, 10°), respectively. In the second numerical simulation using a cyst phantom, lateral resolutions were improved by reconstructing the image using the estimates obtained using the proposed strategy (reciprocal). By the proposed strategy, improvement of the continuity of the lumen-intima interface in the lateral direction was observed in the in vivo experiment., Conclusion: Consequently, the results indicated that the proposed strategy was beneficial for estimation of the average SoS and image reconstruction., (© 2023. The Author(s), under exclusive licence to The Japan Society of Ultrasonics in Medicine.)

Purpose: The contrasts of flowing blood in in vitro experiments using porcine blood and in vivo measurements of human jugular veins were analyzed to demonstrate that the hemorheological property was dependent on the shear rate., Methods: Blood samples (45% hematocrit) suspended in saline or plasma were compared with examine the difference in viscoelasticity. Ultrafast plane-wave imaging at an ultrasonic center frequency of 7.5 MHz was performed on different steady flows in a graphite-agar phantom. Also, in vivo measurement was performed in young, healthy subjects and patients with diabetes. A spatiotemporal matrix of beamformed radio-frequency data was used for the singular value decomposition (SVD) clutter filter. The clutter-filtered B-mode image was calculated as the amplitude envelope normalized at the first frame in the diastolic phase to evaluate contrast. The shear rate was estimated as the velocity gradient perpendicular to the lateral axis., Results: Although nonaggregated erythrocytes at a high shear rate exhibited a low echogenicity, the echogenicity in the plasma sample overall increased due to erythrocyte aggregation at a low shear rate. In addition, the frequency of detection of specular components, defined as components beyond twice the standard deviation of a contrast map obtained from a clutter-filtered B-mode image, increased in the porcine blood at a high shear rate and the venous blood in healthy subjects versus patients with diabetes., Conclusion: The possibility of characterizing hemorheological properties dependent on the shear rate and diabetes condition was indicated using ultrafast plane-wave imaging with an SVD-based clutter filter., (© 2023. The Author(s), under exclusive licence to The Japan Society of Ultrasonics in Medicine.)

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., (© 2021. The Japan Society of Ultrasonics in Medicine.)

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.

Purpose: The backscatter coefficient (BSC) indicates the absolute scatterer property of a material, independently of clinicians and system settings. Our study verified that the BSC differed among the scanners, transducers, and beamforming methods used for quantitative ultrasound analyses of biological tissues., Methods: Measurements were performed on four tissue-mimicking homogeneous phantoms containing spherical scatterers with mean diameters of 20 and 30 µm prepared at concentrations of 0.5 and 2.0 wt%, respectively. The BSCs in the different systems were compared using ultrasound scanners with two single-element transducers and five linear high- or low-frequency probes. The beamforming methods were line-by-line formation using focused imaging (FI) and parallel beam formation using plane wave imaging (PWI). The BSC of each system was calculated by the reference phantom method. The mean deviation from the theoretical BSC computed by the Faran model was analyzed as the benchmark validation of the calculated BSC., Results: The BSCs calculated in systems with different properties and beamforming methods well concurred with the theoretical BSC. The mean deviation was below ± 2.8 dB on average, and within the approximate standard deviation (± 2.2 dB at most) in all cases. These variations agreed with a previous study in which the largest error among four different scanners with FI beamforming was 3.5 dB., Conclusion: The BSC in PWI was equivalent to those in the other systems and to those of FI beamforming. This result indicates the possibility of ultra-high frame-rate BSC analysis using PWI.

Purpose: Radio-frequency (RF) signals from the most dominant scatterer in a dermis, i.e., collagen fibers, are collected as backscattered signals. We aim to confirm the frequency dependence of the spatial distribution of features in ultrasound images, as well as the attenuation coefficient (AC) and backscatter coefficient (BSC) of skin tissue without [LE (-)] and with lymphedema [LE (+)]., Methods: Measurement samples (n = 13) were excised from human skin tissue with LE (-) and middle severity LE (+). A laboratory-made scanner and single-element concave transducers (range 9-47 MHz) were used to measure RF data. A localized AC was computed from the normalized power spectrum using the linear least squares technique. The reflector method and compensation technique of the attenuation of tissue were applied to calculate the BSC. In addition, effective scatterer diameter (ESD), effective acoustic concentration (EAC), and integrated BSC (IBS) were calculated from the BSC as the benchmark to differentiate LE (-) and LE (+) tissues., Results: High-frequency ultrasound displayed different echogenicity and texture compared between LE (-) and LE (+) in all transducers. The AC for LE (-) (0.22 dB/mm/MHz) and LE (+) (0.29 dB/mm/MHz) was comparable. BSC in LE (-) and LE (+) increased linearly with each transducer. The difference of intercept of the BSC between LE (-) and LE (+) indicated that both EAC and IBS of LE (+) were higher than that of LE (-). In contrast, ESD correlated with the slope of the BSC demonstrated the same tendency for both LE (-) and LE (+). These tendencies appeared for each transducer independent of the frequency bandwidth., Conclusion: Frequency independence of AC and BSC in LE (-) and LE (+) was confirmed. Several 9- to 19-MHz ultrasound beams are sufficient for BSC analysis to discriminate LE (-) and LE (+) in terms of the penetration depth of the ultrasound.

Serum, cerebrospinal fluid (CSF), scala vestibuli perilymph, and scala tympani perilymph were collected from 85 normal guinea pigs both before and after i.v. administration of glycerol (1 ml/kg), and the sodium and potassium concentrations were assessed using a microflame photometer. Marked increases in sodium concentrations were observed in scala tympani perilymph and CSF, while there was a slight decrease in the serum and there was no significant change in scala vestibuli perilymph. These increases in sodium concentrations are considered to occur in the dehydration process in the body fluids mentioned above. On the other hand, increase in the potassium concentrations was found only in scala vestibuli perilymph and thus cannot be explained by simple dehydration process. This potassium elevation in scala vestibuli perilymph should be understood by further experiment on endolymph. It became evident that scala vestibuli perilymph differs from scala tympani perilymph and CSF in the dynamics of electrolytes after glycerol administration. In this regard, the nature of the scala vestibuli as fluid space should be studied in future. The above findings obtained in the present study may imply the significance in elucidating the glycerol effect on hearing of endolymphatic hydrops case.