Doppler shift equation and measurement errors affected by spatial variation of the speed of sound in sea water.

Underwater ultrasonic devices using the Doppler effect generally presuppose that the speed of sound is uniform in a propagation medium. Actually, however, the speed of sound in the sea varies with water depth, so that the assumption of such uniform speed has the potential to cause measurement errors. The present study is then involved in theoretically improving the conventional Doppler shift equation by taking into account the fact that the speed of sound is dependent on the propagation path. The study also evaluates measurement errors caused by spatial variation in the speed of sound. Interestingly, the theory predicts that only the speeds of sound at a sound source position and the target position affect the Doppler shift, and the error of the target speed, which is the value that is ultimately measured in ultrasonic Doppler devices, is inversely proportional to the ratio of the speed of sound near the target to that near the source. In order to validate the improved Doppler equation, experiments are conducted using a water tank. The measured Doppler shift data are in agreement with the theoretical predictions within the order of a few tens of hertz.

Feasibility of low-frequency ultrasound imaging using pulse compressed parametric ultrasound.

When using high-frequency (HF) ultrasound in sonography, attenuation due to the viscosity of the medium limits the available imaging depth, and strong reflection and scattering from hard tissue, such as bone, render biological diagnosis very difficult. In order to resolve these problems, the feasibility of low-frequency (LF) parametric ultrasound imaging with high directivity was explored in the present study. A pulse compression technique was applied to chirp-modulated parametric ultrasound waves in the frequency band of 100-500 kHz generated from modulated primary ultrasound waves with a center frequency of 2.8 MHz in order to improve the signal-to-noise ratio (SNR). Low-frequency ultrasound images of brass rods obtained using pulse compressed parametric ultrasound exhibit accurate target distances, a 3-mm range resolution, which agrees well with the theoretical value, and an 8-dB improvement in SNR. Parametric ultrasound imaging with pulse compression makes easy to separate overlapping targets in comparison with HF ultrasound imaging, and indicates the image with brightness independent of distance in comparison with directly radiated LF ultrasound imaging. These results reveal that pulse compressed LF parametric ultrasound is not only a useful method for improving the SNR and providing accurate distance measurements, but also enables imaging of overlapping targets.

In-vivo imaging of blood-brain barrier permeability using positron emission tomography with 2-amino-[3-11C]isobutyric acid.

OBJECTIVE: The blood-brain barrier (BBB) limits the entry of some therapeutics into the brain, resulting in reduced efficacy. BBB-opening techniques have been developed to enhance the entry into the brain. However, a noninvasive, highly sensitive and quantitative method for evaluating the changes in BBB permeability induced by such techniques is needed to optimize treatment protocols. We evaluated 2-amino-[3-C]isobutyric acid ([3-C]AIB) as a PET probe to quantify BBB permeability in model rats. METHODS: BBB opening was induced by a lipopolysaccharide injection or focused ultrasound (FUS) sonication. [3-C]AIB distribution in the brain was evaluated by autoradiography and PET and compared with that of Evans blue, a traditional BBB permeability marker. Kinetics of [3-C]AIB was compared with that of gadolinium-diethylenetriamine pentaacetic acid (Gd-DTPA)-enhanced MRI. The unidirectional blood-brain transfer constant (Ki) of [3-C]AIB was estimated using the Patlak plot. RESULTS: [3-C]AIB uptake in the lesion area was significantly higher than that in the control area and radioactivity colocalized with Evans blue in both models. [3-C]AIB uptake in the FUS-sonicated region decreased over time after sonication. The ratio of [3-C]AIB accumulation in the FUS-treated to the contralateral side increased during the experimental period, whereas that of the Gd-DTPA intensity reached a maximum at 10 min after injection and decreased thereafter. The [3-C]AIB Ki values were significantly higher in the lesion area than the control area. CONCLUSION: [3-C]AIB PET is a promising, highly sensitive and quantitative imaging method for assessment of BBB permeability.

Ultrasound field measurement using a binary lens.

Field characterization methods using a scattering target in the absence of a point-like receiver have been well described, in which scattering is recorded by a relatively large receiver located outside the field of measurement. Unfortunately, such methods are prone to artifacts caused by averaging across the receiver surface. To avoid this problem while simultaneously increasing the gain of a received signal, the present study introduces a binary plate lens designed to focus sphericallyspreading waves onto a planar region having a nearly-uniform phase proportional to that of the target location. The lens is similar to a zone plate, but modified to produce a bi-convexlike behavior, such that it focuses both planar and spherically spreading waves. A measurement device suitable for characterizing narrowband ultrasound signals in air is designed around this lens by coupling it to a target and planar receiver. A prototype device is constructed and used to characterize the field of a highly-focused 400-kHz in-air transducer along 2 radial lines. Comparison of the measurements with numeric predictions formed from nonlinear acoustic simulation showed good relative pressure correlation, with mean differences of 10% and 12% over the center 3-dB full-width at half-maximum drop and 12% and 17% over the 6-dB drop.

Application of the split-step Padé approach to nonlinear field predictions.

We herein propose a new theoretical approach for analyzing the nonlinear propagation of directive sound beams emitted from a planar piston source with a circular aperture. The proposed approach relies on the split-step Padé approximation, which is an efficient method for obtaining wide-angle one-way wave equations, especially in underwater acoustics. Despite including only two Padé terms in the expansion, the theory was applicable to a beam angle of up to ±40° relative to the main propagation direction, the angle of which is approximately twice that of the Khokhlov-Zabolotskaya-Kuznetsov equation, which is based on parabolic approximation. In order to demonstrate the effectiveness of the newly proposed theoretical approach, we performed an experiment using an airborne ultrasonic emitter with a circular aperture of 7.5cm in radius. We drove the emitter powerfully at a 36-kHz and 40-kHz bi-frequency signal and measured the beam patterns of the primary and secondary waves, such as parametric sounds within wide propagation angles. Excellent agreement between measured data and the corresponding numerical simulations supports the validity of the proposed model equations and the computational methods for their numerical solutions.

The feasibility of pulse compression by nonlinear effective bandwidth extension.

Chirp-encoded excitation has been utilized for increased signal-to-noise ratio (SNR) in both linear and harmonic imaging. In either case, it is necessary to isolate the relevant frequency band to avoid artifacts. In contrast, the present study isolates and then combines the fundamental and the higher harmonics, treating them as a single, extended bandwidth. Pulse-inverted sum and difference signals are first used to isolate even and odd harmonics. Matched filters specific to the source geometry and the transmit signal are then separately applied to each harmonic band. Verification experiments are performed using up to the third harmonic resulting from an underwater chirp excitation. Analysis of signal peaks after scattering from a series of steel and nylon wires indicates increased compression using the extended bandwidth, as compared to well-established methods for fundamental and second harmonic chirp compression. Using third harmonic bands, a mean pulse width of 56% relative to fundamental compression and 48% relative to second harmonic compression was observed. Further optimization of the compression by altering the transmission indicated 17% additional reduction in the pulse width and a 47% increase in peak-to-sidelobe ratio. Overall, results establish the feasibility of extended bandwidth signal compression for simultaneously increasing SNR and signal resolution.

Simulation of sound field in a tissue medium generated by a concave spherically annular transducer.

The concave spherically annular transducer is regarded as a negative and a positive concave spherical transducer, and the spheroidal beam equation is used to simulate the linear and nonlinear sound field in a tissue medium generated by this transducer. It is found that the acoustic focus of the ring does not coincide with the acoustic focus of its central part. If the width of the ring increases, its acoustic focus will move toward the geometric focus and the amplitudes of nonlinear harmonics will increase obviously. If there are several coaxial rings placed on the concave spherical surface, more than one peak will appear along the axial direction for the fundamental, and high harmonics focus better. The distribution of sound field will change with the number and the excited signals of rings, so it maybe is a potential approach to treat locally big tumors.

Theoretical and experimental examination of near-field acoustic levitation.

A planar object can be levitated stably close to a piston sound source by making use of acoustic radiation pressure. This phenomenon is called near-field acoustic levitation [Y. Hashimoto et al., J. Acoust. Soc. Am. 100, 2057-2061 (1996)]. In the present article, the levitation distance is predicted theoretically by numerically solving basic equations in a compressible viscous fluid subject to the appropriate initial and boundary conditions. Additionally, experiments are carried out using a 19.5-kHz piston source with a 40-mm aperture and various aluminum disks of different sizes. The measured levitation distance agrees well with the theory, which is different from a conventional theory, and the levitation distance is not inversely proportional to the square root of the surface density of the levitated disk in a strict sense.