Blood flow evaluation in high-frequency, 40 MHz imaging: A comparative study of four vector velocity estimation methods

Ultrasonic imaging is often used to estimate blood flow velocity. Currently, estimates are carried out using Doppler-based techniques. However, there are a number of shortcomings such as the limited spatial resolution and the inability to estimate longitudinal flows. Thus, alternative methods have been proposed to overcome them. Difficulties are notably encountered with high-frequency imaging systems that use swept-scan techniques. In this article, we propose to compare four vector velocity estimation methods that are complementary to Doppler, focusing on 40 MHz, high-frequency imaging. The goal of this study is to evaluate which method could circumvent the limitations of Doppler methods for evaluation of microcirculation, in the vessels having diameter on the order of 1 mm. We used two region-based approaches, one decorrelation-based approach and one spatiotemporal approach. Each method has been applied to seven flow sequences with various orientations and mean velocities. Four sequences were simulated with a system approach based on a 3D set of moving scatterers. Three experimental sequences were carried out by injecting blood-mimicking fluid within a gelatin phantom and then acquiring images with Visualsonics, Vevo 660 system. From velocity estimates, several performance criteria such as the normalized mean error or the normalized mean standard deviation were defined to compare the performance of the four estimators. The results show that region-based methods are the most accurate exhibiting mean errors less than 10% and mean standard deviation less than 13%. However, region-based approaches are those that require the highest calculative cost compared to the decorrelation-based method, which is the fastest. Finally, the spatiotemporal approach appeared to be a trade-off in terms of computational complexity and accuracy of estimates. It provides estimates with errors less than 10% for mean velocity and the CPU time is approximately 17 s for a ROI of size 40 * 80 pixels.     

The preliminary evaluation of a 1 MHz ultrasound probe for measuring the elastic anisotropy of human cortical bone

Our objective is to evaluate an ultrasound probe for measurements of velocity and anisotropy in human cortical bone (tibia). The anisotropy of cortical bone is a known and mechanically relevant property in the context of osteoporotic fracture risk. Current in vivo quantitative ultrasound devices measuring the velocity of ultrasound in long bones can only be applied in the axial direction. For anisotropy measurements a second direction for velocity measurements preferably perpendicular to the axial direction is necessary. We developed a new ultrasound probe which permits axial transmission measurements with a simultaneous second perpendicular direction (tangential). Anisotropy measurements were performed on isotropic and anisotropic phantoms and two excised human female tibiae (age 63 and 82). Anisotropy ratios (AI; ratio of squared ultrasound velocities in the two directions) were for the isotropic phantom 1.06 ± 0.01 and for the anisotropic phantom 1.14 ± 0.03 (mean ± standard deviation). AI was 1.83 ± 0.29 in the tibia from the older donor and 1.37 ± 0.18 in the tibia from the younger donor. The AIs were in the expected range and differed significantly (p < 0.05, t-test) between the tibiae. Measured sound velocities were reproducible (mean standard deviation of short time precision of both channels for phantom measurements 31 m/s) and in agreement with reported velocities of the phantom material. Our results document the feasibility of anisotropy measurements at long bones using a single probe. Further improvements in the design of the probe and tests in vivo are warranted. If this approach can be evaluated in vivo an additional tool for assessing the bone status is available for clinical use.     

Ultrasonic distance and velocity measurement using a pair of LPM signals for cross-correlation method: improvement of Doppler-shift compensation and examination of Doppler velocity estimation

Real-time distance measurement of a moving object with high accuracy and high resolution using an ultrasonic wave is difficult due to the influence of the Doppler effect or the limit of the calculation cost of signal processing. An over-sampling signal processing method using a pair of LPM signals has been proposed for ultrasonic distance and velocity measurement of moving objects with high accuracy and high resolution. The proposed method consists of cross correlation by single-bit signal processing, high-resolution Doppler velocity estimation with wide measurement range and low-calculation-cost Doppler-shift compensation. The over-sampling cross-correlation function is obtained from cross correlation by single-bit signal processing with low calculation cost. The Doppler velocity and distance of the object are determined from the peak interval and peak form in the cross-correlation function by the proposed method of Doppler velocity estimation and Doppler-shift compensation. In this paper, the proposed method of Doppler-shift compensation is improved. Accuracy of the determined distance was improved from approximately within ±140 μm in the previous method to approximately within ±10 μm in computer simulations. Then, the proposed method of Doppler velocity estimation is evaluated. In computer simulations, accuracy of the determined Doppler velocity and distance were demonstrated within ±8.471 mm/s and ±13.87 μm. In experiments, Doppler velocities of the motorized stage could be determined within ±27.9 mm/s.     

Acoustic characterization of a new trisacryl contrast agent. Part II: Flow phantom study and in vivo quantification

The biocompatible trisacryl particles (TMP) are made of a cross-linked acrylic copolymer. Their inherent acoustic properties, studied for a contrast agent application, have been previously demonstrated in a in vitro Couette device. To measure their acoustic behaviour under circulating blood conditions, the TMP backscatter enhancement was further evaluated on a home-made flow phantom at different TMP doses (0.12-15.6 mg/ml) suspended in aqueous and blood media, and in nude mice (aorta and B16 grafted melanoma). Integrated backscatter (IB) was measured by spectral analysis of the Doppler signals recorded from an ultrasound system (Aplio^®) combined with a 12-MHz probe. Doppler phantom experiments revealed a maximal IB of 17 ± 0.88 dB and 7.5 ± 0.7 dB in aqueous and blood media, respectively. IB measured on mice aorta, in pulsed Doppler mode, confirmed a constant maximal value of 7.29 ± 1.72 dB over the first minutes after injection of a 7.8 mg/ml TMP suspension. Following the injection, a 60% enhancement of intratumoral vascularization detection was observed in power Doppler mode. A preliminary histological study revealed inert presence of some TMP in lungs 8 and 16 days after injection.Doppler phantom experiments on whole blood allowed to anticipate the in vivo acoustic behaviour. Both protocols demonstrated TMP effectiveness in significantly increasing Doppler signal intensity and intratumoral vascularization detection. However, it was also shown that blood conditions seemed to shadow the TMP contrast effect, as compared to in vitro observations. These results encourage further investigations on the specific TMP targeting and on their bio-distribution in the different tissues.     

Theoretical and phantom based investigation of the impact of sound speed and backscatter variations on attenuation slope estimation

Quantitative ultrasound features such as the attenuation slope, sound speed and scatterer size, have been utilized to evaluate pathological variations in soft tissues such as the liver and breast. However, the impact of variations in the sound speed and backscatter due to underlying fat content or fibrotic changes, on the attenuation slope has not been addressed. Both numerical and acoustically uniform tissue-mimicking experimental phantoms are used to demonstrate the impact of sound speed variations on attenuation slope using clinical real-time ultrasound scanners equipped with linear array transducers. Radiofrequency data at center frequencies of 4 and 5 MHz are acquired for the experimental and numerical phantoms respectively. Numerical phantom sound speeds between 1480 and 1600 m/s in increments of 20 m/s for attenuation coefficients of 0.3, 0.4, 0.5, 0.6, and 0.7 dB/cm/MHz are simulated. Variations in the attenuation slope when the backscatter intensity of the sample is equal, 3 dB higher, and 3 dB lower than the reference is also evaluated. The sound speed for the experimental tissue-mimicking phantoms were 1500, 1540, 1560 and 1580 m/s respectively, with an attenuation coefficient of 0.5 dB/cm/MHz. Radiofrequency data is processed using three different attenuation estimation algorithms, i.e. the reference phantom, centroid downshift, and a hybrid method. In both numerical and experimental phantoms our results indicate a bias in attenuation slope estimates when the reference phantom sound speed is higher (overestimation) or lower (underestimation) than that of the sample. This bias is introduced via a small spectral shift in the normalized power spectra of the reference and sample with different sound speeds. The hybrid method provides the best estimation performance, especially for sample attenuation coefficient values lower than that of the reference phantom. The performance of all the methods deteriorates when the attenuation coefficient of the reference phantom is lower than that of the sample. In addition, the hybrid method is the least sensitive to sample backscatter intensity variations.     

Water line parameters for weak lines in the range 7400-9600 cm

A long pathlength, Fourier transform spectrum of pure water vapour [J. Mol. Spectrosc. 211 (2002) 169] has been fitted and analysed. Line centres, intensities, and self-broadening parameters have been obtained for about 3900 lines with intensities less then 1 × 10⁻²⁴ cm/molecule. Transitions to 240 newly observed energy levels of H₂O have been identified. 855 lines have been assigned to three other major isotopologues of water. It is pointed out that a new intensity measurements are needed for the stronger lines in this region.     

Suitability of laser Doppler velocimetry for the calibration of pressure microphones

The details of a new approach for absolute calibration of microphones, based on the direct measurement of acoustic particle velocity using laser Doppler velocimetry (LDV), are presented and discussed. The calibration technique is carried out inside a tube in which plane waves propagate and closed by a rigid termination. The method developed proposes to estimate the acoustic pressure with two velocity measurements and a physical model. Minimum theoretical uncertainties on the estimated pressure and minimum measurable pressure are calculated from the Cramer Rao bounds on the estimated acoustic velocity amplitude and phase. These uncertainties and the minimum measurable pressure help to optimize the experimental set up. Acoustic pressure estimations performed with LDV are compared with acoustic pressures obtained with a reference microphone. Measurements lead to a minimum bias of 0.006 dB and a minimum uncertainty of 0.013 dB on the acoustic pressure estimation for frequencies 1360 Hz and 680 Hz.     

A Fabry-Perót type resonator tunable below 2 GHz for use in time domain rotational spectroscopy: Application to the measurement of the radio frequency spectra of bromobenzene and iodobenzene

Two aluminum mirrors with radii of 203.2 mm and radii of curvature also of 203.2 mm have been used to construct a tunable Fabry-Perót type resonator with Q values of ∼200 at frequencies as low as 500 MHz. The resonator has been incorporated into a pulsed nozzle, Fourier transform, Balle-Flygare spectrometer typically used for recording pure rotational spectra in the microwave region. The resonator design allows the instrument to access the radio frequency region (?3 GHz) of the electromagnetic spectrum. The spectrometer is of use in (i) recording low J transitions of large asymmetric molecules where the spectra are often greatly simplified compared to higher frequency regions; (ii) measuring hyperfine constants for heavy molecules with higher accuracy than may be obtained at higher frequencies where hyperfine structure may not be resolvable; and (iii) provides further synchronicity between laboratory based measurements and radio astronomy in the 30 cm region. The resonators use is illustrated by recording the rotational spectra of bromobenzene and iodobenzene. The lowest ΔJ = +1 transition for iodobenzene has been observed at 1130.5292(10) MHz.     

Research on laser Doppler velocimeter for vehicle self-contained inertial navigation system

An idea of using laser Doppler velocimeter (LDV) to measure the velocity for the vehicle inertial navigation system was put forward. The principle of measuring its own velocity with laser Doppler technique was elaborated and reference-beam LDV was designed. Then Doppler signal was processed by tracking filter, frequency spectrum refinement and frequency spectrum correction algorithm. The result of theory and experiment showed that the reference-beam LDV solved the problem that dual-beam LDV cannot be used for measuring when the system was out of focus. Doppler signal was tracked so that signal-to-noise ratio was improved, and the accuracy of the system was enhanced by the technology of frequency spectrum refinement and correction. The measurement mean error was less than 1.5% in velocity range of 0-30 m/s.     

Feasibility of using ultrasound contrast agents to increase the size of thermal lesions induced by non-focused transducers: in vitro demonstration in tissue mimicking phantom

Miniature flat ultrasound transducers have shown to be effective for a large variety of thermal therapies, but the associated superficial heating implicates developing original strategies in order to extend therapeutic depth. The goal of the present paper is to use ultrasound contrast agents (UCA) to increase remote attenuation and heating. Theoretical simulations demonstrated that increasing attenuation from 0.27 to 0.8 Np/cm at 10 MHz beyond a distance of 18 mm from the transducer should result in longer thermal damages due to protein coagulation in a tissue mimicking phantom. Contrast agents (BR14, Bracco, Plan-les-Ouates, Switzerland) were embedded in thermo-sensitive gel and attenuations ranging from 0.27 to 1.33 Np/cm were measured at 10 MHz for concentrations of BR14 between 0 and 4.8%. Thermal damages were then induced in several gels, which had different layering configurations. Thermal damages, 12.8 mm in length, were obtained in homogeneous gels. When mixing contrast agents at a concentration of 3.2% beyond a first 18 mm-thick layer of homogeneous gel, the thermal damages reached 21.5 mm in length. This work demonstrated that contrast agents can be used for increasing attenuation remotely and extending therapeutic depth induced by a non-focused transducer. Additional work must be done in vivo in order to verify the remote-only distribution of bubbles and associated increase in attenuation.     

Progress in developing a thermal method for measuring the output power of medical ultrasound transducers that exploits the pyroelectric effect

Progress in developing a new measurement method for ultrasound output power is described. It is a thermal-based technique with the acoustic power generated by a transducer being absorbed within a specially developed polyurethane rubber material, whose high absorption coefficient ensures energy deposition within a few mm of the ultrasonic wave entering the material. The rate of change of temperature at the absorber surface is monitored using the pyroelectric voltage generated from electrodes disposed either side of a 60 mm diameter, 0.061 mm thick membrane of the piezoelectric polymer polyvinylidene fluoride (pvdf) bonded to the absorber. The change in the pyroelectric output voltage generated by the sensor when the transducer is switched ON and OFF is proportional to the delivered ultrasound power. The sensitivity of the device is defined as the magnitude of these switch voltages to a unit input stimulus of power (watt). Three important aspects of the performance of the pyroelectric sensor have been studied. Firstly, measurements have revealed that the temperature dependent sensitivity increases over the range from approximately 20 °C to 30 °C at a rate of +1.6% °C⁻¹. Studies point to the key role that the properties of both the absorbing backing layer and pvdf membrane play in controlling the sensor response. Secondly, the high sensitivity of the technique has been demonstrated using an NPL Pulsed Checksource, a 3.5 MHz focused transducer delivering a nominal acoustic power level of 4 mW. Finally, proof-of-concept of a new type of acoustic sensor responding to time-averaged intensity has been demonstrated, through fabrication of an absorber-backed hydrophone of nominal active element diameter 0.4 mm. A preliminary study using such a device to resolve the spatial distribution of acoustic intensity within plane-piston and focused 3.5 MHz acoustic fields has been completed. Derived beam profiles are compared to conventional techniques that depend on deriving intensity from acoustic pressure measurements made using the sensor as a calibrated hydrophone.     

A study of the interaction between perylene and the TiO2(1 1 0)-(1 × 1) surface-based on XPS, UPS and NEXAFS measurements

The interaction between a semi-large aromatic hydrocarbon compound (perylene) and the TiO₂(1 1 0)-(1 × 1) surface under ultra high vacuum conditions has been probed by X-ray photoemission spectroscopy (XPS), ultraviolet photoemission spectroscopy (UPS) and near-edge X-ray absorption fine structure (NEXAFS) methods. UPS measurements of the adsorbate system have been compared with an experimental UPS spectrum of perylene in the gas phase and a calculated spectrum obtained by means of density functional theory (DFT) methods. NEXAFS results of perylene molecules adsorbed on TiO₂(1 1 0)-(1 × 1) were compared with data from an α-phase perylene single crystal. A novel analysis of the valence data has been employed to show that no strong chemical interaction takes place between perylene and the TiO₂(1 1 0)-(1 × 1) surface. Furthermore, angle-dependent NEXAFS measurements and the growth curve results suggest that the perylene molecules are oriented flat down onto the TiO₂ substrate due to weak van der Waals interactions.     

Study of the switching phenomena of TlGaS2 single crystals

Single crystals of TlGaS₂ were prepared by a special modified Bridgman technique and used to investigate the switching phenomena. The particular interest shown in switching studies of p-type TlGaS₂ compound is associated with the possibility of its uses as an effective switching and memory elements in electronic devices. The switching effect observed in such crystal shows a memory character. Using a crystal holder and cryostat we measured the switching phenomenon at different ambient conditions such as temperature, light illumination as well as sample thickness. Pronounced parameters for switching for sample of thickness 0.17 cm were determined from the experimental data such as threshold voltage V_th = 400 V, threshold current I_th = 37 μA, holding voltage V_h = 350 V, holding current I_h = 42.3 × 10⁻⁴ A, threshold power P_th = 1.48 × 10⁻² W, threshold field E_th = 196.429 V/cm as well as the ratio between the resistance in the off state R_OFF to the resistance in the conducting state R_ON as 130.253. The factors affecting these parameters have also been investigated.     

Enhanced non-linear optical response in hybrid GSPN crystals: Structural, optical and dielectric analysis

Transparent and well defined crystals of GSPN series have been grown with alpha-glycine in simultaneous presence of two nitrates (NaNO₃ and KNO₃). Transparent and well defined crystals (22 mm × 13 mm × 5 mm) have been obtained in 3-4 weeks time by slow cooling. Addition of a strong acid (0.5% HCl) during growth is seen to enhance solubility, SHG efficiency by 2.78 times, Meyer's hardness index by 1.7 times and M.P by 42 °C of that of GSPN crystals. The SHG efficiency of acid added-GSPN has been found to be more than that of KDP crystal. GSPN is found to crystallize in orthorhombic symmetry and the presence of chemical components/groups has been identified by CHNS, EDAX and NMR analysis. Comparative FTIR and Laser Raman analysis shows the presence of active peaks indicating the molecule with a lack of center of symmetry. The UV spectrum shows existence of wide transparency window suitable for optoelectronic applications with band gap energy of about 5.72 eV. The crystals exhibit linear I-V characteristic followed by switching at 41 V/cm. The dielectric loss was seen to decrease exponentially with applied frequency from 100 Hz to 1 MHz.     

Simultaneous measurements of multiple flow velocity components using frequency modulated lasers and a single molecular absorption cell

Since unsteady, complex flow phenomena play an important role, optical measurements techniques are required for flow investigations, which provide simultaneous measurements of multiple velocity components. Doppler global velocimetry has this potential. It is a flow measurement technique, where the Doppler shift of scattered light is measured by a molecular absorption cell for frequency-to-intensity conversion. However, novel Doppler global velocimeters with laser frequency modulation were only used for single component measurements yet. In order to enhance such a system for the simultaneous measurement of multiple components, a concept based on frequency division multiplexing is introduced for the first time. Besides multiple lasers, only a single molecular absorption cell and a single detector unit is required. Two-component measurements of velocity profiles from nozzle flows are presented as a proof of principle. The designed measurement system provides high measurement rates of up to 20 kHz, which is three orders of magnitude higher than for typical Doppler global velocimetry setups.     

Multilayered perceptron neural networks to compute energy losses in magnetic cores

This paper presents a new approach based on multilayered perceptrons (MLPs) to compute the specific energy losses of toroidal wound cores built from 3% SiFe 0.27 mm thick M4, 0.1  and 0.08 mm thin gauge electrical steel strips. The MLP has been trained by a back-propagation and extended delta-bar-delta learning algorithm. The results obtained by using the MLP model were compared with a commonly used conventional method. The comparison has shown that the proposed model improved loss estimation with respect to the conventional method.     

Comparison of phase velocity in trabecular bone mimicking-phantoms by time domain numerical (EFIT) and analytical multiple scattering approaches

The corrected Waterman-Truell model and the Elastodynamic Finite Integration Technique were used to analyze the ultrasonic wave dispersion in trabecular bones mimicking phantoms. A simple two-phase model of the trabecular bone is assumed; the trabeculae structure and the bone marrow. The phase velocity for frequencies within the range from 400 kHz to 800 kHz were computed for different scatterer arrays varying their dimensions and number. The theoretical and numerical results were compared to experimental published data, obtained from a mimicking phantom composed by a periodic array of nylon shreds (trabeculae array) immersed in a water tank. Our results showed an excellent consistency when compared to experimental data. The negative dispersions of −8.48 m/s/MHz and −9.16 m/s/MHz were computed by the multiple scattering method and the numerical approach, respectively, where the latter is closer to the experimental dispersion of −12.09 m/s/MHz. Similar result has been reported in the literature, where the dispersion predicted by the Generalized Self-Consistent Method [J. Acoust. Soc. Am. 124 (2008) 4047] is −9.96 m/s/MHz.     

Highly accurate laser wavelength meter based on Doppler effect

We have built an accurate wavelength meter based on a Michelson interferometer characterized by a high stability velocity moving system. The unknown wavelength is determined from the Doppler frequency shifts of the output beams of the Michelson interferometer. The reference laser is a frequency stabilized helium-neon laser. A counting resolution of 2.6 × 10⁻⁹ for an integration time of 30 s has been obtained. The apparatus has been used to determine the wavelength of a second frequency stabilized helium-neon laser and the result has been compared to those given by two different methods: frequency beating in regards to the national reference and using a commercially available scanning-Michelson wavemeter. Taking into account the statistical errors, we achieved a relative accuracy on the unknown wavelength of 6.4 × 10⁻⁸ at 1σ.     

Mid-infrared,wide-tunable,continuous-wave Nd:YVO4/PPMgLN intracavity optical parametric oscillator

We obtain a continuous-wave (CW) Nd:YVO₄/PPMgLN intracavity singly resonant optical parametric oscillator (ISRO). The threshold value of the CW-ISRO system is only 2.8 W at 808 nm. When the pump power is 5.5 W, an idle laser output power of 410 mW, and a signal laser output power of 345 mW have been achieved. By changing the grating periods of the domain structure on the PPMgLN wafer, we enable a wide-tunable mid-infrared spectrum of 2.95–4.16 μm and near-infrared spectrum of 1.43–1.66 μm.     

Synthesis,growth and characterization of non-linear optical material: l-Tryptophan p-nitrophenol (LTPNP) single crystal

A new nonlinear optical organic crystal l-tryptophan p-nitrophenol (LTPNP) of dimension 19 mm × 2 mm × 1.5 mm has been grown from an aqueous solution for the first time by slow evaporation technique at ambient temperature. The crystal structure of LTPNP was confirmed by single crystal X-ray diffraction. LTPNP crystallizes in non-centrosymmetric monoclinic system with space group P₂₁. The recorded FTIR spectrum confirms the presence of various functional groups in the grown crystal and confirms the formation of LTPNP. Thermal stability and melting temperature of the LTPNP crystal were identified from TG/DTA analysis. The optical absorption study confirms the suitability of the crystal for device applications. LTPNP exhibits SHG efficiency over 1.7 orders of magnitude higher than that of urea and 4 orders of magnitude higher than that of KDP.