Electromagnetic acoustic imaging

Electromagnetic acoustic imaging (EMAI) is a new imaging technique that uses long-wavelength RF electromagnetic (EM) waves to induce ultrasound emission. Signal intensity and image contrast have been found to depend on spatially varying electrical conductivity of the medium in addition to conventional acoustic properties. The resultant conductivity- weighted ultrasound data may enhance the diagnostic performance of medical ultrasound in cancer and cardiovascular applications because of the known changes in conductivity of malignancy and blood-filled spaces. EMAI has a potential advantage over other related imaging techniques because it combines the high resolution associated with ultrasound detection with the generation of the ultrasound signals directly related to physiologically important electrical properties of the tissues. Here, we report the theoretical development of EMAI, implementation of a dual-mode EMAI/ultrasound apparatus, and successful demonstrations of EMAI in various phantoms designed to establish feasibility of the approach for eventual medical applications.

Reliable estimation of virtual source position for SAFT imaging

The synthetic aperture focusing technique (SAFT), employing a scanned focused transducer as a virtual source, is commonly used to image flaws in immersion testing. The position of a virtual source is estimated from rays emitted from the rim of a focused transducer. However, it is often found that the virtual source position cannot be uniquely determined because of severe focal spot aberration at the focal zone. Based on an analysis of the energy radiated from the focused transducer and the refracted energy varied with the incident angle of ultrasound, we propose that paraxial rays emitted from the focused transducer are the best for estimating the position of a virtual source for incorporation into SAFT. This study results also shows that by using this simple virtual source position estimation for SAFT, the axial resolution and SNR of the reconstructed image can be greatly improved. This new approach minimizes the effect of such factors as refraction at high-velocity-contrast interfaces, distance of the transducer to the couplant-specimen interface, and the focal length of a focused transducer, which may cause focal spot aberration resulting in decreased sensitivity in SAFT imaging.

Ultrasonic Doppler measurements of blood flow velocity of rabbit retinal vessels using a 45-MHz needle transducer

BACKGROUND

The purpose of this study is to measure blood flow velocity of rabbit retinal vessels using a 45-MHz ultrasonic Doppler system with a needle transducer.

METHODS

A high-frequency pulsed Doppler system that utilizes a 45-MHz PMN-PT needle transducer was developed to measure retinal blood flow velocity in situ. The pulsed Doppler allowed the differentiation of retinal from choroidal blood flow velocity. The needle transducer was inserted into the vitreous cavity through a 20-gauge incision port to access the retinal vessels. The first phase of the experiment evaluated the reproducibility of the measurements. The second phase measured velocities at four positions from the optic disc edge to the distal part of each vessel in nine eyes for the temporal and six eyes for the nasal portions. The angle between the transducer and the retinal vessel at each site was measured in enucleated rabbit eyes to estimate and compensate for measurement errors.

RESULTS

In the first phase, the average measurement error was 5.97 +/- 1.34%. There was no significant difference comparing all eyes. In the second phase, the velocities gradually slowed from the disc edge to the distal part, and temporal velocities were faster than nasal velocities at all measurement sites.

CONCLUSION

This study demonstrated the feasibility of reliably measuring retinal blood flow velocity using a 45-MHz ultrasonic Doppler system with a needle transducer.

Multiple matching scheme for broadband 0.72Pb(Mg(13)Nb(23))O(3)-0.28PbTiO(3) single crystal phased-array transducer

Lead magnesium niobate-lead titanate single crystal 0.72Pb(Mg(13)Nb(23))O(3)-0.28PbTiO(3) (abbreviated as PMN-PT) was used to fabricate high performance ultrasonic phased-array transducer as it exhibited excellent piezoelectric properties. In this paper, we focus on the design and fabrication of a low-loss and wide-band transducer for medical imaging applications. A KLM model based simulation software PiezoCAD was used for acoustic design of the transducer including the front-face matching and backing. The calculated results show that the -6 dB transducer bandwidth can be improved significantly by using double lambda8 matching layers and hard backing. A 4.0 MHz PMN-PT transducer array (with 16 elements) was fabricated and tested in a pulse-echo arrangement. A -6 dB bandwidth of 110% and two-way insertion loss of -46.5 dB were achieved.

Self-separated hydrothermal lead zirconate titanate thick films for high frequency transducer applications

Using a simple rapid heating process, Pb(Zr(0.52)Ti(0.48))O(3) (PZT) thick films prepared by hydrothermal method were separated from a Ti substrate. Scanning electron microscopy (SEM) revealed that the self-separated films were crack-free. After solution infiltration and high temperature annealing, the PZT thick films were shown to possess good electric properties. At 1 kHz, the dielectric constant and the loss were 593 and 0.05, respectively. The remnant polarization was 30.0 muCcm(2) at room temperature. A high frequency single element ultrasound transducer fabricated with these films showed a bandwidth at -6 dB of 73% at a center frequency of 67 MHz.

Lead-free KNLNT piezoelectric ceramics for high-frequency ultrasonic transducer application

This paper presents the latest development of a lead-free piezoelectric ceramic and its application to transducers suitable for high-frequency ultrasonic imaging. A lead-free piezoelectric ceramic with formula of (K(0.5)Na(0.5))(0.97)Li(0.03)(Nb(0.9) Ta(0.1))O(3) (abbreviated as KNLNT-0.03/0.10) was fabricated and characterized. The material was found to have a clamped dielectric constant epsilon(33)(S)/epsilon(0)=890, piezoelectric coefficient d(33)=245 pC/N, electromechanical coupling factor k(t)=0.42 and Curie temperature T(c)>300 degrees C. High-frequency (40 MHz) ultrasound transducers were successfully fabricated with the lead-free material. A representative lead-free transducer had a bandwidth of 45%, two-way insertion loss of -18 dB. This performance is comparable to reported performances of popular lead-based transducers. The comparison results suggest that the lead-free piezoelectric material may serve as an alternative to lead-based piezoelectric materials for high-frequency ultrasonic transducer applications.

Piezoelectric PZT thick films on LaNiO(3) buffered stainless steel foils for flexible device applications

In this paper, we report on 4.5µm piezoelectric Pb(Zr(0.52)Ti(0.48))O(3) (PZT) thick films deposited on flexible stainless steel (SS) foils with LaNiO(3) (LNO) buffer layers using a ceramic powder/sol-gel solution modified composite method. The polycrystalline thick films show a hysteresis loop at an applied electric field of 900 kV cm(-1) with remanent polarization and coercive electric field values of 27µC cm(-2) and 85 kV cm(-1), respectively. At 1 kHz, the dielectric constant is 653 and the dielectric loss is 0.052. The leakage current density of the film is lower than 1.55 × 10(-5) Acm(-2) over the range of 0 to ±150V. The conduction current shows ohmic behaviour at a low electric field and space-charge-limited current characteristics at a high electric field.

Hybrid Optical-Ultrasonic Technique for Biomedical Diagnostics

We report the development of a diagnostic system combining time-resolved fluorescence spectroscopy and ultrasound backscatter microscopy and its application in diagnosis of tumors and atherosclerotic disease. This system allows for concurrent evaluation of distinct compositional, functional, and micro-anatomical features of normal and diseased tissues.

Realtime photoacoustic microscopy of murine cardiovascular dynamics

Non-invasive visualization of cardiovascular dynamics in small animals is challenging due to their rapid heart-rates. We present a realtime photoacoustic imaging system consisting of a 30-MHz ultrasound array transducer, receive electronics, a high-repetition-rate laser, and a multicore-computer, and demonstrate its ability to image optically-absorbing structures of the beating hearts of young athymic nude mice at rates of approximately 50 frames per second with 100 microm x 25 microm spatial resolution. To our knowledge this is the first report of realtime photoacoustic imaging of physiological dynamics.

Realtime photoacoustic microscopy of murine cardiovascular dynamics

Non-invasive visualization of cardiovascular dynamics in small animals is challenging due to their rapid heart-rates. We present a realtime photoacoustic imaging system consisting of a 30-MHz ultrasound array transducer, receive electronics, a high-repetition-rate laser, and a multicore-computer, and demonstrate its ability to image optically-absorbing structures of the beating hearts of young athymic nude mice at rates of approximately 50 frames per second with 100 microm x 25 microm spatial resolution. To our knowledge this is the first report of realtime photoacoustic imaging of physiological dynamics.

Multifunctional catheters combining intracardiac ultrasound imaging and electrophysiology sensing

A family of 3 multifunctional intracardiac imaging and electrophysiology (EP) mapping catheters has been in development to help guide diagnostic and therapeutic intracardiac EP procedures. The catheter tip on the first device includes a 7.5 MHz, 64-element, side-looking phased array for high resolution sector scanning. The second device is a forward-looking catheter with a 24-element 14 MHz phased array. Both of these catheters operate on a commercial imaging system with standard software. Multiple EP mapping sensors were mounted as ring electrodes near the arrays for electrocardiographic synchronization of ultrasound images and used for unique integration with EP mapping technologies. To help establish the catheters' ability for integration with EP interventional procedures, tests were performed in vivo in a porcine animal model to demonstrate both useful intracardiac echocardiographic (ICE) visualization and simultaneous 3-D positional information using integrated electroanatomical mapping techniques. The catheters also performed well in high frame rate imaging, color flow imaging, and strain rate imaging of atrial and ventricular structures. The companion paper of this work discusses the catheter design of the side-looking catheter with special attention to acoustic lens design. The third device in development is a 10 MHz forward-looking ring array that is to be mounted at the distal tip of a 9F catheter to permit use of the available catheter lumen for adjunctive therapy tools.

Self-focused ZnO transducers for ultrasonic biomicroscopy

A simple fabrication technique was developed to produce high frequency (100 MHz) self-focused single element transducers with sputtered zinc oxide (ZnO) crystal films. This technique requires the sputtering of a ZnO film directly onto a curved backing substrate. Transducers were fabricated by sputtering an 18 mum thick ZnO layer on 2 mm diameter aluminum rods with ends shaped and polished to produce a 2 mm focus or f-number equal to one. The aluminum rod served a dual purpose as the backing layer and positive electrode for the resultant transducers. A 4 mum Parylene matching layer was deposited on the transducers after housing and interconnect. This matching layer was used to protect the substrate and condition the transfer of acoustic energy between the ZnO film and the load medium. The pulse-echo response for a representative transducer was centered at 101 MHz with a -6 dB bandwidth of 49%. The measured two way insertion loss was 44 dB. A tungsten wire phantom and an adult zebrafish eye were imaged to show the capability of these transducers.

Self-focused ZnO transducers for ultrasonic biomicroscopy

A simple fabrication technique was developed to produce high frequency (100 MHz) self-focused single element transducers with sputtered zinc oxide (ZnO) crystal films. This technique requires the sputtering of a ZnO film directly onto a curved backing substrate. Transducers were fabricated by sputtering an 18 μm thick ZnO layer on 2 mm diameter aluminum rods with ends shaped and polished to produce a 2 mm focus or f-number equal to one. The aluminum rod served a dual purpose as the backing layer and positive electrode for the resultant transducers. A 4 μm Parylene matching layer was deposited on the transducers after housing and interconnect. This matching layer was used to protect the substrate and condition the transfer of acoustic energy between the ZnO film and the load medium. The pulse-echo response for a representative transducer was centered at 101 MHz with a -6 dB bandwidth of 49%. The measured two way insertion loss was 44 dB. A tungsten wire phantom and an adult zebrafish eye were imaged to show the capability of these transducers.

Nano-structured TiO(2) film fabricated at room temperature and its acoustic properties

Nano-structured TiO(2) thin film has been successfully fabricated at room temperature. Using a quarter wavelength characterization method, we have measured the acoustic impedance of this porous film, which can be adjusted from 5.3 to 7.19 Mrayl by curing it at different temperatures. The uniform microstructure and easy fabrication at room temperature make this material an excellent candidate for matching layers of ultra-high frequency ultrasonic imaging transducers.

High frequency broadband PZT thick film ultrasonic transducers for medical imaging applications

A modified sol-gel method is used to prepare PZT thick film on Pt-coated silicon substrate. A new method of vacuum filling sol-gel precursor solution is introduced to improve film quality. The effects of the filling on PZT thick film structure and ferroelectric properties are discussed. The fabrication of a high frequency transducer with the PZT film as the actuating layer is described. The performance of the transducer is measured and results show that the transducer backed by E-Solder without a matching layer has a center frequency of 103 MHz and a bandwidth of 70%. Beam profile measurements show that the transducer has an axial resolution of 9.2 microm and a lateral resolution of 33 microm.

Diagnosis of vulnerable atherosclerotic plaques by time-resolved fluorescence spectroscopy and ultrasound imaging

In this study, time-resolved laser-induced fluorescence spectroscopy (TR-LIFS) and ultrasonography were applied to detect vulnerable (high-risk) atherosclerotic plaque. A total of 813 TR-LIFS measurements were taken from carotid plaques of 65 patients, and subsequently analyzed using the Laguerre deconvolution technique. The investigated spots were classified by histopathology as thin, fibrotic, calcified, low-inflamed, inflamed and necrotic lesions. Spectral and time-resolved parameters (normalized intensity values and Laguerre expansion coefficients) were extracted from the TR-LIFS data. Feature selection for classification was performed by either analysis of variance (ANOVA) or principal component analysis (PCA). A stepwise linear discriminant analysis algorithm was developed for detecting inflamed and necrotic lesion, representing the most vulnerable plaques. These vulnerable plaques were detected with high sensitivity (>80%) and specificity (>90%). Ultrasound (US) imaging was obtained in 4 carotid plaques in addition to TR-LIFS examination. Preliminary results indicate that US provides important structural information of the plaques that could be combined with the compositional information obtained by TR-LIFS, to obtain a more accurate diagnosis of vulnerable atherosclerotic plaque.

The principle of multidimensional arrays

Echocardiography is one of the most important diagnostic tools in cardiology today. One-dimensional phased arrays have been used extensively because they have a small footprint and allow beam steering. Their major limitation lies in that these devices can only be used to acquire images of two-dimensional slices in real-time and that the slice thickness cannot be controlled. To allow real-time three-dimensional imaging of the heart and focusing of the ultrasonic beam in two-dimensional, two-dimensional arrays, the design and fabrication of which are enormous engineering challenges, are required. Before reaching this ultimate goal, limited focusing in the elevational plane can be achieved with 1.5-dimensional arrays. Focusing in the elevational plane allows a reduction in slice thickness and thus an improvement in the image quality over a larger depth of view.

Doppler power variation from porcine blood under steady and pulsatile flow

Although a number of recent studies have demonstrated that the echogenicity of blood varies as a function of time under pulsatile flow, the fundamental mechanisms responsible for it are still uncertain. To better understand this phenomenon, the Doppler power from porcine blood and polystyrene microsphere suspensions was measured at the center of the tube as functions of two crucial parameters, flow velocity and stroke rate (for pulsatile flow), under steady and pulsatile flow in a mock flow loop. In the present study, the experimental results were obtained with a 10-MHz pulsed Doppler system with a frequency response estimated more accurately by electronic injection, and validated by comparing to the radiofrequency (RF) signal acquired from the same Doppler instrument. The results show that the Doppler power from microspheres and porcine red blood cell (RBC) suspensions did not vary appreciably (< 2 dB), with either the speed or stroke rate (for pulsatile flow only) under steady and pulsatile flow. It was found that the Doppler power from porcine whole blood under steady flow decreased with the speed by approximately 13 dB from 3 to 33 cm/s and was only 3 dB higher than that from RBC suspension at 33 cm/s, suggesting minimal RBC aggregation in whole blood at this speed. The apparent cyclic variation from whole blood was observed at 20 and 40 beats/min (BPM). The cyclic variation became more obvious as the speed and stroke rate decreased. The mean Doppler power over a cycle increased as the peak speed decreased. The Doppler power reached a maximum near peak systole and a minimum at late diastole at the center of the tube. This pattern cannot be explained by RBC aggregation due to the shear rate alone, and may be attributed to acceleration and deceleration along with aggregation. The cyclic variation was not observed at 60 BPM, probably because of a lack of time for aggregation to occur.

The "black hole" phenomenon in ultrasonic backscattering measurement under pulsatile flow with porcine whole blood in a rigid tube

The "black hole" phenomenon was further investigated with porcine whole blood under pulsatile flow conditions in a straight rigid tube 120 cm long and of 0.95 cm diameter. A modified Aloka 280 commercial scanner with a 7.5 MHz linear array was used to collect the radio frequency (RF) signal of backscattering echoes from the blood inside the tube. The transducer was located downstream from the entrance and parallel to the longitudinal direction of the tube. The experimental results showed that higher hematocrits enhanced the black hole phenomenon, leading to a more apparent and larger diameter black hole. The black hole was not apparent at hematocrits below 23%. The highest hematocrit used in the experiment was 60%. Beat rates of 20, 40 and 60 beats per minute (bpm) were used, and the black hole became weaker in amplitude and smaller in diameter when the peak flow velocity was increased at each beat rate. These results are consistent with the suggestion in previous work that the black hole arises from insufficient aggregation of red blood cells (RBCs) at the center of the tube because of the low shear rate. At 20 and 40 bpm, the peak flow velocity ranges were 10 approximately 25 cm/s and 18 approximately 27 cm/s, respectively. The black hole was very clear at the minimal peak flow velocity but almost disappeared at the maximal velocities for each beat rate. At 60 bpm, experiments were only performed at one peak flow velocity of 31 cm/s and the black hole was clear. The results showed that the black hole was more pronounced at higher beat rates when the peak velocity was the same. This phenomenon cannot be explained by previous hypotheses. Acceleration seems to be the only flow parameter that varies at different beat rates when peak velocities are the same. Therefore, the influence of acceleration on the structural organization and orientation of RBC rouleaux might be another factor involved in the formation of the black hole in addition to the shear rate. As the entrance length was changed from 110 to 15 diameters (D) in seven steps at the hematocrit of 60%, it was found that a position farther downstream yielded a black hole with a greater contrast relative to the surrounding region, while the backscattering power at the central hypoechoic zone did not increase with increasing entrance length.

In vivo measurements of ultrasonic backscattering in blood

Ultrasonic backscattering in blood including its dependence on the hematocrit, plasma proteins, shear rate, and flow disturbance, has been studied extensively theoretically and experimentally in vitro. However, much of the result has never been validated in vivo. To do so, backscattering measurements were made on pigs using a 10-MHz non-focused intravascular transducer in direct contact with blood. The probe was placed in either the abdominal aorta or the inferior vena cava. The backscattering coefficient (BSC) of blood flowing in these vessels as well as downstream from a stenosis was measured using an approach that was originally developed for measurements with focused transducers. With this approach, 6% porcine red cell saline suspensions prepared immediately after each in vivo measurement were used as the reference medium. Result from seven pigs at hematocrits ranging from 29 to 36% (31.9 +/- 2.5%) demonstrated that BSC of blood in the vena cava, (4.62 +/- 2.06) x 10(-5) cm-sr-1, is consistently higher than that in the aorta, (2.65 +/- 1.22) x 10(-5) cm-sr-1. The difference has been attributed to the lower shear rate and the formation of red cell aggregation in venous blood. These in vivo results are in agreement with those obtained in vitro. In response to stenoses created by ligating the aorta, backscattering of the blood measured downstream from the stenosis showed that the closer the site of measurement relative to the stenosis, the higher the backscatter, presumably resulting from the higher degree of flow disturbance. In vitro backscattering results on porcine whole blood were also acquired at 20 MHz with a Diasonics intravascular scanner.

High frequency properties of passive materials for ultrasonic transducers

The acoustic properties of passive materials for ultrasonic transducers have been measured at room temperature in the frequency range from 25 to 65 MHz using ultrasonic spectroscopy. These materials include alumina/EPO-TEK 301 composites and tungsten/EPO-TEK 301 composites. Experimental results showed that the acoustic impedance of the composites monotonically increased with the volume fraction of the particle filler, which is in agreement with the Denavey model. The attenuation, however, peaked between 7 and 9% volume fraction of particle filler. For comparison, several other passive materials were also fabricated and measured. The results suggest that materials that possess a higher attenuation also appear to have a larger velocity dispersion.

High-frequency backscatter and attenuation measurements of selected bovine tissues between 10 and 30 MHz

There are now diagnostic ultrasonic imaging devices that operate at very high frequencies (VHF) of 20 MHz and beyond for clinical applications in ophthalmology, dermatology, vascular surgery, endoluminal imaging and small animal imaging. To be able to better interpret these images and to further the development of these devices, knowledge of ultrasonic attenuation and scattering of biological tissues in this frequency range is crucial. Attenuation and backscatter coefficients (BSCs) of bovine tissues in the frequency range of 10 to 30 MHz were measured, respectively, using a standard substitution method for attenuation measurements and a modified narrow-band substitution method for scattering measurements. A modified substitution method for scattering measurements has to be used at high frequencies because unfocused transducers due to their decreased sensitivity cannot be used in the simple substitution method. In the modified method, the flat reflector is substituted by a particulate reference medium whose BSC is well-known and documented; in this case, a red cell suspension. In this paper, experimental results on BSC and attenuation coefficient measured between 10 and 30 MHz are reported. The frequency dependence of backscatter of the selected bovine tissues ranges from 2.4 to 3.5, whereas attenuation is observed to be still approximately linearly proportional to frequency. The BSC measured with the modified method is in good agreement with those obtained with the standard method between 10 and 20 MHz.

Ultrasonic backscattering from porcine whole blood of varying hematocrit and shear rate under pulsatile flow

It was shown previously that ultrasonic scattering from whole blood varies during a flow cycle under pulsatile flow both in vitro and in vivo. It has been postulated that this cyclic variation may be associated with the dynamics of red cell aggregation because the shearing force acting on the red cell aggregates across the lumen is a function of time during a flow cycle. In all studies, the local shear rate variation as a function of time is unknown. The effect of shear rate on the red cell aggregation and, thus, on ultrasonic scattering from blood can only be merely speculated. One solution to this problem is to estimate the shear rate in a flow conduit by finite element analysis (FEA). An FEA computational fluid dynamics (CFD) tool was used to calculate local shear rate in a series of experiments in which ultrasonic backscattering from porcine whole blood under pulsatile flow was measured as a function of hematocrit and shear rate intravascularly with a 10-MHz catheter-mounted transducer in a mock flow loop. The results show that, at 20 beats per min (BPM), the magnitudes of the cyclic variation for hematocrits at 30, 40, and 50% were approximately 4 dB. However, at 60 BPM, the magnitude of cyclic variation was found to be minimal. The results also confirm previous findings that the amplitude and the timing of the peak of ultrasonic backscattering from porcine whole blood under pulsatile flow during a flow cycle are dependent upon the shear rate and hematocrit in a complicated way.

Interlaboratory comparison of ultrasonic backscatter, attenuation, and speed measurements

In a study involving 10 different sites, independent results of measurements of ultrasonic properties on equivalent tissue-mimicking samples are reported and compared. The properties measured were propagation speed, attenuation coefficients, and backscatter coefficients. Reasonably good agreement exists for attenuation coefficients, but less satisfactory results were found for propagation speeds. As anticipated, agreement was not impressive in the case of backscatter coefficients. Results for four sites agreed rather well in both absolute values and frequency dependence, and results from other sites were lower by as much as an order of magnitude. The study is valuable for laboratories doing quantitative studies.

The effect of hemodynamics, vessel wall compliance and hematocrit on ultrasonic Doppler power: an in vitro study

Previous in vitro studies in rigid tubes under pulsatile flow conditions have reported a lack of a cyclic variation in blood echogenicity that contradicts in vivo results. To investigate whether or not these variations can be attributed to the compliance of the vessel wall, a series of in vitro experiments with compliant tubes, under pulsatile flow conditions, was performed. Two important factors that may affect the Doppler power were investigated: 1. the dependence on hematocrit and 2. the effect of the vessel wall elasticity. In the present study, it is shown that, at the low beat rates, the peak of the mean Doppler power within the flow cycle depends on the vessel wall compliance. When the vessel becomes more compliant, the peak is shifted from the early to the late systole. Additionally, there is a correlation between the power peak and hematocrit that is more evident in compliant vessels. At a higher pulsation rate of 37 beats/min, a different variation is observed. A drop in the power occurs near peak systole in compliant tube experiments and is more pronounced as the vessel becomes more constricted. The observed power drop agrees with previously reported in vivo results, but is not seen in rigid tube experiments. The results of this study suggest that proper interpretation of cyclic variations in Doppler power requires a knowledge of hemodynamic parameters, such as the modulus of elasticity of the vessel wall, propagation velocity or, possibly, the phase angle of input impedance.

An in vitro study of the effects of Doppler angle, fibrinogen, and hematocrit on ultrasonic Doppler power

For a better understanding of the relationship between the Doppler power and erythrocyte aggregation of whole blood under steady flow in a conduit, the effects of Doppler angle, fibrinogen concentration, and hematocrit were investigated in a mock flow loop. The results show that, at a mean shear rate of 102 s(-1), there was minimal angular dependence; but at a mean shear rate of 52 s(-1), there was a weak angular dependence as the Doppler angle was varied from 40 degrees to 70 degrees . These results suggest that there was, perhaps, no or little alignment of the red cell aggregates at high shear rates. The Doppler power was found to increase nonlinearly as the fibrinogen concentration was increased; and the effect of other plasma proteins on red cell aggregation may not be negligible, although fibrinogen is the dominant factor. The results show that the variation of the Doppler power over the lumen is hematocrit dependent for hematocrits below 26%

In situ measurements of Doppler power vs. flow turbulence intensity in red cell suspensions

Whereas previous studies have shown that ultrasonic backscatter and Doppler power from blood are affected by flow turbulence, turbulence level has only been inferred from the flow Reynolds number and not directly measured. In this study, both ultrasonic Doppler power and flow turbulence intensity were measured in situ to quantify the relationship between Doppler power and flow turbulence. Three grid meshes of different geometries were used in a steady-flow mock loop to generate controlled levels of flow turbulence in porcine red blood cell saline suspensions. Doppler power was measured by a 10-MHz PW Doppler flowmeter, and the turbulence intensity by using constant-temperature hot film anemometry. We showed that Doppler power is affected by turbulence and hematocrit in a complex way. At a fixed hematocrit, Doppler power increases nonlinearly with turbulence intensity and, at fixed turbulence intensity, Doppler power peaks at an optimal hematocrit level that increases with turbulence level. The shape factor, introduced by Lucas and Twersky (1987) to take into account effects of shape and orientation of the scatterers in a dense distribution of small and tenuous scatterers, was estimated by fitting the experimental data to the theoretical model. The results indicate that shape factor decreases with increasing turbulence intensity.

Variation in ultrasonic backscattering from skeletal muscle during passive stretching

The purpose of this study was to further validate the scattering mechanism of ultrasound in the skeletal muscle tissue. It was hypothesized that the endomysial collagen fibers are a major determinant of ultrasonic scattering in the skeletal muscle. Previous studies have found that the ultrasonic backscattering from avian skeletal muscle changed as it was passively stretched from 0 to 40%. In this study, ultrasonic backscattering was measured from chicken breast muscles as they were stretched passively in increments of 10% of original length up to 60%. The integrated backscatter was found to reach a peak as the specimen was stretched to 40% and leveled off as it was further stretched from 40 to 60%. This finding was supported by results from scanning electron microscopy (SEM) of the specimens. SEM images showed that the orientation of the endomysial collagen fibers surrounding muscle fibers became approximately parallel to the axis of the muscle fiber when the muscle was stretched up to 40% of its original length, yielding maximal ultrasonic backscatter and as the muscle was further stretched, no apparent alteration of fiber orientation could be observed.

An approach for measuring ultrasonic backscattering from biological tissues with focused transducers

When the standard substitution method is used with a focused transducer to measure the backscattering coefficient from biological tissues including blood, it yields erroneous results. Extending the backscattering measurements to frequencies beyond 15 MHz necessitates the use of focused transducers because of the worsened signal-to-noise ratio--caused by the increased attenuation and the smaller transducer aperture size--in order to make the measurements close to the transducer. An approach which allows the use of focused transducers in backscattering measurements has been developed. It has been used to measure the backscattering coefficient of red cell suspensions of hematocrit ranging from a few percent to 30% in the frequency range from 5 MHz to 30 MHz. The results at hematocrits below 20% agree well with those obtained with the standard substitution method, although they differ as the hematocrit is increased beyond 20%. The experimental results also show that the fourth-power dependence of backscatter on frequency is in general approximately valid for suspended erythrocytes of hematocrit between 6% and 30%.

Hydrogels with enhanced mass transfer for transdermal drug delivery

The sonophoretic transport rates of monomeric insulin and vasopressin across human skin in vitro in the presence of a 20 kHz ultrasound field are shown to differ substantially depending on whether molecules enter the skin from a saline solution or from a viscous ultrasonic coupling medium (specifically, a methyl cellulose hydrogel or viscous sol). Theoretically, the reduction in sonophoretic transport caused by the hydrogels can be explained by boundary layers that form within the hydrogel owing to the relatively rapid rate of molecular transport across the (ultrasonically) permeated stratum corneum as well as poor diffusive mass transfer between the skin and gel. The results of in vitro experiments performed with an ac current accompanying the ultrasound show that the mass-transfer barrier posed by the hydrogel can be eliminated for both vasopressin and insulin by suppressing the diffusive boundary layers, indicating that relatively high rates of sonophoretic molecular transport across human skin are achievable when hydrogels are used as the ultrasound coupling medium as long as method is used to induce molecular mixing within the gel.

Quantitative measurements of second harmonic Doppler using ultrasound contrast agents

Quantitative measurements of second harmonic and first harmonic Doppler were carried out using two ultrasound contrast agents, Albunex and FS069. The RMS amplitudes of the Doppler shift spectra were measured as a function of the concentration of the agents, frequency and transmitted acoustic pressure. The results showed that, for a given lot of contrast agent investigated, FS069 was able to produce higher levels of first and, especially, second harmonic signals compared to Albunex. Under the same experimental conditions, the RMS Doppler amplitude (RDA) of FS069 was 3.8 +/- 0.8 dB higher than Albunex at first harmonic and 12.6 +/- 0.8 dB higher at second harmonic. The ratio of the second harmonic to first harmonic RDA, which we called R2/R1, decreases at a rate of 7 dB/MHz for both agents with increasing frequency. The difference in the value of R2/R1 between FS069 and Albunex at any frequency was approximately 4.5 dB. R2/R1 was found to increase linearly as a function of the transmitted acoustic pressure for both agents. Simulations using the Rayleigh-Plesset equation show a decrease of R2/R1 at a rate of 5 dB/MHz. Comparison of experimental results with theory indicates that the shell elasticity parameter may be an increasing function of the mean diameter of the bubbles.

Cyclic variation of Doppler power from whole blood under pulsatile flow

The echogenicity and Doppler power from whole blood under pulsatile flow have been found to vary during the flow cycle in previous studies both in vitro and in vivo. The present study was undertaken to better understand this phenomenon. Doppler power from whole blood under pulsatile flow was measured with a pulsed Doppler flowmeter as a function of the flow cycle, radial position and compliance of the vessel in a mock flow loop. It was found that the cyclic variation is more pronounced if the stroke rate is less than 56 beats/min and that the peak of the Doppler power from whole blood flowing near the center stream coincided with the peak of the flow velocity. However, it began to lead the velocity peak as the measurement site was moved away from the center stream. The lead increased as the radial distance was increased. The results also show that the compliance of the vessel can affect, to a certain extent, the magnitude of the cyclic variation. Results from intravascular Doppler measurements rule out the possibility that the cyclic variation is primarily due to the variation in attenuation caused by vessel wall during a flow cycle.

Some considerations on the measurements of mean frequency shift and integrated backscatter following administration of Albunex

Ultrasonic contrast agents have been of heightened interest in recent years. More success has been achieved by agents consisting of micro bubbles, since only a few of these agents are capable of producing very strong ultrasonic backscattered signals for the enhancement of certain tissue structures. Recent investigations also demonstrate that an analysis of the radio frequency (RF) backscattered echoes by the contrast agents may lead to quantitative means for assessing tissue perfusion. In these studies, a parameter, mean frequency shift (MFS) of the RF signal, along with integrated backscatter (IB) has received the most attention. In an effort to better understand the physical mechanisms responsible for the observed mean frequency shift, we have performed experiments on 10 dogs following injections of Albunex (Molecular Biosystems, Inc.) into the left atrium, coronary artery and abdominal aorta, respectively, for investigations in the heart and kidney. The integrated backscatter and mean frequency (MF) of a region of interest (ROI) were calculated from the RF signal acquired with a modified real-time ultrasonic scanner. The results show consistently that the RF signals acquired from all regions of interest are greatly affected by the presence of the contrast agent in the path between the transducer and the ROI, which can cause either an upward or a downward shift of the MF. This could not be observed by video densitometry or a measurement of the IB alone. The MFS is the result of the resonant behavior of the micro bubbles, which is related to the frequency, ambient pressure, and physical properties of the bubbles including size distribution, surface tension and concentration. On the other hand, when there is no contrast agent present in the path, a downward frequency shift is seen.

A study of the relationship between mechanical and ultrasonic properties of dystrophic and normal skeletal muscle

A study has been made of the application of radio frequency (RF) ultrasound to the detection of muscular dystrophy by monitoring passively stretched skeletal muscle. The tests included detection of integrated backscatter changes in response to both static loading, in which muscle samples were stretched and allowed to relax, and stress relaxation. In both static and step strain loading conditions, the dystrophic muscle was found to exhibit little change in backscatter power while normal muscle responded to loading with significant changes in integrated backscatter. The backscatter response is compared with mechanical properties of the tissue (time constants and stress-strain constants). Both mechanical and ultrasonic time constants of relaxation are not significantly different between normal and dystrophic tissue, but stress-strain constants do differ. The difference in response of dystrophic and normal tissue appears to be due to a repression of motion of the constituent anatomy of dystrophic muscle which is responsible for the change of echogenicity with passive stretch.

Time-domain ultrasonic contrast blood flowmetry

Time-domain ultrasonic blood flow estimation methods have recently received considerable attention because of their advantages over conventional Doppler methods. Among them are that they may yield better spatial resolution and that methods based on frame-to-frame speckle tracking do not require a knowledge of the angle between directions of blood flow and the ultrasound beam. These methods, however, suffer from an intrinsic problem of poor signal-to-noise ratio, since the echoes scattered back by blood are much weaker than those of the surrounding structures. In addition, the maximal velocity that can be estimated by frame-to-frame tracking via conventional ultrasonic scanners is limited by the frame rate of the scanner. In this article, we will present experimental results to show that these problems may be alleviated by using a high frame rate scanner in conjunction with the injection of an ultrasonic contrast agent. In this study the contrast agent used was Albunex.

Ultrasound scattering from blood with hematocrits up to 100%

The backscattering coefficient of saline suspensions of porcine red blood cells was measured for hematocrits up to about 90%. It was found that the coefficient peaks at approximately 15%, but then, contrary to what a simple "gap theory" might suggest, it decays smoothly to zero, without showing another peak at high hematocrits. A one-dimensional (1-D) slab scattering model, in which the number of slabs per unit length represents the hematocrit and whose thickness and acoustical properties are similar to red cells/plasma, was also used to investigate the relation between the backscattered power and hematocrit. Monte-Carlo simulations performed for randomized boundary conditions show a similar relation to that of the 3-D system. The experimental data is compared to the Percus-Yevick theory for the packing of hard spheres, and the simulated data is compared to the Percus-Yevick theory for infinite slabs.

A computer model for simulating ultrasonic scattering in biological tissues with high scatterer concentration

Scattering of ultrasonic waves by biological tissues at different scatterer concentrations is investigated using one- and two-dimensional computer simulation models. The backscattered power as a function of scatterer concentrations is calculated using two types of incident waves, a Gaussian shaped pulse and a continuous wave (CW). The simulation results are in good agreement with the Percus-Yevick packing theory within the scatterer concentrations, from 0% to 100% in one-dimensional (1D) space, and 0% to 46% in two-dimensional (2D) space. In all cases, the simulation results from a pulsed incident wave show a much smaller standard deviation (SD) than those from an incident CW. The simulation can serve as a useful tool to verify scattering theories, simulate different experimental conditions, and to investigate the interaction between the scatterer properties and the scattering of ultrasonic waves. More importantly, the 2D simulation procedure serves as an initial step toward the final realization of a true three-dimensional (3D) simulation of ultrasonic scattering in biological tissues.

High frequency ultrasonic backscatter from erythrocyte suspension

Previous studies have shown that ultrasonic backscattering from red blood cells suspended in saline is proportional to the fourth power of frequency for frequencies below 15 MHz, as predicted by Rayleigh scattering theory. Recently, we have extended the measurements up to 30 MHz, because scattering of ultrasound by red blood cells may no longer be negligible at these frequencies and can affect, to a great degree, the operation of intravascular imaging devices. The experimental results show that the fourth power dependence on frequency of the backscattering coefficient for porcine erythrocytes suspended in saline solution appears to be valid up to 30 MHz. To confirm this, backscattering cross-section of porcine red cells was computed as a function of frequency using the T-matrix method. Since at higher frequencies the shape of the scatterers may also play a significant role, its effect was investigated by treating the red cell as a sphere, a disc, and a biconcave disc of the same volume. Good agreement was obtained between the experimental and theoretical results.

Analysis of ultrasonic scattering in blood via a continuum approach

Since the pioneering work by Reid et al. on measuring ultrasonic scattering in blood, this phenomenon has been extensively studied both theoretically and experimentally. The knowledge on ultrasonic scattering properties of blood is needed for the design of ultrasonic methods for measuring blood flow, and a better interpretation of ultrasonic images. The development of high frequency intravascular or intracardiac imaging devices raises the possibility of measuring blood properties, e.g., erythrocyte aggregation and fibrinogen concentration, in situ. A number of theoretical approaches have been developed to analyze this phenomenon where in general ultrasound wavelength is much greater than the erythrocytes. These results show that the backscattering coefficient of blood, defined as power backscattered by a unit volume of blood per steradian per unit incident intensity, is proportional to variance of the erythrocyte number fluctuation and backscattering cross-section of a single erythrocyte. In this paper, we will show that similar results can also be obtained by taking a continuum approach.

Cyclic variation of the power of ultrasonic Doppler signals backscattered by polystyrene microspheres and porcine erythrocyte suspensions

Factors affecting the power of the ultrasonic Doppler signal within the flow cycle have been evaluated experimentally using a pulsatile flow loop model. Polystyrene microspheres and porcine red cells suspended in saline solution for hematocrits between 2 and 40% were used as scattering fluid in the flow model. Experiments were performed at mean flow velocities of 11, 64, and 76 cm/s. In laminar flow experiments performed at a mean velocity of 11 cm/s, no variation of the Doppler power was found for both polystyrene microspheres and red cell suspensions (40% hematocrit). When turbulence was induced in the flow model, the power increased during systole, a maximum was observed early after peak systole, and a decrease was obtained in diastole during deceleration of flow. At higher mean flow velocities (64 and 76 cm/s), a significant cyclic variation of the Doppler power was also measured for all values of hematocrits (between 2 and 40%). The power of the signal scattered by microspheres and red cell suspensions at 4% hematocrit dropped in systole, reached a minimum at peak systole, and then increased during early diastole. For red cells suspended in saline at 40% hematocrit, a slightly different pattern of variation was obtained. The cyclic variations observed at high flow velocities and in the presence of turbulence are believed to be associated with cyclic changes in the correlation among particles. In the present study, the effect of red cell aggregation on the cyclic variation has not been addressed.

Study of red cell aggregation in pulsatile flow from ultrasonic Doppler power measurements

Human red cell aggregability and disaggregability represent important hemorheological parameters of blood. Several techniques have been proposed to evaluate the tendency of red cells to form aggregates and to disrupt in the presence of shear stress. One of the most recent approaches is based on the characterization of the intensity of ultrasonic scattered signals. A pulsatile flow loop model is used in the present study to demonstrate the potential applicability of Doppler ultrasound to detect and characterize the hemodynamic behavior of red cell aggregates. Porcine whole blood specimens collected from 20 different pigs were circulated in the flow model (tube diameter of 0.476 cm) at different mean velocities and pulsation rates. At a pulsation of 70 beats/min for mean velocities of 13 cm/sec and 63 cm/sec, no cyclic variation of the Doppler power was observed, suggesting the absence of rouleaux build-up and rouleaux disruption. At a pulsation of 20 beats/min and mean velocities of 11 cm/sec and 38 cm/sec, statistically significant cyclic variations (p < 0.01) were measured. It is suggested that aggregate size enlargement, rouleaux orientation with the flow field and the effect of shear stress on rouleaux disruption are possible causes for the observed cyclic variation of the Doppler power within the flow cycle at a pulsation of 20 beats/min. A discussion of the potential application of this technique for in vivo study in large vessels is given.

Reverberation reduction in ultrasonic B-mode images via dual frequency image subtraction

The authors demonstrate the feasibility of an approach, dual-frequency subtraction imaging, for suppressing artifacts produced by reverberation of strong echoes among specular reflectors. This method is based upon the principle that specularly reflected echoes from flat boundaries are frequency-independent whereas the diffusely scattered echoes from small scatterers are frequency-dependent. The approach was assessed on phantoms including one consisting of two parallel plastic plates between layers of foam sponges using a prototype experimental system. Preliminary results show that this method is superior to simple thresholding techniques or signal compression and holds great promise for suppressing reverberation artifacts in ultrasonic images.

Scattering of ultrasound from skeletal muscle tissue

The purpose of this work was to explore the mechanisms which are responsible for the scattering of ultrasound from skeletal muscle tissue. It was undertaken in response to an interesting phenomenon observed in the authors' laboratory whereby scattering power from avian skeletal muscle changed in concordance with passive stretch. Ultrasonic scattering from skeletal muscle samples was measured as they were stretched passively in increments of 10% of their original length up to 40%. The samples were illuminated with an ultrasound beam from a transducer which was oriented orthogonally to and at 20 degrees from the normal to the long axis of the muscle sample. It was found that the integrated backscatter increased significantly over the strain range for the orthogonal orientation, but it changed very little after the initial stretch when the orientation was 20 degrees . It is postulated that this phenomenon may be caused by reorientation of the endomysial collagen fibers surrounding each muscle fiber.

A feasibility study on quantitating myocardial perfusion with Albunex, an ultrasonic contrast agent

Quantitating regional myocardial perfusion has been the much sought-after but still elusive goal of many intensive investigations over the years. Videodensitometry of the variation of myocardial echogenicity in two-dimensional (2-D) echocardiograms as a function of time in conjunction with the injection of a bolus of an ultrasound contrast agent has been used clinically as a tool for a direct assessment of regional myocardial perfusion, despite that the precise relationship between tissue echogenicity observed on an image and the echoes detected by the ultrasonic probe is unknown. A study was undertaken to determine whether ultrasonic backscatter calculated from unprocessed radio frequency (RF) echoes returned from myocardium could be used to quantitate regional myocardium perfusion. A real-time ultrasonic scanner has been modified and interfaced to a microcomputer to acquire RF data at a rate up to 10 frames per second. Preliminary experimental data were obtained from four open-chest dogs following intracoronary injection of a bolus of Albunex and two dogs following intravenous injection with this modified scanner. On one hand, these results indicate that the integrated backscatter measured from the region of myocardium perfused by the coronary artery where Albunex is injected and selected for monitoring initially increases, reaches a peak, and then decreases as the contrast agent is washed out and that the magnitude of the peak is approximately linearly proportional to the volume concentration of Albunex microspheres injected, clearly demonstrating the feasibility of this approach for quantitating region myocardial perfusion. On the other hand, intravenous injections did not result in any appreciable change in myocardial backscatter in the left ventricle although a response could be observed in the left ventricular blood pool.

Experimental evaluation of intrinsic and nonstationary ultrasonic Doppler spectral broadening in steady and pulsatile flow loop models

Intrinsic and nonstationary Doppler spectral broadening, and the skewness of the spectral representation, were evaluated experimentally using porcine red cell suspensions as ultrasonic scatterers. Theoretically, the relative Doppler bandwidth, defined as the intrinsic bandwidth divided by the mean Doppler frequency shift, should be velocity independent. The relative Doppler bandwidth invariance theorem was experimentally verified with an in vitro steady laminar blood flow model. It is shown that the relative bandwidth is both independent of the flow velocity and blood hematocrit. Using a pulsatile laminar flow model, the authors demonstrated that the relative Doppler bandwidth invariance theorem did not hold during flow acceleration and deceleration. In addition, a positive skewness of the Doppler spectra was observed during acceleration while a negative skewness was measured during the deceleration of blood. The effect of the window duration used in the Fourier spectral computation, on nonstationary broadening, is characterized.

The effects of hematocrit, shear rate, and turbulence on ultrasonic Doppler spectrum from blood

Previous studies of ultrasonic scattering properties of blood using a pulse-echo experimental arrangement show that ultrasonic backscatter from blood is influenced by a number of factors including hematocrit, shear rate, and the nature of flow (J. Acoust. Soc. Amer., vol. 75, p. 1265, 1984 and J. Acoust. Soc. Amer., vol. 84, p. 1, 1988). Since the Doppler frequency spectrum from a Doppler flowmeter is derived from echoes backscattered by red blood cells in the flowing blood, it is also undoubtedly a function of these parameters. The effects of these parameters on Doppler spectrum from blood have been investigated using a pulsed Doppler flowmeter. The results agree well with those obtained in previous studies. One important conclusion of this study is that the assumption that the Doppler spectral power density at a frequency in Doppler spectrum is linearly proportional to the number of red cells flowing at that velocity used in many theoretical models developed to explain the Doppler phenomenon may be erroneous. An alternative is proposed. It is shown that conclusions derived from these theoretical models would remain valid by making this assumption.

Ultrasonic backscatter from flowing whole blood. II: Dependence on frequency and fibrinogen concentration

Earlier studies showed that ultrasonic backscatter from erythrocytes suspended in saline is a function of hematocrit and frequency and that it can be affected by flow disturbance. The experimental data agree well with the theories. Recently, studies have been extended to flowing whole blood. The results indicated that ultrasonic backscatter from flowing whole blood differs from that from saline suspensions of erythrocytes in that it is shear-rate dependent and species dependent. In the present article, data on the dependence of ultrasonic backscatter from flowing whole blood on frequency and on fibrinogen concentration are reported. It was found that ultrasonic backscatter from flowing whole blood also depends on fibrinogen concentration when red blood cell (RBC) aggregation exists. Moreover, when the blood is under conditions that favor RBC aggregation such as low shear rates, high fibrinogen concentration, or high hematocrits, Rayleigh scattering apparently is no longer sufficient to describe its scattering behavior.

Ultrasonic backscatter from flowing whole blood. I: Dependence on shear rate and hematocrit

Previous results show that ultrasonic backscatter from red blood cells (RBCs) suspended in saline is a function of hematocrit and frequency and that it can be affected by flow disturbance. The experimental data agree well with the theories. In the present article, results on ultrasonic backscatter from flowing whole blood are reported. Studies have been conducted on porcine, bovine, and human blood. Ultrasonic backscatter of flowing whole blood differs from that of RBC suspensions in that it is shear-rate dependent, which means that it is a function of spatial position of the blood in the flow conduit. Moreover, the results indicate that it is also species dependent. This behavior can be readily understood when red cell aggregation is considered.