Search

The objective of this research is to prospectively evaluate the feasibility of vibroacoustography (VA) imaging in monitoring prostate cryotherapy in an ex vivo model. Baseline scanning of an excised human prostate is accomplished by a VA system apparatus in a tank of degassed water. Alcohol and dry ice mixture are used to freeze two prostate tissue samples. The frozen prostates are subsequently placed within the water tank at 27[degrees]C and rescanned. VA images were acquired at prescribed time intervals to characterize the acoustic properties of the partially frozen tissue. The frozen prostate tissue appears in the images as hypoemitting signal. Once the tissue thaws, previously frozen regions show coarser texture than prior to freezing. The margin of the frozen tissue is delineated with a well-defined rim. The thawed cryolesions show a different contrast compared with normal unfrozen prostate. In conclusion, this pilot study shows that VA produces clear images of a frozen prostate at different temperature stages. The frozen tissue appears as a uniform region with well-defined borders that are readily identified. These characteristic images should allow safer and more efficient application of prostatic cryosurgery. These results provide substantial motivation to further investigate VA as a potential modality to monitor prostate cryotherapy intraoperatively. Index Terms--Cryosurgery, prostate cancer, ultrasound, vibroacoustography (VA). more...

The purpose of this paper is to demonstrate a prototype design of a low-frequency multiple-channel hemispherical focused-ultrasound phased-array system for transcranial disruption of the blood-brain barrier (BBB). A 32-channel ultrasound driving system tunable in the frequency range from 200 to 400 kHz was designed for producing a suitable ultrasound output for BBB disruption. The driving system includes a microcontroller/field-programmable gate-array-based control kernel with multiple-channel driving circuits implemented by a high-voltage switching/LC-resonance/impedance-matching circuit module. Three hemispherical phased arrays comprising 22, 31, and 80 elements were fabricated and tested. The pressure distributions at the geometric center and at off-center positions were tested experimentally. The focal performance of the different hemispherical arrays was also evaluated theoretically. The results showed that the developed phased-array system can successfully drive the hemispherical array with multiple-channel ultrasound signals with independent phase control at 8-bit resolution. Good focusing abilities were evident both at the geometric center and at specific off-center target positions. Preliminary animal experiments show that the BBB in rat can be locally disrupted successfully. The system will serve as a reference platform for developing a focused-ultrasound system for clinical use in brain drug delivery applications. Index Terms--Biomedical ultrasonics, brain, drug delivery systems. more...

In freehand 3-D ultrasound (US), the relative positions of US images are usually measured using a position tracking device despite its cumbersome nature. The probe trajectory can instead be estimated from image data, using registration techniques to recover in-plane motion and speckle decorrelation to recover out-of-plane transformations. The relationship between speckle decorrelation and elevational separation is typically represented by a single curve, estimated from calibration data. Distances read off such a curve are corrupted by bias and uncertainty, and only provide an absolute estimate of elevational displacement. This paper presents a probabilistic model of the relationship between correlation measurements and elevational separation. This representation captures the skewed distribution of distance estimates based on high correlations and the uncertainties attached to each measurement. Multiple redundant correlation measurements can then be integrated within a maximum likelihood estimation framework. This paper also introduces a new method based on the traveling salesman problem for resolving sign ambiguities ill data sets resulting from nonmonotonic probe motion and frame intersections. Experiments with real and synthetic US data show that by combining these new methods, out-of-plane US probe motion is recovered with improved accuracy over baseline methods using a deterministic model and fewer measurements. Index Terms--Acoustics, image reconstruction, maximum entropy methods, maximum likelihood estimation, traveling salesman problems. more...

Ultrasound (US) is among the most popular diagnostic techniques today. It is non-invasive, fast, comparably cheap, and does not require ionizing radiation. US is commonly used to examine the size, and structure of the thyroid gland. In clinical routine, thyroid imaging is usually performed by means of 2-D US. Conventional approaches for measuring the volume of the thyroid gland or its nodules may therefore be inaccurate due to the lack of 3-D information. This work reports a semi-automatic segmentation approach for the classification, and analysis of the thyroid gland based on 3-D US data. The images are scanned in 3-D, pre-processed, and segmented. Several pre-processing methods, and an extension of a commonly used geodesic active contour level set formulation are discussed in detail. The results obtained by this approach are compared to manual interactive segmentations by a medical expert in five representative patients. Our work proposes a novel framework for the volumetric quantification of thyroid gland lobes, which may also be expanded to other parenchymatous organs. Index Terms--Geodesic active contour, level set, segmentation, thyroid gland, three-dimensional (3-D) ultrasound. more...

Results are summarized of a finite element comparison of phased array transducers involving a multi-layer composite hybrid versus a single crystal.

Phased array transducers using PZT-5H and two multi-layer composite hybrids are examined and compared. Simulated and experimental results are summarized.

Two new transducer assemblies are described for mechanically scanning of ultrasound beams. The assemblies are made from polyimide films with photolithography that relies on a polyimide micromachine (MEMS) actuator. more...

Subharmonic imaging is used for nondestructive imaging with the contrast agent microbubble MP1950.

Statistical fluctuations of backscatter signals are examined to obtain additional information for characterizing tissues using ultrasound.

An efficient, accurate 3-D image-formation algorithm is obtained by means of simple approximations to the point-spread function for 3-D wave propagation in cylindrical coordinates. Possible applications include high-quality, near-diffraction-limited imaging from small cylindrical platforms, such as needles or catheters. more...

A method of moment (MoM) is employed for designing high-temperature superconductive (HTS) RF surface coils for magnetic resonance imaging (MRI) is described and discussed.

A double-probe reflection ultrasonic nondestructive testing (NDT) imaging method is described. The method is tested and verified by means of laboratory experiments.

A common random input filter model is used to estimate and correct the aberration in ultrasonic b-scan imaging (BSI). Results indicate that the proposed approach can improve both the receive focus and the transmit focus. more...

Ultrasound and albumin-stabilized dodecafluoropentane (DDFP) droplets, injected intravenously and intraarterially, were used to create gas bubbles to potentially cause ischemia or to generate point beacons for phase-aberration correction. Twelve canines were used in this preliminary study, and overall, acoustic droplet vaporization (ADV) worked well in vivo. more...

An analysis is presented of the frequency separation needed to obtain independent values of received power from a random medium in a continuous wave (CW) ultrasound arrangement. The analysis corrrlates well with experimental data. more...

Decorrelation characteristics of transverse blood flow, using an intravascular ultrasound (IVUS) array catheter, are examined by means of computer modeling. Results suggest that decorrelation properties of an IVUS array catheter will not be affected by different transverse blood-flow conditions. more...

Clutter filtering algorithms are investigated, using both experimental data and simulations. It is shown that realistic accelerations of tissue have a significant effect on clutter rejection.

A new method for Doppler angle estimation is proposed. The method, which relies on an autoregressive (AR) model, requires a small number of flow samples.

Diffractionless solutions of the wave equation in the form of X-waves have already been derived, based on the inviscid form of the wave equation. Here, a general solution to the cylindrically symmetric wave equation for a medium with classical viscous is discussed. more...

Efficacy of the Nakagami distribution in tissue characterization is discussed. Results of a receiver operating characteristics (ROC) analysis indicate that the described technique can be used to classify lesions in breast tissue as benign or malignant. more...

Calibration techniques that rely on thermal sensors have a potential to be used for determining acoustic power radiated by ultrasound transducers. Performance of thermoacoustic sensors as a function of operational parameters is investigated. more...

A tissue axial stretching model is described, to study the effects of tissue lateral displacement on echo decorrelation, and a concept called correlation signal-to-noise ratio (CSNR) is used to quantify the decorrelation effect. It is shown that CSNR can be used as a quality indicator of elastography, and a 2-D spatial comprehensive cross-correlation technique can reduce decorrelative noise while slightly decreasing the lateral resolution of the strain image. more...

A new approach for focusing ultrasound energy in brain tissue, through the skull, is examined. The procedure relies on a therapeutic transducer array in combination with a small catheter-inserted hydrophone receiver, used to guide the array's focus. more...

Propagation of low frequency (LF) shear waves can be used to determine tissue parameters such as elasticity. A new technique for measuring the shear velocity in soft tissues is proposed.

Propagation of transient low frequency (LF) shear waves is examined in tissue-equivalent phantoms by means of ultra-fast ultrasonics. A vibrating device is optimized to induce large displacements in the image area, and quantitative information is obtaines by means of a local inversion algorithm. more...

Design, fabrication, and performance testing of two-dimensional (2-D) array transducer, built on a silicon substrate, is discussed. The device fits inside a catheter and can be used for real-time intracardiac volumetric imaging. more...

The feasibility of two-dimensional motion- and strain-rate estimation of the human heart in vivo is investigated.

A method is described for calculating the pressure inside solid, absorbent, cylindrical shells with plane wave harmonic insonation. The effects of refraction and internal lumen reflections are calculated. more...

An architecture is described, for a portable, real-time, 3-D sparse synthetic aperture ultrasound beamformer is discussed. The beamformer can deliver frame rates of up to 1700 frames/s.

Results of an attempt to develop a 30MHz linear array are reported. design goals are achieved, and coupling with an appropriate beamformer should improve performance and make clinical applications possible. more...

A real-time rectilinear volumetric scanning system is described. Such a system could be useful for abdominal, breast, or vascular imaging, especially for real-time interventional radiological procedures. more...

A Fourier-Bessel technique is proposed, for characterizing the field from monochromatic flat annular arrays as a set of limited diffraction Bessel beams. Two transducers, previously described in the literature, are presented as examples, and new insights into their operation are provided. more...

A new compounding approach is described, for reducing speckle brightness variations. The approach makes use of the decorrelation between signals under diverse strain states.

It is proposed that a ligand-targeted, lipid-encapsulated, liquid perfluorodichlorooctane ultrasonic contrast system system with a small nominal particle size be approximated as a simple, thin layer, acoustic transmission line. This model is evaluated, and the ultrasonic reflectivity of different perfluorocarbon formulations with varying acoustic impedances that target either nitrocellulose membranes or plasma thrombi in vitro are compared. more...

A three-parameter model for describing the ultrasonic backscattered echo is proposed. The method uses Raleigh, K, and Nakagami distributions.

It is known that ultrasound-induced lung hemmorrhages can occur in mice, rats, rabbits, pigs and monkeys. Here, the role of beam diameter incident on lung surface on lesion threshold and size is examined. more...

A theory is described that evaluates statistics of the integrated backscatter (IBS) from moving blood, or blood-mimicking fluid (bmf).

Methods for improving the contrast-to-noise ratio (CNR) of low-contrast lesions in medical ultrasound imaging are described. Differences in the frequency spectra and amplitude distributions of the lesion and its surroundings can be used to increase the CNR of the lesion relative to the background. Automated graylevel mapping is used in combination with a contrast-weighted form of frequency-diversity speckle reduction. In clinical studies, the techniques have yielded mean CNR improvements of 3.2 dB above ordinary frequency-diversity imaging and 5.6 dB over sharper conventional images, with no post-processing graylevel mapping. Index Terms - Frequency dependence of ultrasonic backscatter, frequency-diversity ultrasonic imaging, grayscale mapping. more...

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 5MHz 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%. Index Terms - Backscattering, focused transducer, ultrasound. more...

This study was designed to evaluate the feasibility of using cylindrical ultrasound transducers mounted on a catheter for the ablation of cardiac tissues. In addition, the effects of ultrasound frequency and power was evaluated both using computer simulations and in vitro experiments. Frequencies of 4.5, 6, and 10 MHz were selected based on the simulation studies and manufacturing feasibility. These transducers were mounted on the tip of 7-French catheters and applied in vitro to fresh ventricular canine endocardium, submerged in flowing degassed saline at 37 [degrees] C. When the power was regulated to maintain transducer interface temperature at 90-100 [degrees] C, the 10-, 6-, and 4.5-MHz transducers generated a lesion depth of 5.9 [+ or -] 0.2 mm, 4.6 [+ or -] 1.0 mm, and 5.3 [+ or -] 0.9 mm, respectively. The 10-MHz transducer was chosen for the in vivo tests since the maximum lesion depth was achieved with the lowest power. Two dogs were anesthetized and sonications were performed in both the left and right ventricles. The 10-MHz cylindrical transducers caused an average lesion depth of 6.4 [+ or -] 2.5 mm. In conclusion, the results show that cylindrical ultrasound transducers can be used for cardiac tissue ablation and that they may be able to produce deeper tissue necrosis than other methods currently in use. Index Terms - Arrhythmia, cardiac, therapy, ultrasound. more...

A theoretical and numerical study of the acoustic field intensity within a curved flow conduit having 1) diameter similar to the wavelength of the interrogating frequency and 2) speed of sound mismatch with the surrounding medium is presented. The field intensity is shown to vary significantly and in a monotonic fashion across the flow conduit. The resulting insonation of emboli transiting through the Doppler sample volume is explored with a Monte Carlo study of the behavior of the embolus to blood power ratio (EBR). The numerical simulation findings are shown to be in good agreement with previously reported experimental results. A method is explored for estimating embolus diameter when this refraction artifact is present, and shown to yield excellent results when applied to experimental data. Further work toward clinical application of these results is discussed. more...

Backscattering of an incident ultrasound beam by blood gives rise to a Doppler spectrum whose characteristics are affected not only by the velocity distribution, but also by certain basic aspects of the RBC's behavior. Starting from fundamental assumptions, an explicit expression is derived for the Doppler spectrum in terms of the variance in the scatterer number density and the backscattering cross section. This shows that the Doppler power at a given frequency is weighted by the backscattering cross section and the manner in which the red blood cells (RBC's) are packed (packing factor). Since spatial variations in the flow field can result in changes in the variance and backscattering cross section, the mean Doppler frequency will not necessarily be proportional to the mean flow through the sample volume. Experimental results for two different flow fields are used to illustrate these effects. The implications of these findings are discussed in relation to volumetric flow estimation and the power mode display used in some color Doppler flow imaging systems. more...

A method is described for generating ultrasound focus patterns for ultrasound hyperthermia treatment planning for steady state and transient hyperthermia. The solution for placement and intensity of ultrasound focus points is based on two types of temperature constraints: 1) equality constraints on the tumor boundary (temperature is held at maximum safe level), and 2) inequality constraints in the tumor interior (in a therapeutic range of temperatures). The method employs a simplex algorithm to solve a series of linear equations which approximate the heating distribution in tissue. Examples are given for field conjugate acoustic lens applicators capable of generating multiple loci simultaneously. more...

A theory describing pulse Doppler ultrasound signals due to backscattering due to emboli in flowing blood is presented. From this theory, the minimum detectable size of a formed-element embolus can be established as a function of carrier frequency and vessel size. Emboli can be sized and characterized, based on the ratio of the amplitude of the Doppler signal during embolus passage through the sample volume to background bloodflow Doppler signal when no embolus is present. This ratio is defined as the 'embolus to blood ratio' (EBR). Size estimation of emboli can be done by insonating an embolus with a single frequency and measuring the EBR, only if the embolus does not exceed a certain size, and if the vessel diameter and per cent hematocrit are known. Using two different frequencies, the vessel geometry (diameter and sample volume length) and per cent hematocrit can be eliminated from calculation of embolus size. Sources of uncertainty in the EBR and their effect on embolus size estimation are discussed. Discrimination between gas and formed-element emboli is described, given a detector with sufficient dynamic range, and use of three carrier frequencies. The theory presented here is in agreement with experimental findings of other investigators. more...

In order to diagnose ventricular dysfunction based on the acoustic characteristics of the heart muscle of the ventricle, it is necessary to detect vibration signals from various parts of the ventricular wall. This is, however, difficult using previously proposed ultrasonic diagnostic methods or systems. The reason is that the amplitude of the cardiac motion is large during one beat period which produces large fluctuations in the transit time required for ultrasonic waves to travel from the transducer to the heart and back. This paper proposes a new method for overcoming this problem and accurately measuring small vibrations of the ventricle wall using ultrasound. In this method, the demodulated ultrasound signal reflected at the heart wall is converted from analogue to digital (A/D) signal at a high sampling frequency; from the resultant digital signal, the velocity of the wall is accurately obtained over a wide dynamic range based on the Doppler effect. The proposed method is preliminarily applied to the detection of small vibrations on the aortic wall and the interventricular septum. The new method offers potential for research in acoustical diagnosis of heart and artery dysfunction. more...

Phased-array technology offers an incredible advantage to therapeutic ultrasound due to the ability to electronically steer foci, create multiple foci, or to create an enlarged focal region by using phase cancellation. However, to take advantage of this flexibility, the phased-arrays generally consist of many elements. Each of these elements requires its own radio-frequency generator with independent amplitude and phase control, resulting in a large, complex, and expensive driving system. A method is presented here where in certain cases the number of amplifier channels can be reduced to a fraction of the number of transducer elements, thereby simplifying the driving system and reducing the overall system complexity and cost, by using isolation transformers to produce 180 [degrees] phase shifts. Index Terms--Hyperthermia, phased arrays, therapy, surgery, thermal therapy, ultrasound, ultrasound surgery. more...

Suppression of clutter signals in ultrasound color-flow imaging is discussed. Three classes of filters are considered: finite impulse response (FIR), infinite impulse response (IIR), and regression filters. more...