Spatial compounding has been used for years to reduce speckle in ultrasonic images and to resolve anatomical features hidden behind the grainy appearance of speckle. Adaptive imaging restores image contrast and resolution by compensating for beamforming errors caused by tissue-induced phase errors. Spatial compounding represents a form of incoherent imaging, whereas adaptive imaging attempts to maintain a coherent, diffraction-limited aperture in the presence of aberration. Using a Siemens Antares scanner, we acquired single channel RF data on a commercially available 1-D probe. Individual channel RF data was acquired on a cyst phantom in the presence of a near field electronic phase screen. Simulated data was also acquired for both a 1-D and a custom built 8x96, 1.75-D probe (Tetrad Corp.). The data was compounded using a receive spatial compounding algorithm; a widely used algorithm because it takes advantage of parallel beamforming to avoid reductions in frame rate. Phase correction was also performed by using a least mean squares algorithm to estimate the arrival time errors. We present simulation and experimental data comparing the performance of spatial compounding to phase correction in contrast and resolution tasks. We evaluate spatial compounding and phase correction, and combinations of the two methods, under varying aperture sizes, aperture overlaps, and aberrator strength to examine the optimum configuration and conditions in which spatial compounding will provide a similar or better result than adaptive imaging. We find that, in general, phase correction is hindered at high aberration strengths and spatial frequencies, whereas spatial compounding is helped by these aberrators.

Adaptive beamforming has been widely used as a way to correct phase and amplitude aberration errors in medical ultrasound. A less-studied concern in ultrasound beamforming is the deleterious contribution of off-axis bright targets. We describe a new approach, the constrained adaptive beamformer (CAB), which builds on classic array processing methods. Given a desired frequency response in the focal direction, the CAB dynamically imposes an optimal set of time-dependent weights on the receive aperture, reducing signals from directions other than the focal direction. Two implementations of the CAB are presented which differ in their use of calculated weights to form an output image: the Single Iteration CAB and Multiple Iteration CAB. We present results from experiments performed on a Philips SONOS 5500 imaging system operating with an 8.7 MHz linear array and contrast the performance of the two CAB implementations. Data was acquired from wire targets in a water tank and low echogenicity cysts in a grayscale tissue mimicking phantom. The desired system frequency response was specified by a FIR filter with the same center frequency as the transducer. Improvements in lateral resolution for wire targets and contrast for low echogenicity cysts are shown. Simulations are used to demonstrate limitations of the CAB.

The Karmanos Cancer Institute is developing an ultrasound device for measuring and imaging acoustic parameters of human tissue. This paper discusses the experimental results relating to tomographic reconstructions of phantoms and tissue. The specimens were scanned by the prototype scanner at a frequency of 1.5 MHz using 2 microsecond pulses. The receivers and transmitters were positioned along a ring trajectory having a diameter of 20 cm. The ring plane is translated in the vertical direction allowing for 3-D reconstructions from stacked 2-D planes of data. All ultrasound scans were performed at 10 millimeter slice thickness to generate multiple tomographic images. In a previous SPIE paper we presented preliminary results of ultrasound tomographic reconstruction of formalin-fixed breast tissue. We now present new results from data acquired with the scanner. Images were constructed using both reflection-based and transmission based algorithms. The resulting images demonstrate the ability to detect sub-mm features and to measure acoustic properties such as sound speed. Comparison with conventional ultrasound indicates the potential for better margin definition and acoustic characterization of tissue.

A novel system for High-resolution Ultrasonic Transmission Tomography (HUTT) is presented. The critical innovation of the HUTT system includes the use of sub-millimeter transducer elements for both transmitter and receiver arrays and multi-band analysis of the first-arrival pulse. The first-arrival pulse is detected and extracted from the received signal (i.e., snippet) at each azimuthal and angular location of a mechanical tomographic scanner in transmission mode. Each extracted snippet is processed to yield a “multispectral” vector of attenuation values at multiple frequency bands. Other acoustic attributes of the object (such as time-of-flight or wavelet decomposition coefficients) can also be obtained through snippet analysis. These vectors form a 3-D sinogram representing a multispectral augmentation of the conventional 2-D sinogram. A filtered backprojection algorithm is used to reconstruct a stack of multispectral images for each 2-D tomographic slice that may allow tissue characterization and improved image segmentation. We present illustrative examples of 2-D images formed at various frequency bands to demonstrate the high-resolution capability of the system and the potential of multispectral analysis. It is shown that spherical objects with diameter down to 0.3mm can be detected. Reconstruction of 3-D images has been achieved using multiple 2-D slices with sub-mm elevation differences.

In diagnostic ultrasound, tissue differentiation is essential to detect lesions or cancerous tissues from normal tissue. The attenuation characteristics of various tissues will be different at different frequencies, since the propagating ultrasonic pulse undergoes frequency-dependent attenuation, that is characteristic of the material it traverses. These vectors of attenuation values at different frequency bands represent multi-band characteristics of individual pixels (termed “multispectral”) that can be used for tissue differentiation akin to color. In this study, we have developed tissue differentiation methods that utilize the multispectral signatures of different materials in multi-band images produced by a newly built high-resolution ultrasonic transmission tomography (HUTT) system. The HUTT system obtains 3-D multi-band sinograms through FFT analysis of the first arriving pulse (snippet). A filtered backprojection algorithm is utilized to reconstruct a stack of multi-band attenuation images that contain multispectral signatures for each pixel and represent a multispectral augmentation of the 2-D conventional tomographic slice. To differentiate each tissue type according to its characteristic multispectral signature, we adopt the methods of spectral unmixing and spectral target detection. We demonstrate the feasibility of tissue differentiation using multi-band/multispectral signatures of different tissue objects in initial data collected from soft animal tissue phantoms.

Reflectivity tomography is an imaging technique that seeks to reconstruct the reflectivity distribution that characterizes a weakly reflecting object. As in other tomographic imaging modalities, in certain applications of reflectivity tomography it may be necessary to reconstruct an accurate image from measurement data that are incomplete, e.g., reduced-scan measurement data. Recently, we have developed a so-called 'potato peeler' perspective for heuristically demonstrating the possibility of reconstructing accurate images from reduced-scan measurement data. In this work we describe a mathematical formulation of the potato peeler perspective, which provides a theoretical justification for the development and application of reduced-scan reconstruction algorithms in reflectivity tomography. Simulation results are presented to corroborate our theoretical assertions.

A method for reconstructing the image of loss-less object from the measurement of its scattering amplitude is developed for the case of acoustic wave scattering from an inhomogeneous medium consisting of velocity fluctuations (Helmholtz equation). The inversion procedure is exact, and explicitly takes into account all orders of multiple scattering. An interesting feature is that progressively higher resolution of the recovered object is obtained as higher order scattering is progressively incorporated into the inversion procedure. Essentially, the inversion technique, which is based on a consideration of the analyticity properties of the scattering amplitude in the complex scattering angle domain, recovers the Laplace domain representation of the interaction from the angle-scattered data. Although the method described here assumes rotational symmetry, the technique is essentially three-dimensional. However, the method suffers from the disadvantage that the analytic continuation procedures it depends on would be noise sensitive, in practice.

The precise knowledge of the electromechanical properties of an ultrasonic transmit-receive system can be used to optimize the excitation waveform in transmission-mode tomographic imaging. Although a linear system hypothesis is often postulated to model the dynamic transformation of the excitation waveform delivered at the transducer of the transmitter (input) into the received waveform at the receiver (output), linearity may not be appropriate in order to account for the actual dynamic characteristics of the system. In this work, we use a nonlinear system modeling/identification method to find a mathematical model of the nonlinear dynamic transformation between the excitation and received signals. The method employs the Laguerre-Volterra Network that has been successfully applied to modeling dynamic physiological systems. The obtained nonlinear model can be used to derive an optimal excitation waveform that produces the maximum peak value of the received signal for given power of the excitation signal. Using experimental data from a commercial ultrasonic array, we show how an optimal excitation signal can be derived from the obtained nonlinear model that maximizes the peak received value. We also demonstrate that the optimally designed excitation waveform offers significant performance improvement over conventional pulse waveforms (~35 times greater peak value).

2-2 piezocomposite materials are widely used for ultrasonic transducers in medical ultrasound imaging and underwater acoustics. An important issue in 2-2 piezocomposite transducer designs is to avoid spurious lateral modes. We proposed a new method to solve the lateral mode problem in this paper. A 30-element 2-2 piezocomposite transducer composed of PZT-5H and five different types of polymers were studied by using ANSYS finite element software. Using a 2-D model of the transducer the electrical admittances were calculated within the interested frequency range. The results show that there is a strong coupling between the thickness mode and the first lateral mode when any one type of polymer is used in the transducer design. However, the lateral mode is greatly suppressed when all of these polymers are used, and the electromechanical coupling coefficient for the thickness mode is also increased. The analysis further shows that the reduction of the lateral mode is only related tothe shear velocity of the polymer, while the density and longitudinal velocity of the polymer have little effect on it.

Using a scanned laser to generate ultrasound, via the thermoelastic effect, offers an alternative approach for realizing high density, high frequency ultrasound imaging arrays. The approach bypasses the complexity and intricacy required for forming conventional piezoelectric array elements and their associated electrical connections. Thus, it is particularly well suited to 2D arrays. In this paper, the devices considered comprise a carbon black loaded PDMS polymer layer on top of a glass or PDMS substrate. PZFlex Finite Element Analysis (FEA) was used to investigate the impact of a variety of design variables including: laser spot size, substrate material and thermoelastic coupling medium. Predicted single element angular response broadly matched responses obtained by experiment. Specifically, if a low acoustic loss glass substrate is used then measurable sidelobes occur at approximately 40 degrees. However, if the glass substrate is replaced by a PDMS material, then the traveling waves that give rise to sidelobes are no longer supported and a smooth single element angular response is obtained in both experiment and FEA simulation. FEA suggests that there are other modes in addition to the Rayleigh mode observed in the experiment. It is believed that these modes are more quickly damped in the experimental case. Therefore, while FEA provides a very versatile and valuable analysis tool, the accuracy of its predictions are contingent on accurate knowledge of device geometry and relevant material properties.

In this work we have investigated the influence of the backing layer composition and the matching layer thickness in the performance of ultrasound transducers constructed with piezoelectric ceramic disks. We have constructed transducers with backing layers of different compositions, using mixtures of epoxy with alumina powder and/or Tungsten powder and with λ/4 or 3λ/4 thickness epoxy matching layers. The evaluation tests were performed in pulse-echo mode, with a flat target, and in transmission/reception mode, with a calibrated PVDF hydrophone. The acoustical field emitted by each transducer was mapped in order to measure the on-axis and transverse field profiles, the aperture size and the beam spreading. The bandwidths of the transducers were determined in pulse-echo mode. Comparing the evaluation tests results of two transducers constructed with the same backing layer, the one constructed with λ/4 thickness epoxy matching layer showed better performance. The results showed that the transducers constructed with epoxy, alumina and Tungsten powders backing layers have larger bandwidth. The larger depth of field was measured for transducers constructed with epoxy and Tungsten powder backing layers. These transducers and those constructed with epoxy, Tungsten and alumina powders backing layers showed the larger field intensities in the measured transverse profiles.

The characterization and calibration of piezoelectric transducer beam profiles form an essential pre-requisite for ultrasonic applications in non-destructive evaluation (NDE) and medical imaging. Although considerable information is available concerning transducer temporal behavior and spatial field characteristics, the influence of the transmitting electronics, on transmitted pulse and transducer beamforming has been somewhat neglected. This work involves a study of electrical factors which control the beam profiles of wide band piezoelectric transducers usually used in medical and NDE application. The influence of the transmitting electronics, such as; turn-on time and matching circuitry, was examined with respect to pulse shape and frequency spectrum and, more importantly, transducer beam profiles. These factors and method of detection are shown to exert a considerable influence on measured beam profiles.

Trabecular thickness within cancellous bone is an important determinant of osteoporotic fracture risk. Noninvasive assessment of trabecular thickness could potentially yield useful diagnostic information. Faran's theory of elastic scattering from a cylindrical object immersed in a fluid has been employed to predict the dependence of ultrasonic backscatter on trabecular thickness. Methodology to test this theory has been validated in experiments using nylon fishing lines spanning a wide range of diameters. In the case of bone, Faran's theory predicts that, in the range of morphological and material properties expected for trabecular bone, the backscatter coefficient at 500 kHz should be approximately proportional to trabecular thickness to the power of 2.9. Experimental measurements of backscatter coefficient were performed on 43 human calcaneus samples in vitro. Mean trabecular thicknesses on the 43 samples were assessed using micro computed tomography. A power law fit to the data showed that the backscatter coefficient empirically varied as trabecular thickness to the 2.8 power. The 95% confidence interval for this exponent was 1.7 to 3.9. The square of the correlation coefficient for the linear regression to the log transformed data was 0.40. This suggests that 40% of variations in backscatter may be attributed to variations in trabecular thickness. These results (1) reinforce previous studies that offered validation for the Faran cylinder model for prediction of scattering properties of cancellous bone, and (2) provide added evidence for the potential diagnostic utility of the backscatter measurement.

Studies suggest that the composition of atherosclerotic plaque in the carotid arteries is predictive of stroke risk. The goal of this investigation has been to explore how well the true integrated backscatter (IBS) from plaque regions can be measured non-invasively using ultrasound, based on which plaque composition may be inferred. To obtain the true arterial IBS non-invasively, the scattering and aberrating effect of the intervening tissue layers must be overcome. This is achieved by using the IBS from arterial blood as a reference backscatter, specifically the backscatter from a blood volume along the same scan line as and adjacent to the region of interest. The arterial blood IBS is obtained as an estimated mean of a stochastic process, after clutter removal. We have shown that the variance of the IBS estimate of the blood backscatter signal can be quantified and reduced to a tolerable level. The results are in the form of IBS profiles along the vessel. IBS profiles not normalized with the IBS of the blood-mimicking fluid have been measured for vessels phantom, with and without an intervening inhomogeneous medium; these results are contrasted with the corresponding normalized IBS profiles.

The assessment and characterization of the endothelial function is a current research topic as it may play an important role in the diagnosis of cardiovascular diseases. Flow mediated dilatation may be used to investigate endothelial function, and B-mode ultrasonography is a cheap and non-invasive way to assess the vasodilation response. Computerized analysis techniques are very desirable to give higher accuracy and objectivity to the measurements. A new method is presented that solves some limitations of existing methods, which in general depend on accurate edge detection of the arterial wall. This method is based on a global image analysis strategy. The arterial vasodilation between two frames is modeled by a superposition of a rigid motion model and a stretching perpendicular to the artery. Both transformation models are recovered using an image registration algorithm based on normalized mutual information and a multi-resolution search framework. Temporal continuity of in the variation of the registration parameters is enforced with a Kalman filter, since the dilation process is known to be a gradual and continuous physiological phenomenon. The proposed method presents a negligible bias when compared with manual assessment. It also eliminates artifacts introduced by patient and probe motion, thus improving the accuracy of the measurements. Finally, it is also robust to typical problems of ultrasound, like speckle noise and poor image quality.

Most of the quantitative measures from Intravascular Ultrasound (IVUS) images vary with the cardiac cycle. Although ECG-gated acquisition can prevent the pulsations from influencing the measurements, it may extend the acquisition time, and furthermore, very few IVUS systems currently in clinical use incorporate ECG-gated function. In this paper, we present a practical method to retrieve cardiac phase information directly from in vivo clinical IVUS image sequences. In an IVUS image that contains a cross-section of coronary artery, there are three regions annularly distributed from the center of the image - catheter, lumen, and part of the vessel wall. The catheter region exhibits virtually no change from frame to frame during the catheter pullback. While the lumen is a dark region, the vessel wall region appears bright. The change in lumen size and position that accompanies the pulse causes the image intensity of the IVUS images to exhibit a periodic variation along the pullback path. By extracting this signal attributed to the cardiac cycle, a subsequence of frames during pullback at the same phase of the cardiac cycle can be selected. The method was tested by the IVUS images of both a coronary phantom and a patient.

We propose a technique that allows the improvement of lateral resolution in ultrasound imaging using a deconvolution-based strategy. We first derive a formulation for the problem in terms of an arbitrary spatially-variant beam pattern and show that it is possible to optimally estimate the values of the image by solving a large linear inverse problem. This linear system depends on the shape and extent of the point spread function as well as the desired resolution of the resultant image. We show that this linear system is sparse and therefore sparse matrix techniques for storage and algebra are used to make the computational cost reasonable. The strategies used to solve this problem are proposed based on truncated singular value decomposition or regularized conjugate gradient method that allows an equivalent regularization to imposing a quadratic inequality constraint. This allows the condition number of the problem to be kept sufficiently low, thus ensuring a robust solution. For a given ultrasound line with specific transmit and receive focusing characteristics, this problem is solved for the whole image and we show that it is possible to implement the solution in a look-up table form similar to what is used in image reconstruction in current ultrasound systems. This accounts for the variations of the point spread function at different spatial positions.

The use of speckle decorrelation techniques to calculate the displacement of a moving transducer has demonstrated promise. We introduce a technique to estimate distance between image planes without assuming a constant transducer velocity. We developed a theoretical analysis of the uncertainty in estimated plane spacing as a function of speckle size, patch size and the number of planes used for normalization. The best estimates of plane spacing are obtained when the distance between acquired image planes is of the order of half the speckle size. In this region, the uncertainty in estimated plane spacing was < 15% for an 8.1mm (axial) x 9.1mm (lateral) patch, increasing to 33% for an 8.1mm x 1.5mm patch. Patch size is limited to regions of fully developed speckle, and also by brightness gradients in the image. Although brightness gradients seem insignificant on intensity images, they can cause a large bias in plane spacing estimates made using linearized data. Another major source of bias in plane spacing estimates was the number of planes, N_z, used to calculate the normalization factors (averages of brightness and squared brightness). Optimum N_z was found to be 5 to 10 planes, depending on plane spacing.

We have integrated real time volumetric ultrasound imaging and ultrasound ablation in the same intracardiac catheter. This single device could be used to visualize ablation sites in three dimensions immediately prior to inducing necrosis to eliminate cardiac arrythmias. After the course of therapy, the ablated tissue could be examined ultrasonically. The 12 Fr catheter includes a 2D transducer array for imaging and a single element piston for ablation. The imaging transducer consists of 38 active elements built on a multilayer flex circuit operating at 5.2 Mhz. The ablation piston is a 4 mm by 2 mm piece of air backed PZT-4. Our real time 3D scanner (Volumetrics Medical Imaging) and the 2D array were used to image phantom targets. The spatial peak, temporal average intensity (I_SPTA) and acoustic power of the ablation beam were measured using a hydrophone. A 7 mm thick slab of beef was imaged and then ablated for 1 minute. The ultrasound ablation piston produced an I_SPTA of nearly 30 W/cm² and a corresponding acoustic power of 2.6 W. The electrical to acoustic power efficiency of the transducer was 39%. The minute long ablation produced a transmural lesion in the beef 2mm by 4 mm by 7 mm deep.

Recent studies have shown that radio frequency (RF) ablation is a simple, safe and potentially effective treatment for selected patients with liver metastases. Despite all recent therapeutic advancements, however, intra-procedural target localization and precise and consistent placement of the tissue ablator device are still unsolved problems. Various imaging modalities, including ultrasound (US) and computed tomography (CT) have been tried as guidance modalities. Transcutaneous US imaging, due to its real-time nature, may be beneficial in many cases, but unfortunately, fails to adequately visualize the tumor in many cases. Intraoperative or laparoscopic US, on the other hand, provides improved visualization and target imaging. This paper describes a system for computer-assisted RF ablation of liver tumors, combining navigational tracking of a conventional imaging ultrasound probe to produce 3D ultrasound imaging with a tracked RF ablation device supported by a passive mechanical arm and spatially registered to the ultrasound volume.

An approach for acquiring 3D data using a modified transducer array was previously described. This array employs one “Imaging” array and two “Tracking” arrays placed perpendicular to the Imaging array. Any component of diagonal motion due to imperfect elevational scanning has the potential to cause dimensional error in the 3D reconstruction. In this paper, diagonal motion is determined by examining the ratio of cross-correlation values, between successive image frames, from both the Imaging array data sets and the Tracking array data sets. The elevational translation is computed using a speckle tracking method on the Tracking array data if there is more elevational motion than azimuthal motion. Similarly, the speckle tracking is performed on the Imaging array data if there is more azimuthal motion. The angle of motion and the translational component derived from one of the two orientations of arrays allows computation of the component of translation in the other direction. MATLAB simulations and experimental result illustrate that the error in speckle tracking was dependent on the angle of the diagonal motion, and that there were distinct rates of decorrelation from each array for different diagonal motions.

This paper proposes a three-dimensional (3D) region-based segmentation algorithm for extracting a diagnostic tumor from ultrasound images by using a split-and-merge and seeded region growing with a distortion-based homogeneity cost. In the proposed algorithm, 2D cutting planes are first obtained by the equiangular revolution of a cross sectional plane on a reference axis for a 3D volume data. In each cutting plane, an elliptic seed mask that is included tightly in a tumor of interest is set. At the same time, each plane is finely segmented using the split-and-merge with a distortion-based cost. In the result segmented finely, all of the regions that are across or contained in the elliptic seed mask are then merged. The merged region is taken as a seed region for the seeded region growing. In the seeded region growing, the seed region is recursively merged with adjacent regions until a predefined condition is reached. Then, the contour of the final seed region is extracted as a contour of the tumor. Finally, a 3D volume of the tumor is rendered from the set of tumor contours obtained for the entire cutting planes. Experimental results for a 3D artificial volume data show that the proposed method yields maximum three times reduction in error rate over the Krivanek’s method. For a real 3D ultrasonic volume data, the error rates of the proposed method are shown to be lower than 17% when the results obtained manually are used as a reference data. It also is found that the contours of the tumor extracted by the proposed algorithm coincide closely with those estimated by human vision.

Estimation of the mechanical properties of the cardiac muscle has been shown to play a crucial role in the detection of cardiovascular disease. Elastography was recently shown feasible on RF cardiac data in vivo. In this paper, the role of elastography in the detection of ischemia/infarct is explored with simulations and in vivo experiments. In finite-element simulations of a portion of the cardiac muscle containing an infarcted region, the cardiac cycle was simulated with successive compressive and tensile strains ranging between -30% and 20%. The incremental elastic modulus was also mapped uisng adaptive methods. We then demonstrated this technique utilizing envelope-detected sonographic data (Hewlett-Packard Sonos 5500) in a patient with a known myocardial infarction. In cine-loop and M-Mode elastograms from both normal and infarcted regions in simulations and experiments, the infarcted region was identifed by the up to one order of magnitude lower incremental axial displacements and strains, and higher modulus. Information on motion, deformation and mechanical property should constitute a unique tool for noninvasive cardiac diagnosis.

This paper introduces strain-flow imaging as a new technique for investigating vascular dynamics and tumor biology. The deformation of tissues surrounding pulsatile vessels and the velocity of fluid in the vessel are estimated from the same data set. The success of the approach depends on the performance of a digital filter that must separate echo signal components caused by flow from tissue motion components that vary spatially and temporally. Eigenfilters, which are an important tool for naturally separating signal components adaptively throughout the image, perform very well for this task. The method is examined using two tissue-mimicking flow phantoms that provide stationary and moving clutter associated with pulsatile flow.

Acoustic radiation force may be used to induce localized displacements within tissue. This phenomenon is used in Acoustic Radiation Force Impulse Imaging (ARFI), where short bursts of ultrasound deliver an impulsive force to a small region. The application of this transient force launches shear waves which propagate normally to the ultrasound beam axis. Measurements of the displacements induced by the propagating shear wave allow reconstruction of the local shear modulus, by wave tracking and inversion techniques. Here we present in vitro, ex vivo and in vivo measurements and images of shear modulus. Data were obtained with a single transducer, a conventional ultrasound scanner and specialized pulse sequences. Young's modulus values of 4 kPa, 13 kPa and 14 kPa were observed for fat, breast fibroadenoma, and skin. Shear modulus anisotropy in beef muscle was observed.

We have developed a new method of imaging the mechanical properties of tissues based on very brief (<1msec) and localized applications of acoustic radiation force and the ultrasonic measurement of local tissues' responses to that force. Initial results with this technique demonstrate its ability to image mechanical properties of the medial and adventitial layers within ex vivo and in vivo arteries, and to distinguish hard and soft atherosclerotic plaques from normal vessel wall. We have labeled this method Acoustic Radiation Force Impulse (ARFI) imaging. We describe studies to utilize this technique in the characterization of diffuse and focal atherosclerosis. We describe phantom trials and finite element simulations which explore the fundamental resolution and contrast achievable with this method. We describe in vivo and ex vivo trials in the popliteal, femoral and brachial arteries to assess the relationship between the mechanical properties of healthy and diseased arteries provided by this method and those obtained by alternative methods.

Several techniques have been developed in an effort to estimate mechanical properties of tissues. In this paper, we will discuss the advantages of utilizing a new technique that performs RF signal tracking in order to estimate the localized oscillatory motion resulting from a harmonic radiation force produced by two focused ultrasound transducer elements with overlapping beams oscillating at distinct frequencies. Finite-element and Monte-Carlo simulations were performed in order to characterize the range of oscillatory displacements produced by a harmonic radiation force. The frequencies investigated ranged from 200 Hz to 800 Hz and the stiffness between 20 and 80 kPa. In the experimental verification, three transducers were utilized: two focused transducers at 3.75 MHz and a diagnostic transducer 1.1 MHz operating at pulse/receive mode. Agar gels of 7 - 95 kPa stiffness were utilized. Displacement estimates were obtained during the application of the radiation force oscillating at frequencies of 200 - 800 Hz. In experiments, the estimated oscillatory displacement spanned from -800 to 600 microns depending on the magnitude of the applied radiation force. A frequency upshift and an exponential displacement decrease were obtained with stiffness increase in experiments and simulations. These preliminary results demonstrate the feasibility of imaging localized harmonic motion as induced by an oscillatory ultrasound radiation force.

We describe a novel and robust strain estimation method, which is capable of fast, accurate strain estimation even in the presence of large signal decorrelation. Global temporal stretching of post-compression signals compensating for the applied strain significantly improves the "quality" of strain estimates. In a natural extension of this approach (adaptive stretching), a search is performed at each data window for the stretch factor that maximizes the correlation between the pre- and post-compression echo signal segments. Adaptive stretching performs well under harsh signal environments (because the correlation is maximized at each location); however it is computation intensive because many iterations may be required at each location. In contrast, global stretching is a fast algorithm, but performs well only in areas where local strains are close to the applied strain. The proposed method strikes a balance between the speed of global stretching and the performance of adaptive stretching. In this method, global stretching is performed with a range of different stretch factors and strain maps are computed for each stretch factor. The correlation between the pre- and post-compression echo segments is the maximum when the stretch factor corresponds to the local strain. Thus, the area in each computed strain image with strain values closely corresponding to the uniform stretch factor will contain "good quality" strain estimates. (Naturally, this area is different in each image.) To produce a single elastogram at the end, these strain maps are combined as follows. Correlation values quantify the "quality" of strain estimates; thus, at each location we identify the strain map with the maximum correlation, and the strain value in that strain map at that location is chosen for the combined map. Results from data generated by finite-element simulation and phantom experiments show that the variable stretching strain estimator is fast and is significantly less susceptible to signal degradation than conventional strain estimators.

We propose a new nonparametric technique for clutter rejection. We consider the Doppler data sampled using a sufficiently large dynamic range to allow for the clutter rejection to be implemented on the digital side. The Doppler signal is modeled as the summation of the true velocity signal, a clutter component, and a random noise component. To simplify the analysis, the first two components are assumed as deterministic yet unknown signals. The Doppler data are collected from the sample volume of interest as well as from several sample volumes in its neighborhood. Given that the shape of the clutter component will be similar in all these signals and given its relatively higher magnitude, it is possible to separate this component using principal component analysis (PCA). In particular, the clutter component appears as the first eigenvector (principal component) in PCA. Given this principal component, the projection of the Doppler signal of interest onto this component is removed and the remaining signal is subsequently used to derive the Doppler spectrogram. We describe an efficient implementation methodology that allows the added computational complexity of the new system to be reasonable.

We present the results of two studies investigating the optimal aperture configuration for maximized lateral blood flow velocity estimation using Heterodyned Spatial Quadrature. Our objective was to determine the maximum velocities that can be estimated at Doppler angles of 90 degrees and 60 degrees with a bias of less than 5% for both uniform scatterer motion in a tissue-mimicking phantom and blood-mimicking fluid circulated through a wall-less vessel flow phantom. Constant flow rates ranging from 3.0 to 18.0 ml/sec were applied in the flow phantom, producing expected peak velocities of 15.0 to 89.8 cm/sec under laminar flow conditions. Velocity estimates were obtained at each flow rate using 256 trials, with each trial consisting of an ensemble of 32 vectors. For an f/1 receive geometry with bi-lobed Hamming apodization, all peak flow velocities tested were estimated to within 5% of their expected values for both 90 degree and 60 degree Doppler angles. An f/2 receive geometry featuring bi-lobed Blackman apodization generally provided accurate lateral velocity estimates up to 71.9 cm/sec for a Doppler angle of 90 degrees, and accurate lateral component estimates up to 50.1 cm/sec for a 60 degree Doppler angle. The implications of these findings will be discussed.

This paper describes a new kind of clinical instrument designed to allow non-specialists to quantitatively measure blood velocity. The instrument's design utilizes vector continuous-wave (CW) Doppler. Vector CW Doppler insonates a volume with simultaneous multiple-angle beams that define a measurement region; within that region, the velocity vector of the blood can be measured independently of the probe orientation. By eliminating the need for simultaneous imaging and the specially trained technician required for the complicated instrument needed for such imaging, easy and inexpensive blood velocity measurements becomes possible. A prototype for a CW vector Doppler instrument has been used to measure blood velocity in several clinically important arteries: the radial and ulnar in the arm, the femoral in the leg, and the carotid in the neck. We report here on its first clinical use -- monitoring the flow in dialysis access grafts to prevent graft thrombosis. These early clinical results show accuracy and rapid learning of proper instrument use. The design approach presented shows much promise in creating instruments that will provide simple and low-cost-of-use procedures for measurement of blood velocity.

The measure of the regurgitant flow through heart valves provides an indication of the severity of the valve closure dysfunction with diagnostic relevance. The estimation of the volume passing through the closed valve during systole, and its ratio with the ejection volume, can significantly improve the assessment of an ongoing valvular pathology. The noninvasive quantification of flow converging to the valve is still lacking a satisfying degree of precision. The most popular technique is the Proximal Isovelocity Surface Area (PISA), which assumes that, in the flow field upstream of the valve, the surfaces corresponding to the same velocity are spherical, whence the regurgitant flow is estimated by multiplication with the hemispherical surface area. In the present study, a new method is proposed of color Doppler echocardiography image processing, for regurgitant flow measurements. In this method, called Proximal Arbitrary Surface Conservative Assessment of Leakage (PASCAL), the laws of fluid dynamics are used to reconstruct the entire flow field, in the hypothesis of axial symmetry, starting from the echographic Doppler mapping of one component of velocity. In vitro experiments have confirmed that the new method provides better flow estimates than PISA, on account of its more rigorous physical model of the regurgitant flow.

Here, we propose a new technique that allows the improvement of SNR without lowering the frame rate. The basic version of the proposed technique works by exciting multiple non-overlapping apertures at the same time. Assuming N apertures are to be acquired simultaneously, N acquisitions from such arrangement are made with different aperture coding in each acquisition. In particular, the excitation to each aperture is multiplied by a different predetermined factor for each acquisition. By properly selecting these factors, the problem of estimating the different lines become equivalent to solving a well-conditioned linear system of size N. Also, we show that such factors can be chosen to make the system matrix orthogonal, thus enabling simple solution to be obtained. We describe the implementation of such procedure and demonstrate that it is possible to achieve in practice even for systems in which the sign of excitation can only be changed. For more advanced system where coded excitations can be used, more accurate implementation taking into account element sensitivity variations within each probe for closer adherence to the desired characteristics. The proposed system results in noise reduction that is equivalent to averaging N acquisitions similar to phase encoding in magnetic resonance imaging. At the same time, the system maintains the same lateral resolution and frame rate as conventional acquisition strategies. We also discuss the use of sub-optimal overlapped apertures and describe the deterioration of SNR gain as a result of using independent rather than orthogonal apertures.

In many ultrasonic applications, such as ultrasonic imaging and signal processing, it is important that the transmitted pulse shape and ultrasonic beam profile remain consistent throughout the entire radiated field. The principles of diffraction strongly influence the ultrasonic beam profile and pulse shape and causes a considerable fluctuation in the near field region and a significant beam spreading in the far field region. In this work, different voltage distribution such as, logarithmic,linear and Gaussian across the transducer surface was investigated. The effect of a new electrode configuration based on a resistive taper on transducer performance was also discussed. Several transducers have been built to illustrate this design approach with good agreement between theory and experiment.

The ability to accurately track and monitor the progress of lesion formation during HIFU (High Intensity Focused Ultrasound) therapy is important for the success of HIFU-based treatment protocols. To aid in the development of algorithms for accurately targeting and monitoring formation of HIFU induced lesions, we have developed a software system to perform RF data acquisition during HIFU therapy using a commercially available clinical ultrasound scanner (ATL HDI 1000, Philips Medical Systems, Bothell, WA). The HDI 1000 scanner functions on a software dominant architecture, permitting straightforward external control of its operation and relatively easy access to quadrature demodulated RF data. A PC running a custom developed program sends control signals to the HIFU module via GPIB and to the HDI 1000 via Telnet, alternately interleaving HIFU exposures and RF frame acquisitions. The system was tested during experiments in which HIFU lesions were created in excised animal tissue. No crosstalk between the HIFU beam and the ultrasound imager was detected, thus demonstrating synchronization. Newly developed acquisition modes allow greater user control in setting the image geometry and scanline density, and enables high frame rate acquisition. This system facilitates rapid development of signal-processing based HIFU therapy monitoring algorithms and their implementation in image-guided thermal therapy systems. In addition, the HDI 1000 system can be easily customized for use with other emerging imaging modalities that require access to the RF data such as elastographic methods and new Doppler-based imaging and tissue characterization techniques.

We propose two new methods that allow the determination of the phase delays corresponding to phase aberration efficiently. We derive a new optimization methodology to compute the best compensation phase delays in successive steps. In particular, we start with an array consisting of one element with a specific excitation pattern. Then, another element is added and the dynamic receive delays are iteratively computed such that the obtained echoes are optimal in strength. A third element is added and the process is repeated. This process continues until all elements in the aperture are added. Hence, instead of solving the conventional N-dimensional problem of adjusting the delays of N elements together to achieve optimal characteristics, we transform the problem into the one of solving N-1 consecutive one-dimensional optimization problems. Given the fact that the set of available delay values is finite, the one-dimensional problem is shown to be a classical combinatorial optimization problem. The other technique based on Fourier transform tries to align signals based on information from a single frequency selected as the center frequency of the probe. This method is simple, computationally efficient and lends itself to real-time implementation. The proposed methods were implemented to correct real data from a resolution phantom and the results particularly indicate the potential of the second method.

The fast visualisation of cerebral microcirculation supports diagnosis of acute cerebrovascular diseases. However, the commonly used CT/MRI-based methods are time consuming and, moreover, costly. Therefore we propose an alternative approach to brain perfusion imaging by means of ultrasonography. In spite of the low signal/noise-ratio of transcranial ultrasound and the high impedance of the skull, flow images of cerebral blood flow can be derived by capturing the kinetics of appropriate contrast agents by harmonic ultrasound image sequences. In this paper we propose three different methods for human brain perfusion imaging, each of which yielding flow images indicating the status of the patient's cerebral microcirculation by visualising local flow parameters. Bolus harmonic imaging (BHI) displays the flow kinetics of bolus injections, while replenishment (RHI) and diminution harmonic imaging (DHI) compute flow characteristics from contrast agent continuous infusions. RHI measures the contrast agents kinetics in the influx phase and DHI displays the diminution kinetics of the contrast agent acquired from the decay phase. In clinical studies, BHI- and RHI-parameter images were found to represent comprehensive and reproducible distributions of physiological cerebral blood flow. For DHI it is shown, that bubble destruction and hence perfusion phenomena principally can be displayed. Generally, perfusion harmonic imaging enables reliable and fast bedside imaging of human brain perfusion. Due to its cost efficiency it complements cerebrovascular diagnostics by established CT/MRI-based methods.

This paper proposes a method of imaging the stiffness of soft tissue to help diagnose cancers or tumors which have been difficult to detect with ultrasound B-mode imaging modality. To measure the soft tissue stiffness, sinusoidal vibrations are applied to it, and the magnitude of its mechanical vibration is determined by estimating the temporal variation of speckle pattern brightness in ultrasound B-mode images. It is verified by simulation and experiment that the proposed method can estimate the relative tissue stiffness from B-mode images with a modest amount of computation.

Ultrasonic contrast agents are used to assist perfusion assessment based on evaluation of the time intensity curve (TIC). Previous results showed that such techniques are susceptible to the shadowing effect. To overcome this problem, a new TIC based technique using time-intensity relationships from both the inflow and the outflow of a perfused area is proposed. The technique is called the input-output TIC technique (IOTIC). It contrasts with conventional TIC techniques in that conventional techniques measure the time intensities directly from a perfused area. With both the inflow and the outflow time intensities, the microbubble concentration within the perfused area can be derived based on the fact that the difference between the number of microbubbles at the outflow and the inflow equals the time derivative of the number of microbubbles inside the perfused area. In this study, efficacy of the IOTIC technique is experimentally tested and compared with conventional techniques. Results show that the shadowing effect significantly affected flow estimation with conventional techniques. The IOTIC technique eliminates the shadowing effect and provides a good correlation between the actual flow rate and measured flow-related parameters, thus making quantitative estimation of perfusion feasible. Potential clinical applications of the IOTIC technique were also explored. One example is brain perfusion assessment where the poor acoustic access of the brain tissue may be avoided with the IOTIC technique.

Synthetic sinc wave employs pulsed plane waves as a transmit beam with linear time delay curve. The received echoes in different transmit directions at different transmit times are superposed at imaging points with proper time delay compensation using synthetic focusing scheme. This scheme, which uses full aperture on transmit, obtains a high SNR image, and also features high lateral resolution by using two-way dynamic focusing at all imaging depths. In this paper, we consider the realization problem of the synthetic sinc wave. It is experimentally explored by obtaining phantom and in vivo data with a linear array of 5 MHz. The phantom experiments indicate that the synthetic sinc wave maintains a high resolution over a more extended imaging depth than conventional fixed point transmit and dynamic receive focusing schemes. In vivo images show that the resolution of the synthetic sinc wave does not exceed the conventional focusing systems because of tissue motion, phase aberration, or both, but that the frame rate can be increased by a factor of more than five compared to the conventional focusing schemes, while maintaining competitive resolution at all imaging depths.

In this paper, we present a motion compensated frame rate up-conversion method for real-time three-dimensional (3-D) ultrasound fetal image enhancement. The conventional mechanical scan method with one-dimensional (1-D) array converters used for 3-D volume data acquisition has a slow frame rate of multi-planar images. This drawback is not an issue for stationary objects, however in ultrasound images showing a fetus of more than about 25 weeks, we perceive abrupt changes due to fast motions. To compensate for this defect, we propose the frame rate up-conversion method by which new interpolated frames are inserted between two input frames, giving smooth renditions to human eyes. More natural motions can be obtained by frame rate up-conversion. In the proposed algorithm, we employ forward motion estimation (ME), in which motion vectors (MVs) ar estimated using a block matching algorithm (BMA). To smooth MVs over neighboring blocks, vector median filtering is performed. Using these smoothed MVs, interpolated frames are reconstructed by motion compensation (MC). The undesirable blocking artifacts due to blockwise processing are reduced by block boundary filtering using a Gaussian low pass filter (LPF). The proposed method can be used in computer aided diagnosis (CAD), where more natural 3-D ultrasound images are displayed in real-time. Simulation results with several real test sequences show the effectiveness of the proposed algorithm.

The Doppler velocimeter developed allows determining the angle between the ultrasonic beam and the velocity vector of the flow, and utilizes this angle to calculate the blood flow in a vessel. Four piezoelectric transducers constitute the Doppler velocimeter. Three of these transducers are positioned to form an equilateral triangle (base of a pyramid). When these transducers move simultaneously, backward or forward from the initial position, the emitted ultrasonic beams focalize on a position (peak of the pyramid) closer or farther from the transducers faces, according to the depth of the vessel where we intend to measure de flow. The angle between the transducers allows adjusting the height of this pyramid and the position of the focus (where the three beams meet). A forth transducer is used to determine the diameter of the vessel and monitor the position of the Doppler velocimeter relative to the vessel. The results showed that with this technique it is possible to accomplish measurement of blood flow and to reduce Doppler measurements subjectivity.

Articular cartilage (AC) is a biological weight-bearing tissue covering the ends of articulating bones within synovial joints. Its function very much depends on the unique multi-layered structure and the depth-dependent material properties, which have not been well invetigated nondestructively. In this study, transient depth-dependent material properties of bovine patella cartilage were measured using ultrasound elastomicroscopy methods. A 50 MHz focused ultrasound transducer was used to collect A-mode ultrasound echoes from the articular cartilage during the compression and subsequent force-relaxation. The transient displacements of the cartilage tissues at different depths were calculated from the ultrasound echoes using a cross-correlation technique. It was observed that the strains in the superficial zone were much larger than those in the middle and deep zones as the equilibrium state was approached. The tissues inside the AC layer continued to move during the force-relaxation phase after the compression was completed. This process has been predicted by a biphasic theory. In this study, it has been verified experimentally. It was also observed that the tissue deformations at different depths of AC were much more evenly distributed before force-relaxation. AC specimens were also investigated using a 2D ultrasound elastomicroscopy system that included a 3D translating system for moving the ultrasound transducer over the specimens. B-mode RF ultrasound signals were collected from the specimens under different loading levels applied with a specially designed compressor. Preliminary results demonstrated that the scanning was repeatable with high correlation of radio frequency signals obtained from the same site during different scans when compression level was unchanged (R² > 0.97). Strains of the AC specimens were mapped using data collected with this ultrasound elastomicroscope. This system can also be potentially used for the assessment of other biological tissues, bioengineered tissues or biomaterials with fine structures.

We have developed a technique to measure backscattered ultrasound from lumbar vertebrae using a commercial ultrasound scanner. The spectral centroid shift between the spine and reference phantom data is an index of attenuation within the spine. From measurements from 11 vertebrae, we found a correlation coefficient of r = -0.61 between spectral centroid shift and bone mineral density (BMD). This negative correlation is expected as denser, more highly attenuating bone would be expected to produce greater downshifts in spectral centroid. This is the first technique performing quantitative ultrasound measurements on trabecular bone in the spine. This study shows that (1) acquisition of ultrasonic backscatter data from human spine in vivo is feasible, and (2) spectral centroid shift exhibits a moderate negative correlation with BMD in accordance with expectations.

B-mode ultrasound images are characterized by speckle artefact, which results from interference effects between returning echoes, and may make the interpretation of images difficult. Consequently, many methods have been developed to reduce this problematic feature. One widely used method, popular in both medical and non-destructive-testing applications, is a 1D method known as Split Spectrum Processing (SSP), or also as Frequency Diversity. Although this method was designed for speckle reduction applications, the final image experiences a resultant loss of resolution, impinging a trade-off between speckle reduction and resolution loss. In order to overcome this problem, we have developed a new method that is an extension of SSP to 2D data using directive filters, called Split Phase Processing (SPP). Instead of using 1D narrow band-pass filters as in the SSP method, we use 2D directive filters to split the RF ultrasound image in a set of wide band images with different phases. The use of such filters substantially avoids the resolution loss usually associated with SSP for speckle reduction, because they effectively have the same bandwidth as the original image. It is concluded that the Split Phase Processing, as introduced here, provides a significant improvement over the conventional Split Spectrum Processing.

An efficient algorithm is proposed for interactive ultrasound image retrieval using magnitude frequency spectrum (MFS). The interactive retrieval is especially intended to be useful for training an intern to diagnose with ultrasound images. In the retrieval process, information on which are relevant to a query image among object images retrieved in the previous iteration is fed back by user interaction. In order to improve discrimination between a query image and each of object images in a database (DB) by using the MFS, which is powerful for ultrasound image retrieval, we incorporate feature vector normalization and root filtering in feature extraction. To effectively integrate the feedback information, we use a feedback scheme based on Rocchio equation, where the feature of a query image is replaced with the weighted average of the feature of a query image and those of object images. Experimental results for real ultrasound images show that while yielding a precision of about 75% at a recall of about 8% in the initial retrieval, the interactive procedure yields a great performance improvement, that is, a precision of about 95% in the third iteration.

Reflection imaging has the potential to produce higher-resolution breast images than transmission tomography; however, the current clinical reflection imaging technique yields poor-quality breast images due to speckle. We present a new ultrasonic breast imaging method for obtaining high-resolution and clear breast images using ultrasonic reflection data acquired by a new ultrasonic scanning device that provides a better illumination of targets of interest than the clinical B-scan. The new imaging method is based on the solution of the wave equation in Cartesian coordinates and is implemented using Fast Fourier Transform algorithms. We apply the new ultrasonic breast imaging method to two ultrasonic data sets obtained using an experimental ultrasound scanner recently developed by the Karmanos Cancer Institute. One data set was acquired for a "cyst" phantom using 360 transmitter positions and 321 receiver positions along a 20-cm diameter ring. Another data set was collected with 180 transmitter positions and 1601 receiver positions along a 30-cm diameter ring with the breast specimen located at the center of the ring. We report on the breast imaging results for these two data sets using the new breast imaging method. The results demonstrate that the wave-equation-based ultrasonic breast imaging has the potential to produce high-resolution breast images.

Ultrasound characterization of biological tissues is facing the challenge of accurate sound velocity determination in soft materials of poorly controlled shape. In this work, an original method based on a through-transmission configuration is proposed to accurately determine the sound velocity c_s in non-parallel bio-mimetic specimens. To account for the possible lack of parallelism of the specimen, a set of 8 geometrical parameters is introduced. In a three-step process, the transducer spacing and mis-orientation are deduced from a pair of reference echoes with no specimen in the burst path. Then, main and intermediate echoes in presence of the specimen are processed to yield the characteristic distances of the through-transmission configuration. Finally, the orientations of the specimen faces are determined through a minimization algorithm. Once the system geometry is fully determined, the specimen sound velocity is obtained. A scan of the specimen along the acoustic path (Z-scan) provides the necessary information to completely determine the equation set. The proposed method has been applied to determine the sound velocity in oil-in-gel emulsions as well as in their pure constituents. It is expected that the above method could provide accurate sound velocity data for a better understanding of complex material acoustic response to achieve a more efficient biological tissues characterization.

The objective of this work was to optimize the process of apodization of piezoelectric ceramic discs, looking for the better relationship between oil bath temperature and time of electrical field application. The apodization was performed to reduce the diffraction in the acoustic field generated by ultrasonic transducers. We used 12.7mm diameter and 1mm thickness PZT-5A discs. The apodization field (2kV/mm thickness) has been shaped by a 5mm radius spherical electrode throughout the apodization processes we have used. The apodized ceramic discs which showed electromechanical coupling coefficient value, for the thickness mode of vibration, equal or larger than 0.37, were considered well apodized. We used initial oil bath temperatures from 120°C to 250°C and the electric field was applied for periods of at least 2 minutes to up to 4 hours. The results showed that if the poling electric field was applied to the piezoelectric ceramic even before the oil was heated, we obtained larger piezoelectric coupling coefficients; in higher temperatures (250°C) this was not necessary. We concluded that using higher temperatures (250°C) it was possible to reduce the apodization process, with satisfactory results, from 4-5 hours to 2 minutes. Ultrasound transducers were built with apodized ceramics and their acoustic fields showed larger depth of field relative to non apodized ones.

A number of researchers have previously shown that the ultrasound RF echo of tissue exhibits (1/f)^-β characteristics and developed tissue characterization methods based on the fractal parameter β. In this paper we propose Fractional Differencing Autoregressive Moving Average (FARMA) process for modeling RF ultrasound echo and develop breast tissue characterization method based on the FARMA model parameters. This model has been used to capture statistical self-similarity and long-range correlations in image textures, in wide ranging engineering and science applications, including communication network traffic. Here, we present estimation techniques to extract the model parameters, namely features, for classification purposes and tissue characterization. We show the performance of our tissue characterization procedure on several in vivo ultrasound breast images including benign and malignant tumors. The area of the receiver operator characteristics (ROC) based on 60 in vivo images yields a value of 0.79, which indicates that proposed tissue characterization method is comparable in performance with other successful methods reported in the literature.

This study characterizes the Doppler signal from simulated microemboli of various sizes in blood mimicking fluid using spectral energy parameters. The goal of this research is to detect microemboli as a non-invasive diagnostic tool, or intra-operatively as a surgical aid. A dual beam diffraction-grating ultrasound probe operating at 10 MHz (Model Echoflow BVM-1, EchoCath, Inc., Princeton, NJ) was used with a flow phantom. Microemboli were polystyrene microspheres in 200 to 1000 micron diameters, in concentrations of 0.1, 0.5, and 1.0 per ml. Average flow velocities were 25, 50, 75, and 100 cm/sec. The distribution of peak values of the power spectrum at 2.5 msec windows was plotted over 15 seconds. The means of the distributions corresponding to the microspheres and background fluid were averaged for the four velocity conditions. Embolic peak spectral power ranged from approximately 12 to 25 dB relative to the background. A detection method based on these measurements is currently being developed.

This paper presents parameter estimation of general ultrasound backscatter models, such as the generalized Nakagami and generalized K distributions, via entropy maximization. Parameters of these distributions are related to scatterer density and regularity, and therefore accurate parameter estimation techniques are needed. Parameter estimation based on entropy maximization shows promising results in terms of accuracy for simulated K data and high goodness-of-fit values for the two general backscatter models, especially for the generalized Nakagami distribution.

Modern diagnostic ultrasound beamformers require delay information for each sample along the image lines. In order to avoid storing large amounts of focusing data, delay generation techniques have to be used. In connection with developing a compact beamformer architecture, recursive algorithms were investigated. These included an original design and a technique developed by another research group. A piecewise-linear approximation approach was also investigated. Two imaging setups were targeted -- conventional beamforming with a sampling frequency of 40 MHz and subsample precision of 2 bits, and an oversampled beamformer that performs a sparse sample processing by reconstructing the in-phase and quadrature components of the echo signal for 512 focal points. The algorithms were synthesized for a FPGA device XCV2000E-7, for a phased array image with a depth of 15 cm. Their performance was as follows: (1) For the best parametric approach, the gate count was 2095, the maximum operation speed was 131.9 MHz, the power consumption at 40 MHz was 10.6 mW, and it requires 4 12-bit words for each image line and channel. (2) For the piecewise-linear approximation, the corresponding numbers are 1125 gates, 184.9 MHz, 7.8 mW, and 15 16-bit words.

Articular cartilage (AC) is a biological weight-bearing tissue covering the bony ends of articulating joints. Subtle changes in structure or composition can lead to degeneration of AC such as in osteoarthritis. Currently, there is a lack of reliable diagnostic techniques for early signs of osteoarthritis. The objective of this study was to use ultrasound to probe the transient depth-dependent swelling of AC in vitro, and ultimately to develop a new approach for the early assessment of osteoarthritis. A 50 MHz ultrasound system was used to collect reflected and scattered echoes from AC specimens. The displacements of selected portions of ultrasound signals were measured using a cross-correlation tracking approach. Osteochondral cylinders prepared from fresh bovine patellae were used in this study. During a test, the AC specimen was fixed in a testing chamber filled with saline solution. AC swelling was induced by either changing the concentration of the saline solution or emerging dehydrated AC specimens into the saline solution. Our preliminary results demonstrated that ultrasound could be used to reliably monitor the transient depth-dependent swelling induced by both approaches. It was found that water was gradually absorbed by the AC, first in the superficial layer, and then deep layer. The ultrasound speeds of AC tissues bathed in different saline solutions were different.

Ultrasound computer-tomography (USCT) is a novel ultrasound imaging method capable of producing volume images with both high spatial and temporal resolution. Several thousand ultrasound transducers are arranged in a cylindrical array around a tank containing the object to be examined coupled by water. Every single transducer is small enough to emit an almost spherical sound-wave. While one transducer is transmitting, all others receive simultaneously. Our experimental setup, using only a few transducers simulating a ring-shaped geometry, showed even nylon threads (0.1 mm) with an image quality superior to clinical in-use ultrasound scanners. In order to build a complete circular array several thousand transducers, with cylindrical sound field characteristics, are needed. Since such transducer arrays are hardly available and expensive, we developed inexpensive transducer arrays consisting of 8 elements. Each array is based on a plate of lead titanate zirconate ceramics (PZT) sawn into 8 elements of 0.3 mm width, 3.8 mm height and 0.5 mm pitch. Each element has a mean frequency of 3.8 MHz and can be triggered separately. The main challenge was the development of production steps with reproducible results. Our transducer arrays show only small variances in the sound field characteristics which are strongly required for ultrasound tomography.

Conventional freehand 3D ultrasound (US) is a complex process, involving calibration, scanning, processing, volume reconstruction, and visualization. Prior to calibration, a position sensor is attached to the probe for tagging each image with its position and orientation in space; then calibration process is performed to determine the spatial transformation of the scan plan with respect to the position sensor. Finding this transformation matrix is a critical, but often underrated task in US-guided surgery. The purpose of this study is to enhance previously known calibration methods by introducing a novel calibration fixture and process. The proposed phantom is inexpensive, easy to construct, easy to scan, while yielding more data points per image than previously known designs. The processing phase is semi-automated, allowing for fast processing of a massive amount of data, which in turn increases accuracy by reducing human errors.