Behind the algebra of statistical signal processing there lies a fascinating geometry. Our aim in this keynote talk is to develop this geometry, and use it to illuminate a number of problems in beamforming, detection, and estimation. In the realm of beamforming, we revisit the Capon, or MVDL beamformer, and establish its connections to more conventional beamformers like the Bartlett. We then offer several extensions to the Capon beamformer, making it suitable for the imaging of distributed targets from broadband data. For detection, we review the geometry of matched and adaptive subspace detectors. These detectors are matched to multidimensional subspaces, making them suitable for detecting distributed or broadband sources. They generalize much of what is known about matched filters, which are matched only to one-dimensional subspaces. Our review of estimation exploits the geometry of oblique projections. These processors have the virtue that they image sources with perfect nulling of nearby sources. Of course there is a price to pay in noise gain. This price depends on principal angles between subspaces, explaining why super-resolution is a risky enterprise.

Methods for fabricating and modeling of a high frequency linear ultrasonic array are presented. This array was designed primarily for human eye imaging, and features elements mechanically diced out of a fine grain high density PZT-5H ceramic. Array elements were spaced with a 50 micron pitch, interconnected via a flexible circuit and matched to the 50 Ohm system electronics via a 85 Ohm transmission line coaxial cable. The current array design was based upon tradeoffs between the time domain response bandwidth sensitivity, as well as, the level of array encapsulation and suppression acoustic crosstalk. A prototype four element array was constructed with promising results. An average center frequency of 34 MHz with a -6 dB bandwidth of at least 45% per element was achieved with a 20 dB pulse length of 105 ns. The array performance was compared to a time domain finite element analysis program and excellent agreement between theory and experiment was achieved.

We describe a very low cost handheld ultrasound system that we are currently developing for routine applications such as image guided needle insertion. We provide a system overview and focus discussion on our beamforming strategy, direct sampled I/Q (DSIQ) beamforming. DSIQ beamforming is a low cost approach that relies on phase rotation of in-phase/quadrature (I/Q) data to implement focusing. The I/Q data are generated by directly sampling the received radio frequency (RF) signal, rather than through conventional baseband demodulation. We describe our efficient hardware implementation of the beamformer, which results in significant reductions in beamformer size and cost. We also present the results of experiments and simulations that compare the DSIQ beamformer to more conventional approaches, namely time delay beamforming and traditional complex demodulated I/Q beamforming. Results that show the effect of an error in the direct sampling process, as well as dependence on signal bandwidth and system f number (f#) are presented. These results indicate that the image quality and robustness of the DSIQ beamformer are adequate for routine applications.

Synthetic transmit aperture (STA) imaging has previously been investigated and compared to traditional imaging techniques in simulations and phantom studies. However, a full in-vivo study evaluating its clinical potential has yet to be conducted. This paper presents a preliminary in-vivo study of STA imaging in comparison to conventional imaging. The purpose is to evaluate whether STA imaging is feasible in-vivo, and whether the image quality obtained is comparable to traditional scanned imaging in terms of penetration depth, spatial resolution, contrast resolution, and artifacts. Acquisition was done using our RASMUS research scanner and a 5.5 MHz convex array transducer. STA imaging applies spherical wave emulation using multi-element subapertures and a 20 µs linear FM signal as excitation pulse. For conventional imaging a 64 element aperture was used in transmit and receive with a 1.5 cycle sinusoid excitation pulse. Conventional and STA images were acquired interleaved yielding ensuring exact same anatomical location. Image sequences were recorded in real-time, and processing was done offline. Male volunteers were scanned abdominally, and resulting images were compared by medical doctors using randomized blinded presentation. Penetration and image quality were scored and evaluated statistically. Results show that in-vivo imaging using STA imaging is feasible with improved image quality compared to conventional imaging.

B-flow techniques introduced in commercial scanners have been useful is visualizing places of flow. The method is relatively independent of flow angle and can give a good perception of vessel location and turbulence. This paper introduces a technique for making a synthetic aperture B-flow system. Data is acquired over a number of pulse emissions, where a set of elements synthesizes a spherical wave and the received signal on all elements are acquired. The sequence is repeated and a full new image can always be formed from the last set of emissions, thus making the frame rate very high. The data is continuously available at all places in the image and any kind of echo canceling filter can therefore be used without the usual initialization problems. The B-flow images are then formed by displaying the gray level image after echo canceling. A fast moving scatterer will give a bright echo and slower moving scatterers will yield a dark echo. The approach is demonstrated through in-vivo images. A 128 elements 7 MHz probe with lambda pitch is used together with the RASMUS experimental scanner. Eleven elements are used per emission and the eight emissions are spread evenly over the 128 elements of the array. The signal received by the 64 elements closets to the emission are sampled at 40 MHz and 12 bits at a pulse repetition frequency of 3 kHz. A full second of data is acquired from a healthy 29 years old male volunteer from the carotid artery. The data is beamformed, combined, and echo canceled off-line. High-pass filters designed by the Remez exchange algorithm, have been used for the B-flow processing. The image is displayed after each set of emissions yielding 375 frames per second. Both the flow in the carotid artery and the jugular vein can be seen along with an indication of the acceleration and spatial variation of the velocity.

Angular scatter offers a new source of tissue contrast and an opportunity for tissue characterization in ultrasound imaging. We have previously described the application of the translating apertures algorithm (TAA) to coherently acquire angular scatter data over a range of scattering angles. While this approach works well at the focus, it suffers from poor depth of field (DOF) due to a finite aperture size. Furthermore, application of the TAA with large focused apertures entails a tradeoff between spatial resolution and scattering angle resolution. While large multielement apertures improve spatial resolution, they encompass many permutations of transmit/receive element pairs. This results in the simultaneous interrogation of multiple scattering angles, limiting angular resolution. We propose a synthetic aperture imaging scheme that achieves both high spatial resolution and high angular resolution. In backscatter acquisition mode, we transmit successively from single transducer elements, while receiving on the same element. Other scattering angles are interrogated by successively transmitting and receiving on different single elements chosen with the appropriate spatial separation between them. Thus any given image is formed using only transmit/receive element pairs at a single separation. This synthetic aperture approach minimizes averaging across scattering angles, and yields excellent angular resolution. Likewise, synthetic aperture methods allow us to build large effective apertures to maintain a high spatial resolution. Synthetic dynamic focusing and dynamic apodization are applied to further improve spatial resolution and DOF. We present simulation results and experimental results obtained using a GE Logiq 700MR system modified to obtain synthetic aperture TAA data. Images of wire targets exhibit high DOF and spatial resolution. We also present a novel approach for combining angular scatter data to effectively reduce grating lobes. With this approach we have been able to push the grating lobes below -50 dB in simulation and effectively eliminate their presence in the experimental wire target images.

The purpose of this study is to investigate the feasibility of generating 3D projection ultrasound computed tomography images using a transmission ultrasound system via a piezoelectric material coated CMOS ultrasound sensing array. There are four main components in the laboratory setup: (1) a transducer operated at 5MHz frequency generating unfocused ultrasound plane waves, (2) an acoustic compound lens that collects the energy and focuses ultrasound signals onto the detector array, and (3) a CMOS ultrasound sensing array (Model I100, Imperium Inc. Silver Spring, MD) that receives the ultrasound and converts the energy to analog voltage followed by a digital conversion, and (4) a stepping motor that controls the rotation of the target for each projection view. The CMOS array consists of 128×128 pixel elements with 85μm per pixel. The system can generate an ultrasound attenuation image similar to a digital image obtained from an x-ray projection system. A computed tomography (CT) study using the ultrasound projection was performed. The CMOS array acquired ultrasound attenuation images of the target. A total of 400 projections of the target image were generated to cover 180o rotation of the CT scan, each with 0.45° increment. Based on these 400 projection views, we rearranged each line profile in the corresponding projection views to form a sinogram. For each sinogram, we computed the cross section image of the target at the corresponding slice. Specifically, the projection ultrasound computed tomography (PUCT) images were reconstructed by applying the filtered back-projection method with scattering compensation technique. Based on the sequential 2D PUCT images of the target, we generated the 3D PUCT image.

Ultrasound computer tomography is an imaging method capable of producing volume images with both high spatial and temporal resolution. The promising results of a 2D experimental setup of an ultrasound computer tomography system with at least 0.25 mm resolution encouraged us to build a new 3D demonstration system. It consists of three parts: a tank containing the sensor system, a data acquisition hardware and a computer workstation for image reconstruction and visualization. For the sensor system we developed and manufactured our own low-cost transducer array emitting or receiving ultrasound signals in three dimensions. To optimize the transducer geometry in respect to aperture angle and pressure amplitude the pressure field was simulated using the ultrasound simulation program Field II. Each transducer arrays system carries 8 emitting and 32 receiving elements with integrated amplifier and address electronics. 192 A-scans can be recorded in parallel by the data acquisition hardware. 48 multiplexing steps are needed to store all A-scans of the 1536 receiving transducers. After recording the data is transmitted to the computer workstation.

Thermoacoustic tomography (TAT) is an emerging imaging technique with great potential for a wide range of biomedical imaging applications. In this work, we propose and investigate reconstruction approaches for TAT that are based on the half-time reflectivity tomography paradigm. We demonstrate that half-time reconstruction approaches can produce images in TAT that possess better statistical properties than images produced by use of conventional reconstruction approaches.

Recently it was shown that soft tissue can be differentiated with spectral unmixing and detection methods that utilize multi-band information obtained from a High-Resolution Ultrasonic Transmission Tomography (HUTT) system. In this study, we focus on tissue differentiation using the spectral target detection method based on Constrained Energy Minimization (CEM). We have developed a new tissue differentiation method called “CEM filter bank”. Statistical inference on the output of each CEM filter of a filter bank is used to make a decision based on the maximum statistical significance rather than the magnitude of each CEM filter output. We validate this method through 3-D inter/intra-phantom soft tissue classification where target profiles obtained from an arbitrary single slice are used for differentiation in multiple tomographic slices. Also spectral coherence between target and object profiles of an identical tissue at different slices and phantoms is evaluated by conventional cross-correlation analysis. The performance of the proposed classifier is assessed using Receiver Operating Characteristic (ROC) analysis. Finally we apply our method to classify tiny structures inside a beef kidney such as Styrofoam balls (~1mm), chicken tissue (~5mm), and vessel-duct structures.

One factor used to tell if a tumor is benign or malignant is by examining the underlying structures of the tumor. In ultrasound images, these underlying structures are often buried in speckle noise. The proposed technique utilizes a median-anisotropic diffusion interactive algorithm to “dissolve” speckle noise and enhance images. This technique has two important differences from conventional anisotropic diffusion techniques. First, there is a two resolution level process, which converts the speckle to quasi-impulsive noise, making it more easily removable. Second, a median related reaction term is applied to the anisotropic diffusion equation, thus making it well suited for removing impulse type noise. In addition, the reaction term will make speckle reduction and image enhancement more adaptive to the local features in the image. The experimental data showed the proposed technique improved the legibility of the breast tumor images as important structures were emphasized.

Segmentation of deformable structures remains a challenging task in ultrasound imaging especially in low signal-to-noise ratio applications. In this paper a fully automatic method, dedicated to the luminal contour segmentation in intracoronary ultrasound imaging is introduced. The method is based on an active contour with a priori properties that evolves according to the statistics of the ultrasound texture brightness, determined as being mainly Rayleigh distributed. However, contrary to classical snake-based algorithms, the presented technique neither requires from the user the pre-selection of a region of interest tight around the boundary, nor parameter tuning. This fully automatic character is achieved by an initial contour that is not set, but estimated and thus adapted to each image. Its estimation combines two statistical criteria extracted from the a posteriori probability, function of the contour position. These criteria are the location of the function maximum (or maximum a posteriori estimator) and the first zero-crossing of the function derivative. Then starting form the initial contour, a region of interest is automatically selected and the process iterated until the contour evolution can be ignored. In vivo coronary images from 15 patients, acquired with a 20 MHz central frequency Jomed Invision ultrasound scanner were segmented with the developed method. Automatic contours were compared to those manually drawn by two physician in terms of mean absolute difference. Results demonstrate that the error between automatic contours and the average of manual ones (0.099±0.032mm) and the inter-expert error (0.097±0.027mm) are similar and of small amplitude.

Atherosclerotic lesions have been shown to have different mechanical properties than the non-diseased artery. Calculating vessel wall strain from cross-sectional vessel wall motions allows for the measurement of local stiffness. In this paper, a robust method is developed to track cross-sectional displacements of an artery wall using two different intravascular ultrasound (IVUS) images acquired at two different pressure levels respectively. First, the vessel wall region in one image is segmented semi-automatically by refining two spline-based contours to the locations of inner and outer vessel wall borders. Then the ringlike wall region in one image is registered to its counterpart in the other image in polar coordinates. The registration is performed by minimizing an energy function of the 2D motion field based on a spline-deformable-model. Both intensity and gradient information of the images are used to construct the energy function so that an accurate registration can be achieved. Registration accuracy was tested on simulated motions using IVUS images of a human coronary artery and a porcine carotid. The wall displacement fields calculated from real motion images are also demonstrated.

The registration of preoperative CT to intra-operative reality systems is a crucial step in Computer Assisted Orthopedic Surgery (CAOS). The intra-operative sensors include 3D digitizers, fiducials, X-rays and Ultrasound (US). Although US has many advantages over others, tracked US for Orthopedic Surgery has been researched by only a few authors. An important factor limiting the accuracy of tracked US to CT registration (1-3mm) has been the difficulty in determining the exact location of the bone surfaces in the US images (the response could range from 2-4mm). Thus it is crucial to localize the bone surface accurately from these images. Moreover conventional US imaging systems are known to have certain inherent inaccuracies, mainly due to the fact that the imaging model is assumed planar. This creates the need to develop a bone segmentation framework that can couple information from various post-processed spatially separated US images (of the bone) to enhance the localization of the bone surface. In this paper we discuss the various reasons that cause inherent uncertainties in the bone surface localization (in B-mode US images) and suggest methods to account for these. We also develop a method for automatic bone surface detection. To do so, we account objectively for the high-level understanding of the various bone surface features visible in typical US images. A combination of these features would finally decide the surface position. We use a Bayesian probabilistic framework, which strikes a fair balance between high level understanding from features in an image and the low level number crunching of standard image processing techniques. It also provides us with a mathematical approach that facilitates combining multiple images to augment the bone surface estimate.

Turbulence is ubiquitous to many systems in nature, except the human vasculature. Development of turbulence in the human vasculature is an indication of abnormalities and disease. A severely stenosed vessel is one such example. In vitro modeling of common vascular diseases, such as a stenosis, is necessary to develop a better understanding of the fluid dynamics for a characteristic geometry. Doppler ultrasound (DUS) is the only available non-invasive technique for in vivo applications. Using Doppler velocity-derived data, turbulence intensity (TI) can be calculated. We investigate a realistic 70% stenosed bifurcation model in pulsatile flow and the performance of this model for turbulent flow. Blood-mimicking fluid (BMF) was pumped through the model using a flow simulator, which generated pulsatile flow with a mean flow rate of 6 ml/s. Twenty-five cycles of gated DUS data were acquired within regions of laminar and turbulent flow. The data was digitized at 44.1 kHz and analyzed at 79 time-points/cardiac cycle with a 1024-point FFT, producing a 1.33 cm/s velocity resolution. We found BMF to exhibit DUS characteristics similar to blood. We demonstrated the capabilities to generate velocities comparable to that found in the human carotid artery and calculated TI in the case of repetitive pulsatile flow.

Noninvasive approaches for measuring anatomical and physiological changes resulting from myocardial ischemia / reperfusion injury in the mouse heart have significant value since the mouse provides a practical, low-cost model for modeling human heart disease. In this work, perfusion was assessed before, during and after an induced closed- chest, coronary ischemic event. Ultrasound contrast agent, similar to MP1950, in a saline suspension, was injected via cannulated carotid artery as a bolus and imaged using a Siemens Sequoia 512 scanner and a 15L8 intraoperative transducer operating in second harmonic imaging mode. Image sequences were transferred from the scanner to a PC for analysis. Regions of interest were defined in septal and anterior segments of the myocardium. During the ischemic event, when perfusion was diminished in the anterior segment, mean video intensity in the affected segment was reduced by one half. Furthermore, following reperfusion, hyperemia (enhanced blood flow) was observed in the anterior segment. Specifically, the mean video intensity in the affected segment was increased by approximately 50% over the original baseline level prior to ischemia. Following the approach of Kaul et al., [1], gamma variate curves were fitted to the time varying level of mean video intensity. This foundation suggests the possibility of quantifying myocardial blood flow in ischemic regions of a mouse heart using automated analysis of contrast image data sets. An improved approach to perfusion assessment using the destruction-reperfusion approach [2] is also presented.

This work presents a new approach to lateral strain estimation in the field of tissue elasticity imaging with ultrasound. A particular beamforming is used to produce a point spread function (PSF) with lateral oscillations. Lateral RF signals can then be considered as the juxtaposition of RF samples coming from the same depth. This enables to estimate the lateral strain with a scaling factor estimator applied to the lateral signals. The approach is validated in simulation on a medium stretched only in the lateral direction. The estimation is unbiased for lateral strain values from 0.5 to 7 % with standard deviation less than 0.5 %.

We are summarizing new research aimed at forming spatially and temporally registered combinations of strain and color-flow images using echo data recorded from a commercial ultrasound system. Applications include diagnosis of vascular diseases and tumor malignancies. The challenge is to meet the diverse needs of each measurement. The approach is to first apply eigenfilters that separate echo components from moving tissues and blood flow, and then estimate blood velocity and tissue displacement from the filtered-IQ-signal phase modulations. At the cost of a lower acquisition frame rate, we find the autocorrelation strain estimator yields higher resolution strain estimate than the cross-correlator since estimates are made from ensembles at a single point in space. The technique is applied to in vivo carotid imaging, to demonstrate the sensitivity for strain-flow vascular imaging.

Non-invasive ultrasound elastography (NIVE) was recently introduced to characterize mechanical properties of superficial arteries. In this paper, the feasibility of NIVE for the purpose of studying small vessels in humans and small animals is investigated. The experiments were performed in vitro on vessel-mimicking phantoms of 1.5-mm lumen diameter and 1.5-mm wall thickness. Polyvinyl alcohol cryogel (PVA-C) was used to create double layer vessel walls. The stiffness of the interior portion of the vessels was made softer. The vessels were insonified at 32 MHz with an ultrasound biomicroscope. Radial stress was applied within the lumen of the phantom by applying incremental static pressure steps with a column of a flowing mixture of water-glycerol. The Lagrangian speckle tissue model estimator was used to assess the 2D-strain tensor, and the composite Von Mises elastograms were then computed. The two-layer vessel walls were clearly identifiable. Strain values close to 3% were measured for the interior portion, whereas strains around 1% were noted for the stiffer outside layer. In conclusion, the feasibility of NIVE for small vessel elasticity imaging was demonstrated in vitro.

Recently, it was suggested that ultrasound elasticity imaging can be used to age deep vein thrombosis (DVT) since blood clot hardness changes with fibrin content. The main components of ultrasound elasticity imaging are deformation of the object, speckle or internal boundary tracking and evaluation of tissue motion, measurement of strain tensor components, and reconstruction of the spatial distribution of elastic modulus using strain images. In this paper, we investigate a technique for Young's modulus reconstruction to quantify ultrasound elasticity imaging of DVT. In-vivo strain imaging experiments were performed using Sprague-Dawley rats with surgically induced clots in the inferior vena cavas (IVC). In this model, the clot matures from acute to chronic in less than 10 days. Therefore, nearly every 24 hours the strain imaging experiments were performed to reveal temporal transformation of the clot. The measured displacement and strain images were then converted into maps of elasticity using model-based elasticity reconstruction where the blood clot within an occluded vein was approximated as a layered elastic cylinder surrounded by incompressible tissue. Results of this study demonstrate that Young's modulus gradually increases with clot maturity and can be used to differentiate clots providing a desperately needed clinical tool of DVT staging.

Ultrasonic Mechanical Relaxation (UMR) imaging is a new research technique for visualizing viscoelastic properties of tumors. Tissues behave mechanically as water-based polymers, similar to gelatin, with time-varying viscoelastic properties that depend on the chemical environment. We hypothesized that changes in pH, alter the polymer-fiber surface charge density that determines extent of polymer cross-linking. Gelatin samples with similar material properties and variable pH were prepared. A cone-plate viscometer measured the elastic as well as the viscous response of the polymer to a shear stress stimulus in the pH range of 6 to 8. To image local pH changes, two homogeneous gelatin samples were constructed, one made from buffered saline and the other was unbuffered. 0.05ml NaOH (pH 12) was injected into both samples and subsequent dynamic changes were imaged using UMR methods at 5, 20 and 50 minutes. UMR images include elastic strain and viscous creep relaxation maps produced by applying a compressive step-stress stimulus while recording RF echo frames at a high rate. Estimated local displacements occurring between frames in the echo sequence yield strain images. Relaxation parameters are estimated and mapped for each pixel using the strain time series to produce parametric UMR images. Viscometer experiments indicate that the viscoelastic properties of gelatin vary with pH. Also, elastic strain and viscous creep UMR images show contrast in the region of pH change. These results suggest that UMR methods can be used to explore the microenvironments of living tumors, where their viscoelastic properties are influenced by changes in pO₂, pH and collagen density that predict metastatic potential and resistance to treatments.

This paper reports on a new ultrasound device for noninvasive assessment of bone. The device, known as the QRT 2000 -for Quantitative Real-Time-is entirely self-contained, portable, and handheld. The QRT 2000 is powered by 4 “AA” rechargeable batteries and permits near real-time evaluation of a novel set of ultrasound parameters and their on-line display to the user. The parameters have been studied both in vitro and clinically with a laboratory unit that measured the calcaneus in through transmission and computed the ultrasound features off-line. The data related the ultrasound parameters to the bone mineral density (BMD) of the calcaneus, spine and hip, as determined by x-ray absorptiometry, and demonstrated that the parameters were superior to the standard ones known as BUA and SOS (broadband ultrasound attenuation and speed-of-sound, respectively). The QRT 2000 was then constructed to compute the same parameters; however as noted about it does this in near real-time and provides visual feedback to the user while the measurements are being made. The compactness and portability of the unit make it also ideal for spaceflight applications. Finally, the QRT 2000 was designed to be manufactured at relatively low cost, and therefore should enable the significant expansion of quantitative ultrasound measurements to, for example, primary care physicians in this country and abroad, and including for use in the developing world.

Our research is intended to develop ultrasonic methods for characterizing cancerous prostate tissue and thereby to improve the effectiveness of biopsy guidance, therapy targeting, and treatment monitoring. We acquired radio-frequency (RF) echo-signal data and clinical variables, e.g., PSA, during biopsy examinations. We computed spectra of the RF signals in each biopsied region, and trained neural network classifers with over 3,000 sets of data using biopsy data as the gold standard. For imaging, a lookup table returned scores for cancer likelihood on a pixel-by-pixel basis from spectral-parameter and PSA values. Using ROC analyses, we compared classification performance of artificial neural networks (ANNs) to conventional classification with a leave-one-patient-out approach intended to minimize the chance of bias. Tissue-type images (TTIs) were compared to prostatectomy histology to further assess classification performance. ROC-curve areas were greater for ANNs than for the B-mode-based classification by more than 20%, e.g., 0.75 +/- 0.03 for neural-networks vs. 0.64 +/- 0.03 for B-mode LOSs. ANN sensitivity was 17% better than the sensitivity range of ultrasound-guided biopsies. TTIs showed tumors that were entirely unrecognized in conventional images and undetected during surgery. We are investigating TTIs for guiding prostrate biopsies, and for planning radiation dose-escalation and tissue-sparing options, and monitoring prostrate cancer.

We have conducted a general study that relates calibrated 2-D ultrasonic spectral parameters to the physical properties of sub-resolution tissue scatterers. Our 2-D spectra are computed form digital radio-frequency echo data obtained as the transducer linearly scans along the cross-range (scan direction) with increments smaller than the half beam width. Acquired data are Fourier transformed with respect to range (beam) and cross-range (scan) directions. To quantitatively measure and classify the physical properties of tissues, we have defined two spectral functions and four spectral parameters. The 2-D spectral functions are: radially integrated spectral power (RISP) and angularly integrated spectral power (AISP). The summary parameters are: peak value and 3-dB width of the RISP, slope and intercept of the AISP. These parameter are understood in terms of the beam properties, transducer parameters and the physical properties of the tissue microstructures including size, shape, orientation, concentration and acoustic impedance. Our theoretical model indicates that 1) the 3-dB width of the RISP is predominantly determined by the scatterer size along the beam direction; 2) the slope of the linear fit of the AISP is predominantly determined by the scatterer size along range direction; 3) the concentration and the relative acoustic impedance fluctuation of the scatterers change the overall spectrum magnitude. The predictions of the theoretical model have been verified using beef muscle fibers examined with 40 MHz center frequency.

Femtosecond pulsed laser beams can induce precise photodisruption in tissue and tissue-like materials. Both geometrical and biochemical manipulation of laser-induced optical breakdown (LIOB) produces highly localized photodisruption without residual damage to surrounding tissue. Measurable effects associated with LIOB are shock wave emission and microbubble formation. In previous work, we presented techniques for monitoring site-targeted, LIOB microbubbles with high-frequency (>50MHz) ultrasonic imaging. In this study, we used these techniques to study the stability of LIOB-induced bubbles in water-based gelatin. Successive recordings taken before, during, and after laser exposure illustrated bubble creation and stability. Bubbles with a range of lifetimes (20 - 400 ms) and dissolution behaviors were produced by varying either laser fluence (0.7 - 2.1 J/cm²/pulse) or total number of laser pulses delivered (30 - 500 pulses at 18kHz repetition rate). While both increases in pulse fluence and pulse number lengthened bubble lifetime by an order of magnitude and decreased the rate of bubble dissolution, bubble stability was nonlinearly related to total laser exposure. A few pulses at high laser fluence created initially large bubbles with long lifetimes and slow dissolution rates. In contrast, pulses at near-threshold laser fluence created initially smaller, shorter lifetime bubbles that were stabilized with subsequent pulses. Increased stability could be maintained only above a threshold bubble size. Below that critical size, dissolution rate rapidly increased, causing bubble collapse. Ultimately, these results demonstrated an ability to control the size, lifetimes, and stability of laser-induced microbubbles with various optical parameters, increasing their utility as site-activated contrast agents that can be sensitively monitored with high-frequency ultrasound.

For three-dimensional echocardiography (3DE) to have greater clinical use, there will need to be automated means for estimating cardiac function parameters such as left ventricular (LV) volume directly from the 3DE data set. A prerequisite for estimation of LV volume is the accurate extraction of the endocardium over a cardiac cycle. In this paper we present a semi-automated algorithm that, with minimal operator input, effectively tracks the LV boundary through the spatial and temporal sequences of 2D frames generated by 3DE. Variations in imaging conditions and heart motion make it difficult to develop effective prior geometric and dynamic models for the LV. However, operators can easily locate a few landmark points on the boundary in a given 2D frame. Our algorithm begins with the operator marking some highly visible points along the boundary in a few spatially separated frames at end-systole. This takes a few seconds to complete, and is the only operator input. Full boundary estimates in these initial frames are completed by spline fitting to the selected points. These estimates are used to establish search regions for the intermediate frames at end-systole, within which boundary points are specified as those having highest edge probability. The use of search regions avoids matches to non-endocardial edges. A similar procedure is then used for the temporal sequence of frames at each spatial location: the boundary is tracked by finding points of high edge probability within search regions initialized by the end-systole estimate at that location. LV volume as a function of time is then calculated from the set of estimated boundaries using a modified version of planimetry.

The image quality in medical ultrasound scanners is determined by several factors, one of which is the ability of the receive beamformer to change the aperture weighting function with depth and beam angle. In digital beamformers, precise dynamic apodization can be achieved by representing that function by numeric sequences. For a 15 cm scan depth and 100 lines per image, a 64-channel, 40 MHz ultrasound beamformer may need almost 50 million coefficients. A more coarse representation of the aperture relieves the memory requirements but does not enable compact and precise beamforming. Previously, the authors have developed a compact beamformer architecture which utilizes sigma-delta A/D conversion, recursive delay generation and sparse sample reconstruction using FIR filters. The channel weights were here fixed. In this paper, a compact implementation of dynamic receive apodization is presented. It allows precise weighting coefficient generation and utilizes a recursive algorithm which shares its starting parameters with the recursive delay generation logic. Thus, only a separate calculation block, consisting of 5 adders and 5 registers, is necessary. A VHDL implementation in a Xilinx XCV2000E-7 FPGA has been made for the whole receive beamformer for assessing the necessary hardware resources and the achievable performance for that platform. The code implements dynamic apodization with an expanding aperture for either linear or phased array imaging. A complete 32-channel beamformer can operate at 129.82 MHz and occupies 1.28 million gates. Simulated in Matlab, a 64-channel beamformer provides gray scale image with around 55 dB dynamic range. The beamformed data can also be used for flow estimation.

Spatial compound imaging via beam-steering aims to improve image quality through signal averaging. However, compounding techniques are vulnerable to speed-of-sound and refraction distortions in non-homogeneous tissue. We have developed a system to perform image-based non-rigid registration in real time. The goal is to increase image quality by improving the alignment of the ultrasound frames before compounding. Frames are acquired by a PC-based ultrasound machine (Ultrasonix Inc, Vancouver, Canada), and transmitted to a Windows-based workstation through a high-speed network. Robust image-to-image registration (warping) is performed using block-based estimation of local shifts and thin-plate spline interpolation. Compound images are computed as a rolling average of the nine most recent warped frames. The procedure runs at 20 frames per second on a dual-processor Xeon workstation, demonstrating the feasibility of sophisticated real-time image processing on a standard PC platform. High speed is achieved through algorithm refinements, approximations in speed-critical sections, and low-level optimizations. The result is a fully-automatic real-time spatial compounding system with a demonstrated improvement in image quality. Tests of registration accuracy were performed on simulated data with realistic speckle patterns, using a 10% speed-of-sound variation and an 8° beam-steering angle. The average misalignment across the image was reduced by 70%, from of 0.22 mm to 0.07 mm; in the deepest parts of the image, alignment was improved by 91%. Improved quality is demonstrated on images of a human forearm, which show visibly improved edge sharpness. This work is one of a series of projects demonstrating the ability of a new open-architecture ultrasound system.

Most modern ultrasound scanners use the so-called pulsed-wave Doppler technique to estimate the blood velocities. Among the narrowband-based methods, the autocorrelation estimator and the Fourier-based method are the most commonly used approaches. Due to the low level of the blood echo, the signal-to-noise ratio is low, and some averaging in depth is applied to improve the estimate. Further, due to velocity gradients in space and time, the spectrum may get smeared. An alternative approach is to use a pulse with multiple frequency carriers, and do some form of averaging in the frequency domain. However, the limited transducer bandwidth will limit the accuracy of the conventional Fourier-based estimator; this method is also known to have considerable variance. More importantly, both the mentioned methods suffer from the maximum axial velocity bound, v_zmax = ^cfprf/_4fc, where c is the speed of propagation. In this paper, we propose a nonlinear least squares (NLS) estimator. Typically, NLS estimators are computationally cumbersome, in general requiring the minimization of a multidimensional and often multimodal cost function. Here, by noting that the unknown velocity will result in a common known frequency distorting function, we reformulate the NLS estimator as an one-dimensional minimization problem confirmed by extensive simulations. The results show that the NLS method not only works better than both the autocorrelation estimator and Periodogram method for high velocities, it will also not suffer from the maximum velocity bound.

In this paper, we propose an efficient method for implementing bi-directional pixel-based focusing (BiPBF) based on a sparse array imaging technique. The proposed method can improve spatial resolution and frame rate of ultrasound imaging with reduced hardware complexity by synthesizing a large transmit aperture with sparsely distributed small subapertures. As the distance between adjacent subapertures increases, however, the image resolution tends to decrease due to the elevation of grating lobes. Such grating lobes can be eliminated in conventional synthetic aperture imaging techniques. On the contrary, the grating lobes of the sparse BiPBF scheme can not be eliminated, which is to be proven analytically in this paper. We also propose the condition and method for suppressing the grating lobes below -40dB, which can be achieved by placing the transmit focal depth at a near depth and properly selecting the subaperture distance in proportion to receive aperture size. The results of both the phantom and in vivo experiments show that the proposed method implements two-way dynamic focusing using a smaller number of subapertures, resulting in reduced system complexity and increased frame rate.

In conventional synthetic transmit aperture imaging (STA) the image is built up from a number of low resolution images each originating from consecutive single element firings to yield a high resolution image. This may result in motion artifacts making flow imaging problematic. This paper describes a method in which all transmitting centers can be excited at the same time and separated at the receiver. Hereby the benefits from traditional STA can be utilized and a high fframe rate can be maintained and the images are not influenced by motion artifacts. The different centers are excited using mutually orthogonal codes. The total signal at the receiver is then a linear combination of the transmitted signals convolved with the corresponding pulse-echo impulse response. The pulse-echo impulse responses for the different elements are modeled as FIR channels and estimated using a maximum likelihood technique. The method was verified using Field II. A 7 MHz transducer was simulated with 128 receiving elements and 64 transmitting elements divided into subapertures so that 4 virtual transmission centers were formed. The point spread function was measured and the axial resolution was 0.2312 mm (-3dB) and 0.3083 mm (-6dB), lateral resolution 0.5301 mm (-3dB) and 0.7068 mm (-6dB) and maximum lateral sidelobe level less than 44 dB. Conventional STA is given as a reference with the same setup excited with a single cycle sinusoid at 7 MHz with axial resolution 0.2312 mm (-3dB) and 0.3083 mm (-6dB), lateral resolution 0.5301 mm (-3dB) and 0.7068 mm (-6dB) and maximum lateral sidelobe level less than 44 dB.

Conventional ultrasound scanners are restricted to display the blood velocity component in the ultrasound beam direction. By introducing a laterally oscillating field, signals are created from which the transverse velocity component can be estimated. This paper presents velocity and volume flow estimates obtained from flow phantom and in-vivo measurements at 90° relative to the ultrasound beam axis. The flow phantom experiment setup consists of a SMI140 flow phantom connected to a CompuFlow 1000 programmable flow pump, which generates a flow similarly to that in the femoral artery. A B-K medical 8804 linear array transducer with 128 elements and a center frequency of 7 MHz is emitting 8 cycle ultrasound pulses with a pulse repetition frequency of 7 kHz in a direction perpendicular to the flow direction in the phantom. The transducer is connected to the experimental ultrasound scanner RASMUS, and 1.4 seconds of data is acquired. Using 2 parallel receive beamformers a transverse oscillation is introduced with an oscillation period 1.2 mm. The velocity estimation is performed using an extended autocorrelation algorithm. The volume flow can be estimated with a relative standard deviation of 13.0% and a relative mean bias of 3.4%. The in-vivo experiment is performed on the common carotid artery of a healthy 25 year old male. The same transducer and setup is used as in the flow phantom experiment, and the data is acquired using the RASMUS scanner. The peak velocity of the carotid flow is estimated to 1.2 m/s and the volume flow to 290 ml/min. This is within normal physiological range.

In this paper, a novel method for simultaneous transmit multi-zone (STMZ) focusing along multiple scan lines using modulated orthogonal codes is presented. In this method, M mutually orthogonal Golay codes are transmitted along M respective directions to obtain M scan lines simultaneously, where each Golay code consists of M complementary sequences. This implies that along each scan direction M complementary sequences comprising one of the Golay code are to be fired sequentially. Therefore, total M transmit and receive (T/R) events are required to obtain M scan lines. In the proposed method, however, each complementary code is convolved with the sum of L orthogonal chirps that are focused at L different depths. Consequently, the proposed method requires M T/R events to obtain M scan lines with L transmit foci each, whereas ML T/R events are required in conventional pulse echo imaging. After M firings, first the M orthogonal Golay codes are separately compressed, secondly each compressed Golay signal is correlated with L respective orthogonal chirps, and finally the L compressed chirp signals are independently focused and combined to provide the dynamically focused beams along M scan lines, each with multi-zone transmit foci. Experimental results with a 7.5MHz linear array, for M=L=2, showed that the proposed method can provide ultrasound images with higher frame rates and significantly improved SNR compared to conventional multi-zone focusing methods using short pulses.

Various features based on qualitative description of shape, contour, margin and echogenicity of solid breast nodules are used clinically to classify them as benign or malignant. However, there continues to be considerable overlap in the sonographic findings for the two types of lesions. This is related to the lack of precise definition of the various features as well as to the lack of agreement among observers, among other factors. The goal of this investigation is to define clinical features quantitatively and evaluate if they differ significantly in malignant and benign cases. Features based on margin sharpness and continuity, shadowing, and attenuation were defined and calculated from the images. These features were tested on digital phantoms. Following the evaluation, the features were measured on 116 breast sonograms of 58 biopsy-proven masses. Biopsy had been recommended for all of these breast lesions based on physical exams and conventional diagnostic imaging of ultrasound and/or mammography. Of the 58 masses, 20 were identified as malignant and 38 as benign histologically. Margin sharpness, margin echogenicity, and angular margin variation were significantly different for the two groups (p<0.03, two-tailed student t-test). Shadowing and attenuation of ultrasound did not show significant difference. The results of this preliminary study show that quantitative margin characteristics measured for the malignant and benign masses from the ultrasound images are different and could potentially be useful in identifying a subgroup of solid breast nodules that have low risk of being malignant.

Three-dimensional ultrasound machines based on matrix phased-array transducers are gaining predominance for real-time dynamic screening in cardiac and obstetric practice. These transducers array acquire three-dimensional data in spherical coordinates along lines tiled in azimuth and elevation angles at incremental depth. This study aims at evaluating fast filtering and scan conversion algorithms applied in the spherical domain prior to visualization into Cartesian coordinates for visual quality and spatial measurement accuracy. Fast 3d scan conversion algorithms were implemented and with different order interpolation kernels. Downsizing and smoothing of sampling artifacts were integrated in the scan conversion process. In addition, a denoising scheme for spherical coordinate data with 3d anisotropic diffusion was implemented and applied prior to scan conversion to improve image quality. Reconstruction results under different parameter settings, such as different interpolation kernels, scaling factor, smoothing options, and denoising, are reported. Image quality was evaluated on several data sets via visual inspections and measurements of cylinder objects dimensions. Error measurements of the cylinder's radius, reported in this paper, show that the proposed fast scan conversion algorithm can correctly reconstruct three-dimensional ultrasound in Cartesian coordinates under tuned parameter settings. Denoising via three-dimensional anisotropic diffusion was able to greatly improve the quality of resampled data without affecting the accuracy of spatial information after the modification of the introduction of a variable gradient threshold parameter.

This study investigates the instationary flow field in human femoarl arteries. The flow fiel is measured before and after the implantaion of five different metal stent implants in elastic and scaled silicone models of femoral arteries. The pulsating flow field is investigated under physiological conditions within the silicone vessel. For the simulation of the physilogical hemodynamics a computer controlled pump for the reproducible generation of flow patterns and a fluid with flow properties similar to human blood is used. At significant positions distal, proximal and inside the stent dopplersonographic measurements are performed with stationary and pulsatile flow. Via fast fourier analysis the sampled doppler audio signal, gained from the ultrasound stereo output, is converted into velocity profiles and displayed as color coded 3D spectrograms. By subtracting the spectra of the unstented model of the stented models differential spectra are obtained and compared. These differential spectra are used for a semiquantitative analysis of the flow field change caused by stents and are easy to interpret for the examining physician.

In this paper, we introduce new diagnosis tool to observe carotid artery based on ultrasonic volume data. The main components and applied algorithms of the developed diagnosis tool are explained. As one of main components, the semi-automatic segmentation method includes an effective speckle reducing filter and an automatic ROI tracking scheme. Furthermore, we present the reconstruction method that is effective for Y-typed carotid artery and the navigation path generation method that applies interpolation of medial points of ROI. To support the objective diagnosis, we provide the automatic measurement method of artery’s diameter. To show usefulness of the developed tool, we constructed 3D model for carotid artery of 34-year-old person and the diameter of carotid artery was automatically measured.

The aim of this paper is to propose a new Markov Random Field (MRF) for textured ultrasound image which use is more relevant than the use of the classic MRF, such as the gaussian markovian model. The main difference is that our model is based on κ-distribution. We have built this κ-MRF with reference to the Product Model. This latter means that the observed intensities of ultrasound image are the product of a degraded perfect image by a multiplicative noise called speckle. When the construct of κ-MRF is already described, we propose in this paper a validation on synthetic and medical B-scan textured image. The synthetic textures are obtained by stimulating the κ-MRF. For medical texture, we estimate the parameters of the model from tissues. The estimated parameters are simulated and compared to medical texture. The resemblance is a first validation of the κ-MRF and the tissue can be then characterized by the parameters of the model.

Doppler ultrasound (DUS) is widely used to diagnose and plan treatments for vascular diseases, but the relationship between complex blood flow dynamics and the observed DUS signal is not completely understood. In this paper, we demonstrate that Doppler ultrasound can be realistically simulated in a real-time manner via the coupling of a known, previously computed velocity field with a simple model of the ultrasound physics. In the present case a 3D computational fluid dynamics (CFD) model of physiologically pulsatile flow a stenosed carotid bifurcation was interrogated using a sample volume of known geometry and power distribution. Velocity vectors at points within the sample volume were interpolated using a fast geometric search algorithm and, using the specified US probe characteristics and orientation, converted into Doppler shifts for subsequent display as a Doppler spectrogram or color DUS image. The important effect of the intrinsic spectral broadening was simulated by convolving the velocity at each point within the sample volume by a triangle function whose width was proportional to velocity. A spherical sample volume with a Gaussian power distribution was found to be adequate for producing realistic Doppler spectrogram in regions of uniform, jet, and recirculation flow. Fewer than 1000 points seeded uniformly within a radius comprising more than 99% of the total power were required, allowing spectra to be generated from high resolution CFD data at 100Hz frame rates on an inexpensive desktop workstation.