Medical Imaging : Ultrasound


New Technology for Measuring Stress in Tendons, Ligaments and Muscles

UW–Madison researchers have developed a new device and technique for dynamically, noninvasively and accurately measuring longitudinal stress in tendons, muscles and ligaments in vivo.

The inventors use skin-mounted accelerometers to measure transverse wave speeds in superficial tissues under time-varying loading scenarios. Such wave speed propagation metrics are then used to determine tissue stress based on a wave propagation model.

Ultrasound System Solves Attenuation Problem

The researchers have enhanced their previous work and overcome the attenuation problem associated with ARFI. They have determined that in tissue with high acoustic attenuation, conventional focusing of ultrasound energy to the region of interest results in substantial acoustic force being misapplied above the intended region.

Their new system is based on an ultrasound transducer array with independently controlled elements. The system divides ultrasound energy into two separate angled beams that converge at the target region to generate push-pulses. A set of varying apodization functions can be applied to the separate beams to improve uniformity and intensity in the focal region.

Real-Time 3-D Elastography

The researchers have now developed an enhancement to their technique that works especially well with a 2-D ultrasound array to provide real-time 3-D imaging. The improvement derives from a new reconstruction scheme that uses sparse data.

The new scheme imposes two key requirements – interpolation and smoothing. Essentially, raw ultrasonic echo data is acquired over many imaging planes. Then, an efficient algorithm tracks frame to frame displacement of the underlying tissue at each pixel in the imaging plane. Mechanical properties such as strain can be estimated by a calculation along the ultrasound scan line direction. The 3-D reconstruction algorithm rapidly reconstructs a complete 3-D visualization from a sparse collection of scattered data points.

Composite Images for Clearer Ultrasound

UW–Madison researchers have developed an algorithm that combines ultrasonic data from multiple images into a high-resolution image or video.

To combine images taken at different times, each of the images is first subdivided into corresponding regions. These are separately registered in rotation and translation, and then combined into a high-resolution image. The process is repeated to create video.

The method can be extended to combine images obtained at different frequencies. This takes advantage of the fact that higher frequencies provide sharper detail closer to the ultrasound machine while lower frequencies are better with distance. Accordingly, acoustic distance is considered when weighting frequency data and combining images.

Cervical Probe Predicts Preterm Delivery Risk

UW–Madison researchers have developed a method to evaluate cervical tissue using ultrasound.

An ultrasound beam is generated and steered to assess the collagenous microstructure of the cervix, revealed by backscatter power variation at a range of angles and depths. The distribution of power loss in reference to the structure of the cervix can provide diagnostic information to the operator.

Rapid Three-Dimensional Elasticity Imaging

UW–Madison researchers have developed an ultrasonic probe assembly and a reconstruction technique for rapid three-dimensional elasticity imaging using limited data.

The probe sends an ultrasonic beam of energy into tissue and receives echoes from the displaced material generally along an axis. Ultrasound data is acquired over a set of planes (between four and six in number) angularly spaced and sharing a common axis. A computer receives the ultrasound data and determines elasticity of the material at multiple points within each plane. A three-dimensional reconstruction then is generated. This reconstruction is faster than the traditional sequential data acquisition for three-dimensional visualization.

Ultrasound Machine and Image Segmentation Algorithm to Improve Longitudinal Tissue Analysis

UW–Madison researchers have developed an ultrasound apparatus and method to automatically segment or isolate tissues being studied by ultrasonic techniques so that the same tissue can be identified at different times. When combined with acoustoelastic techniques, the segmentation permits the detection of subtle changes in acoustic properties that would otherwise be lost if averaged with the acoustic properties of surrounding tissue. This could enable measurement of tissue properties of extremely small regions such as a portion of a tendon or the wall of a blood vessel.

Measurement of Stress, Strain and Stiffness in Functionally Loaded Tissues

UW-Madison researchers have developed an apparatus and technique that can provide axial properties of material when only lateral access is available for ultrasound probes. The device provides a transverse directed ultrasonic transducer that determines transverse properties using the acoustoelastic techniques described in WARF reference number P06115US. The inclusion of a pair of angled transducers, one of which is a transmitter and the other a receiver, provides an angled measurement of both transverse and axial properties. Mathematical combination of these two measurements allows extraction of isolated axial properties.

Intelligent, Real-Time Tracking Method to Enhance Ultrasound-Based Strain and Elasticity Imaging

UW–Madison researchers have developed a new process that combines a regularized speckle tracking algorithm and a quality-based seeding strategy.  The method improves the identification of high quality seed displacement vectors by evaluating similarity and correlation of seed kernel displacement calculations as well as local continuity. This combined approach greatly reduces the risk of the selected seed kernels having peak hopping errors while preserving the benefits of a quality-based seeding strategy.

The method improves image quality and reduces image reconstruction time by using an algorithm that chooses the highest quality seeds possible with the highest probability of accuracy. Seed quality is improved by factoring in specific organs and types of transducers. As a result, noise in the reconstructed image is reduced.

Improved Method and Apparatus for Monitoring Tissue Ablation in Minimally Invasive Tumor Treatment

UW-Madison researchers now have developed an ablation electrode that can vibrate ablated tissue and utilize the propagating shear wave velocities to obtain quantitative stiffness measurements.  The electrode is used in a new method that improves definition of tissue boundaries and quantization of tissue stiffness by measuring both conventional axial compression and perpendicular shear wave velocity changes.  The change in shear wave velocity provides direct measurement of Young’s modulus, the ratio of tensile stress to tensile strain, which may be used to define the stiffness of the treated region. 

Specifically, the device for monitoring the progress of ablation comprises an RF or microwave electrode to ablate tissue, an actuator to produce ultrasonic vibration of the RF or microwave electrode, a tissue imager to detect axial displacement data and a computer to receive and analyze displacement data.  The displacement data is used to compute the velocity change in the orthogonal shear wave, which characterizes the ablated lesion.  Analysis of the displacement data allows a real-time tissue image to be generated, indicating the size of the ablated and non-ablated regions. 

The new electrode displacement imaging method to assist tumor ablation provides accurate quantization of a tissue’s Young’s modulus through an improved computer algorithm that calculates shear wave velocity.  Further analysis of discontinuities in the Young’s modulus data enables multidimensional imaging of the tumor and ablated lesion boundaries.  The improved technique can be coupled with conventional quasi-static elastography monitoring methods to greatly enhance the quality of elastographic images and quantization of tissue stiffness to assist minimally invasive ablation procedures.

Non-Invasive Ultrasound of Cervical Tissue Predicts Preterm Delivery Risk & Labor Induction Success

UW-Madison researchers have developed a non-invasive method using backscattered ultrasound to measure changes in cervical microstructure that can predict the likelihood of preterm birth or the success of inducing labor at full term.  The method involves applying ultrasound to the cervical canal at different angles to assess cervical remodeling, a process which occurs prior to delivery.  A correlation then can be found between the amount of backscattering and the ultrasound angle. The method also allows scanning with different ultrasound frequencies to generate a more robust measurement of the microstructure.

Besides evaluating the microstructure, the method also can be used to measure the elasticity of the cervical tissue through acoustic radiation force. Since this assessment does not depend on compressive force by the operator, it is not user-dependent.  In addition, a very small ultrasound transducer can be used inside the cervical canal without disrupting the tissue.  The combination of the elasticity and backscattering/angle measurements relating to the microstructure may indicate the likelihood of a preterm birth or the success of inducing labor at full term.

Highly Constrained Image Reconstruction (HYPR) for Ultrasound Imaging

UW-Madison researchers have now applied HYPR to ultrasound, resulting in higher quality ultrasound imaging frames with improved spatial resolutions and signal-to-noise ratios. A composite image is formed by combining acquired ultrasound image frames that are higher resolution and have a lower signal-to-noise ratio. The resulting composite image has a much higher signal-to-noise ratio than the individual ultrasound image frames. Individual image frames then are processed using the composite image as a priori information, resulting in a highly constrained image frame that retains both the high signal-to-noise ratio of the composite image and the temporal and spatial resolution of the acquired image. This enhancement method results in higher quality ultrasound imaging.

Optimizing Ultrasonic Elasticity Imaging with Selectable Inputs

UW-Madison researchers have developed an improved method of ultrasonic elasticity imaging that uses cost functions, a type of mathematical optimization, to weight the differences between the correlation and continuity for different forms of tissue.  The correlation of the tissue refers to the restoration of signal coherence, where as the continuity refers to the correctness of kernel block matching with respect to other matches.  This method also will allow the operator and the computer program to fine-tune the image based on a priori knowledge of the tissues or imaging situation and information acquired during the scanning process.

The new method improves upon previous techniques of elasticity imaging, namely the use of empirical equations and parallel computer processing to enhance image acquisition, by providing additional input parameters from both the ultrasound operator and the imaging software.  Specifically, the operator can input information about the type of tissue, specific imagining task, appropriate cost functions, tissue boundaries and other imaging protocol.  The computer software can select specific or general cost functions, shorten the computational process with a Viterbi algorithm, integrate equations into the block matching process and manage calculations across tissue boundaries.  Together, the operator input and computer analysis greatly improve the speed and precision of the ultrasonic elasticity imaging process, increasing the probability of tumor identification and accurate diagnosis.

Rapid Multidimensional Strain Imaging for Cardiac, Abdominal and Other Applications

UW-Madison researchers have developed an improved method of accurately and rapidly providing an elastographic image of tissue strain.  This method estimates axial, lateral and elevational displacements for data acquired using an ultrasonic beam in a phased array, sector or fan-shaped geometry. 

Pre- and post- deformation data in the radial direction are acquired using ultrasound.  These data are processed from a coarse-to-fine scale, on the pre- and post-deformation echo signals.  This approach improves the spatial resolution of the displacement and strain estimates as well as the computational efficiency. 

Coarse displacement estimates guide the tracking and estimation of fine displacement estimates on radiofrequency echo signals, typically performed in two to four stages. The signals from pre-deformation data segments in the radial direction are correlated to corresponding post-deformation echo signals to indicate movement during deformation.  Pre-deformation echo-signals from each radial line also are compared to those lines laterally or elevationally adjacent to them. 

Then displacements from the segment pairs with the maximum correlation are defined as the displacement peak, and sub-sample displacement estimates are obtained by interpolating the displacements about the peak, using a sector or phased array grid to estimate displacement vector images.  That image may incorporate a color scale and can represent either tissue displacement or tissue strain.

Identifying Stroke-Causing Plaque

UW–Madison researchers have developed a new method to differentiate vulnerable and stable plaques. The new method predicts rupture risk based on strain and deformation measurements of the plaque and blood vessels.

Specifically, an imaging system (ultrasound, MRI, etc.) produces images that distinguish between plaque and the arterial wall. A computer receives the images and isolates plaque movement from arterial wall movement caused by blood flow. Then plaque movement is analyzed (plaques that undergo large deformations and strains are of particular interest).

Finally, the computer assesses risk by comparing the observed plaque deformation to deformations associated with increased risk of stroke and cognitive decline.

Arm Brace for Sonographers to Reduce Wrist Injuries

UW-Madison researchers have developed a spiral splint that acts as a kind of lever to transfer at least some of the force required for medical ultrasound imaging from the hand and wrist to the arm and forearm. The padded splint is fixed to the forearm with two Velcro straps. An ultrasound probe can be flexibly connected to the splint via a lockable, universal ball and socket joint mounted above the sonographer’s hand.

Method and Apparatus for Acoustoelastic Extraction of Strain and Material Properties

UW-Madison researchers have developed an analysis technique that uses reflected ultrasound data from the near and far surfaces of a material, thereby including wave propagation data from when the wave passed through the tissue, to calculate both material properties and strain. When acoustic waves propagate through deformed elastic materials, characteristics such as acoustic impedance and wave velocity depend on the material properties and the magnitude of the stress. This phenomenon is known as “acoustoelasticity.” These data can be combined into acoustoelastic analyses to accurately calculate strain and material stiffness without additional information about tissue properties or loads.

Elastography Method for Parallel Processing of Tissue Displacement Estimates

UW-Madison researchers have developed a method to calculate the displacement between pre- and post-deformation data sets that is suited to parallel computer processing and can thus provide real-time elastographic images. The technique’s fundamental innovation is that it tracks motion between pre- and post-deformation signals along columns of tissue running parallel to the ultrasound beam, rather than along rows as in previous methods. Unlike row-by-row calculations, which must occur in sequence, column-by-column computations are largely independent of one another, making parallel processing possible.

Two-Step Strain Estimation Method to Improve Ultrasonic Elasticity Imaging

UW-Madison researchers have developed an improved method for computing local strain components in ultrasonic elasticity imaging.  The new two-step cross-correlation technique allows a smaller window to be used, enhancing image resolution while improving the signal-to-noise and contrast-to-noise ratios.  In the first step of the process, coarse local displacement estimates with a high signal-to-noise ratio are obtained via a window length equal to or greater than 10 wavelengths.  Then the displacement estimates are interpolated with a second order polynomial fitted to the coarse data to produce a fine local displacement map.  The local displacement map guides the second correlation step using one to two wavelength overlapping windows to attain accurate, precise and highly detailed strain images.

The new method improves upon previous techniques by utilizing a refining step to guide a more exact analysis of the tissue strain.  The image resolution for the new two-step method is five to 10 times greater than that of traditional techniques.  The processing time is doubled, approximately five to seven frames per second for the new technique compared to 10 to 15 frames per second for traditional algorithms.  However, given the speed of current processors the increase in time is negligible.  With improved image resolution and signal-to-noise and contrast-to-noise ratios, the new two-step cross-correlation technique will make ultrasonic elasticity imaging a more accurate, precise and explicit medical diagnostic tool.

Method and Apparatus Providing Improved Ultrasonic Strain Measurements of Soft Tissue

UW-Madison researchers have developed a method of modeling the variation in acoustic properties of soft tissues as a function of strain to improve elastographic measurements and to obtain direct measurements of strain. In general, this method exploits the recognition that strain in biological tissue fundamentally affects the acoustic properties of the biological tissue.

Typically, strain is deduced by measuring the motion of the tissue under implicit assumptions about constant stress fields and acoustic properties. Using this method, tissue strain is deduced directly from the modification of the ultrasonic signal caused by changes in the acoustic properties of the material.

Automated Evaluation of Ultrasonic Elasticity Images

UW-Madison researchers have developed a novel quantitative method for automatically evaluating the quality of images used in ultrasonic elasticity imaging. The method uses an empirical equation to combine different types of image quality measurements into a single quantitative descriptor of overall performance. For an operator manually deforming tissue, it may be used to provide a real-time corrective signal to improve the quality of the data acquired. It may also be used to automatically select images for averaging or animation.

New Elastography Technique That Provides Direct Estimates of All Strain Tensor Components

The UW-Madison researchers have now extended this method to ultrasonic elastography to provide direct measurements of strain tensor components, such as lateral, elevational and shear strain, allowing complete characterization of the tissues being imaged.

Parametric Ultrasound Imaging by Using Angular Compounding to Reduce Statistical Variability

UW-Madison researchers have developed a method of significantly reducing the statistical variability inherent in parametric ultrasound imaging by employing a multiple angle (angular compounding) acquisition strategy that has recently become available on ultrasound scanners. Rather than sweeping parallel or quasi-parallel beams over a scanned region as in traditional ultrasound, this technique interrogates a scanned volume with beams traveling at various directions from the scanner transducer’s surface. Multiple echo signals at various angles are then acquired from the same tissue volume. These statistically independent measurements are then averaged to provide the parametric measurement of interest.

Elastographic Imaging of the Cervix and Uterine Wall

A group of UW-Madison researchers has now developed an elastographic method and device for producing diagnostic images of the cervix, uterus and pelvic floor. Their invention includes a number of ways to achieve the controlled tissue compression needed for imaging, such as using the ultrasound probe itself to compress the uterus or cervix, or inflating a balloon (similar to the balloons used in angioplasty) inside these organs to compress them.

Ultrasound Determination of Vascular Age

A UW-Madison researcher has combined direct measurements of atherosclerotic burden with existing risk paradigms to determine an individual’s “vascular age.” Atherosclerotic burden is determined from measurements of carotid artery intimal-media thickness (CIMT) acquired by high-resolution ultrasound -- a non-invasive, highly reproducible technique for detecting and quantifying atherosclerosis. CIMT was combined with population-based nomograms from the Atherosclerosis Risk in Communities Study (1993. Stroke 24:1297-1304) to create mathematical algorithms for determining vascular age, which in turn is used in conjunction with traditional risk assessment models to improve evaluation of individual coronary heart disease risk.

Method and Apparatus for Cardiac Elastography

UW-Madison researchers have developed an improved device and method for producing in vivo elastographic images of the heart to diagnose cardiac disease. The system collects a two-dimensional array of strain data points from heart tissue, each of which has an associated magnitude and sign (i.e., positive or negative value). The data array is then used to create a color image of the heart in which color brightness indicates the magnitude of the strain at any particular point in the tissue, and the hue represents the strain’s positive or negative sign, i.e., whether the tissue is contracted or relaxed.

Elastographic Imaging of Soft Tissue in Vivo

UW-Madison researchers have now discovered that by using an RF ablation probe to internally compress tissue, they can generate 3-D elastographic images of the liver in vivo. Thus, this technique provides a simple and effective way of monitoring the RF ablation of soft tissue inside the body, without the lateral slippage caused by external compression. Elastography may be performed either during RF ablation or after the procedure is complete.

Rapid, Precise Ultrasonic Elasticity Imaging

UW-Madison researchers have developed an improved method of estimating strain that overcomes these limitations by using an adaptive search strategy that uses motion estimates at one place in the image to predict the motion in the neighboring region. Predicting motion allows the size of each search region to be reduced, allowing the rapid creation of strain images.

In this method, an ROI is repeatedly scanned with an ultrasound transducer. Varying stress is applied to the ROI, and ultrasound echoes are acquired at first and second stress levels. Sets of corresponding samples from the echoes at the first and second stress levels are compared, and the displacement vector for each comparison is estimated using block matching. After displacement vectors are estimated for samples throughout the ROI, corresponding strain values that indicate the degree of elasticity of the respective portions of the tissue are estimated. The system then displays an image that shows the strain distribution within the ROI as it is stressed.

Improved Liquid and Solid Tissue Mimicking Material for Ultrasound Phantoms

UW-Madison researchers have developed tissue-mimicking materials that can be used to determine exposure parameters more accurately. The tissue-mimicking material is composed of an ultrafiltered aqueous mixture of large, organic, water-soluble molecules in water, with a low concentration of lipids.

Tissue-Mimicking Material for MRI, CT and Ultrasound Imaging Phantoms

UW–Madison researchers have developed a tissue-mimicking material that may be adjusted to depict particular human tissues such as organs, skeletal muscle and fat, and is applicable to several imaging methods, including ultrasound, CT and MRI. The phantom is especially useful for simulating prostate tissue.

The material comprises an aqueous mixture of large organic water soluble molecules, a copper salt, a chelating agent for binding copper ions in the salt and a gel-forming agent. Glass beads also may be intermixed and treated to have low effect on the MRI T1 and T2. The materials mimicking the various tissues can be in direct contact with one another and remain stable in their multiple imaging properties over time.

Quality Assurance Ultrasound Phantom

UW–Madison researchers have developed an ultrasound imaging phantom for use in quality assurance testing of ultrasound scanners. The device comprises an ultrasound phantom container and a tissue mimicking background material contained within the phantom container. It may be utilized to provide a rapid test of significant imaging performance characteristics of a scanner by taking scans of the various sections of the phantom and recording the information, and where appropriate, quantifying the results.

Various sections within the phantom allow low contrast resolution of large objects, spatial resolution regarding lateral and axial dimensions, maximum visualization depth, image gray level uniformity and distance measurement accuracy to be determined. The phantom may be utilized to provide comparative tests of various scanners and to monitor the performance of a particular scanner over time to determine any changes in the performance of the scanner.

Improved Ultrasound Phantom

UW–Madison researchers have developed an improved ultrasound phantom that can contain water-based liquid or solid gel tissue mimicking material. The container comprises a window covered by an ultrasound transmitting window cover that seals and protects a water-based tissue mimicking material within the container. The window cover comprises a metal layer that is adhered to a layer of plastic and is essentially impervious to moisture and air molecules; this prevents both desiccation of the water-based material within the phantom and oxidation or contamination of the tissue mimicking materials. Multiple windows may be formed in the container or the phantom may be formed entirely of the multilayer film.