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Medical ultrasound imaging uses pulsed acoustic waves that are transmitted and received by a hand-held transducer. This is a mature technology that it is widely used around the world. Among its advantages are that it is cost-effective, flexible, and does not require ionizing radiation. However, the image quality is affected by degradation of ultrasound signals when propagating through biological tissues. Many efforts have been done in the last three decades to improve the quality of the images. This paper reviews some of the most important methods for ultrasound enhancement. We classified these techniques into two groups: preprocessing and post-processing, analyzed their benefits and limitations, and presented our beliefs about where ultrasound research could be directed to, in order to improve its effectiveness and broaden its applications.

Level set methods have been widely used in image processing and computer vision. In conventional level set formulations, the level set function typically develops irregularities during its evolution, which may cause numerical errors and eventually destroy the stability of the evolution. Therefore, a numerical remedy, called reinitialization, is typically applied to periodically replace the degraded level set function with a signed distance function. However, the practice of reinitialization not only raises serious problems as when and how it should be performed, but also affects numerical accuracy in an undesirable way. This paper proposes a new variational level set formulation in which the regularity of the level set function is intrinsically maintained during the level set evolution. The level set evolution is derived as the gradient flow that minimizes an energy functional with a distance regularization term and an external energy that drives the motion of the zero level set toward desired locations. The distance regularization term is defined with a potential function such that the derived level set evolution has a unique forward-and-backward (FAB) diffusion effect, which is able to maintain a desired shape of the level set function, particularly a signed distance profile near the zero level set. This yields a new type of level set evolution called distance regularized level set evolution (DRLSE). The distance regularization effect eliminates the need for reinitialization and thereby avoids its induced numerical errors. In contrast to complicated implementations of conventional level set formulations, a simpler and more efficient finite difference scheme can be used to implement the DRLSE formulation. DRLSE also allows the use of more general and efficient initialization of the level set function. In its numerical implementation, relatively large time steps can be used in the finite difference scheme to reduce the number of iterations, while ensuring sufficient numerical accuracy. To demonstrate the effectiveness of the DRLSE formulation, we apply it to an edge-based active contour model for image segmentation, and provide a simple narrowband implementation to greatly reduce computational cost.

Most of the devices for acquisition and display of medical images use rectangular lattices even though there are other sampling strategies that can be more efficient in terms of resolution. This paper proposes an approach for ultrasound image enhancement that uses a hexagonal sampling scheme to display and process the images. The images were resampled on an interlaced grid. Interlaced sampling uses square pixels shifted half a pixel on alternate rows. Two types of hexagonal filters were designed and tested on ultrasound images: a statistical adaptive filter and an anisotropic diffusion filter. Results show improvements in signal-to-noise ratio and more natural representation of curved structures. © 2014 SPIE and IS&T (DOI: 10.1117/1.JEI.23.4.043022)

A system has been developed based on the measurement of skin surface vibration that can be used to detect the underlying vascular wall motion of superficial arteries and the chest wall. Data obtained from tissue phantoms suggested that the detected signals were related to intravascular pressure, an important clinical and physiological parameter. Unlike the conventional optical Doppler techniques that have been used to measure blood perfusion in skin layers and blood flow within superficial arteries, the present system was optimized to pick up skin vibrations. An optical interferometer with a 633-nm He:Ne laser was utilized to detect micrometer displacements of the skin surface. Motion velocity profiles of the skin surface near each superficial artery and auscultation points on a chest for the two heart valve sounds exhibited distinctive profiles. The theoretical and experimental results demonstrated that the system detected the velocity of skin movement, which is related to the time derivative of the pressure. The system also reduces the loading effect on the pulsation signals and heart sounds produced by the conventional piezoelectric vibration sensors. The system’s sensitivity, which could be optimized further, was 366.2 mm/s for the present research. Overall, optical cardiovascular vibrometry has the potential to become a simple noninvasive approach to cardiovascular screening. © 1997 Society of Photo-Optical Instrumentation Engineers. [S1083-3668(97)00604-7]

This paper describes an approach to improve the contrast and signal to noise ratio on ultrasound images. Images with sub-pixel lateral displacements were re-sampled using a hexagonal grid, registered and compounded. The resultant image was filtered using a hexagonal adaptive masking filter. This approach was evaluated with simulated images and real images from a breast phantom. The results show total improvements in signal to noise ratio of up to 313% in simulated images, and 182% in phantom images. Contrast to noise ratio was improved by 286% in simulated images and 56% in phantom images.

Arterial pulsatility was measured using an optical interferometer. As opposed to laser Doppler flow meters, the prototype system the authors evaluated can detect pulsation profiles of major arteries with potentially useful information including pulse wave velocity, profile of pulse pressure, etc. >

This paper describes a method for estimating small displacements in ultrasound images. The method is based on the addition of a “pseudo-phase” to the intensity ultrasound images and subsequent calculation of the cross-correlation. The peak of the cross-correlation function is interpolated to add precision to the estimates.

Ultrasound is a widely used medical imaging methodology because it is safe and relatively inexpensive. However, the quality of the images is affected by the point spread function of the system and coherent wave interference or speckle. The present research studies the averaging of images that have been displaced laterally and displays them using an interlaced grid. The main goals are to reduce speckle and improve contrast and resolution. The point spread function of the ultrasound scanner was estimated using a thin nylon thread within a water bath. Then, a set of eight images of a breast phantom (having lateral displacements smaller than the width of the point spread function) were averaged and interlaced. The results show a total improvement of 4% in signal to noise ratio and 7% in contrast to noise ratio.

The resolution of ultrasound imaging is restricted primarily by the blurring process of the imaging system embedded in the point spread function (PSF). Supercompounding has been found to be a highly effective way to enhance the resolution of ultrasound imaging. Here we utilize a spatial ultrasound compounding technique using a B-mode array rotated around a target in a range encompassing 180° or greater which we term "supercompounding." For some ultrasound imaging modalities, the PSF is unknown and is space variant, caused by a mono-focus imaging device. To use linear algorithms to enhance the resolution, images must be assumed to have a uniform PSF which is space invariant; otherwise, it is necessary to use complicated non-linear algorithms. Under the above circumstances, an image with a uniform PSF is the key element to more effective resolution enhancement. The supercompounding technique as here can create an image with a uniform PSF from 214 B-scan images thus allowing the use of linear algorithm enhancement. Once a supercompound image with a uniform PSF is constructed, the resolution of the image was further improved with Weiner deconvolution. The processing technique was demonstrated on imaging a dissected porcine aortic root at 5 different critical height levels both with and without inflated pressure into to the sinus. The results can be analyzed to acquire the mechanical properties and geometry of the aortic valve for future use. The resolution as measured by -6dB width of the sinus wall shows a 14% improvement.

Ultrasound compounding is a method of combining multiple images from different angles in order to create a single image with improved resolution and reduced angular-dependent artifactual detail. Compounding methods traditionally calculate each pixel in the compound image as a simple functional relationship between pixel elements in the component image set. In order to achieve an even better resolution and further reduce angular-dependent artifacts, this paper investigates a new type of compounding we call paired angle multiplicative compounding (PAMC), in which compound images are produced by a summation of multiplied pairs of component images acquired at different angles. A PAMC image of a breast phantom demonstrates improved delineation of microcalcifications in comparison to the mean operation. Images of the forearm are used to investigate larger angles of paired multiplication, the best improving contrast ratio (28%) and signal to noise ratio (24%) when compared to the mean method. The PAMC method is found to be similar to the MEM and median compounding operations, the largest difference being that it is better preserving contributions from normal incidence but at the expense of needing a larger angular range.

Aortic valve stenosis is a significant cause of morbidity and mortality [1]. Currently, the preferred treatment of severe aortic stenosis is aortic valve replacement, which carries a significant risk for patients with comorbidities [2]. Recently, percutaneous aortic valve (PHV) replacement represents an endovascular alternative to conventional open heart surgery without the need for sternotomy, aortotomy, or cardiopulmonary bypass [3]. However, there are significant serious adverse events associated with the percutaneous procedure, such as myocardial infarction, peripheral embolism, injury to the aorta, perivalvular leak and access site injury [3–5]. Furthermore, long-term durability and safety of these valves need to be evaluated and studied carefully. We hypothesize that the device (dys)function could be more accurately predicted if a better understanding of the biomechanical interaction between the native aortic valve/root and the PHV were available. In this study, our objective is to characterize the mechanical properties of the aortic root such that its interaction with the PHV device can be quantified.

Accurate measurement of structural features represented in medical images is important in clinical trials and patient diagnosis. A key factor for precision is spatial resolution, which in ultrasonic imaging is limited by transducer array arrangements, transmitting frequency, and data acquisition firmware. In this paper, a variation of pixel compounding is proposed to enhance ultrasound resolution using acquired cine loops. The technique operates on a sequence of ultrasound B-scan images acquired with random motion. Subpixel registration is estimated and a maximum a posteriori (MAP) approach with the shift information is used to reconstruct a high-resolution single image. A nonhomogeneous anisotropic diffusion algorithm follows from the estimation process and is implemented to enhance the high-resolution edges. Preliminary tests using simulations and phantom studies show promising results. Pixel compounding can be a powerful preprocessing tool to assure accurate segmentation, measurement, and analysis of ultrasound images.

Accurate and fast image segmentation algorithms are of paramount importance for a wide range of medical imaging applications. Level set algorithms based on narrow band implementation have been among the most widely used segmentation algorithms. However, the accuracy of standard level set algorithms is compromised by the fact that their evolution schemes deteriorate the signed distance level set functions required for accurate computation of normals and curvatures. The most common remedy is to use an ad-hoc reinitialization step to rebuild the signed distance function frequently. Meanwhile, complex upwind finite difference schemes are required for stable evolution. They together make the overall computation expensive. In this paper, we propose a novel fast narrow band distance preserving level set evolution algorithm that eliminates the need for both reinitialization and complex upwind finite difference schemes. This is achieved by incorporating into a variational level set formulation with a signed distance preserving term that regularizes the evolution. As a result, stable, accurate, fast evolution could be obtained using a simple finite difference scheme within a very narrow band, defined as the union of all 3times3 pixel blocks around the zero crossing pixels. Also, our method allows the use of larger time step to speed up the convergence while ensuring accurate result, as well as the use of more general and computational efficient initial level set functions rather than the signed distance functions required by standard level set methods. The proposed algorithm has been applied on both synthetic and real images of different modalities with promising results

Elevated blood cholesterol levels and coronary heart disease (CHD) have a direct relationship that has been widely investigated and well documented. Commercially available instrumentation use enzymatic method to measure cholesterol from filtered plasmalsem and not from whole blood. We have tested the applicability of infrared (IR) spectroscopy for measuring cholesterol in whole, human blood. Our preliminary results indicate consistent occurrence of a characteristic cholesterol absorption spectrum, in four independently tested blood samples, extending from 1200 cm-1 to 1080 cm-1. TIX significant peaks and shoulders occur at approximately 8.55.8.86.9.04 and 9.24 microns in the mid-IR. Our data also suggests a relation between higher plasma/serum cholesterol concentrations and a stronger absorption at the above frequencies

Tracking of isophase lines in a structured light pattem projected on the human face was used to acquire the three dimensional coordinates of the face in real time (1/30th of a second). The resultant data was rendered in a three dimensional computer graphic representation and utilized to construct a physical model. Preliminary experimental results yielded an accuracy of 3.4% in quantitation of surface area.

Different classifications have been proposed in the literature for the characterization of atherosclerotic plaque morphology, resulting in considerable confusion. For example plaques containing medium of high level uniform echoes were classified as homogeneous by others and correspond closely to dense and calcified plaques, other types. This survey is to understand different types of plaque when imaged using ultrasound and MR.

The intima-media thickness (IMT) of the carotid artery is an important biomarker for the clinical prognosis and diagnosis of atherosclerosis and stroke. This paper presents a new approach, pixel compounding, to enhance the resolution of the intima-media vascular layers in ultrasound B-scan images and provide increased image resolution for a more precise measurement. First, homomorphic transformation is used to estimate the lumped point spread function (PSF) of the images, then, the images are deblurred with the estimated PSF, and finally, a non-homogeneous anisotropic diffusion algorithm is used to further enhance the resolution of the image. The homogeneous part of the algorithm is used to suppress speckle while enhancing the coherent structures, specifically the edges; the non-homogeneous part (likelihood estimator) progressively adds the details from succeeding frames in the image sequence for an optimal and sub-pixel resolved image. Phantom studies have shown 300% improvement on Peak Distance Standard Deviation and nearly 100% improvement on Average Half Peak Width, indicating significant resolution enhancement.

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.

Spin fusion ultrasound imaging is introduced. In this approach, multiple images of the same slice section of an object are taken with equal angular displacement at each step. A full view of slice image is obtained by combining all the images. The potential advantages of this approach include reduction of speckle noise, reduction of shadowing and attenuation effects, improvement of overall resolution and improved 360/spl deg/ visualization. This image modality can be particularly useful in small animal laboratory research and human extremities, where there is easy full 360/spl deg/ acoustic access.

A simple method is described for adding texture to the relatively smooth surface of the human face by projecting fine textural patterns drawn on transparency. Experiments show that these fine patterns can improve the accuracy of cross-correlation data and thus the accuracy of surface height calculated from such cross correlation. The average percent error of height determination on a face phantom improved from 79% without a projected pattern to 18% with such a pattern, or a 438% improvement. >

Projected electronic dissection (PED) creates the effect of actually dissecting a tissue structure under examination by operating on a 3-D digitized data set. Advanced computer techniques have made it possible to range through a 3-D MRI (magnetic resonance image) data set in real time. In actual dissection, however, if a portion of a volume is sliced off, is it possible to see both the remaining external surface and any internal surfaces revealed by the removal of superficial layers. PED refers to a technique that displays not only tissue paraenchyma at the level of cut, but also these internal and external surfaces, which can be important both for orientation and diagnosis. Preliminary PED results were obtained on MRI volumetric data set of proton densities of the brain. The initial results suggest that the partial shaded projection technique described offers the prospect of providing a natural way to display and analyze experimental data from the brain and other organ systems with applications in medical diagnosis, the study of anatomy and the planning of surgery. >

Preliminary results of applying the advanced three-dimensional visualization techniques of volumetric rendering to represent state-of-the art magnetic resonance images of the ventricular system are reported. Cinematic loops provide the subjective impression of a rotating 3-D object in space, thus enhancing the appreciation of the 3-D structure. Different images were obtained by changing the color/opacity parameters and transparency factors. Only minimal editing was required to obtain the judges. It was not necessary to trace the precise ventricular borders as is usually required in computer graphics approaches. Initial results suggest that volumetric display of magnetic resonance images of the brain can be of value for the assessment of the ventricular system deep with the brain paraenchyma. >

The Hartley and Cas-Cas transforms which are discrete and real-to-real, are compared in terms of times taken, compression ratios and quality of the reconstructed image, for the purpose of comparing their suitability for transform domain compression applications. A thresholding method for quantization is introduced where threshold values are adaptively selected according to the correlation factor of each block of equally divided blocks in the image. Additionally in some pixels it was found advantageous to code by retaining only the sign bit. Preliminary results suggest that compression ratios of 10-15 to 1 were readily achievable for projection radiographs with no subjective loss of image quality. The fast Cas-Cas transform ran about 25% faster than the Hartley transform with equal output quality. >

The authors present an electronic optical contouring method and its biomedical application to three-dimensional quantitation of the craniofacial surface. These optical but noninterferometric contours, representing depth of a surface, are generated by projection optics, which utilize incoherent light and are thus relatively inexpensive. With the aid of digital image processing techniques, the process of contour mapping is automated for improved speed and accuracy. Preliminary experiments on a volunteer demonstrate the capability for accurate three-dimensional real-time cephalometry. >

The performance of the binary joint transform correlator for defining a tumor in a mammographic image as cancerous or benign is investigated and compared to that of the classical correlator. The areas of comparison are the correlation peak and the discrimination sensitivity. The study is a computer simulation of coherent optical processing and includes three separate patient mammograms, two with carcinomas and one with a fibroadenoma. It is shown that the performance of the binary correlator surpassed that of the classical correlator. The binary joint transform correlator is able to differentiate noninvasively and quantitatively between benign and malignant tumors in this experiment. >

The major practical problem in applying Fourier domain techniques to medical imagery has been the large number of calculations required for substantial image sizes with resultant non-real-time operation. The recent development of an imaging supercomputer with associated two-dimensional fast Fourier transform software has now made possible the calculation of (120*128)-pixel two-dimensional Fourier transforms, multiplication by an arbitrary filter function, and inverse two-dimensional Fourier transformation possible in real time. Here, plain-film projection radiographs are digitized and a variety of high-pass, low-pass and bandpass filters are applied in the transform domain. The preliminary results suggest that such real-time spatial filtering may be particularly attractive to radiologists because of the ability to see simultaneously both the unprocessed and processed radiograph and to utilize rapidly and assess a variety of available filters. >

A three-dimensional formulation was used to quantitate the maximal velocity in a flow jet produced by circulating a Doppler acoustic fluid through a Pasteur pipette in an open recirculating system. The flow jet velocity, as determined from the three-dimensional vector magnitude, was 119.7+or-3.17 cm/s; the actual velocity of 119.6 cm/s was determined with an Aalborg ball flowmeter. Doppler velocities for calculation of pressure using the Bernoulli equation were within 3.9% of actual jet stream velocities. The three-dimensional ultrasound Doppler velocity of a flow jet detemined at the site of the maximal pressure gradient should lead to a more accurate determination of pressure drop through a structure, stenosis, or septal defect using the Bernoulli equation. >

In previous work, tracking of isophase lines in a structured light pattern projected on the human face was used to acquire the 3-D coordinates of the face in real time (1/30th of a second) (see K. Cheung and M.D. Fox, Proc. IEEE EMBS 11th Ann. Conf., p.1655-6, June 1989). In the present work, the author automated tracing of an isophase line to achieve submillimeter 3-D resolution. In contrast to work previously reported, the results obtained here indicate that a fully automated high-resolution system for non-contact morphometry with submillimeter resolution is feasible using readily available hardware and software. Preliminary experimental results suggest that average resolution on the order of .5 mm can be readily obtained through this approach, which should be adequate for many important surgical procedures, including craniofacial and plastic surgical planning and following the healing of facial surgery and trauma. >