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Biomedical subjects

J F Greenleaf

Publications and source records attributed to J F Greenleaf.

At least 55 records · Page 3Linked to original sources
PubMed1996

Statistics of the log-compressed echo envelope.

Log compression of A lines to produce B-scan images in clinical ultrasound imaging systems is a standard procedure to control the dynamic range of the images. The statistics of such compressed images in terms of underlying scatterer statistics have not been derived. The statistics are analyzed for partially formed speckle using a general K distribution model of envelope statistics to derive the density function for the log-compressed envelope. This density function is used to elucidate the relation between the moments of the compressed envelope, the compression parameters, and the statistics of the scatterers. The analysis shows that the mean of the log-compressed envelope is an increasing function of both the backscattered energy and the effective scatterer density. The variance of the log-compressed envelope is a decreasing function of the effective scatterer density and is independent of the backscattered energy.

Computer Simulation↗
PubMed1996

Acoustic radiation pressure in a three-dimensional lossy medium.

Acoustic radiation pressure exerted by an arbitrary acoustic wave in a three-dimensional lossy medium is calculated by extending an indirect approach developed by Chu and Apfel [B-T. Chu and R.E. Apfel, "Acoustic radiation pressure produced by a beam of sound," J. Acoust. Soc. Am. 72, 1673-1687 (1982)]. Without appealing to the detailed solutions of equations governing fluid motion, a general analytic expression for the radiation pressure in lossy media with arbitrary waves is obtained. When an infinite lossy medium is considered, the expression states that the radiation pressure, to the lowest order of approximation (i.e., second order), is equal to corresponding total energy density. For a special class of confined spaces, the expression leads to a rather general formula for the radiation pressure, in which the radiation pressure is given in terms of various energy densities in the field. Furthermore, a relationship among these energy densities is generalized to the case of lossy media, which enables one to compute the radiation pressure in the class of spaces with the knowledge of the first-order perturbation solution only.

Acoustics↗
PubMed1995

Magnetic resonance elastography by direct visualization of propagating acoustic strain waves.

A nuclear magnetic resonance imaging (MRI) method is presented for quantitatively mapping the physical response of a material to harmonic mechanical excitation. The resulting images allow calculation of regional mechanical properties. Measurements of shear modulus obtained with the MRI technique in gel materials correlate with independent measurements of static shear modulus. The results indicate that displacement patterns corresponding to cyclic displacements smaller than 200 nanometers can be measured. The findings suggest the feasibility of a medical imaging technique for delineating elasticity and other mechanical properties of tissue.

Acoustic Stimulation↗
PubMed1995

Speckle analysis using signal to noise ratios based on fractional order moments.

The SNR (signal-to-noise ratio) of the echo envelope image is a monotonically-increasing function of scatterer number density. Various SNRs, like amplitude SNR and intensity SNR, can be used to quantify the scatterer density. The problem of using a SNR based on higher order moments like the intensity SNR is that they require large sample sizes to obtain estimates with high confidence (the variance of the estimate becomes large for higher moments). In this paper, we consider SNRs based on fractional order moments (moments of order less than 1), and obtain mathematical analyses of their properties using the K distribution, which has been shown to be a good model for the density function of backscatter echo envelope signal. Statistics of SNRs based on fractional moment are derived and appear to be more robust and useful than the amplitude and intensity SNRs previously studied. The SNRs based on fractional order moments have greater dynamic range and the sample size requirements are smaller than those for integral order moment SNRs, like amplitude SNR or intensity SNR. Thus, SNRs based on fractional order moments could be used to better quantify the variations in scatterer density which can be used for tissue classification problems.

Humans↗
PubMed1995

Congenital heart disease: wide-field, three-dimensional, and four-dimensional ultrasound imaging.

The next significant advance for cardiovascular ultrasound will be the introduction of clinical three-dimensional (3-D) imaging. With increasing computer power and software and hardware, 3-D ultrasound imaging will become a reality over the next few years. Of all cardiovascular abnormalities, congenital heart disease is one of the most logical entities to lend itself to wide-field and 3-D presentation. Tomographic two-dimensional (2-D) echocardiography has in great part replaced cardiac catheterization as the means of accurately visualizing congenital cardiac defects. However, two distinct limitations exist with current 2-D presentations: (1) limited field of view (ie, 90 degrees sector) and (2) tomographic slices that must be assimilated by the examiner into a 3-D or four-dimensional diagnosis. True 3-D imaging has the ultimate capability of rendering anatomy in a format comparable to looking at the actual cardiac specimen. If electronic rendering were really feasible and of suitable quality, one could envision electronically extracting the heart from a living human and examining abnormalities much as one might examine a cardiac specimen (ie, "electronic vivisection"). This article reviews the state of the art of wide-field and 3-D cardiovascular ultrasound in the assessment of congenital heart disease.

Adult↗
PubMed1994

Low intensity ultrasound treatment increases strength in a rat femoral fracture model.

Bilateral closed femoral shaft fractures were made in 22 male Long-Evans rats. In 16 animals, ultrasound was applied to one limb for 15 minutes daily 10 times within the first 14 postoperative days. The treated limbs received a 200 microseconds burst of 1.5 or 0.5 MHz sine waves repeated at 1.0 kHz at a spatial average and temporal average intensity of 30 mW/cm2. The contralateral limb of each animal served as a nontreated control. Six remaining animals with fractures and six additional animals without fractures received sham ultrasound treatment to control for the effects of anesthesia and handling. Fracture repair was evaluated on postoperative day 21 by radiography, mechanical testing in torsion, and histology. Five of 16 ultrasound-treated fractures showed obliteration of the fracture gap on radiographs, whereas none of the 28 controls did. The average maximum torque of fractures treated with either signal was 22% greater than that of the contralateral controls (p < 0.05). The stiffness of treated fractures was greater than that of control fractures, but the difference was significant only in animals treated with the 1.5 MHz signal (p < 0.02). Sham treatment did not affect repair in the control group. These results indicate that low-intensity pulsed ultrasound at either 0.5 or 1.5 MHz can accelerate fracture repair at 21 days in this highly controlled model.

Animals↗
PubMed1994

Biomedical ultrasound beam forming.

The principles of biomedical ultrasound beam forming control the quality of diagnostic imaging. Beam parameters associated with imaging quality are: (1) lateral and axial resolutions; (2) depth of field; (3) contrast and (4) frame rate. In this paper, we review some of the current beam forming techniques and their principles. We focus on trade-offs among the above four aspects of beam forming and relate them to system parameters such as aperture size, f-number (the ratio between focal length and aperture diameter), central frequency (wavelength), system bandwidth and sidelobes. Methods for steering conventional and limited diffraction beams with array transducers are also reviewed.

Ultrasonics↗
PubMed1994

Diagnostic performance of two-dimensional versus three-dimensional transesophageal echocardiographic images of selected pathologies evaluated by receiver operating characteristic analysis.

UNLABELLED: The sensitivity and specificity of 2-D and 3-D echocardiographic images for the detection of selected morphological abnormalities were compared using receiver operating characteristic (ROC) analysis. Five experienced clinical echocardiographers blinded to the patients' diagnoses evaluated the 20 original static 2-D image sets and 20 corresponding 3-D reconstructions using a five point categorical scale that ranged from definitely abnormal to definitely normal. The ROC curve for the 3-D images was significantly (P < 0.05) closer to the ideal discrimination function than was the ROC curve for the 2-D transesophageal images (i.e., the sensitivity of the 3-D images was higher than that of the 2-D sequential images at the same specificity). IN CONCLUSION: 3-D transesophageal images provided better visual clues for the identification of morphological abnormalities than did serial 2-D echocardiographic images despite the same input information in both image formats. The use of ROC analysis assisted in the comparison of these two imaging techniques.

Adolescent↗
PubMed1994

Ultrasound echo envelope analysis using a homodyned K distribution signal model.

The statistics of ultrasound echo envelope signals can be used to characterize scattering media. The Rayleigh distribution and its generalized forms, the K and Rice distributions, have been previously used to model the echo signal. A more generalized statistical model, the homodyned K distribution, combines the K and Rice distribution features to better account for the statistics of the echo signal. We show that this model can give two parameters that are useful for media characterization: k, the ratio of coherent to diffuse signals, and, beta, which characterizes the clustering of scatters in the medium.

Computer Simulation↗
PubMed1993

Three-dimensional ultrasound imaging of the atrial septum: normal and pathologic anatomy.

OBJECTIVES: This study investigated the feasibility of producing three-dimensional gray scale ultrasound images of the atrial septum to demonstrate normal and pathologic anatomy. BACKGROUND: Two-dimensional echocardiography is the principal technique used for imaging the atrial septum. Although the diagnostic accuracy of two-dimensional echocardiography is high, its capability for displaying complex three-dimensional relations is limited. METHODS: Three-dimensional ultrasound images were reconstructed from tomographic images obtained during routine transesophageal echocardiographic examinations. Custom-made semi-automatic algorithms for image enhancement, interpolation and segmentation were used to produce volumetric gray scale images. Volume-rendered displays of the atrial septum were generated for analysis. Sequential three-dimensional images were generated through the cardiac cycle and displayed cinematographically to permit assessment of motion. RESULTS: The three-dimensional images obtained from six patients clearly demonstrated normal and pathologic anatomy of the atrial septum, including atrial septal defects, atrial septal aneurysm and aortic valve ring abscess. The images could be manipulated electronically to demonstrate spatial relations and internal structural details. CONCLUSIONS: Three-dimensional gray scale reconstruction of ultrasound images obtained by transesophageal echocardiography is feasible. These images clearly demonstrate anatomic details and spatial relations. The gray scale images may be interactively manipulated to optimize the clinician's visualization of the atrial septum and its associated pathologic conditions.

Cardiomyopathies↗
PubMed1993

Multidimensional visualization in echocardiography: an introduction.

X-ray films depict three-dimensional objects as shadows in a two-dimensional plane; thus, objects become superimposed. Computed tomography and other types of tomographic imaging, such as ultrasonography, acquire two-dimensional images of a material property within a thin slice. Sequential adjacent two-dimensional tomograms can be used to construct three-dimensional displays of objects. Visualization, a field of computer science, enables scientists to measure image attributes (extraction of features), identify features (classification), separate objects from one another (segmentation), and produce comprehensible, information-dense images from three-dimensional data sets (rendering). A three-dimensional rendering of the heart can be used to represent only one component of the heart, such as the atrial septum or the ventricular chamber, and can be shaded or colored to enhance comprehension. Three-dimensional images rendered sequentially over time result in a dynamic four-dimensional display. This report describes multidimensional visualization of objects and tissues and specifically discusses examples from echocardiography.

Echocardiography↗
PubMed1993

Three- and four-dimensional cardiovascular ultrasound imaging: a new era for echocardiography.

Three-dimensional and four-dimensional ultrasonography were pioneered in the 1960s yet have been used little clinically. Only recently have advances in cardiovascular ultrasound equipment and in digital image storage, manipulation, and display techniques made three- and four-dimensional imaging clinically feasible. In this report, we review the historical development of these technologies during 3 decades to their culmination in current state-of-the-art technology. Examples of such multidimensional images are presented, with special emphasis on clinical applications. Although several limitations persist, three-dimensional cardiovascular ultrasonography seems likely to enhance imaging of the heart and vessels in a manner similar to the advent of two-dimensional echocardiography in the M-mode era. Clinician-scientists will soon be able to extract an object, such as the heart, from the body electronically for the purpose of anatomic, functional, and histologic analysis without adverse effect on the patient.

Echocardiography↗
PubMed1993

Modified X waves with improved field properties.

A method to obtain a good compromise between the depth of field and the lateral resolution of "X waves" is proposed. The original X waves are theoretically nondiffracting beams generated by a specially phased infinite transmit aperture. When generated by a finite aperture, X waves are diffracting beams but have a large depth of field, maintaining uniform lateral field profiles. The proposed modification of the wave equation solution for X waves replaces a constant parameter representing the propagation angle of ultrasound with a function of radial distance at the aperture surface, and results in modified X waves that have a larger depth of field than the original X waves. Computer simulations show that a proper choice of the modification function can produce a new beam with improved field properties compared with the original X waves, promising images with higher lateral resolution and increased contrast over a large depth of field in high frame rate medical imaging. Experimental results are presented to verify the simulation results of the proposed method.

Humans↗
PubMed1993

Producing deep depth of field and depth-independent resolution in NDE with limited diffraction beams.

Limited diffraction beams, such as Durnin's J0 Bessel beam, are a class of nonspreading solutions to the isotropic/homogeneous scalar wave equation. These beams can be approximately produced with finite aperture and energy over a deep depth of field. In this paper, we report the application of a broadband J0 Bessel beam to nondestructive evaluation (NDE) of materials. Pulse-echo images of a stainless steel block phantom were obtained with both the J0 Bessel beam and a conventional focused Gaussian beam. Results show that uniformly high resolutions were obtained with the J0 Bessel beam over a large distance. In addition, the lateral resolution of the J0 Bessel beam is almost independent of the speed of sound of the materials inspected. In contrast, the lateral resolution of images obtained with the conventional focused Gaussian beam changes dramatically with the distance and the focal length of the beam in water is greatly reduced by the steel block. Therefore, limited diffraction beams could be useful for nondestructive evaluation of materials of different speeds of sound. Restoration of pulse-echo images obtained with these beams could be simplified.

Image Interpretation, Computer-Assisted↗
PubMed1992

Recursive RF excitation.

We have investigated the properties of a recursive process in which the output signal from a given RF excitation pulse may be used as the input (excitation) pulse of a subsequent iteration. This recursive excitation technique increases contrast and improves feature segmentation for the purpose of motion tracking.

Agar↗
PubMed1991

Pulse-echo imaging using a nondiffracting beam transducer.

Conventional ultrasonic transducers generate beams that diffract as they travel. This phenomenon causes images produced in B-mode to be degraded in the far-field of the transducers. Focused transducers are used to improve image quality. Unfortunately, focused transducers have short depth of field. Although multiple pulse transmissions focused at several depths are used to increase the effective depth of field, imaging frame rate is reduced dramatically leading to blurred images of moving objects such as the heart. We present a family of transducers that produce nondiffracting beams of large depth of field. Therefore, uniformly high resolution throughout the imaging area can be obtained without sacrificing the imaging frame rate. In addition, the nondiffracting property of these beams makes the correction for beam diffraction negligible in tissue characterization. This paper reports the results of computer simulations as well as in vitro and in vivo pulse-echo imaging experiments with a nondiffracting transducer. Images are compared to those obtained by conventional focused Gaussian shaded beam transducers and a commercial ACUSON 128 B-scanner. The new transducer has much longer depth of field with higher sidelobes than conventional transducers of the same aperture. Sidelobes can be reduced using the new transducer to transmit and the dynamically focused transducer to receive.

Calibration↗