Biomedical subjects

M Bilgen

Publications and source records attributed to M Bilgen.

12 recordsLinked to original sources

PubMed2001

Dynamic contrast-enhanced MRI of experimental spinal cord injury: in vivo serial studies.

The progression of experimental spinal cord injury (SCI) was followed with in vivo dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) and neurobehavioral studies on postinjury days 0, 2, 4, 7, 10, 14, 17, 21, 28, 35, and 42. Gadopentate dimeglumine (Gd) was administered IV and postcontrast, T(1)-weighted, axial images were acquired repetitively for up to 60 min. Images were analyzed to determine the spatial and temporal evolution of the intensity enhancement. A statistical decision mechanism was developed to objectively detect the enhancement. Strong and rapid enhancement was observed at the epicenter of injury, indicating a significant compromise in blood spinal cord barrier. The enhanced regions in each slice were combined to estimate the area and volume of the lesion. On the day of injury, around 85% of the total cord area at the epicenter showed enhancement within the first 15 min of Gd administration. At the same time, the enhanced volumes attained nearly 40% of the total cord volume and extended axially over 8 mm along the cord. These quantities decreased steadily with time, with a concomitant improvement in the motor functions. The volume of enhancement correlated highly with the neurobehavioral tests (r = -0.87). DCE-MRIs revealed small hyperintense regions distributed inside white matter about two weeks postinjury. Based on histology, these enhancements appear to represent new vessels with "leaky endothelium." Magn Reson Med 45:614-622, 2001.

Animals↗

PubMed2001

Phase aberration effects in elastography.

In sonography, phase aberration plays a role in the corruption of sonograms. Phase aberration does not have a significant impact on elastography, if statistically similar phase errors are present in both the pre- and postcompression signals. However, if the phase errors are present in only one of the pre- or postcompression signal pairs, the precision of the strain estimation process will be reduced. In some cases, increased phase errors may occur only in the postcompression signal due to changes in the tissue structure with the applied compression. Phase-aberration effects increase with applied strain and may be viewed as an image quality derating factor, much like frequency-dependent attenuation or undesired lateral tissue motion. In this paper, we present a theoretical and simulation study of the effects of phase aberration on the elastographic strain-estimation process, using the strain filter approach.

Biomechanical Phenomena↗

PubMed2000

Spatial and temporal evolution of hemorrhage in the hyperacute phase of experimental spinal cord injury: in vivo magnetic resonance imaging.

To follow the spatial and temporal evolution of hemorrhage, in vivo MRI studies of experimental spinal cord injury (SCI) were performed on 17 rats in the very acute phase (hyperacute), starting as early as 9 min and continued up to 400 min posttrauma. Axial MR images were processed slice by slice over a 21 mm length around the epicenter of the injury. The data were analyzed statistically and fitted to an empirically derived function to characterize the spatial and temporal evolution of hemorrhage. The results indicated that 1) the initial hemorrhage in the very early phase of the injury area covered 12.5% of the total cord area and subsequently increased with a time constant of 700 min, 2) a major portion of the hemorrhage was concentrated spatially within the 4 mm distance from the epicenter, 3) the volume of hemorrhage normalized to its initial value increased linearly at a rate of approximately 0.0015 min(-1), and 4) edema was observed at the gray- and white-matter junction as early as 12 min postinjury. In general, edema appeared to be focal and scattered in this phase of the injury, which made its quantification unreliable on MRI.

Acute Disease↗

PubMed2000

Direct strain estimation in elastography using spectral cross-correlation.

Spectral estimation of tissue strain has been performed previously by using the centroid shift of the power spectrum or by estimating the variation in the mean scatterer spacing in the spectral domain. The centroid shift method illustrates the robustness of the direct, incoherent strain estimator. In this paper, we present a strain estimator that uses spectral cross-correlation of the pre- and postcompression power spectrum. The centroid shift estimator estimates strain from the mean center frequency shift, while the spectral cross-correlation estimates the shift over the entire spectrum. Spectral cross-correlation is shown to be more sensitive to small shifts in the power spectrum and, thus, provides better estimation for smaller strains when compared to the spectral centroid shift. Spectral cross-correlation shares all the advantages gained using the spectral centroid shift, in addition to providing accurate and precise strain estimation for small strains. The variance and noise properties of the spectral strain estimators quantified by their respective strain filters are also presented.

Acoustics↗

PubMed2000

Dynamics of errors in 3D motion estimation and implications for strain-tensor imaging in acoustic elastography.

For the purpose of quantifying the noise in acoustic elastography, a displacement covariance matrix is derived analytically for the cross-correlation based 3D motion estimator. Static deformation induced in tissue from an external mechanical source is represented by a second-order strain tensor. A generalized 3D model is introduced for the ultrasonic echo signals. The components of the covariance matrix are related to the variances of the displacement errors and the errors made in estimating the elements of the strain tensor. The results are combined to investigate the dependences of these errors on the experimental and signal-processing parameters as well as to determine the effects of one strain component on the estimation of the other. The expressions are evaluated for special cases of axial strain estimation in the presence of axial, axial-shear and lateral-shear type deformations in 2D. The signals are shown to decorrelate with any of these deformations, with strengths depending on the reorganization and interaction of tissue scatterers with the ultrasonic point spread function following the deformation. Conditions that favour the improvements in motion estimation performance are discussed, and advantages gained by signal companding and pulse compression are illustrated.

Acoustics↗

PubMed2000

Maximum-likelihood approach to strain imaging using ultrasound

A maximum-likelihood (ML) strategy for strain estimation is presented as a framework for designing and evaluating bioelasticity imaging systems. Concepts from continuum mechanics, signal analysis, and acoustic scattering are combined to develop a mathematical model of the ultrasonic waveforms used to form strain images. The model includes three-dimensional (3-D) object motion described by affine transformations, Rayleigh scattering from random media, and 3-D system response functions. The likelihood function for these waveforms is derived to express the Fisher information matrix and variance bounds for displacement and strain estimation. The ML estimator is a generalized cross correlator for pre- and post-compression echo waveforms that is realized by waveform warping and filtering prior to cross correlation and peak detection. Experiments involving soft tissuelike media show the ML estimator approaches the Cramer-Rao error bound for small scaling deformations: at 5 MHz and 1.2% compression, the predicted lower bound for displacement errors is 4.4 microns and the measured standard deviation is 5.7 microns.

Journal Article↗

PubMed1998

Elastostatics of a spherical inclusion in homogeneous biological media.

A three-dimensional spherical inclusion model that approximates a lesion bonded to a tissue matrix is proposed for biomedical elastography. Analytical formulae describing spatial strain and stress distributions generated in infinite media by uniform loading are given under a linear, homogeneous, isotropic elasticity assumption. Strain and stress distributions are also calculated using finite-element analysis (FEA) for a variety of cases to determine the effects of shear modulus distribution, external loading conditions (uniform stress versus uniform displacement), compressor size and matrix dimensions on the elastostatics of the tissue. Analytical strain and stress predictions are shown to agree with the FEA results to within 10% accuracy provided that the matrix dimensions are at least ten times that of the inclusion. Also for these cases, uniform-stress boundary conditions can be equivalently represented by uniform displacement of the boundary. Spherical inclusions exhibit a lower efficiency for transferring elastic shear modulus contrast into strain contrast than cylindrical or planar inclusions. Additional compression will increase the strain contrast. However, large compressions also lead to increases in ultrasonic signal decorrelation and strain and stress concentrations in the homogeneous matrix around the inclusion. Although strain concentrations may help describe the boundaries of the inclusion more clearly, they also increase the risk of damaging the tissue. Understanding the strain and stress distributions in a biological tissue containing a lesion is necessary for optimizing the experimental configurations and consequently improving the diagnostic values of elasticity imaging.

Cysts↗

PubMed1997

The nonstationary strain filter in elastography: Part II. Lateral and elevational decorrelation.

The nonstationary evolution of the strain filter due to lateral and elevational motion of the tissue scatterers across the ultrasound beam is analyzed for the 1-D cross-correlation-based strain estimator. The effective correlation coefficient that includes the contributions due to lateral and elevational signal decorrelation is used to derate the upper bound of the signal-to-noise ratio in the elastogram (SNRe) predicted by the ideal strain filter. In the case of an elastically homogeneous target, if the transducer is on the axis of symmetry of such target in the elevational direction, the motion of the scatterers out the imaging plane is minimized. In addition, the ultrasound beam along the elevational direction is broader, allowing scatterers to stay longer within the beam during tissue compression. Under these conditions, lateral signal decorrelation becomes the primary contributor to the nonstationary behavior of the strain filter. Both the elastographic SNRe and the dynamic range are reduced, with an increase in lateral decorrelation. Finite element simulations and phantom experiments are presented in this paper to corroborate the theoretical strain filter. The nonstationary behavior of the strain filter is reduced by confining the tissue in the lateral direction (minimizing motion of tissue scatterers), thereby improving the quality of the elastogram.

Computer Simulation↗

PubMed1997

Error analysis in acoustic elastography. I. Displacement estimation.

Correlation between acoustic echo signals obtained before and after application of an external compressional force provides information about the internal deformation of an elastic medium. In this paper, the variance for displacement estimated from an echo data segment and the covariance between two windowed segments that may overlap are derived. The signal and noise spectra are Gaussian and independent. The dependence of the displacement variance on input signal-to-noise ratio (SNRi), time-bandwidth product W, fractional bandwidth Y-1, and the rate of displacement variation with depth a is investigated. The relationship between a and the other experimental parameters is crucial for understanding how signal decorrelation affects displacement error. The expression for displacement variance reduces to the Cramer-Rao lower bound result when a = 0 and W > > 1 for both bandpass and base-band signals. When a not equal to 0 displacement variance increases, and there is an optimal window length at W = square root of 20/a square root of 1 + Y2 for which the displacement variance is minimum. Narrow-band signals produce larger errors than broadband signals for long observation windows when a not equal to 0 and just the opposite when a = 0. Errors are greatest for displacements estimated from the envelope of narrow-band signals. Finally, a general expression for the minimum displacement variance for arbitrary signal and noise spectra is derived as a function of the experimental parameters. These results form a framework for analyzing strain estimates in elastography, the subject of a companion paper.

Acoustics↗

PubMed1997

Error analysis in acoustic elastography. II. Strain estimation and SNR analysis.

Accurate displacement estimates are required to obtain high-quality strain estimates in elastography. In this paper the strain variance is derived from the statistical properties of the displacement field to define a point signal-to-noise ratio for elastography (SNR0). Displacements caused by compressional forces applied along the axis of the transducer beam are modeled by scaling and shifting the axial reflectivity profile of the tissue. The strain variance is given as a function of essential experimental parameters, such as the amount of tissue compression, echo waveform window length, and the amount of window overlap. SNR0 is defined in terms of applied compression and strain variance and normalized by the input signal-to-noise ratio (SNRi) for echo signals, to formulate the performance metric SNR0/SNRi. This quantity characterizes the noise properties, dynamic range, and sensitivity of strain images based on the spatial resolution requirements. The results indicate that low noise, high sensitivity, and limited dynamic range strain images are obtained for high-frequency bandpass signals when the applied strain is small. For large strains, however, one strategy for low-noise strain imaging employs base-band signals to obtain images with large dynamic range but limited peak sensitivity and noise figure. A better strategy includes companding, which eliminates the average strain in the echo signal before cross-correlation to reduce the dynamic range requirement and increase peak sensitivity for strain estimates.

Acoustics↗

PubMed1997

Statistical analysis of strain images estimated from overlapped and filtered echo signals.

The visibility of soft-tissue lesions in strain imaging is currently limited by strain noise from waveform decorrelation. Attempts to balance noise reduction with concerns for contrast and spatial resolution rely on accurate models of time delay covariance for guidance. The most useful analytical models describe the covariance of time-varying time delay estimates in terms of experimental parameters and tissue deformation patterns. Assuming compressed tissue deforms linearly along the axis of the sound beam, we derived a delay covariance expression for echo data with Gaussian spectra that were filtered by a Gaussian window function before correlation. The Gaussian filter reduced the number of assumptions needed to obtain closed-form expressions and minimized the effects of strain within the correlation window. However, strain images are often made using uniformly weighted (sinc filtered) window functions. This paper compares time delay covariances for these two window functions, and describes an equivalent window duration at which delay variances for Gaussian and uniform windows are equal. At the equivalent window length, the analysis can be used to predict strain errors for either window function. Finally, this paper uses the delay covariance data to show how strain noise and image sharpness vary depending on the amount of overlap between correlation windows. For an applied strain less than 5%, an overlap near 50% offers an adequate compromise. These results can guide the selection of experimental parameters for improving the visibility of lesions in strain images.

Acoustics↗

PubMed1996

Deformation models and correlation analysis in elastography.

Cross-correlation functions are derived with the purpose of determining how strain inhomogeneities affect the displacement estimates used in ultrasound-based elastography. Variations in the strain profile occur in most imaging situations and are caused by fluctuations in the stress field or elastic modulus of the sample. An analytical framework for developing signal processing strategies in elastography is described, and the limitations of correlation-based methods for measuring displacements in tissuelike media caused by static compression are emphasized. This paper includes (1) an accurate approximation for an inverse coordinate transformation that release pre- and postcompression reflectivity profiles of the media, (2) a derivation of the echo-signal cross-correlation function in media with deterministic or stochastic strain profiles; (3) mathematical and graphical descriptions of the consequences that nonuniformities in the strain profile impose upon the uncertainty of displacement estimation; and (4) a demonstration of the advantages of echo signal conditioning and ultrasonic-pulse shaping to reduce the nonstationary effects that attenuate the cross-correlation peak and reduce the signal-to-noise ration for displacement estimation.

Elasticity↗