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Biomedical subjects

M F Insana

Publications and source records attributed to M F Insana.

At least 19 recordsLinked to original sources
PubMed2001

Ultrasonic properties of random media under uniaxial loading.

Acoustic properties of two types of soft tissue-like media were measured as a function of compressive strain. Samples were subjected to uniaxial strains up to 40% along the axis of the transducer beam. Measurements were analyzed to test a common assumption made when using pulse-echo waveforms to track motion in soft tissues--that local properties of wave propagation and scattering are invariant under deformation. Violations of this assumption have implications for elasticity imaging procedures and could provide new opportunities for identifying the sources of backscatter in biological media such as breast parenchyma. We measured speeds of sound, attenuation coefficients, and echo spectra in compressed phantoms containing randomly positioned scatterers either stiffer or softer than the surrounding gelatin. Only the echo spectra of gel media with soft scatterers varied significantly during compression. Centroids of the echo spectra were found to be shifted to higher frequencies in proportion to the applied strain up to 10%, and increased monotonically up to 40% at a rate depending on the scatterer size. Centroid measurements were accurately modeled by assuming incoherent scattering from oblate spheroids with an eccentricity that increases with strain. While spectral shifts can be accurately modeled, recovery of lost echo coherence does not seem possible. Consequently, spectral variance during compression may ultimately limit the amount of strain that can be applied between two data fields in heterogeneous media such as lipid-filled tissues. It also appears to partially explain why strain images often produce greater echo decorrelation in tissues than in commonly used graphite-gelatin test phantoms.

Animals↗
PubMed1999

Strain imaging with a deformable mesh.

Ultrasonic strain imaging has drawn much attention recently because of its ability to noninvasively provide information on spatial variation of the elastic properties of soft tissues. Traditionally, local strain is estimated by scaling and cross correlating pre- and postcompression ultrasound echo fields. However, when the motion field generated by compression is more complex, scaling and cross correlation can no longer provide precise displacement estimates because of signal decorrelation. We introduce a new algorithm based on the deformable mesh method. This algorithm can accommodate more general forms of motion, namely, the motion that can be described by bilinear transformations. We applied the new algorithm to three sets of data in order to evaluate its performance. In the first set of data, primitive motions such as shearing and rotation are simulated. The second set of data is collected by compressing a tissue-mimicking phantom with three hard inclusions. The third experiment involves an ex vivo pig kidney embedded in a block of gelatin. The results from all three experiments show improvements with the new algorithm over other methods.

Algorithms↗
PubMed1998

Ultrasonic and elasticity imaging to model disease-induced changes in soft-tissue structure.

Ultrasonic techniques are presented for the study of soft biological tissue structure and function. Changes in echo waveforms caused by microscopic variations in the mechanical properties of tissue can reveal disease mechanism, in vivo. On a larger scale, elasticity imaging describes the macroscopic mechanical properties of soft tissues. We summarize the approach and present preliminary results for studying disease-induced changes in renal tissue using these two acoustic imaging techniques.

Algorithms↗
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↗
PubMed1998

Errors in biased estimators for parametric ultrasonic imaging.

Maximum likelihood (ML) methods are widely used in acoustic parameter estimation. Although ML methods are often unbiased, the variance is unacceptably large for many applications, including medical imaging. For such cases, Bayesian estimators can reduce variance and preserve contrast at the cost of an increased bias. Consequently, including prior knowledge about object and noise properties in the estimator can improve low-contrast target detectability of parametric ultrasound images by improving the precision of the estimates. In this paper, errors introduced by biased estimators are analyzed and approximate closed-form expressions are developed. The task-specific nature of the estimator performance is demonstrated through analysis, simulation, and experimentation. A strategy for selecting object priors is proposed. Acoustic scattering from kidney tissue is the emphasis of this paper, although the results are more generally applicable.

Acoustics↗
PubMed1998

Comparison of a cathode-ray-tube and film for display of computed radiographic images.

The goal of the study was to compare a cathode-ray-tube (CRT) digital display with film by using task-dependent image quality assessment methods. Contrast-detail analysis was utilized. Human observers performed a simple detection task, specifically, detecting a pillbox target in a uniform Poisson field, using either film or a digital display that employed a CRT monitor. Observers performed equally well on both film and CRT when the window settings of the digital display were established subjectively by a radiologist. Changing the window settings of the digital display to match the average background luminance of a film-illuminator combination decreased the luminance contrast of the targets and observer performance was reduced, though these effects were probably not linked. The "gold standard" film had lower luminance contrast than the CRT displayed images, yet observer performance was never lower for film than for the CRT. Therefore we concluded that luminance contrast was not a limiting factor for observer performance in this study. The CRT monitor changed fairly rapidly after it was calibrated. During a period of six months the gamma of the display increased from 1.82 to 2.42 and the maximum luminance decreased from 319 to 228 cd/m2. Low luminance output demonstrated a larger percentage decrease (approximately equal to 85%) than high luminance output (approximately equal to 29%) over the same time period. These observations suggest that standard window settings should be reviewed from time to time to ensure that the display is used optimally. No special look-up table setup such as perceptual linearization was used.

Calibration↗
PubMed1997

Mathematical model of progressive renal disease.

A simple mathematical model is proposed that predicts the dynamics of chronic progressive renal disease. The model consists of coupled linear differential equations formed from three state variables, four control parameters, and three parameters related to initial conditions. All have straightforward physical interpretations. Applied to a population of nephrons, the model predicted the hypertrophic and sclerotic features of parenchyma progressing towards end-stage renal disease. Simulation results compared favorably with measurements obtained from the literature involving the subtotal nephrectomy rat model for renal disease. The time course of disease progression and treatment were considered. Also, the implications of the model for designing new diagnostic techniques using ultrasonic analysis are discussed.

Disease Progression↗
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 properties of estimates of signal-to-noise ratio and number of scatterers per resolution cell.

Elementary theory underlying the relationship between the number of scatterers per resolution cell (N) and echo intensity signal-to-noise ratio (SNR) is reviewed. A relationship between the probability density functions for estimates of N and SNR2 is derived. This relationship is validated using a computer simulation. Phantom and in vitro experiments are described. In one set of experiments on phantoms, empirical distributions of estimates of N and SNR2 are measured and compared to theoretical predictions. The utility of SNR2 for discrimination of phantoms with different values for N is assessed using receiver operating characteristic (ROC) analysis. In another set of experiments, the frequency dependence of the SNR2 estimate is investigated for a two-component phantom and for excised dog kidney. It is shown that the frequency dependence of the SNR can help to identify the presence of two or more scattering components that are spatially mixed. With regard to kidney data, measurements performed both parallel and perpendicular to the predominant nephron orientation are reported. The observed anisotropy is compared to the anisotropy of backscatter coefficient encountered in previous investigations.

Animals↗
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

Ultrasonic measurement of glomerular diameters in normal adult humans.

The average size of acoustic scatterers and the integrated backscatter coefficient of kidney cortex were measured in vivo from 2-4 MHz for a group of 50 normal adult volunteers. Our goal was to determine the sensitivity of the ultrasonic measurements under clinical conditions by identifying biologic sources of estimation uncertainty. Based on 10 measurements on each kidney of each volunteer, the average glomerular diameter for the group was found to be 216 +/- 27 microns (SD). Glomerular size was found to correlate with body surface area (r = 0.4), and the ratio of glomerular surface area to body surface area (GSA/BSA) was found to be constant throughout normal adult life with GSA/BSA = (8.24 +/- 1.35) x 10(-8) (SD). These results are consistent with histologic analyses found in the literature. Within an individual, 7% standard errors in GSA/BSA are typical. Biologic variability dominates the variance in scatterer size estimates in a group not matched for BSA, where it accounts for 47% of the variance. In a group of individuals matched for BSA, biologic variability accounts for only 21% of the variance; day-to-day variability accounts for 35% of the variance; and experimental parameters account for the remainder. If a deviation greater than 2 x SD is considered abnormal, then this technique can potentially detect changes in glomerular diameter as small as 30 microns within an individual. To detect abnormal GSA/BSA values for an individual, GSA/BSA would have to differ from the mean for that group by about 3.6 x 10(-8) or about 40%. Therefore, at this time scatterer size estimates appear most reliable for tracking the progression of disease and treatment for an individual over time.

Adolescent↗
PubMed1996

The effects of lossy compression on the detection of subtle pulmonary nodules.

We examined the ability of radiologists to detect pulmonary nodules in computed radiographic (CR) chest images subjected to lossy image compression. Low-contrast 1-cm diameter targets simulating noncalcified pulmonary nodules were introduced into clinical images and presented to ten radiologists in a series of two-alternative forced-choice (2AFC) observer experiments. The percentages of correct observer responses obtained while viewing noncompressed images (1:1) were compared with those obtained for the same images compressed 7:1, 16:1, 44:1, and 127:1. The images were compressed using a standard full-frame discrete cosine transform (DCT) technique. The degree of compression was determined by quantizing Fourier components in various frequency channels and then Huffman encoding the result. The data show a measurable decline in performance for each compression ratio. Through signal-to-noise ratio (SNR) analysis, we found that the reduction in performance was due primarily to the compression algorithm that increased image noise in the frequency channels of the signals to be detected.

Algorithms↗
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↗
PubMed1996

Error bounds on ultrasonic scatterer size estimates.

Precision errors that occur in estimating the average scatterer size from pulse-echo ultrasound waveforms are examined in detail. The method-independent lower bound on estimation error is found from the Cramér-Rao inequality for comparison with the predicted error for the measurement technique currently used to estimate scatterer sizes in soft biological tissues. The probability density function for the estimate is also derived. From these statistical analyses, strategies for designing experiments that minimize the error are discussed. It is shown that compared with biological variability, measurement errors in scatterer size estimates are relatively large. Consequently, there is reason to continue searching for more efficient estimators. Although the analysis and results are derived for Gaussian correlation models that have been used to study the function and structure of kidneys, generalization to include correlation models for other tissues is straightforward.

Models, Biological↗
PubMed1996

Autoregressive spectral estimation in ultrasonic scatterer size imaging.

An autoregressive (AR) spectral estimation method was considered for the purpose of estimating scatterer size images. The variance and bias of the resulting estimates were compared with those using classical FFT periodograms for a range of input signal-to-noise ratios and echo-signal durations corresponding to various C-scan image slice thicknesses. The AR approach was found to produce images of significantly higher quality for noisy data and when thin slices were required. Several images reconstructed with the two techniques are presented to demonstrate difference in visual quality. Task-specific guidelines for empirical selection of the AR model order are also proposed.

Algorithms↗
PubMed1996

Ultrasonic imaging of microscopic structures in living organs.

It is possible to estimate acoustic parameters that describe the microscopic anatomy of live tissues. The same rf echo signals used to form diagnostic ultrasound images undergo additional signal processing to compute D, the average scatterer size; SNR, a measure of the number density of scatterers; and IBC, a measure of tissue echogenicity. With these three parameters it is possible to identify the sources of ultrasonic scattering in kidneys, observe functional changes in live organs, and produce images of each parameter. Although this chapter has emphasized the more basic research elements of possible applications, similar methods have been used to study changes in tissue morphometry during the progress of disease and during the course of therapy in several organ systems. Advances in diagnostic ultrasound are providing safe and low-cost method for studying soft tissues in vivo.

Acute Disease↗