Thomas A. Krouskop

Elastic moduli of breast and prostate tissues under compression

To evaluate the dynamic range of tissue imaged by elastography, the mechanical behavior of breast and prostate tissue samples subject to compression loading has been investigated. A model for the loading was validated and used to guide the experimental design for data collection. The model allowed the use of small samples that could be considered homogeneous; this assumption was confirmed by histological analysis. The samples were tested at three strain rates to evaluate the viscoelastic nature of the material and determine the validity of modeling the tissue as an elastic material for the strain rates of interest. For loading frequencies above 1 Hz, the storage modulus accounted for over 93 percent of the complex modulus. The data show that breast fat tissue has a constant modulus over the strain range tested while the other tissues have a modulus that is dependent on the strain level. The fibrous tissue samples from the breast were found to be 1 to 2 orders of magnitude stiffer than fat tissue. Normal glandular breast tissue was found to have an elastic modulus similar to that of fat at low strain levels, but the modulus of the glandular tissue increased by an order of magnitude above fat at high strain levels. Carcinomas from the breast were stiffer than the other tissues at the higher strain level; intraductal in situ carcinomas were like fat at the low strain level and much stiffer than glandular tissue at the high strain level. Infiltrating ductal carcinomas were much stiffer than any of the other breast tissues. Normal prostate tissue has a modulus that is lower than the modulus of the prostate cancers tested. Tissue from prostate with benign prostatic hyperplasia (BPH) had modulus values significantly lower than normal tissue. There was a constant but not significant difference in the modulus of tissues taken from the anterior and posterior portions of the gland.

Elastography: Ultrasonic estimation and imaging of the elastic properties of tissues

Abstract The basic principles of using sonographic techniques for imaging the elastic properties of tissues are described, with particular emphasis on elastography. After some preliminaries that describe some basic tissue stiffness measurements and some contrast transfer limitations of strain images are presented, four types of elastograms are described, which include axial strain, lateral strain, modulus and Poissons ratio elastograms. The strain filter formalism and its utility in understanding the noise performance of the elastographic process is then given, as well as its use for various image improvements. After discussing some main classes of elastographic artefacts, the paper concludes with recent results of tissue elastography in vitro and in vivo.

ELASTOGRAPHIC CHARACTERIZATION OF HIFU-INDUCED LESIONS IN CANINE LIVERS

The elastographic visualization and evaluation of high-intensity focused ultrasound (HIFU)-induced lesions were investigated. The lesions were induced in vitro in freshly excised canine livers. The use of different treatment intensity levels and exposure times resulted in lesions of different sizes. Each lesion was clearly depicted by the corresponding elastogram as being an area harder than the background. The strain contrast of the lesion/background was found to be dependent on the level of energy deposition. A lesion/background strain contrast between -2.5 dB and -3.5 dB was found to completely define the entire zone of tissue damage. The area of tissue damage was automatically estimated from the elastograms by evaluating the number of pixels enclosed inside the isointensity contour lines corresponding to a strain contrast of -2.5, -3 and -3.5 dB. The area of the lesion was measured from a tissue photograph obtained at approximately the same plane where elastographic data were collected. The estimated lesion areas ranged between approximately 10 mm2 and 110 mm2. A high correlation between the damaged areas as depicted by the elastograms and the corresponding areas as measured from the gross pathology photographs was found (r2 = 0.93, p value < 0.0004, n = 16). This statistically significant high correlation demonstrates that elastography has the potential to become a reliable and accurate modality for HIFU therapy monitoring.

Elastography : Imaging the Elastic Properties of Soft Tissues with Ultrasound

Elastography is a method that can ultimately generate several new kinds of images, called elastograms. As such, all the properties of elastograms are different from the familiar properties of sonograms. While sonograms convey information related to the local acoustic backscatter energy from tissue components, elastograms relate to its local strains, Youngs moduli or Poissons ratios. In general, these elasticity parameters are not directly correlated with sonographic parameters, i.e. elastography conveys new information about internal tissue structure and behavior under load that is not otherwise obtainable. In this paper we summarize our work in the field of elastography over the past decade. We present some relevant background material from the field of biomechanics. We then discuss the basic principles and limitations that are involved in the production of elastograms of biological tissues. Results from biological tissues in vitro and in vivo are shown to demonstrate this point. We conclude with some observations regarding the potential of elastography for medical diagnosis.

Poroelastography: imaging the poroelastic properties of tissues.

In the field of elastography, biological tissues are conveniently assumed to be purely elastic solids. However, several tissues, including brain, cartilage and edematous soft tissues, have long been known to be poroelastic. The objective of this study is to show the feasibility of imaging the poroelastic properties of tissue-like materials. A poroelastic material is a material saturated with fluid that flows relative to a deforming solid matrix. In this paper, we describe a method for estimating the poroelastic attributes of tissues. It has been analytically shown that during stress relaxation of a poroelastic material (i.e., sustained application of a constant applied strain over time), the lateral-to-axial strain ratio decreases exponentially with time toward the Poissons ratio of the solid matrix. The time constant of this variation depends on the elastic modulus of the solid matrix, its permeability and its dimension along the direction of fluid flow. Recently, we described an elastographic method that can be used to map axial and lateral tissue strains. In this study, we use the same method in a stress relaxation case to measure the time-dependent lateral-to-axial strain ratio in poroelastic materials. The resulting time-sequenced images (poroelastograms) depict the spatial distribution of the fluid within the solid at each time instant, and help to differentiate poroelastic materials of distinct Poissons ratios and permeabilities of the solid matrix. Results are shown from finite-element simulations.

ELASTOGRAPHIC IMAGING OF LOW-CONTRAST ELASTIC MODULUS DISTRIBUTIONS IN TISSUE

Elastography is a new ultrasonic imaging technique that produces images of the strain distribution in compliant tissues. This strain distribution is derived from ultrasonically estimated longitudinal internal motion induced by an external compression of the tissue. The displayed two-dimensional (2-D) images are called elastograms. In this paper, it is demonstrated that, when signal-to-noise ratio-enhancing techniques are used, elastography is capable of imaging low-contrast elastic modulus tissue structures with high contrast-to-noise ratios. This is demonstrated using both computer simulations and data obtained from 3 days postmortem ovine kidneys in vitro. The elastograms of such organs suggest that the modulus slowly decays from the renal cortex (RC) to the interior of the renal sinus (RS). Such modulus variation is corroborated by independent measurements of the Youngs moduli. It is found that the RC is approximately twice as hard as the interior of the RS. We invoke our previous results on elastographic contrast-transfer efficiency to demonstrate that, at low contrast, the elastogram may be interpreted as a quantitative image of the relative Youngs modulus distribution.

Elastographic imaging of thermal lesions in soft tissue: a preliminary study in vitro

The use of elastography for the visualization of thermal lesions in biological soft tissue in vitro was investigated. Thermal lesions were created in samples of postmortem ovine kidney using a surgical neodymium: YAG (Nd:YAG) laser. The kidney samples were cast in gel, and elastographic images of the lesions were constructed using sonographic information and external markers to locate the region of interest. Gross pathology of the kidney samples confirmed the dimensions of the lesions. Good correlation between the lesion length along the laser fiber axis and maximum diameter measured off of the fiber axis determined from elastographic images and gross pathology photographs was found.

Nonlinear stress-strain relationships in tissue and their effect on the contrast-to-noise ratio in elastograms

The practice of elastography is generally limited to small applied compressions (typically 1%), under the assumption of a linear stress-strain relationship in biological tissue. However, the recent reports of larger applied compressions and precompression levels to increase the strain contrast violate the above assumption. The nonlinear stress-strain relationships in different breast tissue types significantly alter the contrast in elastography, especially for large applied compression. The moduli of normal fibrous and glandular breast tissue (along with cancerous lesions) are strain-dependent, with tissue stiffness increasing with applied compression. In this paper, we illustrate that the strain-dependence of the modulus has a significant impact on the elastographic contrast and on the contrast-to-noise ratio, and may even cause a reversal of the contrast in certain situations. This paper also emphasizes the effect of the precompression strain level on the strain contrast.

Effectiveness of mattress overlays in reducing interface pressures during recumbency.

This study evaluates the pressure-reduction characteristics of seven mattress overlays. Thirty subjects were evaluated on each support surface to determine the interface pressures that are generated under the most common pressure sore sites. The results of this study indicate that there is great variability in the effectiveness of traditional mattress overlays. The most effective overlays are the Roho and Akros DFD mattresses; whereas 2-inch thick convoluted foam provides no significant protection for the trochanter when the subjects were lying on their sides (lateral position).

Enhancement of subthreshold sensory nerve action potentials during muscle tension mediated noise

In certain nonlinear systems, of which neurons are an example, the addition of random fluctuations, or “noise”, can enhance the detectability or transmission efficiency of a weakly applied signal. This counterintuitive statistical process, called stochastic resonance, —first advanced as a possible explanation for the observed periodic recurrences of the Earth’s ice ages [Benzi et al., 1981; Nicolis, 1993] —has by now been well established in a variety of physical systems [Wiesenfeld & Moss, 1995]. Recently it has been observed in the sensory nervous systems of two different arthropods [Douglass et al., 1994; Levin & Miller, 1996; Pei et al., preprint] and may be deeply linked to the evolution of all sensory systems. We report here the results of an experiment—the first in a human modality—designed to study the possibility of SR in the median nerve. Moreover, two additional features of this work are unique: first, our study makes use of the internal noise (as opposed to previous protocols wherein the noise was introduced externally) and second, we study the transmission of an aggregate stimulus through a bundle of individual neurons which are thought to be connected in parallel, a subject of recent theoretical [Collins et al., 1995] and experimental [Bezrukov & Vodynoy, 1995] interest. In our experiment, near subthreshold electrical stimuli were applied periodically to the median nerve, and responses were detected and signal averaged by electromyography. The internal noise intensity was mediated by muscle tension. The results are consistent with the hypothesis that internally generated electrical noise can enhance the signal transmission efficiency in this modality.