[Evaluation of viscoelastic property inhomogeneities in soft tissues exposed to low frequency perturbations]. / Diagnostika neodnorodnostei raspredeleniia viazkouprugikh svoistv miagkikh biologicheskikh tkanei pri ikh nizkochastotnom vozmushchenii.

This work presents the methods of recognition of inhomogeneities of tissue shear viscoelastic properties using partial data on the internal displacements in an object exposed to low-frequency perturbation. An approach to detect tissue inhomogeneities using the single displacement component is presented.

Strain rate imaging using two-dimensional speckle tracking.

Strain rate images (SRI) of the beating heart have been proposed to identify non-contracting regions of myocardium. Initial attempts used spatial derivatives of tissue velocity (Doppler) signals. Here, an alternate method is proposed based on two-dimensional phase-sensitive speckle tracking applied to very high frame rate, real-time images. This processing can produce high resolution maps of the time derivative of the strain magnitude (i.e., square root of the strain intensity). Such images complement traditional tissue velocity images (TVI), providing a more complete description of cardiac mechanics. To test the proposed approach, SRI were both simulated and measured on a thick-walled, cylindrical, tissue-equivalent phantom modeling cardiac deformations. Real-time ultrasound images were captured during periodic phantom deformation, where the period was matched to the data capture rate of a commercial scanner mimicking high frame rate imaging of the heart. Simulation results show that SRI with spatial resolution between 1 and 2 mm are possible with an array system operating at 5 MHz. Moreover, these images are virtually free of angle-dependent artifacts present in TVI and simple strain rate maps derived from these images. Measured results clearly show that phantom regions of low deformation, which are difficult to identify on tissue velocity-derived SRI, are readily apparent with SRI generated from two-dimensional phase-sensitive speckle tracking.

Three-dimensional static displacement, stimulated echo NMR elasticity imaging.

This article presents a method for measuring three-dimensional mechanical displacement and strain fields using stimulated echo MRI. Additional gradient pulses encode internal displacements in response to an externally applied deformation. By limiting the mechanical transition to the stimulated echo mixing time, a more accurate static displacement measurement is obtained. A three-dimensional elasticity reconstruction within a region of interest having a uniform shear modulus along its boundary is performed by numerically solving discretized elasticity equilibrium equations. Data acquisition, strain measurements and reconstruction were performed using a silicone gel phantom containing an inclusion of known elastic properties. A comparison between two-dimensional and three-dimensional reconstructions from simulated and experimental displacement data shows higher accuracy from the three-dimensional reconstruction. The long-term objective of this work is to provide a method for remotely palpating and elastically quantitating manually inaccessible tissues.

[Mechanical properties of soft biological tissues]. / Mekhanicheskie svoistva miagkikh biologicheskikh tkanei.

Odd published data concerning the shear mechanical properties of some soft tissues in norm and pathology are reviewed.

Reconstructive ultrasound elasticity imaging for renal transplant diagnosis: kidney ex vivo results.

It may be possible to diagnose and monitor scarring, inflammation and edema in transplant kidney using reconstructive ultrasound elasticity imaging. Kidney elasticity is expected to change dramatically with scar, and to a lesser degree, with acute inflammation and edema. The hypothesis that changes in kidney elasticity can be imaged using a clinical ultrasound scanner was experimentally tested with an ex vivo canine kidney model, and results on a single pair of kidneys are reported in this paper. A cross-linking agent affected kidney elasticity both globally and locally. Elasticity changes were monitored with accurate estimates of internal displacement and strain followed by Young's modulus reconstruction. The results of this study strongly suggest that ultrasound elasticity imaging can detect elasticity changes in complex structures such as the kidney. Moreover, it has the potential to become an important clinical tool for renal transplant diagnosis.

High-resolution elasticity imaging for tissue engineering.

An elasticity microscope provides high resolution images of tissue elasticity. With this instrument, it may be possible to monitor cell growth and tissue development in tissue engineering. To test this hypothesis, elasticity micrographs were obtained in two model systems commonly used for tissue engineering. In the first, strain images of a tissue-engineered smooth muscle sample clearly identified a several hundred micron thick cell layer from its supporting matrix. Because a one-dimensional mechanical model was appropriate for this system, strain images alone were sufficient to image the elastic properties. In contrast, a second system was investigated in which a simple one-dimensional mechanical model was inadequate. Uncultured collagen microspheres embedded in an otherwise homogeneous gel were imaged with the elasticity microscope. Strain images alone did not clearly depict the elastic properties of the hard spherical cell carriers. However, reconstructed elasticity images could differentiate the hard inclusion from the background gel. These results strongly suggest that the elasticity microscope may be a valuable tool for tissue engineering and other applications requiring the elastic properties of soft tissue at high spatial resolution (75 microm or less).

[The allowable degree of compression of biological soft tissues for tumor diagnosis]. / O dopustimoi stepeni szhatiia miagkikh biologicheskikh tkanei pri diagnostike opukholei.

The quantitative estimation of safe contraction for the soft tissue cancer detection procedure is presented, which is based on the classical analytical solution for the concentration of stress around spherical inclusions and flaws in elastic media.

Reconstructive elasticity imaging for large deformations.

A method is presented to reconstruct the elastic modulus of soft tissue based on ultrasonic displacement and strain images for comparatively large deformations. If the average deformation is too large to be described with a linear elastic model, nonlinear displacement-strain relations must be used and the mechanical equilibrium equations must include high order spatial derivatives of the displacement. Numerical methods were developed to reduce error propagation in reconstruction algorithms, including these higher order derivatives. Problems arising with the methods, as well as results using ultrasound measurements on gel-based, tissue equivalent phantoms, are given. Comparison to reconstructions using a linear elastic model shows that equivalent image quality can be produced with algorithms appropriate for finite amplitude deformations.

Measuring the elastic modulus of small tissue samples.

Independent measurements of the elastic modulus (Young's modulus) of tissue are necessary step in turning elasticity imaging into a clinical tool. A system capable of measuring the elastic modulus of small tissue samples was developed. The system tolerates the constraints of biological tissue, such as limited sample size (< or = 1.5 cm3) and imperfections in sample geometry. A known deformation is applied to the tissue sample while simultaneously measuring the resulting force. These measurements are then converted to an elastic modulus, where the conversion uses prior calibration of the system with plastisol samples of known Young's modulus. Accurate measurements have been obtained from 10 to 80 kPa, covering a wide range of tissue modulus values. In addition, the performance of the system was further investigated using finite element analysis. Finally, preliminary elasticity measurements on canine kidney samples are presented and discussed.

[The reconstruction of mechanical properties of layered viscoelastic media based on impedance measurements]. / Opredelenie mekhanicheskikh svoistv mnogosloinoi viazkouprugoi sredy po dannym izmereniia inpedansa.

The possibility for the reconstruction of unknown mechanical properties of layered media is investigated using the displacement data of rigid circular piston based on the extension of the solution to classical Lamb problem. The examples of published experimental data processing are given.

Elasticity reconstructive imaging by means of stimulated echo MRI.

A method is introduced to measure internal mechanical displacement and strain by means of MRI. Such measurements are needed to reconstruct an image of the elastic Young's modulus. A stimulated echo acquisition sequence with additional gradient pulses encodes internal displacements in response to an externally applied differential deformation. The sequence provides an accurate measure of static displacement by limiting the mechanical transitions to the mixing period of the simulated echo. Elasticity reconstruction involves definition of a region of interest having uniform Young's modulus along its boundary and subsequent solution of the discretized elasticity equilibrium equations. Data acquisition and reconstruction were performed on a urethane rubber phantom of known elastic properties and an ex vivo canine kidney phantom using <2% differential deformation. Regional elastic properties are well represented on Young's modulus images. The long-term objective of this work is to provide a means for remote palpation and elasticity quantitation in deep tissues otherwise inaccessible to manual palpation.

Nonlinear estimation of the lateral displacement using tissue incompressibility.

Using the incompressibility property of soft tissue, high quality lateral displacement distributions can be reconstructed from accurate axial displacement measurements and noisy lateral displacement estimates. Previous methods appropriate for small deformations have been extended for high magnitude deformations requiring a nonlinear model. Problems arising in incompressibility processing for large deformations are considered. Applications of nonlinear incompressibility methods to ultrasonic measurements on gel-based, tissue equivalent phantoms are given. Lateral displacement images reconstructed with nonlinear methods are compared to those reconstructed with linear methods for both small and large deformations.

[Reconstruction of elastic properties of soft biological tissues exposed to low frequency disruption]. / O rekonstruktsii uprugikh svoistv miagkikh biologicheskikh tkanei pri ikh nizkochastotnom vozmushchenii.

This work is part of investigation devoted to developing new noninvasive methods for diagnosing the pathology of soft biological tissues. The experimental basis of these methods is an ultrasound examination of statically or dynamically deformed object with the aim to obtain data for reconstruction of its yet unknown mechanical properties. A critical analysis of the dynamic approach to the problem is carried out.

Magnetic-resonance imaging techniques for detection of elasticity variation.

The relative success of manual palpation in the detection of breast cancer would suggest that a method for remote palpation resulting in a measurement of tissue elasticity could provide a diagnostic tool for detecting cancerous lesions deeper within the breast. This presumption is based in part on the excellent contrast between neoplastic and normal tissue due to the large (orders of magnitude) relative variation in the shear elastic modulus. By comparison, the bulk deformational modulus maintains the same value to within 20% for most soft tissues. A specific method of magnetic-resonance imaging (MRI) which measures tissue displacements has been used in experiments with a phantom containing regions of increased Young's modulus as a demonstration. The spatial modulation of magnetization technique uses the displacement of a spatial grid pattern caused by spin saturation to track regional motion. Mathematical reconstruction of the distribution of elastic moduli is shown for select examples. Any modality, e.g., MRI, ultrasound, etc., which can detect local tissue motion with sufficient spatial resolution can be used and therefore the results presented here should give an indication of the utility of such motion tracking techniques to future measurement of tissue elasticity.

Elasticity imaging for early detection of renal pathology.

Early detection of renal pathology may be possible with elasticity imaging. This hypothesis was experimentally tested by quantitatively imaging internal mechanical strain due to surface deformations in an in vitro animal model of nephritis. Preliminary data support the hypothesis that kidney elasticity changes with renal damage and concomitant scarring before problems are detectable by traditional diagnostic techniques such as laboratory measurements of renal function.