Measurement Of Tissue Properties Research Articles

Simulations of the thermo-acoustic lens effect during focused ultrasound surgery.

Laboratory measurements of soft tissue properties show a dependence of background propagation properties on temperature. For typical focused ultrasound surgery (FUS) applications, only the slow variations in tissue background parameters need to be accounted for when computing the outcome of a FUS sonication. The cumulative effect of slowly varying sound speed has been referred to in the literature as a thermal lens, or a thermo-acoustic lens because of its beam-distorting properties. An algorithm to solve the coupled acoustic-thermal problem is described, and numerical results are presented to illustrate the effects of dynamic sound-speed profiles in layered tissues undergoing FUS. The results of simulations in liver with and without a fat layer indicate that the thermal-acoustic interaction results in more complex dynamics in FUS than a simple model will predict. Both the size and the position of the lesions predicted from the simulations are affected by the thermo-acoustic lens effect. However, the overall effect from short sonications at high power from sharply focused single element sources (F-no. from 0.8 to 1.3) around 1 MHz similar to those used in clinical setups is found to be small.

A Cell-Based Constitutive Relation for Bio-Artificial Tissues

By using a combination of continuum and statistical mechanics we derive an integral constitutive relation for bio-artificial tissue models consisting of a monodisperse population of cells in a uniform collagenous matrix. This constitutive relation quantitatively models the dependence of tissue stress on deformation history, and makes explicit the separate contribution of cells and matrix to the mechanical behavior of the composite tissue. Thus microscopic cell mechanical properties can be deduced via this theory from measurements of macroscopic tissue properties. A central feature of the constitutive relation is the appearance of “anisotropy tensors” that embody the effects of cell orientation on tissue mechanics. The theory assumes that the tissues are stable over the observation time, and does not in its present form allow for cell migration, reorientation, or internal remodeling. We have compared the predictions of the theory to uniaxial relaxation tests on fibroblast-populated collagen matrices (FPMs) and find that the experimental results generally support the theory and yield values of fibroblast contractile force and stiffness roughly an order of magnitude smaller than, and viscosity comparable to, the corresponding properties of active skeletal muscle. The method used here to derive the tissue constitutive equation permits more sophisticated cell models to be used in developing more accurate representations of tissue properties.

On the evolution of some endoscopic light delivery systems for photodynamic therapy.

Progress in photodynamic therapy (PDT) depends on the development of: (1) photosensitizers, (2) optical devices among which are lasers and light delivery systems, and (3) clinical procedure. The light delivery systems which are the focus of this article are fiberoptic devices developed in Lausanne, Switzerland for use in the endoscopic treatment of cancer or precancerous lesions in the bronchi, the esophagus, the uterus, the cervix, the upper aerodigestive tract and thoracic cavity. Light delivery systems for both surface and interstitial application are presented, together with some of the physical principles on which they are based. Incorporation in these devices of the possibility for in-situ measurement of reflected therapeutic light and/or fluorescence emitted by endogenous and/or exogenous dyes allows for improved light and drug dosimetry, as well as the measurement of photobleaching, local oxygenation and other tissue properties. The necessity of information on tissue optical parameters, as well as the use of simple mathematical models and tissue phantoms, for optimizing light distributing devices is underlined. The devices are optimized for delivering the desired light intensity distribution to the targeted region with minimal losses. In some cases this implies using the device to modify the shape of the hollow organ during PDT, an example of which is given for the case of the esophagus. In another strategy, one adapts the shape of the device to that of the organ, using an elastic balloon catheter. Here examples are given for the uterus, the bronchi and the thoracic cavity. The mechanical properties, the sizes, shapes and materials of the light delivery systems must be optimized for safe use while retaining low cost. Furthermore, the devices must whenever possible be rendered compatible with existing medical technology. A significant improvement in clinical efficacy has been demonstrated in the testing of some of these new fiberoptic light delivery systems. For endoscopic PDT in the hollow organs, the design and optimalization of multiple new approaches to light distribution will continue to lead to improved clinical results.

A low noise, high resolution silicon temperature sensor

High resolution temperature measurement is essential for determination of blood perfusion in biomaterials. A compact, low noise, high resolution temperature sensor designed for use in an invasive tissue property measurement probe is presented. The circuit is based on traditional proportional-to-absolute-temperature (PTAT) principles. A feedback technique is used to improve linearity and reduce noise. Data from test chips shows temperature resolution of 3 m/spl deg/C. The chips were fabricated using a 1.75 /spl mu/m double poly, single metal modified CMOS process designed for this project.

New models of the oculomotor mechanics based on data obtained with chronic muscle force transducers.

Several phenomenological models of the oculomotor mechanics that produce saccadic eye movements have been developed. These models have been based on measurements of macroscopic muscle and orbital tissue properties and measurements of eye kinematics during saccades. We recorded the forces generated by the medial and lateral recti during saccades in an alert, behaving monkey using chronically implanted force transducers. With this new data, we tested the ability of the classic saccade models to generate realistic muscle force profiles. Errors in the predictions of the classic saccade models led to a reexamination of the current models of extraocular muscle. Both a phenomenological, Hill-type muscle model and an approximation to Huxley's molecular level muscle model based on the cross-bridge mechanism of contraction (distribution moment model) were derived and studied for monkey extraocular muscle. Simulations of the distribution moment model led to insights suggesting (i) specific modifications in the lumped force/velocity relationship in the Hill-type model that resulted in this type of phenomenological model being able to generate realistic dynamics in extraocular muscle during saccades; (ii) the distribution of activity in the different fiber types in extraocular muscle may be central to the characteristics exhibited by the muscle during saccades; (iii) the transient properties of lengthening muscle such as yielding are not significant during saccades; and (iv) the series elastic component in active muscle may be predominantly generated by the elastic properties of the cross-bridges.

Morphology-mechanical property relations in trabecular bone of the osteoarthritic proximal tibia

Alteration of morphologic and mechanical properties of trabecular bone in the osteoarthritic proximal tibia may be a contributing factor in tibial component loosening. To explore this issue, the authors performed tissue property measurements, morphologic analysis, and mechanical testing of subchondral, epiphyseal, and metaphyseal trabecular bone specimens retrieved from six human proximal tibias exhibiting a range of medial unicondylar osteoarthritic degeneration. Apparent density in the proximal tibia was altered according to varus misalignment and medial subluxation associated with medial osteoarthritis of the knee. In subchondral bone, a decrease in tissue mineralization contributed to a significant reduction in axial mechanical properties with degenerative disease ( P < .0005). In epiphyseal and metaphyseal bone, trabecular thickness and the number of trabeculae increased linearly with volume fraction, providing a power law relationship between axial elastic modulus and apparent density ( R 2 = .84). Average elastic properties of the tibial epiphysis and metaphysis were not reduced by degenerative disease ( P < .05). The results suggest that absolute minimization of tibial resection might not be an optimal strategy for tibial component fixation and that mechanical properties of the tibial resection surface are more homogeneous in planes parallel to the joint surface than in a plane normal to the longitudinal axis of the tibia.

Dielectric measurements on reference liquids using automatic network analysers and calculable geometries

Knowledge of the in vivo dielectric properties of human tissues is essential to the development of biomedical techniques which make use of radiofrequency and microwave radiation. The sensors used for measurement of tissue properties can be conveniently calibrated using dielectric reference liquids, but this in turn requires that the dielectric properties of such liquids be accurately known. Many complex permittivity measurements on suitable liquids, such as the primary alcohols and saline solutions, have been reported in the literature, but unfortunately there are in some cases significant discrepancies between reported values. The authors describe a program of work undertaken to resolve some of these anomalies. A number of techniques which make use of 14 mm coaxial cells and sensors coupled to an automatic network analyser are under investigation. Comparison of the different techniques should facilitate evaluation of systematic uncertainties. The liquid measurement geometries and the associated computational methods are described. Details are given of the uncertainty analysis, and examples of mappings from the complex permittivity plane on to the scattering coefficient plane are shown. Some preliminary results are presented, which highlight the value of the comparative approach that has been adopted.

Indirect versus direct techniques for the measurement of the optical properties of tissues.

Abstract Knowledge of the optical absorption and scattering properties of tissues is required to calculate the spatial distribution of light in vivo for applications such as photodynamic therapy. In many soft tissues in the visible wavelength range the optical albedo is high, the scattering is highly forward peaked, and the mean free path between photon interactions is small. This regime presents severe technical difficulties in measuring the absorption and scattering coefficients. Two classes of technique may be used: 1. indirect methods in which, by applying specific models of light propagation, the properties are deduced from measurements of bulk tissue properties, and 2. direct, model‐free measurements using optically thin tissue samples. These two approaches are described and the relative advantages and disadvantages discussed. Data for the optical properties of model soft tissues at 630 nm are presented to illustrate the applications of indirect and direct methods.

Miniature piezoelectric polymer hydrophones in biomedical ultrasonics

Abstract The performance properties of miniature piezoelectric polymer ultrasonic hydrophone probes are described, their applications as reference hydrophones and for in vivo ultrasonic measurement of tissue properties are discussed, and the advantages of a needle-like construction are pointed out. A novel calibration technique, based on Time Delay Spectrometry, allows the hydrophone probe free field parameters to be determined as a continuous function of frequency in reverberant environments, while maintaining outstanding signal-to-noise ratio.