Homogeneous Tissue Phantoms Research Articles

Identifying the mechanical properties of tissue by ultrasound strain imaging

A new elastography method is presented that images the mechanical properties of soft tissue. The tissue is externally vibrated over a range of frequencies simultaneously and the resulting displacement is recorded at multiple locations and time instants with a sequence of ultrasound images. Two methods are proposed for estimating the mechanical properties from the recorded data. In the first method, equations of motion are written for a tissue model of masses, springs and dampers. The equations are solved to identify the model parameters and construct an image. The second method performs a transfer function analysis of the tissue motion. The tissue properties are identified from the shape the of transfer function. Simulations show good performance of the new method compared with static elastography. Initial experimental results from homogeneous and layered tissue phantoms also demonstrate the ability quantitatively to image tissue stiffness. Preliminary results are obtained for viscosity and density estimation. Further work is needed to extend the formulation to 3-D and improve robustness of the viscosity and density estimates. (E-mail: tims@ece.ubc.ca)

Method of calculating percentage depth dose for diagnostic X-rays

Data on the percentage depth-dose of diagnostic X-rays are important in evaluating patient dose from medical exposure. We developed a new method of calculating central-axis depth-dose in a homogeneous tissue phantom irradiated by diagnostic X-rays. First, primary and scattered components of percentage depth-dose for mono-energetic X-rays were calculated by means of Monte Carlo simulation, and the data were stored in data tables. Then, percentage depth-doses for individual X-rays were calculated by making use of the data tables, the photon spectrum of X-rays, and exposure conditions such as field size. This method, which can calculate depth-dose under any condition of irradiation by combining with the approximating equation of the X-ray spectrum, is useful in evaluating the percentage depth-dose of diagnostic X-rays.

Optical tomography using the time-independent equation of radiative transfer — Part 1: forward model

The presented study consists of two parts. The overall goal is to introduce and experimentally test a novel optical tomographic imaging algorithm that is based on the equation of radiative transfer. Using the equation of radiative transfer rather than the diffusion equation permits the consideration of highly scattering media that contain void-like regions that have very low absorption and scattering coefficients. In part I we concentrate on the detailed description and evaluation of a numerical forward model that accurately describes photon propagation in such media. In part II we focus on the inclusion of this forward model into a model-based iterative image reconstruction (MOBIIR) scheme. Using the MOBIIR scheme one can determine the spatial distribution of optical properties inside highly scattering media from measurements acquired on the surface of the medium. The mathematical and numerical background for the reconstruction algorithm, especially the adjoint differentiation scheme for the gradient calculation, will be presented. The code is tested with experimental data from tissue-phantoms that contain water-filled, void-like regions. The forward model to be described in part I is based on an upwind-difference discrete-ordinate formulation of the time-independent equation of radiative transfer. The upwind-difference representation has the advantage that it provides a convenient mathematical framework for calculating the derivative of the fluence with respect to the optical parameters using an adjoint differentiation technique, to be described in part II. The performance of the forward model is tested with experimental data obtained from homogeneous tissue-phantoms and from phantoms that contain void-like regions. We find good agreement between experimental measurements and theoretical predictions of the measurements.

Experimental verification of a theory for the time-resolved fluorescence spectroscopy of thick tissues

Fluorescence spectroscopy provides potential contrast enhancement for near-infrared tissue imaging and physiologically correlated spectroscopy. We present a fluorescence photon migration model and test its quantitative predictive capabilities with a frequency-domain measurement that involves a homogeneous multiple-scattering tissue phantom (with optical properties similar to those of tissue in the near infrared) that contains a fluorophore (rhodamine B). After demonstrating the validity of the model, we explore its ability to recover the fluorophore's spectral properties from within the multiple-scattering medium. The absolute quantum yield and the lifetime of the fluorophore are measured to within a few percent of the values measured independently in the absence of scattering. Both measurements are accomplished without the use of reference fluorophores. In addition, the model accurately predicts the fluorescence emission spectrum in the scattering medium. Implications of these absolute measurements of lifetime, quantum yield, concentration, and emission spectrum from within multiple-scattering media are discussed.

Multilayer model of photon diffusion in skin

A diffusion model describing the propagation of photon flux in the epidermal, dermal, and subcutaneous tissue layers of the skin is presented. Assuming that the skin is illuminated by a collimated, finite-aperture source, we develop expressions relating photon flux density within the skin and intensities re-emitted from the skin surface to the optical properties of the individual layers. Model simulations show that the rate at which re-emitted intensities diminish with radial distance away from the source can provide information about absorption and scattering in underlying tissues. Re-emitted intensities measured from homogeneous and two-layer tissue phantoms compare favorably with model predictions. We demonstrate potential applications of the model by estimating the absorption (sigma a) and transport-corrected scattering (sigma's) coefficients of dermis and subcutis from intensities measured from intact skin and by predicting the magnitude of the optical-density variations measured by a photoplethysmograph.

The differential scatter-air ratio and differential backscatter factor method combined with the density scaling theorem.

Using the O'Connor density scaling theorem, two basic equations have been derived to express scatter dose calculations in both homogeneous and heterogeneous phantoms: (i) the differential scatter-air ratio for calculating the frontscatter, and (ii) the differential backscatter factor for calculating the backscatter. In the derivation of both equations the relative electron density along the line between the scattering element and the calculation point has been averaged to account for scatter attenuation. Each equation expresses the amount of front or backscatter at the point of calculation per unit primary dose, per unit relative electron density, per unit volume at the scattering element. Primary dose calculations in both a homogeneous and a heterogeneous phantom have been carried out using a simple exponential attenuation law. Except in the area of or near interfaces and the area around field borders where there is electron disequilibrium, satisfactory dose calculations have been obtained using the primary and scatter dose calculation procedure for experiments done with Co-60 gamma rays in homogeneous soft tissue phantoms and in heterogeneous cork and aluminum phantoms.

Thermal distribution of magnetic-loop induction hyperthermia in phantoms and animals: Effect of the living state and velocity of heating

Hyperthermia has shown promise for the treatment of cancer. The magnetrode radio-frequency magnetic-loop induction applicator was recently introduced and is being evaluated at many centers with encouraging initial results. This investigation was undertaken to delineate the thermal distribution patterns of magnetrode hyperthermia in both normal tissue phantoms and healthy dogs. Temperatures on homogeneous and heterogeneous tissue equivalents and animal tissue phantoms, a dead animal extremity, a dead dog and a live dog subjected to rapid high-dose heating showed preferential and potentially injurious peripheral heating with a central “cold spot”. However, sequential moderate-dose heating in live dogs showed virtually uniform (<1°C) and potentially tumoricidal temperatures ≥42°C in both peripheral and central internal organs without injury to surface tissues. These data indicate that magnetic-loop induction can produce potentially safe and effective deep central heat in living, normal animals, but that safety and effectiveness are significantly influenced by the velocity of heat deposition. Furthermore, normal tissue phantom models and dead animals are unreliable predictors of magnetrode heat patterns in living systems.