M. A. von Krüger

Software didático para modelagem do padrão de aquecimento dos tecidos irradiados por ultra-som fisioterapêutico

INTRODUCTION: Ultrasound is a resource commonly used in Physical Therapy. However, its inadequate application may produce insufficient heating or cause damage to biological tissues. Therefore, the knowledge on the optimum parameters for achieving the appropriate temperature, within safe limits, is necessary. Heat generation depends on equipment parameters and the physical properties of tissues. This study presented a software that simulates the energy and temperature variation in tissues over time, thus allowing users to view the heating patterns in tissues as a function of these parameters. METHODS: The software was implemented based on the bioheat transfer equation for four layers (skin, fat, muscle and bone), in which the user can change the thickness and thermal or acoustic properties of these tissues. The intensity, frequency and time of application can also be chosen. Graphs showing the percentage energy absorption in relation to depth and the respective temperature variation per millimeter of tissue are presented. RESULTS: Simulations were produced to give examples of situations of interest for therapy, by varying the time of application, thickness and ultrasound frequency. Differences in heating patterns are seen, especially at the interfaces. CONCLUSIONS: The software made it possible to study the heating of biological tissues by ultrasound and can be used both for teaching purposes and for planning heating doses for continuous waves. In the future, the software will be adapted, in order to estimate which dose should be regulated in the apparatus to maintain the desired temperature for the time chosen. Software available in: http://www.peb.ufrj.br/lus.htm.

Feasibility of non-invasive temperature estimation by the assessment of the average gray-level content of B-mode images.

This paper assesses the potential of the average gray-level (AVGL) from ultrasonographic (B-mode) images to estimate temperature changes in time and space in a non-invasive way. Experiments were conducted involving a homogeneous bovine muscle sample, and temperature variations were induced by an automatic temperature regulated water bath, and by therapeutic ultrasound. B-mode images and temperatures were recorded simultaneously. After data collection, regions of interest (ROIs) were defined, and the average gray-level variation computed. For the selected ROIs, the AVGL-Temperature relation were determined and studied. Based on uniformly distributed image partitions, two-dimensional temperature maps were developed for homogeneous regions. The color-coded temperature estimates were first obtained from an AVGL-Temperature relation extracted from a specific partition (where temperature was independently measured by a thermocouple), and then extended to the other partitions. This procedure aimed to analyze the AVGL sensitivity to changes not only in time but also in space. Linear and quadratic relations were obtained depending on the heating modality. We found that the AVGL-Temperature relation is reproducible over successive heating and cooling cycles. One important result was that the AVGL-Temperature relations extracted from one region might be used to estimate temperature in other regions (errors inferior to 0.5 °C) when therapeutic ultrasound was applied as a heating source. Based on this result, two-dimensional temperature maps were developed when the samples were heated in the water bath and also by therapeutic ultrasound. The maps were obtained based on a linear relation for the water bath heating, and based on a quadratic model for the therapeutic ultrasound heating. The maps for the water bath experiment reproduce an acceptable heating/cooling pattern, and for the therapeutic ultrasound heating experiment, the maps seem to reproduce temperature profiles consistent with the pressure field of the transducer, and in agreement with temperature maps developed by COMSOL®MultiPhysics simulations.

Ultrasound method applied to characterize healthy femoral diaphysis of Wistar rats in vivo.

A simple experimental protocol applying a quantitative ultrasound (QUS) pulse-echo technique was used to measure the acoustic parameters of healthy femoral diaphyses of Wistar rats in vivo. Five quantitative parameters [apparent integrated backscatter (AIB), frequency slope of apparent backscatter (FSAB), time slope of apparent backscatter (TSAB), integrated reflection coefficient (IRC), and frequency slope of integrated reflection (FSIR)] were calculated using the echoes from cortical and trabecular bone in the femurs of 14 Wistar rats. Signal acquisition was performed three times in each rat, with the ultrasound signal acquired along the femurs central region from three positions 1 mm apart from each other. The parameters estimated for the three positions were averaged to represent the femur diaphysis. The results showed that AIB, FSAB, TSAB, and IRC values were statistically similar, but the FSIR values from Experiments 1 and 3 were different. Furthermore, Pearsons correlation coefficient showed, in general, strong correlations among the parameters. The proposed protocol and calculated parameters demonstrated the potential to characterize the femur diaphysis of rats in vivo. The results are relevant because rats have a bone structure very similar to humans, and thus are an important step toward preclinical trials and subsequent application of QUS in humans.

Development of silicon-based materials for ultrasound biological phantoms

Ultrasound (US) phantoms are made of materials that mimic acoustic properties of human soft tissue. Ideally, such materials should mimic US velocity (1435 to 1631 m.s-1), attenuation (0.22 to 1.47 dB.cm-1MHz-n with ranging from 1.09 to 1.15), density (916 to 1100 kg.m-3) and scattering. Many of the tested materials have some of the desired properties like Agar, polyvinyl alcohol gel and polyacrylamide gel, however, they are susceptible to dehydration and biological attacks. This fact imposes a frequent material renewal, being other source for inaccuracies and uncertainties for measurements. Silicone rubbers are stable materials, but present higher attenuation and lower US velocity compared to tissues. This work investigates the mixing of silicone rubber with other substances to bring its properties close to biological tissues, and evaluates its stability along time.

In vivo characterization of long-bone in animal model by ultrasonic reflection parameters: IRC and FSIR

In this work, an ultrasound (US) pulse-echo method was proposed to characterize long bone diaphysis of in vivo animal model. Two quantitative parameters (Integrated Reflection Coefficient ?? IRC and Frequency slope integrated reflection - FSIR) related to US reflection, were applied to the echo from the muscle/bone interface in both femurs of five Wistar rats. FSIR is proposed here is the fraction of the apparent reflection corresponding to each frequency. The US signal is acquired from 3 previous chosen positions, one millimeter apart from each other, along the femur central region. The parameters were estimated for the three positions than averaged to represent the femur. The results showed that their values statistically belong to the same group. This is indication that the proposed method (from acquisition protocol to parameter estimation) has potential to characterize normal in vivo long bones in animal models.

On the reproducibility of the average gray-level for noninvasive temperature estimation in the hyperthermia range

The thermal treatment of deep-sited tissues requires temperature feedback by minimally invasive or even by completely noninvasive methods. Ultrasound has been pointed as a viable technology for noninvasive temperature assessment. More precisely, parameters from B-Mode images were pointed to vary monotonically with temperature and able to estimate temperature with appropriate resolution. In this paper, a bovine muscle sample is subjected to several heating and cooling periods in order to analyze how the average gray-level from B-Mode images is reproducible with temperature. This is an important aspect concerning the real-time application on a clinical setting. Results point that for formaldehyde fixed tissue the average gray-level was reproducible with temperature. In non-fixed (unstable) media reproducibility was achieved by considering relative (temperature and average gray-level) changes to the starting of a particular heating/cooling phase.

In vivo characterization of long-bone in animal model by two ultrasonic scattering parameters: AIB and FSAB

Quantitative Ultrasound (QUS) is a new possibility for non-ionizing diagnosis and monitoring of bone diseases which are usually monitored by radiographs. In this work, an ultrasound (US) pulse-echo method was proposed to in vivo signal acquisition for long bone characterization in an animal model. Two parameters, one related to US backscattering from bone and the other to US reflection at the muscle/bone interface, were calculated in both femurs of seven rats and the results showed that their values statistically belong to the same group. This is indication that the proposed protocol has potential to characterize normal in vivo long bones in animal models.

Influence of temperature on Mean Scattering Spacing estimation of in vitro bovine muscle

Periodicity of structures can be investigated with backscattered ultrasound, usually by applying spectral methods and estimating the Mean Scatterer Spacing (MSS). Biological tissues, such as skeletal muscle, or liver or trabecular bone, can present periodic or quasi-periodic structures. In this work it is studied the behavior of MSS with temperature and frequency for in vitro samples of bovine muscular tissues. MSS was obtained in the range of 37ºC to 43ºC, and between 1 MHz to 10.0 MHz Results show that, for the studied sample, MSS decreased with temperature for the muscle. It seems to be in agreement with the expected physiologic behavior of muscular fibers. Further complementary experiments are being envisaged.

Is the Average Gray-Level from Ultrasound B-Mode Images Able to Estimate Temperature Variations in Ex-Vivo Tissue?

This paper presents the first results of a method to estimate temperature change by monitoring the gray-scale average value of standard ultrasound (US) images. It was carried out an experiment with a bovine muscle sample with four thermocouples along its depth, spaced of 1 cm. The sample was immersed in a reservoir to which water at 50°C was added. Images were recorded with a commercial ultrasound equipment during sample heating and cooling (2.5 h experiment). Temperature was acquired every 10 s. Each image was then divided in four horizontal regions-of-interest (ROIs), corresponding to each thermocouple positioning. Average gray-scale values versus temperature change were obtained and have shown a clear linear pattern within the range 0.5 to 8°C for all ROIs. A linear model was proposed to fit the curves with a maximum prediction error of approximately 0.6°C, which is close to the accepted error for hyperthermia treatment (0.5°C). The next step is to use ultrasound also as the heating source.

A thermochromic test body and its relationship with the Effective Radiation Area (ERA) of Therapeutic Ultrasound devices

This paper presents a thermochromic test body to qualitative evaluate the Effective Radiation Area (ERA) of Therapeutic Ultrasound (TUS) devices. The thermal areas produced by three transducers are measured and compared to the ERA of the same transducers measured according to the IEC 61689 standard. A linear relationship was obtained between both kinds of areas suggesting that a quantitative relation can be found and used to easily monitor TUS working conditions.