Guillermo A. Cortela

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.

Application of the Schlieren pulsed method for the observation of simple and multiple scattering of ultrasonic waves

The Schlieren pulsed method uses short-term lighting triggered by an acoustic pulse. This allows for an observation of elastic deformation fields in pulsed regime and for an evaluation of the evolution of the pulse in the interior of homogenous and heterogeneous media. In this paper we apply the Schlieren pulsed method to determine the conditions of the change from simple to multiple scattering. An ultrasound transducer put in water emits a wide-band pulse. The pulse goes through a region of parallel cylindrical wires, uniformly spaced and perpendicular to the acoustic beam. Varying the width of the zone of the scatterer cylinders, it is possible to optically observe the transmitted acoustic field and to calculate the transmission coefficient. By studying the behavior of this coefficient in function of the medium width, we obtained the zone in which the transition between simple and multiple scattering happens.

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.

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.

Influence of ultrasonic scattering in the calculation of thermal dose in ex-vivo bovine muscular tissues.

This study explores the effect of ultrasound scattering on the temperature increase in phantoms and in samples of ex-vivo biological tissue through the calculation of the thermal dose (TD). Phantoms with different weight percentages of graphite powder (0-1%w/w, different scattering mean free paths, ℓS) and ex-vivo bovine muscle tissue were isonified by therapeutic ultrasound (1 MHz). The TD values were calculated from the first 4 min of experimental temperature curves obtained at several depths and were compared with those acquired from the numerical solution of the bio-heat transfer equation (simulated with 1 MHz and 0.5-2.0 W cm(-2)). The temperature curves suggested that scattering had an important role because the temperature increments were found to be higher for higher percentages of graphite powder (lower ℓS). For example, at a 30-mm depth and a 4-min therapeutic ultrasound application (0.5 W cm(-2)), the TDs (in equivalent minutes at 43 °C) were 7.2, 17.8, and 58.3 for the phantom with ℓS of 4.35, 3.85, and 3.03 mm, respectively. In tissue, the inclusion of only absorption or full attenuation in the bio-heat transfer equation (BHTE) heat source term of the simulation leads to under- or overestimation of the TD, respectively, as compared to the TD calculated from experimental data. The experiments with phantoms (with different scatterer concentrations) and ex-vivo samples show that the high values of TD were caused by the increase of energy absorption due to the lengthening of the propagation path caused by the changing in the propagation regime.

Ultrasonic attenuation and speed in phantoms made of polyvinyl chloride-plastisol and graphite powder

Biological phantoms are very useful for controlled experiments on biomedical ultrasound. Nevertheless they are normally made of organic materials with short time-duration. We have studied the ultrasonic properties of test-blocks made of polyvinyl chloride-plastisol (PVCP) that are very stable in time. In this work, we analyzed ultrasonic (US) attenuation and speed at 1 MHz, as a function of temperature (15–45°C) of five phantoms made with PVCP and different concentrations of graphite powder (0, 0.5, 1, 2, and 5%) using the classical transmission method. US speed diminishes almost linearly (from 1408 to 1333 m.s−1) as temperature increases. In general attenuation lied between 0.73 and 0.09 dB.cm−1, but presenting a more complex behavior. For graphite concentrations of 0.5 and 1%, attenuation was lower than for 0% and for the other two phantoms (2 and 5% concentrations) attenuation was higher. This behavior can be perhaps due to the fact that the fabrication temperature for 0.5 and 1% was 140°C and for the ...

Ex vivo determined experimental correction factor for the ultrasonic source term in the bioheat equation

HIGHLIGHTSThe influence of the scattering on the ultrasonic absorption and its effect on the thermal dose is evidenced.An effective absorption coefficient that considers the ultrasonic scattering is proposed.An empirical correction factor for the source term of the bioheat equation is proposed.The robustness of the model is presented by the concordance between measured and simulated temperature curves. ABSTRACT The objective of this work is to propose an effective absorption coefficient (&agr;effec) as an empirical correction factor in the source term of the bioheat equation. The temperature rise in biological tissue due to ultrasound insonification is produced by energy absorption. Usually, the ultrasonic absorption coefficient (&agr;A) is used as a source term in the bioheat equation to quantify the temperature rise, and the effect of scattering is disregarded. The coefficient &agr;effec includes the scattering contribution as an additional absorption term and should allow us to make a better estimation of the thermal dose (TD), which is important for clinical applications. We simulated the bioheat equation with the source term considering &agr;A or &agr;effec, and with heating provided by therapeutic ultrasound (1 MHz, 2.0 W cm−2) for about 5.5 min (temperature range 36–46 °C). Experimental data were obtained in similar heating conditions for a bovine muscle tissue (ex vivo) and temperature curves were measured for depths 7, 30, 35, 40 and 45 mm. The TD values from the experimental temperature curves at each depth were compared with the numerical solution of the bioheat equation with the classical and corrected source terms. The highest percentual difference between simulated and experimental TD was 42.5% when assuming the classical &agr;A, and 8.7% for the corrected &agr;effec. The results show that the effective absorption coefficient is a feasible parameter to improve the classical bioheat transfer model, especially for depths larger than the mean free propagation path.

Comparison of diabetic foot diagnosis between thermography infrared methods and the elastography techniques

In this paper the results of pathology detection in the diabetic foot using an ultrasonic elastography technique and thermography technique using an infrared camera are compared. Detection infrared camera is based on the temperature difference of the soles of the feet, especially in areas that are more prone to ulcerations, which are the big toe and heel. For ultrasonic elastography speckle interferometry system ultrasonic signals obtained at passage one low frequency wave is used. To determine the temperature of the soles of the feet a thermal camera “Fluke Ti32” is used; a system for activation and capture of thermal imaging was performed. Comparative study of the results of both techniques and their possible applications in early detection of diabetic foot pathology is performed.

Elastograms of the diabetic foot by ultrasonic impulse elastography

Elastography allows an elastic map from a soft biological tissue using shear waves. The technique consists of exciting the tissue or organ to be analyzed with an external mechanical low frequency pulse and detecting the displacements of each small region of tissue when receiving the shear wave. This is done by obtaining the records of the echographic signals of the region analyzed with a transducer ultrasound. The speckle interferometry of the Ascan signals is then performed. Thus the displacement produce by the shear wave of the each small region of tissue is determined. In this way a map of the shear wave velocity is obtained across the scanned area. The Youngs modulus of the analyzed region is computed from this velocity value. In this work, this new technique is applied to study the diabetic foot, making an elastic map of the sole, allowing detect diseased areas.

Effect of the ultrasound wave propagation regime in the heat source term of Penne’s bio-heat transfer equation

This work analyses the influence of the ultrasound wave propagation regime on the temperature increase on biological media. Simulated and experimental temperature values from bovine ex-vivo tissue in two different depths, 5 and 40 mm, respectively smaller and bigger than the mean free propagation path (lS = 8.52 mm), are obtained. Temperature curves measured during US therapeutic application (High Intensity Therapeutic Ultrasound, HITU, 1 MHz, 1.5 – 2.0 Wcm−2) for 10 min, are compared to simulated ones using the Penne’s bio-heat transfer equation (BHTE) at the same frequency and intensities and using the standard source term (ultrasonic absorption coefficient). At 5 mm, estimated and measured temperature diverge no more than 1%, while at 40 mm, when the scattering starts being not negligible, the simulated temperature is 20% smaller than the measured one. The results indicate that absorption increases when the wave propagation regime changes for depths greater than the lS value. This work analyses the influence of the ultrasound wave propagation regime on the temperature increase on biological media. Simulated and experimental temperature values from bovine ex-vivo tissue in two different depths, 5 and 40 mm, respectively smaller and bigger than the mean free propagation path (lS = 8.52 mm), are obtained. Temperature curves measured during US therapeutic application (High Intensity Therapeutic Ultrasound, HITU, 1 MHz, 1.5 – 2.0 Wcm−2) for 10 min, are compared to simulated ones using the Penne’s bio-heat transfer equation (BHTE) at the same frequency and intensities and using the standard source term (ultrasonic absorption coefficient). At 5 mm, estimated and measured temperature diverge no more than 1%, while at 40 mm, when the scattering starts being not negligible, the simulated temperature is 20% smaller than the measured one. The r...