Institute of Fundamental Technological Research

A tissue thermal conductivity (Ks) is an important parameter which knowledge is essential whenever thermal fields induced in selected organs are predicted. The main objective of this study was to develop an alternative ultrasonic method for determining Ks of tissues in vitro suitable for living tissues. First, the method involves measuring of temperature-time T(t) rises induced in a tested tissue sample by a pulsed focused ultrasound with measured acoustic properties using thermocouples located on the acoustic beam axis. Measurements were performed for 20-cycle tone bursts with a 2 MHz frequency, 0.2 duty-cycle and 3 different initial pressures corresponding to average acoustic powers equal to 0.7 W, 1.4 W and 2.1 W generated from a circular focused transducer with a diameter of 15 mm and f-number of 1.7 in a two-layer system of media: water/beef liver. Measurement results allowed to determine position of maximum heating located inside the beef liver. It was found that this position is at the same axial distance from the source as the maximum peak-peak pressure calculated for each nonlinear beam produced in the two-layer system of media. Then, the method involves modeling of T(t) at the point of maximum heating and fitting it to the experimental data by adjusting Ks. The averaged value of Ks determined by the proposed method was found to be 0.5±0.02 W/(m·°C) being in good agreement with values determined by other methods. The proposed method is suitable for determining Ks of some animal tissues in vivo (for example a rat liver).

Acoustics, Sound pressure, Beef, Thermal conductivity, Thermocouples, Nonlinear systems, Sound waves, Bioacoustics

Therapeutic and surgical applications of focused ultrasound require monitoring of local temperature rises induced inside tissues. From an economic and practical point of view ultrasonic imaging techniques seem to be the most suitable for the temperature control. This paper presents an implementation of the ultrasonic echoes displacement estimation technique for monitoring of local temperature rise in tissue during its heating by focused ultrasound The results of the estimation were compared to the temperature measured with thermocouple. The obtained results enable to evaluate the temperature fields induced in tissues by pulsed focused ultrasonic beams using non-invasive imaging ultrasound technique

HIFU, therapeutic ultrasound, ultrasonic imaging, echo strain estimation

Many therapeutic applications of pulsed focused ultrasound are based on heating of detected lesions which may be localized in tissues at different depths under the skin. In order to concentrate the acoustic energy inside tissues at desired depths a new approach using a planar multi-element annular array transducer with an electronically adjusted time-delay of excitation of its elements, was proposed. The 7-elements annular array transducer with 2.4 MHz center operating frequency and 20 mm outer diameter was produced. All its elements (central disc and 6 rings) had the same radiating area. The main purpose of this study was to investigate thermal fields induced in bovine liver in vitro by pulsed focused ultrasonic beams with various acoustic properties and electronically steered focal plane generated from the annular array transducer used. The measurements were performed for the radiating beams with the 20 mm focal depth. In order to maximize nonlinear effects introducing the important local temperature rise, the measurements have been performed in two-layer media comprising of a water layer, whose thickness was specific for the transducer used and equal to 13 mm, and the second layer of a bovine liver with a thickness of 20 mm. The thickness of the water layer was determined numerically as the axial distance where the amplitude of the second harmonics started to increase rapidly. The measurements of the temperature rise versus time were performed using a thermocouple placed inside the liver at the focus of the beam. The temperature rise induced in the bovine liver in vitro by beams with the average acoustic power of 1W, 2W, and 3W and duty cycle of 1/5, 1/15 and 1/30, respectively, have been measured. For each beam used the exposure time needed for the local tissue heating to the temperature of 43◦C (used in therapies based on ultrasonic enhancement of drug delivery or in therapies involving stimulation of immune system by enhancement of the heat shock proteins expression) and to the temperature of 56◦C (used in HIFU therapies) was determined. Two sets of measurements were done for each beam considered. First, the thermocouple measurement of the temperature rise was done and next, the real-time monitoring of dynamics of growth of the necrosis area by using ultrasonic imaging technique, while the sample was exposed to the same acoustic beam. It was found that the necrosis area becomes visible in the ultrasonic image only for beams with the average acoustic power of 3 W, although after cutting the sample the thermally ablated area was visible with the naked eye even for the beams with lower acoustic power. The quantitative analysis of the obtained results allowed to determine the exposure time needed to get the necrosis area visible in the ultrasonic image.

annular array transducer, pulsed focused nonlinear ultrasound, electronically moved focus, tissue heating, biological effects, tissue necrosis

Terapeutyczne i chirurgiczne zastosowania ogniskowych ultradźwięków wymagają monitorowania lokalnych zmian temperatury w tkance. Najkorzystniejsze z punktu widzenia użytkowego i ekonomicznego byłoby zastosowanie do tych celów technik ultradźwiękowych.
Praca przedstawia próbę zastosowania metody estymacji przemieszczenia ech do monitorowania zmian temperatury podczas ultradźwiękowego nagrzewania tkanki in vitro. Dane uzyskane drogą pomiarów ultradźwiękowych zostały przetworzone w celu wyznaczenia mapy przemieszczeń ech i odniesione do pomiarów rozkładu temperatury przeprowadzonych za pomocą termopar. Uzyskane wyniki umożliwiają ocenę pola temperatury i pozytywnie rokują połączeniu ultradźwiękowych technik nagrzewania i szacowania lokalnej temperatury tkanki.

obrazowanie temperatury, prędkość akustyczna

Therapeutic and surgical applications of focused ultrasound require monitoring of local temperature rises induced inside tissues. From an economic and practical point of view ultrasonic imaging techniques seem to be the best for a temperature control. In this work an attempt to apply the method of the ultrasonic echoes displacement estimation for monitoring local temperature rises in tissues during their heating by focused ultrasound is presented. The estimated temperature rise was compared with this measured by a thermocouple. The obtained results enable to evaluate the temperature fields induced in tissues by pulsed focused ultrasonic beams using non-invasive imaging ultrasound technique.

HIFU, therapeutic ultrasound, ultrasonic imaging, echo strain estimation

Many therapeutic applications of focused ultrasound are based on heating of a detected lesion which may be located inside tissues at different depths under a skin. In order to raise the tumor temperature the focal spot of the ultrasonic beam is guided to it. The focusing of acoustic energy in tissues at the desired depth can be achieved using single-element circular concave piezoelectric transducers with the selected diameters and radii of curvature. This solution is inefficient because it requires as many transducers as there are depths at which tumors are localized. In order to solve this problem a new approach, allowing to concentrate the acoustic energy at different depths inside tissues using a planar multi-element phased annular array with electronically steered focus, was proposed in this work. The main purpose of this study was to investigate the thermal fields induced in bovine liver in vitro by the pulsed focused ultrasound beam with various acoustic properties generated from the planar multi-element phased annular array with electronically steered focal spot. The array with 20 mm outer diameter and 2.4 MHz centre frequency has 7 elements with the same radiating surface. The electronic steering of the time-delay of excitation of each array element allowed to obtain the focal spot of the beam at any distance from the transducer surface. This paper presents measurement results performed for the beam focused at 20 mm distance. In order to maximize nonlinear propagation effects being one of the reason of the local temperature rise induced in tissues by focused ultrasound the measurements have been performed in two-layer parallel media of propagation comprising of a water layer, whose thickness was specific for the transducer used and equal to 13 mm, and of a bovine liver in vitro layer with a thickness of 27 mm. It was determined numerically as the axial distance at which the second harmonics amplitude for the tone burst generated by the transducer used in water starts to increase rapidly. The measurements of temperature rise versus time were performed using a thermocouple placed inside the liver at the beam focus. The temperature rises induced in the bovine liver in vitro for beams with the average acoustic power of 1W, 2W and 3W and duty cycle of 1/5, 1/15 and 1/30 have been measured. For each beam the exposure time needed for local heating of the liver to a temperature of 43 °C (used in therapies based on ultrasonic enhancement of drug delivery or therapies involving stimulation of immune system by enhancement of the heat shock proteins expression) and to a temperature of 56 °C (used in HIFU therapies) was determined. In each sample of liver for each considered beam at first, the measurement of temperature rise (with thermocouple) was carried out, and then, exposing the sample (without thermocouple) to the same beam the real-time monitoring of dynamics of the thermo ablation area growth was performed using the ultrasonic imaging technique. The necrosis spot becomes visible in the ultrasonic image only when the beam has sufficient acoustic power. The quantitative analysis of the obtained results allowed to determine the beam acoustic power and exposure time that are sufficient to visualize necrosis spot in the ultrasonic image.

phased annular array, pulsed focused nonlinear ultrasound beams, electronically movable focus, tissue heating, biological effects, tissue necrosis