Temperature monitoring using ultrasound contrast agents: in vitro investigation on thermal stability

Method for estimating average grey-level's measurement uncertainty from ultrasound images for non-invasive estimation of temperature in different tissue types

The objective of this work is to assess, on metrological basis, the average grey-levels (AVGL) calculated from B-Mode images for estimating temperature variations non-invasively in different kinds of tissues. Thermal medicine includes several thermal therapies, being hyperthermia the most noted and well known. Recently, efforts have been made to understand the benefits of ultrasound hyperthermia at mild temperature levels, i.e., between 39 °C and 41 °C. Moreover, the best practices on ultrasound bio-effects research have been encouraged by recommending that temperature rise in the region of interest should be measured even when a thermal mechanism is not being tested. In this work, the average grey-levels (AVGL) calculated from B-Mode images were assessed for non-invasive temperature estimation in a porcine tissue sample containing two different tissue types, fat and muscle, with temperature varying from 35 °C to 41 °C. The sample was continuously imaged with an ultrasound scanner, and simultaneously the temperature was measured. The achieved results were assessed under the light of the measurement uncertainty in order to allow comparability among different ultrasound thermometry methods. The highest expanded uncertainty of estimating temperature variation using AVGL was determined as 0.68 °C.

Effect of Temperature on the Size Distribution, Shell Properties, and Stability of Definity®

Physical characterization of an ultrasound contrast agent (UCA) aids in its safe and effective use in diagnostic and therapeutic applications. The goal of this study was to investigate the impact of temperature on the size distribution, shell properties, and stability of Definity^®, a U.S. Food and Drug Administration-approved UCA used for left ventricular opacification. A Coulter counter was modified to enable particle size measurements at physiologic temperatures. The broadband acoustic attenuation spectrum and size distribution of Definity^® were measured at room temperature (25 °C) and physiologic temperature (37 °C) and were used to estimate the viscoelastic shell properties of the agent at both temperatures. Attenuation and size distribution was measured over time to assess the effect of temperature on the temporal stability of Definity^®. The attenuation coefficient of Definity^® at 37 °C was as much as 5 dB higher than the attenuation coefficient measured at 25 °C. However, the size distributions of Definity^® at 25 °C and 37 °C were similar. The estimated shell stiffness and viscosity decreased from 1.76 ± 0.18 N/m and 0.21 × 10^-6 ± 0.07 × 10^-6 kg/s at 25 °C to 1.01 ± 0.07 N/m and 0.04 × 10^-6 ± 0.04 × 10^-6 kg/s at 37 °C, respectively. Size-dependent differences in dissolution rates were observed within the UCA population at both 25 °C and 37 °C. Additionally, cooling the diluted UCA suspension from 37 °C to 25 °C accelerated the dissolution rate. These results indicate that although temperature affects the shell properties of Definity^® and can influence the stability of Definity^®, the size distribution of this agent is not affected by a temperature increase from 25 °C to 37 °C.

Non-invasive Estimation of Temperature during Physiotherapeutic Ultrasound Application Using the Average Gray-Level Content of B-Mode Images: A Metrological Approach

Healing therapies that make use of ultrasound are based on raising the temperature in biological tissue. However, it is not possible to heal impaired tissue by applying a high dose of ultrasound. The temperature of the tissue is ultimately the physical quantity that has to be assessed to minimize the risk of undesired injury. Invasive temperature measurement techniques are easy to use, despite the fact that they are detrimental to human well being. Another approach to assessing a rise in tissue temperature is to derive the material's general response to temperature variations from ultrasonic parameters. In this article, a method for evaluating temperature variations is described. The method is based on the analytical study of an ultrasonic image, in which gray-level variations are correlated to the temperature variations in a tissue-mimicking material. The physical assumption is that temperature variations induce wave propagation changes modifying the backscattered ultrasound signal, which are expressed in the ultrasonographic images. For a temperature variation of about 15°C, the expanded uncertainty for a coverage probability of 0.95 was found to be 2.5°C in the heating regime and 1.9°C in the cooling regime. It is possible to use the model proposed in this article in a straightforward manner to monitor temperature variation during a physiotherapeutic ultrasound application, provided the tissue-mimicking material approach is transferred to actual biological tissue. The novelty of such approach resides in the metrology-based investigation outlined here, as well as in its ease of reproducibility.

Determining temperature distribution in tissue in the focal plane of the high (>100 W/cm(2)) intensity focused ultrasound beam using phase shift of ultrasound echoes

In therapeutic applications of High Intensity Focused Ultrasound (HIFU) the guidance of the HIFU beam and especially its focal plane is of crucial importance. This guidance is needed to appropriately target the focal plane and hence the whole focal volume inside the tumor tissue prior to thermo-ablative treatment and beginning of tissue necrosis. This is currently done using Magnetic Resonance Imaging that is relatively expensive. In this study an ultrasound method, which calculates the variations of speed of sound in the locally heated tissue volume by analyzing the phase shifts of echo-signals received by an ultrasound scanner from this very volume is presented. To improve spatial resolution of B-mode imaging and minimize the uncertainty of temperature estimation the acoustic signals were transmitted and received by 8 MHz linear phased array employing Synthetic Transmit Aperture (STA) technique. Initially, the validity of the algorithm developed was verified experimentally in a tissue-mimicking phantom heated from 20.6 to 48.6 °C. Subsequently, the method was tested using a pork loin sample heated locally by a 2 MHz pulsed HIFU beam with focal intensity ISATA of 129 W/cm(2). The temperature calibration of 2D maps of changes in the sound velocity induced by heating was performed by comparison of the algorithm-determined changes in the sound velocity with the temperatures measured by thermocouples located in the heated tissue volume. The method developed enabled ultrasound temperature imaging of the heated tissue volume from the very inception of heating with the contrast-to-noise ratio of 3.5-12 dB in the temperature range 21-56 °C. Concurrently performed, conventional B-mode imaging revealed CNR close to zero dB until the temperature reached 50 °C causing necrosis. The data presented suggest that the proposed method could offer an alternative to MRI-guided temperature imaging for prediction of the location and extent of the thermal lesion prior to applying the final HIFU treatment.

Temperature-dependent differences in the nonlinear acoustic behavior of ultrasound contrast agents revealed by high-speed imaging and bulk acoustics

Previous work by the authors has established that increasing the temperature of the suspending liquid from 20°C to body temperature has a significant impact on the bulk acoustic properties and stability of an ultrasound contrast agent suspension (SonoVue, Bracco Suisse SA, Manno, Lugano, Switzerland). In this paper the influence of temperature on the nonlinear behavior of microbubbles is investigated, because this is one of the most important parameters in the context of diagnostic imaging. High-speed imaging showed that raising the temperature significantly influences the dynamic behavior of individual microbubbles. At body temperature, microbubbles exhibit greater radial excursion and oscillate less spherically, with a greater incidence of jetting and gas expulsion, and therefore collapse, than they do at room temperature. Bulk acoustics revealed an associated increase in the harmonic content of the scattered signals. These findings emphasize the importance of conducting laboratory studies at body temperature if the results are to be interpreted for in vivo applications.

Heat enhances gas delivery and acoustic attenuation in CO(2) filled microbubbles

Thermo-responsive chitosan microbubbles were developed as new therapeutic device for vehiculating gases to tissues concomitantly to hyperthermic treatments. Aiming at applications to non-invasive temperature monitoring, microbubbles were characterized for acoustic attenuation properties in the 1-15 MHz range both by direct methods and by B-mode Ultrasound imaging up to 43 degrees C, which is the temperature used in clinical hyperthermia. The chitosan microbubbles showed a mean diameter of 1 microm at room temperature, which slightly decreases after heating, enhancing gas delivery. Acoustic attenuation monotonically increases with temperature, being the extent of such variation larger than that observed in tissues. Both the physico-chemical and the acoustic profiles showed reversible variations of microbubbles approaching 43 degrees C, which might be of interest for applications in hyperthermic therapies.

Ultrasound monitoring of temperature change in liver tissue during laser thermotherapy: 10 degrees C intervals

In thermal tissue ablation, it is very important to control the increase in the temperature for having an efficient ablation therapy. We conducted this study to determine the efficacy of measuring pixel shift of ultrasound B-mode images as a function of change in tissue temperature. By fixing some micro thermocouples in liver tissues, temperature at different points was monitored invasively in vitro during laser-induced thermotherapy. According to our results optimum power and exposure time were determined for ultrasound temperature monitoring. Simultaneously, noninvasive temperature monitoring was performed with ultrasound B-mode images. These images were saved on computer from 25 degrees C to 95 degrees C with 10 degrees C steps. The speed of sound changes with each 10 degrees C temperature change that produce virtual shifts in the scatter positions. Using an image processing method, the pixel shift due to 10 degrees C temperature change was extracted by motion detection. The cubic regression function between the mean pixel shifts on ultrasound B-mode images caused by the change in speed of sound which in turn was a function of the mean change in temperature was evaluated. When temperature increased, pixel shift occurs in ultrasound images. The maximum pixel shift was observed between 60 to 70 degrees C. After 70 degrees C, the local pixel shift due to change in the speed of sound in liver tissue had an irregular decreasing. Pearson correlation coefficient between invasive and non-invasive measurements for 10 degrees C temperature changes was 0.93 and the non-linear function was suitable for monitoring of temperature. Monitoring of changes in temperature based on pixel shifts observed in ultrasound B-mode images in interstitial laser thermotherapy of liver seems a good modality.

Assessment of pixel shift in ultrasound images due to local temperature changes during the laser interstitial thermotherapy of liver: in vitro study

Laser interstitial thermotherapy (LITT) is an internal ablation therapy consisting of percutaneous or intraoperative insertion of laser fibers directly into the liver tumor with maximum diameter of 5 cm. It is very important to control the temperature increase, because tissue carbonization occurs with high temperatures, which can damage normal tissues. In this research, pixel shift changes on ultrasound B-mode images with temperature changes were measured. LITT in vitro was performed on 12 freshly excised sheep liver tissues using a Nd:YAG laser with a bare-tip optical fiber. The 1 W power setting was used for 700 s exposure time (2477 J/mm2). Invasive temperature monitoring was performed during the heating and cooling by attaching microthermocouples to the tissue. At the same time, ultrasound B-mode images were saved on the computer for each 5 degrees C temperature increase from 25 degrees C to 100 degrees C, for noninvasive temperature monitoring. These pixel shifts were measured by an echo-tracking algorithm. Linear and nonlinear regression analyses between the independent variable (temperature change) and the dependent variable (pixel shift on images) were performed. Regression functions and correlation coefficients were determined. It was shown that with a correlation coefficient of 0.998, the cubic function was suitable. Pixel shift increased for each 5 degrees C temperature increase and the maximum shift was observed during 60 to 70 degrees C. Beyond these temperatures, the pixel shift decreased. In this method, because of evaporation of tissue water and bubble formation and tissue carbonization, monitoring greater than 100 degrees C was difficult. It is possible to monitor temperature changes on the ultrasound B-mode images in interstitial laser thermotherapy of liver. Also, with the improvement of image processing, this method could be used for noninvasive temperature monitoring for a large number of patients during LITT.