Ultrasound Thermometry Research Articles

Temperature dependence of acoustic harmonics generated by nonlinear ultrasound beam propagation in ex vivo tissue and tissue-mimicking phantoms

Purpose: Hyperthermia is a cancer treatment technique that could be delivered as a stand-alone modality or in conjunction with chemotherapy or radiation therapy. Noninvasive and real-time temperature monitoring of the heated tissue improves the efficacy and safety of the treatment. A temperature-sensitive acoustic parameter is required for ultrasound-based thermometry. In this paper the amplitude and the energy of the acoustic harmonics of the ultrasound backscattered signal are proposed as suitable parameters for noninvasive ultrasound thermometry.Materials and methods: A commercial high frequency ultrasound imaging system was used to generate and detect acoustic harmonics in tissue-mimicking gel phantoms and ex vivo bovine muscle tissues. The pressure amplitude and the energy content of the backscattered fundamental frequency (p1 and E1), the second (p2 and E2) and the third (p3 and E3) harmonics were detected in pulse-echo mode. Temperature was increased from 26° to 46 °C uniformly through both samples. The amplitude and the energy content of the harmonics and their ratio were measured and analysed as a function of temperature.Results: The average p1, p2 and p3 increased by 69%, 100% and 283%, respectively as the temperature was elevated from 26° to 46 °C in tissue samples. In the same experiment the average E1, E2 and E3 increased by 163%, 281% and 2257%, respectively. A similar trend was observed in tissue-mimicking gel phantoms.Conclusions: The findings suggest that the harmonics generated due to nonlinear ultrasound beam propagation are highly sensitive to temperature and could potentially be used for noninvasive ultrasound tissue thermometry.

MR-guided focused ultrasound for painful bone metastases: safety when combined with chemotherapy

Magnetic Resonance guided Focused Ultrasound (MRgFUS) is a non-invasive, non-ionizing treatment producing thermal ablation using high intensity focused ultrasound and MR thermometry to denervate pain from bone metastases. A recent phase III study comparing MRgFUS to sham treatment for a painful bone metastasis allowed patients to be treated with concurrent chemotherapy as long as the regimen was stable for at least 4 weeks. We performed a retrospective analysis of the safety of combination MRgFUS with active systemic chemotherapy in patients treated on this study.

Ultrasound Thermometry for Optimizing heat Supply During a Hyperthermia Therapy of Cancer Tissue

Monitoring the temperature during a hyperthermia therapy allows optimizing the heat supply to destroy the cancer whereby the damage in the surrounding tissue is minimized. This contribution presents the fundamental research and current work to realize a locally resolved, noninvasive and intra-surgically applicable temperature measurement in tissue. This is realized by measuring the sound velocity locally resolved by an annular array, which allows noninvasive measurements although the observed tissue is not accessible from all directions. The method had been already qualified for fluids and analyses the echoes of moving scattering particles to obtain the time of flight to the focus of the transducer. As the parameters of the transducer are known the focus position (and thus the time of flight) can be calculated as a function of the sound velocity distribution of the propagation medium. Hence the measured time of flight allows determining the focus position and mean sound velocity simultaneously by means of this function. Varying the time lags of the signals for each element allows moving the focus and so measuring locally resolved. This contribution presents first ex-vivo measurements in tissue and thus proves the adaptability of this technique for tissue.

Creation and Characterization of an Ultrasound and CT Phantom for Noninvasive Ultrasound Thermometry Calibration

Ultrasound thermometry provides noninvasive 2-D temperature monitoring, and in this paper, we have investigated the use of computed tomography (CT) radiodensity to characterize tissues to improve the accuracy of ultrasound thermometry. Agarose-based tissue-mimicking phantoms were created with glyceryl trioleate (a fat-mimicking material) concentration of 0%, 10%, 20%, 30%, 40%, and 50%. The speed of sound (SOS) of the phantoms was measured over a temperature range of 22.1-41.1 °C. CT images of the phantoms were acquired by a clinical dedicated breast CT scanner, followed by calculation of the Hounsfield units (HU). The phantom was heated with a therapeutic acoustic pulse (1.54 MHz), while RF data were acquired with a 10-MHz linear-array transducer. Two-dimensional speckle tracking was used to calculate the thermal strain offline. The tissue-dependent thermal strain parameter required for ultrasound thermometry was analyzed and correlated with CT radiodensity, followed by the validation of the temperature prediction. Results showed that the change in SOS with the temperature increase was opposite in sign between the 0%-10% and 20%-50% trioleate phantoms. The inverse of the tissue-dependent thermal strain parameter of the phantoms was correlated with the CT radiodensity (R(2) = 0.99). A blinded ultrasound thermometry study on phantoms with a trioleate range of 5%-35% demonstrated the capability to estimate the tissue-dependent thermal strain parameter and estimate temperature with error less than ~1 °C. In conclusion, CT radiodensity may provide a method for improving ultrasound thermometry in heterogeneous tissues.

Use of Ultrasound Thermometry to Study Natural Convection in Opaque Nanofluids

Abstract Nanofluids are being evaluated as alternative heat transfer fluids, and thus their behavior during storage or low velocity applications in which natural convection can be significant has to be known and documented. Buoyancy induced flows in rectangular enclosures using nanofluids can be studied experimentally using thermocouples, thermistors, surface heat flux sensors, and ultrasound thermometry. The effects of the mass fraction concentration of nanoparticles, the enclosure aspect ratio, and the inclination have been studied experimentally, but more could be done. The opacity of nanofluids does not permit the use of particle image velocimetry, laser induced fluorescence, or any other means of flow visualization or visual temperature measurement of the local fluid temperature. However, the temperature field can be observed using a non-invasive method such as ultrasound thermometry. The experimental enclosure here was validated using water as the initial fluid; measured values of the local fluid temperature were compared with numerical simulations utilizing commercial software. Nanofluid mass fractions of 10% and 25% were used for comparative purposes to study the effects of concentration on the temperature field. Buoyancy force reversal effects were witnessed in both 10% and 25% concentrations. The nanofluid also prolonged the multicellular effects that occur in buoyancy inversion flows. A Rayleigh number inversion was observed with the 25% mass fraction nanofluid. The multicellular regime transitions to a boundary layer regime at about Ra = 1 × 107 when the aspect ratio is 2.625 and at about Ra = 2 × 108 when the aspect ratio is 1.000 for different concentrations of nanofluid. The observations could be physically explained. The current work confirms that temperature measurements of the flow field can be made to assess convective regimes and flow phenomena that induce significant temperature variations. The use of ultrasound thermometry is successfully demonstrated for opaque nanofluid.

Noninvasive thermometry assisted by a dual-function ultrasound transducer for mild hyperthermia

Mild hyperthermia is increasingly important for the activation of temperature-sensitive drug delivery vehicles. Noninvasive ultrasound thermometry based on a 2-D speckle tracking algorithm was examined in this study. Here, a commercial ultrasound scanner, a customized co-linear array transducer, and a controlling PC system were used to generate mild hyperthermia. Because the co-linear array transducer is capable of both therapy and imaging at widely separated frequencies, RF image frames were acquired during therapeutic insonation and then exported for off-line analysis. For in vivo studies in a mouse model, before temperature estimation, motion correction was applied between a reference RF frame and subsequent RF frames. Both in vitro and in vivo experiments were examined; in the in vitro and in vivo studies, the average temperature error had a standard deviation of 0.7°C and 0.8°C, respectively. The application of motion correction improved the accuracy of temperature estimation, where the error range was 1.9 to 4.5°C without correction compared with 1.1 to 1.0°C following correction. This study demonstrates the feasibility of combining therapy and monitoring using a commercial system. In the future, real-time temperature estimation will be incorporated into this system.

A reflex transmission method for ultrasound thermometry

It is of great interest to develop ultrasound‐based thermal imaging for monitoring thermal ablation procedures and as a tissue characterization technique. A phase contrast analysis for temperature mapping has been investigated, based on the temperature dependence of sound speed of water and tissue. The method employs reflex transmission imaging principles, planar projection and tomographic reconstruction techniques, for interrogating a region featuring a temperature contrast. Through‐transmission and pulse‐echo configurations were both used to image phase contrasts caused from a change in sound speed, in experimental measurements and numerical simulations. Tissue phantoms were used as a medium in which to create a steady‐state sound speed contrast, and a raster scan was performed to acquire the sound field with a hydrophone. Two scans were acquired, a reference sound field and a scan featuring a phase contrast which corresponded to a 7 ‐ 10°C peak temperature rise. The through‐transmission configuration employed a 0.2‐mm diameter needle hydrophone, while the pulse‐echo measurement employed a custom‐built thin‐wire hydrophone to acquire the sound field between the interrogation transducer and tissue phantom. The method successfully located the location and amplitude of the phase contrast, and future steps necessary for tracking dynamic temperature changes will be discussed.

MR thermometry characterization of a hyperthermia ultrasound array designed using the k-space computational method

BackgroundUltrasound induced hyperthermia is a useful adjuvant to radiation therapy in the treatment of prostate cancer. A uniform thermal dose (43°C for 30 minutes) is required within the targeted cancerous volume for effective therapy. This requires specific ultrasound phased array design and appropriate thermometry method. Inhomogeneous, acoustical, three-dimensional (3D) prostate models and economical computational methods provide necessary tools to predict the appropriate shape of hyperthermia phased arrays for better focusing. This research utilizes the k-space computational method and a 3D human prostate model to design an intracavitary ultrasound probe for hyperthermia treatment of prostate cancer. Evaluation of the probe includes ex vivo and in vivo controlled hyperthermia experiments using the noninvasive magnetic resonance imaging (MRI) thermometry.MethodsA 3D acoustical prostate model was created using photographic data from the Visible Human Project®. The k-space computational method was used on this coarse grid and inhomogeneous tissue model to simulate the steady state pressure wavefield of the designed phased array using the linear acoustic wave equation. To ensure the uniformity and spread of the pressure in the length of the array, and the focusing capability in the width of the array, the equally-sized elements of the 4 × 20 elements phased array were 1 × 14 mm. A probe was constructed according to the design in simulation using lead zerconate titanate (PZT-8) ceramic and a Delrin® plastic housing. Noninvasive MRI thermometry and a switching feedback controller were used to accomplish ex vivo and in vivo hyperthermia evaluations of the probe.ResultsBoth exposimetry and k-space simulation results demonstrated acceptable agreement within 9%. With a desired temperature plateau of 43.0°C, ex vivo and in vivo controlled hyperthermia experiments showed that the MRI temperature at the steady state was 42.9 ± 0.38°C and 43.1 ± 0.80°C, respectively, for 20 minutes of heating.ConclusionUnlike conventional computational methods, the k-space method provides a powerful tool to predict pressure wavefield in large scale, 3D, inhomogeneous and coarse grid tissue models. Noninvasive MRI thermometry supports the efficacy of this probe and the feedback controller in an in vivo hyperthermia treatment of canine prostate.

Design of double ultrasound pulse transmission and receiving circuit used in ultrasound thermometry

Hyperthermia treatments of tumours are attracting more and more attention from physical doctors and researchers. In particular, invasive microwave coagulation (IMC) therapy is a newly developed and effective method for the treatment of liver tumours. Generally successful applications of IMC are based on accurate temperature measurements of the tissues as the microwaves take effect. As a noninvasive technology, ultrasonic thermometry is based on the correlation between temperature and ultrasound speed or ultrasound decay coefficients of tissues. This paper presents the design of a double ultrasonic pulse transmission/receiving circuit used in ultrasonic thermometry. The circuit can synchronously produce the double ultrasonic pulse required by ultrasonic thermometry and can successfully receive weak ultrasonic echo signals. Some key wave-shapes used in experiments are presented. The experimental results indicate that the circuit is reliable and has good functionality.

C309 超音波とCTを用いた流体内の温度場計測に関する研究

Ultrasound thermometry is based on the variation of the speed of sound with temperature in medium. The speed of sound is measured using the pulse-transit-time technique. A transducer on one side of the test section is used to generate a pulse, which travels across the test section and is monitored by a receiver on the opposite side at a known separation distance. The time taken for the transit of the pulse is related to the average temperature along the signal path. To visualize the temperature distribution within the test section, the principle of Computer Tomography is applied.In the case of a gas, a non-uniform temperature field was measured by this method. It was compared with measurement data from a thermocouple array. The thermocouple data. indicated that the temperature of the central part of the heat source was lower than that of the region surrounding it. However, the ultrasound results yielded a uniform temperature for the heat source.

Ultrasound thermometry in transparent and opaque fluids

We have exploited the temperature dependence of sound velocity to measure the thermal fields in transparent and opaque fluids. A chamber containing glycerol undergoing Rayleigh–Benard convection was probed with an ultrasound transducer operating in the pulse-echo mode. The times-of-flight for the ultrasound pulse to traverse the fluid at several transducer locations were converted into a temperature profile that is in qualitative agreement with simultaneous thermochromic liquid crystal visualization of the flow pattern. Temperature profiles in a mercury-filled stainless steel chamber have also been obtained, both for quiescent and turbulent flows, thereby validating the ultrasound thermometry concept for opaque fluids as well.