Ultrasound Hyperthermia Treatments Research Articles

Development and testing of a system for controlled ultrasound hyperthermia treatment with a phantom device.

Hyperthermia is the process of raising tissue temperatures in the range 40 - 45 °C for a prolonged time (up to hours). Unlike in ablation therapy, raising the temperature to such levels does not cause necrosis of the tissue but has been postulated to sensitize the tissue for radiotherapy. The ability to maintain a certain temperature in a target region is key to a hyperthermia delivery system. The aim of this work was to design and characterize a heat delivery system for ultrasound hyperthermia able to generate a uniform power deposition pattern in the target region with a closed-loop control which would maintain the defined temperature over a defined period. The hyperthermia delivery system presented herein is a flexible design with the ability to strictly control the induced temperature rise with a feedback loop. The system can be reproduced elsewhere with relative ease and is adaptable for various tumor sizes/locations and for other temperature elevation applications, such as ablation therapy. The system was fully characterized and tested on a newly-designed custom-built phantom with controlled acoustic and thermal properties and containing embedded thermocouples. Additionally, a layer of thermochromic material was fixed above the thermocouples and the recorded temperature increase was compared to the RGB (red, green, and blue) color-change in the material. The transducer characterization allowed for input voltage to output power curves to be generated, thus allowing for comparison of power deposition to temperature increase in the phantom. Additionally, the transducer characterization generated a field map of the symmetric field. The system was capable of increasing the temperature of the target area by 6 °C above body temperature and maintain the temperature to within ±0.5 °C over a defined period. The increase in temperature correlated with the RGB image analysis of the thermochromic material. The results of this work have the potential to contribute towards increasing confidence in the delivery of hyperthermia treatment to superficial tumors. The developed system could potentially be used for phantom or small animal proof-of-principle studies. The developed phantom test device may be used for testing other hyperthermia systems.

Offset-Free Model Predictive Temperature Control for Ultrasound-Based Hyperthermia Cancer Treatments

Heating cancer cells over an extended period of time, referred to as hyperthermia, has been proven to enhance the effects of chemotherapy and radiotherapy without inducing additional toxicity or undesirable side effects, and is therefore considered a highly valuable adjuvant therapy in cancer treatment. In this work, a model predictive control (MPC) setup is developed for improving performance and robustness in regulating the temperature for magnetic-resonance-guided high-intensity focused ultrasound (MR-HIFU) hyperthermia treatments. The proposed control design incorporates a disturbance estimator as encountered in offset-free MPC that is able to remove the steady-state temperature error caused by plant-model mismatch. For the considered healthcare application, such modeling errors are inevitable in practice due to the high variability of tissue properties in patients, some of which even exhibit time- and temperature-dependent behavior due to the body’s thermoregulatory response, combined with the fact that extensive model identification is undesirable in the clinic. The controller’s performance is demonstrated by means of <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">in vivo</i> experiments on a porcine thigh muscle using a clinical MR-HIFU treatment setup.

Research and implementation of single point focusing system based on planar array ultrasonic transducer

Ultrasound hyperthermia treatment is an advanced method of tumor treatment developed in recent years. Different from traditional tumor cutting and chemotherapy, it has the advantages of non-invasive, reducing patients’ pain and recovery time. The focus of ultrasound is one of the key technologies of this treatment technology. In this paper, a phased array ultrasound focus system is designed. Combined with the 256 element planar array ultrasonic transducer, the single point focus and three-point focus of ultrasound are realized by using time inversion method. The intensity distribution map is drawn by MATLAB, and the actual intensity distribution is detected by using the horizontal plane, which verifies the correctness of the method it can enhance the energy density of the emitted ultrasound and shorten the treatment time.

Ultrasound hyperthermia induces apoptosis in head and neck squamous cell carcinoma: an in vitro study

BackgroundHyperthermia is considered an efficient complement in the treatment of head and neck squamous cell carcinoma (HNSCC). Hyperthermia induces cell apoptosis in a temperature- and time-dependent manner. However, the molecular mechanism of hyperthermia remains unclear. The aim of this study was to investigate the mechanism of apoptosis induced by ultrasound hyperthermia in HNSCC cell lines HN-30 and HN-13.Material and MethodsWe examined the dynamic changes of early apoptosis and secondary necrosis in HN-30 and HN-13 cells treated by hyperthermia at 42°C for 10 min. We further examined mitochondrial membrane potential in vitro by ultrasound hyperthermia for different heating temperatures (38-44°C, 10 min) and heating times (42°C, 10-50 min). After heating by ultrasound at 42°C for 10 min, the apoptosis index achieved its highest level at 8 h after treatment, decreased rapidly and remained constant at a reduced level at 12 h.ResultsThe level of secondary necrosis increased with the level of early apoptosis but remained at a higher level until 14 h. The level of secondary necrosis correlated with the level of early apoptosis (HN-13: r=0.7523, P=0.0030; HN-30: r=0.6510, P=0.016). The fractions of cells with low mitochondrial membrane potential (Δψ) in the heating-temperature grads group and heating-time grads group decreased significantly over time. Therefore, HN-30 and HN-13 cells developed apoptosis after ultrasound hyperthermia treatment with decreases in the mitochondrial transmembrane potential level.ConclusionsUltrasound hyperthermia induces apoptosis in HN-30 and HN-13 cells, possibly via the mitochondrial caspase pathway. Key words:Head and neck squamous cell carcinoma, apoptosis, mitochondrial membrane potential, ultrasound hyperthermia.

Temperature superposition for fast computation of 3D temperature distributions during optimization and planning of interstitial ultrasound hyperthermia treatments

Purpose: A temperature superposition method has been developed for fast optimisation and planning of interstitial hyperthermia treatments with convectively cooled multi-transducer ultrasound applicators integrated within high dose rate (HDR) brachytherapy catheters.Methods: Steady-state temperature distributions produced by individual tubular transducers capable of directional heating were pre-computed using finite element models (FEM) methods. The composite temperature distributions generated by multi-applicator implants were approximated as superposition sums of the pre-computed temperature profiles. Composite temperature distributions produced by the multi-applicator implants were also computed using accurate but computationally expensive FEM methods (considered here as the validation standard). Both methods were used for temperature calculation on a range of test implant geometries and representative patient cases (HDR implants in prostate (n = 13) and cervix (n = 2)), with optimised treatment plans created for the latter.Results: Difference between temperatures calculated by the superposition and FEM methods was below 0.37°C (95% confidence interval) in test implants at clinically relevant acoustic intensities (0.3–2.0 W/cm2) and blood perfusion (2 kg/m3/s). Difference in 41°C isothermal volumes was below 8.3%. Superposition-based optimisations followed by FEM forward calculations (hybrid plans) were completed 4–7 times faster than FEM-only plans (FEM optimisation + FEM forward). Mean T90, T50 and T10 values from both plans were within 0.3°C, 0.4°C and 0.45°C respectively, and the mean acoustic intensities were within 0.23 W/cm2.Conclusions: Temperature superposition provides a fast technique for forward or optimised planning of interstitial ultrasound hyperthermia treatments with calculations comparable to more accurate but time consuming FEM methods.

Dual-mode IVUS catheter for intracranial image-guided hyperthermia: feasibility study.

In this study, we investigated the feasibility of modifying 3-Fr IVUS catheters in several designs to potentially achieve minimally-invasive, endovascular access for image-guided ultrasound hyperthermia treatment of tumors in the brain. Using a plane wave approximation, target frequencies of 8.7 and 3.5 MHz were considered optimal for heating at depths (tumor sizes) of 1 and 2.5 cm, respectively. First, a 3.5-Fr IVUS catheter with a 0.7-mm diameter transducer (30 MHz nominal frequency) was driven at 8.6 MHz. Second, for a low-frequency design, a 220-μm-thick, 0.35 x 0.35-mm PZT-4 transducer--driven at width-mode resonance of 3.85 MHz--replaced a 40-MHz element in a 3.5-Fr coronary imaging catheter. Third, a 5 x 0.5-mm PZT-4 transducer was evaluated as the largest aperture geometry possible for a flexible 3-Fr IVUS catheter. Beam plots and on-axis heating profiles were simulated for each aperture, and test transducers were fabricated. The electrical impedance, impulse response, frequency response, maximum intensity, and mechanical index were measured to assess performance. For the 5 x 0.5-mm transducer, this testing also included mechanically scanning and reconstructing an image of a 2.5-cm-diameter cyst phantom as a preliminary measure of imaging potential.

Hyperthermia combined with radiation for the treatment of locally advanced prostate cancer

AbstractBACKGROUND:The authors present long‐term results from a phase 2 study that assessed the efficacy of transrectal ultrasound hyperthermia plus radiation with or without androgen suppression for the treatment of locally advanced prostate cancer.METHODS:Patients with clinical T2b‐T3bN0M0 disease (according to 1992 American Joint Committee on Cancer [AJCC] criteria) received radiation plus 2 transrectal ultrasound hyperthermia treatments. After the first 4 patients, 6 months of androgen suppression were allowed. The study was designed to assess absolute improvement in the 2‐year disease‐free survival rate compared with the short‐term androgen suppression arm in Radiation Therapy Oncology Group (RTOG) study 92‐02.RESULTS:Thirty‐seven patients received a total of 72 hyperthermia treatments. The mean cumulative equivalent minutes (CEM) T9043°C was 8.4 minutes. According to the 1992 AJCC classification, there were 19 patients with T2b tumors, 8 patients with T2c tumors, 5 patients with T3a tumors, and 5 patients with T3b tumors. The median Gleason score was 7 (range, 6‐9), and the median prostate‐specific antigen (PSA) level was 13.3 ng/mL (range, 2‐65 ng/mL). Thirty‐three patients received androgen suppression. At a median follow‐up of 70 months (range, 18‐110 months), the 7‐year overall survival rate was 94%, and 61% of patients remained failure free (according to the American Society for Therapeutic Radiology and Oncology definition for failure free survival). The absolute rate of disease‐free survival at 2 years, which was the primary study endpoint, improved significantly (84%) compared with a rate of 64% for similar patients on the 4‐month androgen suppression arm of RTOG 92‐02. When Phoenix criteria (PSA nadir + 2 ng/mL) were used to define biochemical failure, 89% of patients were failure free at 2 years.CONCLUSIONS:Hyperthermia combined with radiation for the treatment of locally advanced prostate cancer appeared to be promising. The current results indicated that further study of hyperthermia for the treatment of prostate cancer with optimal radiation and systemic therapy is warranted. Cancer 2010. © 2010 American Cancer Society.

Hyperthermia combined with radiation in treatment of locally advanced prostate cancer is associated with a favourable toxicity profile

Purpose: Hyperthermia is used to treat several pelvic tumours. An important step in establishing a broader role for hyperthermia in treatment of prostate cancer is verification of an acceptable toxicity profile. In this report, short- and long-term toxicity profiles of a completed phase II trial of transrectal ultrasound hyperthermia combined with radiation in treatment of locally advanced prostate cancer are presented.Methods and materials: Thirty-seven patients enrolled on a phase II study of external beam radiation ± androgen suppression with two transrectal ultrasound hyperthermia treatments were assessed for short- and long-term toxicity. Prostatic and anterior rectal wall temperatures were monitored. Rectal wall temperatures were limited to 40°C (19 patients), 41°C (three patients) and 42°C (15 patients). Univariate logistic regression was used to estimate the log hazard of developing NCI CTC Grade 2 toxicity based on temperature parameters. Hazard ratios, 95% confidence intervals, p-values for statistical significance of each parameter and proportion of variability explained for each of the parameters were calculated.Results: Median follow-up was 42 months. Both short- and long-term GI toxicity were limited to grade 2 or less. Acute grade 2 proctitis was greater for patients with allowable rectal wall temperature of >40°C. Eleven of 18 patients in this group had acute grade 2 proctitis vs three of 19 patients with rectal wall temperatures limited to 40°C (p = 0.004). Long-term grade 2 GI and GU toxicity occurred in 5% and 19% of patients. No late grade 3 or greater toxicity occurred. Late GI and GU toxicity were not associated with the allowable rectal wall temperature.Conclusion: Transrectal ultrasound hyperthermia combined with radiation for treatment of advanced clinically localized prostate cancer is safe and well tolerated.

INTENSITY DISTRIBUTION AND TEMPERATURE RISE FOR TRANSCRANIAL BRAIN ULTRASOUND HYPERTHERMIA

The main purpose of this paper is to investigate the intensity distribution and the temperature responses when ultrasound beams are used for the intact brain tumor hyperthermia. A search method is developed to search for the appropriate transducer diameter and the input power level that induce a well focused high intensity region and an effective thermal dose region to enhance the goodness of the brain ultrasound hyperthermia treatments. This work employs a simplified model of a scanned ultrasound transducer power deposition (a cone with convergent/divergent shape). Multiple reflections and transmissions occurring at the interfaces are taken into consideration of calculating the intensity distribution as an ultrasound plane wave propagates through skull bone into brain tissue. The distributions of ultrasound intensity, specific absorption rate ratio, and temperature are used to determine the appropriateness of the transducer diameter and the input power level for a yielded set of tumor conditions. Simulation results demonstrate that (1) the maximum intensity ratio in the tumor region is larger than that in the bone region for all frequencies; (2) the treatments for larger tumors become more difficult to solve if the ratio of the diameter of transducer to the tumor diameter is less than 10 when one tries to use only high intensity ultrasound exposure to cause small lesions due to the cavitation effects. Results demonstrate the feasibility of inducing both a well focused high intensity region to produce local cavitation lesions and an effective thermal dose region to enhance the goodness of the intact brain ultrasound hyperthermia treatments. The results of this study can be a guideline for designing an optimal ultrasound heating system, arranging the transducers, and implementing further treatment planning for the transcranial brain tumor hyperthermia.

Association of rectal toxicity with thermal dose parameters in treatment of locally advanced prostate cancer with radiation and hyperthermia

Purpose: Although hyperthermia has been used for more than two decades in the treatment of pelvic tumors, little is known about the potential impact of heat on rectal toxicity when combined with other treatment modalities. Because rectal toxicity is a concern with radiation and may be exacerbated by hyperthermia, definition of the association of thermal dose parameters with rectal toxicity is important. In this report, we correlate rectal toxicity with thermal dose parameters for patients treated with hyperthermia and radiation for prostate cancer.Methods and Materials: Thirty patients with T2b-T3b disease (1992 American Joint Committee On Cancer criteria) enrolled in a Phase II study of external beam radiation ± androgen-suppressive therapy with two transrectal ultrasound hyperthermia treatments were assessed for rectal toxicity. Prostatic and anterior rectal wall temperatures were monitored for all treatments. Rectal wall temperatures were limited to 40°C in 19 patients, 41°C in 3 patients, and 42°C in 8 patients. Logistic regression was used to estimate the log hazard of developing National Cancer Institute Common Toxicity Criteria Grade 2 toxicity based on temperature parameters. The following were calculated: hazard ratios, 95% confidence intervals, p values for statistical significance of each parameter, and proportion of variability explained for each parameter.Results: Gastrointestinal toxicity was limited to Grade 2. The rate of acute Grade 2 proctitis was greater for patients with an allowable rectal wall temperature of >40°C. In this group, 7 of 11 patients experienced acute Grade 2 proctitis, as opposed to 3 of 19 patients in the group with rectal wall temperatures limited to 40°C (p = 0.004). Preliminary assessment of long-term toxicity revealed no differences in toxicity. Hazard ratios for acute Grade 2 proctitis for allowable rectal wall temperature, average rectal wall Tmax, and average prostate Tmax were 9.33 (p = 0.01), 3.66 (p = 0.03), and 2.29 (p = 0.08), respectively. A model combining these three parameters explained 48.6% of the variability among groups.Conclusion: Rectal toxicity correlates with maximum allowable rectal wall temperature, average rectal wall Tmax, and average prostate Tmax for patients undergoing transrectal ultrasound hyperthermia combined with radiation for treatment of advanced clinically localized prostate cancer. Further definition of this association of thermal dose parameters with rectal toxicity in treatment of pelvic malignancies with hyperthermia should advance the goal of delivering thermal therapy in an effective yet safe manner.

Study of the cause of the temperature rise at the muscle-bone interface during ultrasound hyperthermia.

Undesirable temperature rise at the muscle-bone interface has been one of the major problems during ultrasound hyperthermia treatment. In this study, we examined by both computer calculation and phantom experiment the cause of this problem. Ultrasound penetrates a bone in two different waveforms, longitudinal and transversal. The transmission coefficient of these two waves vary greatly with the incident angle. From both theoretical and experimental results, the incident angle dependency of the interface heat was confirmed. When the incident angle is less than the critical angle of the longitudinal wave, the main cause of the temperature elevation is the absorption of the longitudinal wave in the bone. When the incident angle is larger than the critical angle of the longitudinal wave, the transversal wave becomes the major cause of the heat generation. At the incident angles larger than the critical angle of the transversal wave, no temperature rise is produced by the absorption of the ultrasound at the bone; the incident longitudinal wave, strengthened by the reflected wave, is absorbed in the muscle just in front of the bone. The heat generated in the muscle is transported to the interface so that the temperature of the interface and bone increases slightly.

Noninvasive real-time multipoint temperature control for ultrasound phased array treatments

A method for noninvasively estimating spatiotemporal temperature changes in samples using diagnostic ultrasound, and using these as inputs to a multipoint ultrasound phased array temperature controller, is presented in this paper. This method is based on a linear relationship between the apparent tissue echo pattern displacements and temperature, as seen along A-lines acquired with diagnostic ultrasound when the sample is heated by external heating fields. The proportionality constant between echo displacement and temperature is determined by the local change in speed of sound due to temperature and the linear coefficient of thermal expansion of the material. Accurate estimation of the displacements and proportionality constant yields accurate calibrated high-resolution (1 mm spatial, sub-/spl deg/C) noninvasive sample temperature estimates. These are used as inputs to a multipoint temperature controller, capable of controlling ultrasound phased array treatments in real-time. Phantom and in vitro results show that the noninvasively estimated temperature values can effectively be used to control ultrasound hyperthermia treatments, almost replacing invasive thermocouple measurements. The mathematical background and assumptions of the noninvasive temperature estimator and the controller are presented in this paper, together with experimental results showing estimator and controller performance and limitations. To the best of our knowledge, this paper presents the first demonstration of real-time treatment control based entirely on noninvasive temperature estimates.

Optimizing ultrasound focus distributions for hyperthermia.

A method is described for generating ultrasound focus patterns for ultrasound hyperthermia treatment planning for steady state and transient hyperthermia. The solution for placement and intensity of ultrasound focus points is based on two types of temperature constraints: 1) equality constraints on the tumor boundary (temperature is held at maximum safe level) and 2) inequality constraints in the tumor interior (in a therapeutic range of temperatures). The method employs a simplex algorithm to solve a series of linear equations which approximate the heating distribution in tissue. Examples are given for field conjugate acoustic lens applicators capable of generating multiple foci simultaneously.

Dynamic focusing in ultrasound hyperthermia treatments using implantable hydrophone arrays

A prototype 16-element needle hydrophone array has been designed, fabricated and characterized. The primary use of this array is to provide acoustic feedback during ultrasound hyperthermia treatments. This feedback can be used to compensate for patient motion and tissue inhomogeneities by controlling the phased array driving patterns. It can also be used in adaptive dynamic focusing, a procedure which enables the phased array to focus at points away from specified control points. The hydrophone array consists of a PVDF sheet, which covers a silicon substrate carrier that contains the signal electrodes of the individual acoustic sensors. A complete description of the hydrophone array and its characteristics is given in this paper. The aberration correction and motion compensation algorithms are also described, and some experimental results are shown. Finally, a Taylor series based adaptive dynamic focusing method for phased arrays based on a set of discrete hydrophone array measurements is described. This algorithm does not require any prior knowledge of the applicator geometry and all the parameters needed for correction can be measured directly at the hydrophone array sensor locations.

Simulation of bidirectional ultrasound hyperthermia treatments of neck tumours.

The temperature distributions produced in neck tumours by using either a single, scanned transducer (a unidirectional scan) or two separate transducers whose axis are perpendicular (a bidirectional scan) were simulated. The three-dimensional neck model included separate anatomical regions for the normal neck muscle tissue, the tumour, the spinal column and the trachea (no large blood vessels). The effects of variations in the transducer frequency and f number, the tumour size and location, and the normal and tumour blood perfusion rates were studies. The best simulated temperature distributions were produced by bidirectionally scanned, 2 MHz, f number 2.0 ultrasound transducers whose powers were modulated as a function of position. The simulated temperature distributions from such modulated bidirectional scans were significantly better than those of both unidirectional and unmodulated bidirectional scans. The 1-MHz transducers generally produced hot spots at the tissue-spine and/or tissue-trachea interface. The 3-MHz transducers eliminated those deep hot spots but created other hot spots close to the skin surface, and did not adequately heat the deeper regions of the tumour. These results from the simplified computer simulations may be used to guide the construction of improved ultrasound hyperthermia systems for the treatment of neck tumours.

An analytic solution for energy deposition in model spherical tumours undergoing ultrasound-hyperthermia treatments

The authors develop a method to predict the energy deposition in tumours undergoing ultrasound-hyperthermia treatments. Energy depositions are calculated using an exact, analytic solution to the problem of ultrasound scattering from a spherical tumour. The biological tissues are modelled as solid, lossy elastic media so that (i) transverse-wave modes, in addition to longitudinal-wave modes, are considered, and (ii) mode coupling is fully accounted for during the scattering. The model tumour is of arbitrary size and no restrictions are placed on its material parameters relative to the surrounding tissue. Simpler analytic results are given for tumours that differ in densities and rigidities only slightly from the surrounding tissue. The authors briefly discuss how the above analysis could be extended to more complex systems, i.e., irregularly shaped tumours.

The effect of wave reflection and refraction at soft tissue interfaces during ultrasound hyperthermia treatments.

An improved numerical model for calculating the ultrasonic power deposition in layered medium was developed and experimentally tested. The new model takes into account the ultrasound wave reflection and refraction at the tissue interfaces thereby providing improved accuracy in ultrasound hyperthermia treatment planning. The model was compared with a simplified model to evaluate when the tissue interfaces could be ignored in the hyperthermia treatment planning and evaluation. The effect of variations in water and tissue temperatures, the fat layer thicknesses, and the beam entrance angle were also investigated to establish guidelines for treatment execution. It was found that in most cases the effects of the soft tissue interfaces can be ignored. However, in some instances the acoustic focus may be shifted several millimeters off axis in layered medium. This is important when sharply focused transducers are used for ultrasound surgery or under the condition of pulsed, high-temperature hyperthermia treatments.

A comparison of theoretical and experimental ultrasound field distributions in canine muscle tissue in vivo

Relative ultrasound field distributions were measured using thermal techniques in canine thighs in vivo and in water. The experimental results were compared with distributions obtained from a numerical model based on the one-dimensional integration of the Rayleigh-Sommerfeld diffraction integral. The comparisons showed that the theoretical model is a good approximation to the distributions measured in water, with the agreement decreasing for regions in front of the acoustic focus. The main lobe profiles obtained in the muscle tissue also agreed well with both theoretical results and results measured in water (focussing was not lost). However, these in vivo distributions showed enlargement of the side lobes indicating scattering of the waves. It was also found that the interfaces between muscle groups produced considerable beam distortions as well as increased side lobe levels. Scattering of energy from the main lobe to the side lobes was verified by measurements of the peak intensity and the total acoustic power attenuation coefficients for passage of the beams through the thighs which showed that the former was about 40% higher than the latter. Also, absolute intensity values at the acoustic focus were measured in water using a hydrophone (0.5 mm active diameter) for 11 transducers ranging in frequency from 0.246 to 3.54 MHz. When these absolute values were compared with the model predictions, it was found that the model consistently overestimated the experimental data by a factor of less than 2. That is, the model can also be used to obtain upper bounds for absolute intensity values. Consequences of these results on ultrasound hyperthermia treatments are discussed.

The role of nonlinear ultrasound propagation during hyperthermia treatments

In this study the feasibility of utilizing nonlinear ultrasound propagation to control the power deposition patterns during ultrasound hyperthermia treatments of malignant tumors was investigated. From the in vivo results it appears that the power deposition pattern and the temperature distributions can be significantly altered by increasing the temporal peak power of the source while reducing the duty cycle to maintain the same acoustic time average power. The maximum temperature gains obtained during hyperthermia sonication (between 1.5 and 2) were significantly less than one could obtain by varying the frequency and the other parameters of transducers and thus, the linear characteristics of the ultrasound fields should be used when hyperthermia systems are designed and treatments are planned. Then during the treatment nonlinear propagation can be used to provide on-line control of the power deposition patterns. The transducer characteristics determined the magnitude of the temperature gain and the nonlinear propagation region could not be reached when sharply focused (F number = 1) transducers were used at the frequencies between 0.25 to 1.68 MHz due to the high pressure amplitudes that exceeded the transient cavitation threshold.

Temperature distributions during clinical scanned, focused ultrasound hyperthermia treatments

In this study a scanned focused ultrasound (SFUS) system was used to heat 66 tumours at various anatomical locations in 52 patients. A total of 160 treatments were given. On average, temperatures were measured in 14 or 15 locations in the scanned volume. The time-averaged temperatures over the 30 min treatment period in the best treatment of each tumour were 44.0 +/- 2.4 degrees C (mean +/- SD) and 39.6 +/- 1.5 degrees C at the location of the highest and lowest sensor, respectively. On average, 39% of the sensors were above 42.5 degrees C. When only the cases that were judged to be good candidates for the hyperthermia device were analysed, 64% of the sensors reached a temperature over 42.5 degrees C with the highest temperature achieved being 45.9 +/- 2.3 degrees C and the lowest 40.7 +/- 1.4 degrees C. Although the system tested has many technical limitations (for example, fixed frequency, beam geometry and power during the scan cycle), the results demonstrate that therapeutic temperatures can be achieved in many tumours. Significantly better temperatures are expected when all of the theoretical potential of scanned focused ultrasound systems has been used.