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ABSTRACT: Referenceless proton resonance frequency (PRF) shift thermometry provides a means to measure temperature changes during minimally invasive thermotherapy that is inherently robust to motion and tissue displacement. However, if the referenceless method is used to determine temperature changes during prostate ablation, phase gaps between water and fat in image regions used to determine the background phase can confound the phase estimation. We demonstrate an extension to referenceless thermometry which eliminates this problem by allowing background phase estimation in the presence of phase discontinuities between aqueous and fatty tissue. In this method, images are acquired with a multi-echo sequence and binary water and fat maps are generated from a Dixon reconstruction. For the background phase estimation, water and fat regions are treated separately and the phase offset between the two tissue types is determined. The method is demonstrated feasibile in phantoms and during in vivo thermal ablation of canine prostate.
IEEE Transactions on Medical Imaging 07/2007; · 3.64 Impact Factor
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IEEE Trans. Med. Imaging. 01/2007; 26:813-821.
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ABSTRACT: High-temperature thermal therapy is emerging as a feasible treatment option for prostate cancer and benign prostatic hyperplasia. Previous investigations have demonstrated distinct advantages of catheter-based ultrasound technology over other heating modalities for thermal ablation therapies, with significant potential for better spatial control and faster heating times. The purpose of this study was to develop ultrasound devices and techniques specifically for treating prostate cancer in conjunction with magnetic resonance thermal imaging (MRTI) to monitor and control treatment progression. Directional transurethral applicators have been designed with arrays of sectored tubular (90 degrees active acoustic sector) or with narrow planar transducer segments and integrated with a flexible delivery catheter with a cooling balloon. This applicator can be rotated within the prostatic urethra to target specific regions during treatment. MRI compatible catheter-cooled interstitial ultrasound applicators with 180 degrees active acoustic sectors were developed specifically to treat the prostate. These applicators may be implanted through the perineum into the posterior portion of the prostate, with their heating energy directed away from the rectum. Both heating strategies were evaluated via biothermal simulations and in vivo experiments within canine prostate (n = 3). During the in vivo studies, MRTI was used to monitor treatment temperatures, cytotoxic thermal doses (t43 > 240 min) and corresponding maximum temperature thresholds (Tmax > 52 degrees C) within three imaging planes simultaneously. Urethral and endorectal cooling was employed with both treatment strategies to provide further protection of the urethral mucosa and rectum from thermal damage. Results using the transurethral applicators demonstrated that narrow zones of coagulation (approximately 30 degrees sector for planar, approximately 90 degrees for tubular), extending up to 20 mm from the urethra to the periphery of the prostate gland, could be produced within 10-15 min. Further, rotation of the applicator during treatment could be used to destroy larger regions in the prostate. Experiments using multiple interstitial directional applicators (approximately 180 degrees active sectors), implanted within the posterior margin of the prostate with the energy directed away from the rectum, produced contiguous zones of thermal coagulation which extended from the posterior prostate toward the anterior-lateral periphery of the gland. Both transurethral and interstitial treatment strategies demonstrated significant potential for thermal ablation of localized prostate cancer, particularly when MRTI is used to guide and assess treatment.
International Journal of Hyperthermia 12/2004; 20(7):739-56. · 1.92 Impact Factor
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ABSTRACT: High-temperature thermal therapy for the treatment of prostate cancer is currently being applied as a minimally-invasive alternative over traditional forms of treatment. Catheter-based interstitial and transurethral ultrasound applicators are being developed for controlled and selective thermal ablation of prostaric tissues with concurrent MR thermal imaging. As part of this treatment strategy we have devised a transurethral cooling catheter and a cooling jacket to be placed over the endorectal MR imaging coil to protect the urethral mucosa and rectal wall from thermal damage during treatment. The cooling efficiencies and protective abilities of these devices were evaluated in vivo within three canine prostate glands. Invasive and MR derived temperature measurements within the prostate and rectal wall indicate that the protective influence of the endorectal cooling extends 5-10 mm from the rectal wall into the dorsal prostate. The urethral cooling extends ∼5 mm from the cooling balloon. The protective capabilities were further verified with subsequent histological analysis with TTC stained tissue sections and contrast enhanced T1-weighted MR images post treatment. Both of these cooling devices are compatible with the MR thermometry and can be used to protect the urethral mucosa and rectal wall during prostate thermal ablation with interstitial and transurethral ultrasound devices.
Engineering in Medicine and Biology Society, 2004. IEMBS '04. 26th Annual International Conference of the IEEE; 10/2004
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ABSTRACT: Thermal ablation has been investigated as a treatment for a variety of cancers. Heat treatments have not gained large-scale clinical acceptance due to inconsistencies in controlling heat deposition in vivo and the lack of precise temperature measurement. Interstitial ultrasound provide a good method of controlling the radial depth of a thermal lesion and the applicator designs evaluated in this study allow for dynamic angular control of the shape of the lesion. A trisectored internally water-cooled applicator (TriAD) and a rotating catheter water-cooled applicator (RIUS) angularly controlled thermal dose to a target area. Both devices were small in diameter (1.8 mm-2.4 mm), making them clinically feasible for minimally invasive treatment in device size-sensitive tissues. A biothermal model accounting for changes in acoustic attenuation and perfusion as a function of thermal dose was used to evaluate and predict applicator performance. The MR susceptibility artifact of the applicators was examined with MR temperature imaging (MRTI) sequences at 1.5 T and 0.5 T. Ex vivo experiments in turkey and beef muscle with realtime MRTI correlated well with results from the biothermal model. These results display the feasibility of thermally treating tumors with controllable interstitial ultrasound applicators under real-time MRTI and bracket the applicators' predicted performance in vivo.
Engineering in Medicine and Biology Society, 2004. IEMBS '04. 26th Annual International Conference of the IEEE; 10/2004
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ABSTRACT: A catheter-based transurethral ultrasound applicator with angularly directional heating patterns has been designed for prostate thermal therapy and evaluated in canine prostate in vivo using MRI to monitor and assess performance. The ultrasound transducer array (3.5 mm diameter tubular transducers, 180 degrees active sectors, approximately 7.5 MHz) was integrated to a flexible delivery catheter (4 mm OD), and encapsulated within an expandable balloon (35 mm x 10 mm OD, 80 ml min(-1) ambient water) for coupling and cooling of the prostatic urethra. These devices were used to thermally coagulate targeted portions of the canine prostate (n = 2) while using MR thermal imaging (MRTI) to monitor the therapy. MRI was also used for target definition, positioning of the applicator, and evaluation of target viability post-therapy. MRTI was based upon the complex phase-difference mapping technique using an interleaved gradient echo-planar imaging sequence with lipid suppression. MRTI derived temperature distributions, thermal dose exposures, T1-contrast enhanced MR images, and histology of sectioned prostates were used to define destroyed tissue zones and characterize the three-dimensional heating patterns. The ultrasound applicators produced approximately 180 degrees directed zones of thermal coagulation within targeted tissue which extended 15-20 mm radially to the outer boundary of the prostate within 15 min. Transducer activation lengths of 17 mm and 24 mm produced contiguous zones of coagulation extending axially approximately 18 mm and approximately 25 mm from base to apex, respectively. Peak temperatures around 90 degrees C were measured, with approximately 50 degrees C-52 degrees C corresponding to outer boundary t43 = 240 min at approximately 15 min treatment time. These devices are MRI compatible, and when coupled with multiplanar MRTI provide a means for selectively controlling the length and sector angle of therapeutic thermal treatment in the prostate.
Medical Physics 03/2004; 31(2):405-13. · 2.83 Impact Factor
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ABSTRACT: High-temperature thermal therapy for the treatment of prostate cancer is currently being applied as a minimally-invasive alternative over traditional forms of treatment. Catheter-based interstitial and transurethral ultrasound applicators are being developed for controlled and selective thermal ablation of prostaric tissues with concurrent MR thermal imaging. As part of this treatment strategy we have devised a transurethral cooling catheter and a cooling jacket to be placed over the endorectal MR imaging coil to protect the urethral mucosa and rectal wall from thermal damage during treatment. The cooling efficiencies and protective abilities of these devices were evaluated in vivo within three canine prostate glands. Invasive and MR derived temperature measurements within the prostate and rectal wall indicate that the protective influence of the endorectal cooling extends 5-10 mm from the rectal wall into the dorsal prostate. The urethral cooling extends approximately 5 mm from the cooling balloon. The protective capabilities were further verified with subsequent histological analysis with TTC stained tissue sections and contrast enhanced T1-weighted MR images post treatment. Both of these cooling devices are compatible with the MR thermometry and can be used to protect the urethral mucosa and rectal wall during prostate thermal ablation with interstitial and transurethral ultrasound devices.
Conference proceedings: ... Annual International Conference of the IEEE Engineering in Medicine and Biology Society. IEEE Engineering in Medicine and Biology Society. Conference 02/2004; 4:2492-5.
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ABSTRACT: Human cadaver lumbar spines were used to assess the acute effects of intradiscal electrothermal therapy in vitro.
To determine whether intradiscal electrothermal therapy produces acute changes in disc histology and motion segment stability.
Intradiscal electrothermal therapy has been introduced as an alternative for the treatment of discogenic low back pain. Several hypothesized mechanisms for the effect of intradiscal electrothermal therapy have been suggested including shrinkage of the nucleus or sealing of the anulus fibrosus by contraction of collagen fibers, and thermal ablation of sensitive nerve fibers in the outer anulus.
Intradiscal electrothermal therapy was performed with the Spinecath by Oratec on 19 fresh, frozen human lumbar cadaver specimens. In a separate study, eight specimens were tested biomechanically and instrumented to map the thermal distribution, whereas five specimens were tested only biomechanically, both before and after intradiscal electrothermal therapy. Six additional specimens were heated with intradiscal electrothermal therapy, and the resulting canal was backfilled with a silicone rubber compound to allow colocalization of the catheter and anular architecture.
A consistent pattern of increased motion and decreased stiffness was observed. For the specimens in which only biomechanical measurements were taken, a 10% increase in the motion, on the average, at 5 Nm torque was observed after intradiscal electrothermal therapy. No apparent alteration of the anular architecture was observed around the catheter site in the intradiscal electrothermal therapy-treated discs.
The data from this study suggest that the temperatures developed during intradiscal electrothermal therapy are insufficient to alter collagen architecture or stiffen the treated motion segment acutely.
Spine 11/2001; 26(20):2198-207. · 2.08 Impact Factor
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ABSTRACT: Catheter-cooled (CC) interstitial ultrasound applicators were evaluated for their use in high-temperature coagulative thermal therapy of tissue. Studies in ex vivo beef muscle were conducted to determine the influences of applied electrical power levels (5-20 W per element), catheter flow rate (20-60 ml min(-1)), circulating water temperature (7-40 degrees C), and frequency (7-9 MHz) on temperature distribution and thermal lesion geometry. The feasibility of using multiple interstitial applicators to thermally coagulate a predetermined volume of tissue was also investigated. Results of these studies revealed that the directional shape of the thermal lesions is maintained with increasing time and power. Radial depths of the thermal lesions ranged from 10.7 +/- 0.7 mm after heating for 4 min with an applied power level of 5 W, to 16.2 +/- 1.4 mm with 20 W. The axial length of the thermal lesions is controlled tightly by the number of active transducers. A catheter flow rate of 20 to 40 ml min(-1) (52.2 +/- 5.5 kPa at 40 ml min(-1)) with 22 degrees C water was determined to provide sufficient cooling of the transducers for power levels used in this study. In vivo temperatures measured in the center of a 3-cm-diam peripheral implant of four applicators in pig thigh muscle reached 89.3 degrees C after 4 min of heating, with boundaries of coagulation clearly defined by applicator position and directivity. Conformability of heating in a clinically relevant model was demonstrated by inserting two directional CC applicators with a 2 cm separation within an in vivo canine prostate, and generating a thermal lesion measuring 3.8 cm x 2.2 cm in cross section while directing energy away from, and protecting the rectum. Maximum measured temperatures at midgland exceeded 90 degrees C within 20 min of heating. The results of this study demonstrate the utility of single or multiple CC applicators for conformal thermal coagulation and high temperature thermal therapy, with potential for clinical applications in sites such as prostate, liver, breast, or uterus.
Medical Physics 08/2001; 28(7):1525-34. · 2.83 Impact Factor
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ABSTRACT: This study presents a comparative evaluation of the control of heating and thermal coagulation with microwave (MW) and ultrasound (US) interstitial applicators. Helical coil MW antennas (17 mm and 25 mm length radiating antennae) were tested using an external implant catheter (2.2 mm o.d.) with water-cooling. US applicators with tubular transducers (2.2 and 2.5 mm o.d., 10 mm length, single-element and 3-element) were utilized with a direct-coupled configuration and internal water-cooling. Measurements of E-field distributions (for MW) and acoustic beam distributions (for US) were used to characterize the applicator energy output. Thermal performance was evaluated through multiple heating trials in vitro (bovine liver) and in vivo (porcine thigh muscle and liver) at varied levels of applied power (20-40 W for microwave, 15-35 W for ultrasound) and heating times (0.5-5 min). Axial temperature distributions in the tissue were recorded during heating, and dimensions of the resulting lesions of thermal coagulation were measured. Both MW and US applicators produced large volumes of tissue coagulation ranging from 8 to 20 cm3 with singular heating times of 5 min. Radial depth of lesions for both MW and US applicators increased with heating duration and power levels, though US produced notably larger lesion diameters (30-42 mm for US vs 18-26 mm for MW, 5 min heating). Characteristic differences between the applicators were observed in axial energy distribution, tissue temperatures, and thermal lesion shapes. MW lesions increased significantly in axial dimensions (beyond the active applicator length) as applied power level and/or heating duration was increased, and lesion shapes were generally not uniform. US provided greater control and uniformity of heating, with energy deposition and axial extent of thermal lesions corresponding to the length of the active transducer(s). The improved ability to control the extent of thermal coagulation demonstrated by the US applicators provides greater potential to target a specific region of tissue.
Medical Physics 02/2001; 28(1):104-17. · 2.83 Impact Factor
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ABSTRACT: Internal water-cooling of direct-coupled ultrasound (US) applicators for interstitial thermal therapy (hyperthermia and coagulative thermal therapy) was investigated. Implantable applicators were constructed using tubular US sources (360 angular acoustic emittance, approximately 7 MHz) of 10 mm length and 1.5, 1.8, 2.2, and 2.5 mm outer diameter (OD). Directional applicators were also constructed using 2.2 mm OD tubes sectored to provide active acoustic sectors of 90 degrees and 200 degrees. A water-cooling mechanism was integrated within the inner lumen of the applicator to remove heat from the inner transducer surface. High levels of convective heat transfer (2100-3800 W/m2K) were measured for practical water flow rates of 20-80 mL/min. Comparative acoustic measurements demonstrated that internal water-cooling did not significantly degrade the acoustic intensity or beam distribution of the US transducers. Water-cooling allowed substantially higher levels of applied electrical power (> 45 W) than previous designs (with air-cooling or no cooling), without detriment to the applicators. High-temperature heating trials performed with these applicators in vivo (porcine liver and thigh muscle) and in vitro (bovine liver) showed improved thermal penetration and coagulation. Radial depth of coagulation from the applicator surface ranged from 12 to 20 mm for 1-5 min of sonication with 28-W applied power. Higher powers (41 W) demonstrated increased coagulation depths (approximately 9 mm) at shorter times (15 s). Thermal lesion dimensions (angular and axial expanse) produced with directional applicators were controlled and directed, and corresponded to the active zone of the transducer. These characteristic lesion shapes were also generally unchanged with different sonication times and power, and were found to be consistent with previous coagulation studies using air-cooled applicators. The implementation of water-cooling is a significant advance for the application of ultrasound interstitial thermal therapy (USITT), providing greater treatment volumes, shorter treatment times, and the potential for treatment of highly perfused tissue with shaped lesions.
IEEE Transactions on Biomedical Engineering 11/2000; 47(10):1356-65. · 2.28 Impact Factor
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ABSTRACT: Six aperture array dual concentric conductor (DCO) microwave hyperthermia applicators were studied using theoretical models to characterize power deposition (SAR) and steady state temperature distributions in perfused tissue. SAR patterns were calculated using the finite difference time domain (FDTD) numerical method, and were used as input to a finite difference thermal modeling program based on the Pennes Bio-Heat Equation in order to calculate corresponding temperature distributions. Numerous array configurations were investigated including the use of different size DCC apertures (2, 3, and 4 cm), different spacing between apertures (1.0-2.0 cm), and different water bolus thicknesses (5-15 mm). Thermal simulations were repeated using blood perfusion values ranging from 0.5 to 5 kg/m3 s. Results demonstrate the ability of DCC array applicators to effectively and uniformly heat tissue down to a depth of 7.5-10 mm below the skin surface for a large number of different combinations of DCC element size, spacing, and water bolus thickness. Results also reveal the close correlation between SAR patterns and corresponding temperature distributions, verifying that design studies of the applicator can be performed confidently by analysis of SAR, from which the thermal behavior can be estimated. These simulations are useful in the design optimization of large microwave DCC array applicators for superficial tissue heating and for identifying appropriate aperture spacing and bolus thickness parameters for different size DCC aperture arrays and tissue blood perfusion conditions.
Medical Physics 05/2000; 27(4):745-53. · 2.83 Impact Factor
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ABSTRACT: A multi-element, direct-coupled ultrasound (US) applicator with internal water cooling was investigated for axial control of interstitial thermal coagulation. A prototype implantable applicator was constructed with a linear array of three tubular PZT ultrasound transducers (each 2.5 mm OD, 10 mm length, 360 degrees emittance). Acoustic beam distributions from each element were measured and found to be collimated within the transducer length. The internally cooled applicator could sustain high levels of applied power to each transducer (0 to 40 W) and maintain acceptable applicator surface temperatures (<100 degrees C). Thermal performance of the applicator was investigated through heating trials in vivo (porcine thigh muscle and liver) and in vitro (bovine liver). The radial depth of thermal lesions produced was dependent on the applied power and sonication time and was controlled independently with power levels to each transducer element. With 18 W per element (applied electrical power) for 3 min, cylindrical thermal lesions were produced with a diameter of ~3 cm and a length ranging from 1.2 cm (with one element) to 3.5 cm (three elements). Higher powers (24 to 30 W) for 3 to 5 min provided increased depths of coagulation (~4 cm diameter lesions). Analysis of axial lesion shapes demonstrated that individual variation of power to each transducer element provided control of axial heating and depth of coagulation (for custom lesion shapes); lesion lengths corresponded to the number of active transducers. This ability to control the heating distribution dynamically along the length of the applicator has potential for improved target localization of thermal coagulation and necrosis in high temperature thermal therapy.
IEEE Transactions on Ultrasonics Ferroelectrics and Frequency Control 02/2000; 47(1):170-8. · 1.69 Impact Factor
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ABSTRACT: Reviews recent advances in interstitial and intracavitary
ultrasound applicators designed for high-temperature thermal therapy and
hyperthermia. Design and performance characteristics determined by
acoustic measurements, in vitro and in vivo studies are presented. These
results demonstrate the ability of this ultrasound technology to provide
highly controllable zones of thermal coagulation or temperature
elevation
BMES/EMBS Conference, 1999. Proceedings of the First Joint; 11/1999
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ABSTRACT: This effort describes the clinical realization of a large area
microwave vest applicator for hyperthermia treatment of the entire
chestwall. The performance of this applicator is analyzed in terms of
theoretical and experimental SAR patterns in realistic heterogeneous
tissue loads, temperature distributions in clinical patient treatments,
and success at achieving the design goals of this microwave vest
applicator development project
BMES/EMBS Conference, 1999. Proceedings of the First Joint; 11/1999
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ABSTRACT: Hyperthermia (HT) is used in the clinical management of cancer and benign disease. Numerous biological and clinical investigations have demonstrated that HT in the 41-45 degrees C range can significantly enhance clinical responses to radiation therapy, and has potential for enhancing other therapies, such as chemotherapy, immunotherapy and gene therapy. Furthermore, high-temperature hyperthermia (greater than 50 degrees C) alone is being used for selective tissue destruction as an alternative to conventional invasive surgery. The degree of thermal enhancement of these therapies is strongly dependent on the ability to localize and maintain therapeutic temperature elevations. Due to the often heterogeneous and dynamic properties of tissues, most notably blood perfusion and the presence of thermally significant blood vessels, therapeutic temperature elevations are difficult to spatially and temporally control during these forms of HT therapy. However, ultrasound technology has significant advantages that allow for a higher degree of spatial and dynamic control of the heating compared to other commonly utilized heating modalities. These advantages include a favorable range of energy penetration characteristics in soft tissue and the ability to shape the energy deposition patterns. Thus, heating systems have been developed for interstitial, intracavitary, or external approaches that utilize properties such as multiple transducer arrays, phased arrays, focused beams, mechanical and/or electrical scanning, dynamic frequency control and transducers of various shapes and sizes. This article provides a general review of a selection of ultrasound hyperthermia systems that are either in clinical use or currently under development, that utilize these advantages as a means to better localize and control HT for the aforementioned therapies.
Ultrasound in Medicine & Biology 08/1999; 25(6):871-87. · 2.29 Impact Factor
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ABSTRACT: The performance characteristics and thermal coagulation of tissue produced by directional air-cooled, direct-coupled interstitial ultrasound (US) applicators were evaluated. Prototype applicators (2.2 mm o.d.) were constructed using cylindrical transducers sectored into angular active zones of 90 degrees, 200 degrees, 270 degrees, and 360 degrees. Acoustic characterization of the applicators showed the beam output to be angularly directed from the active sector of the transducer and collimated within the axial extent. Empirical determination of the average convective heat transfer coefficient, resulting from airflow cooling the inner surface of the transducer, showed significantly high levels of transfer (> 700 W m(-2) degrees C(-1)) with a flow rate of 5.6 L min(-1). Thermal performance of the applicators was characterized through high temperature heating in vivo (porcine thigh muscle, 11 trials) and in vitro (bovine liver, 46 trials). Results demonstrated directional coagulation of tissue, with good correlation between the angular extent of the lesions and the active acoustic sector. Radial depth of coagulation with a 200 degrees applicator extended 8-17 mm, with a heating time of 1-10 min, respectively. Angular and axial lesion shape remained similar over the course of 1-10 min heating trials. Implementation of air-cooling within direct-coupled interstitial US applicators provided enhanced directivity of heating in angular and axial dimensions, and significantly increased the power handling and radial depth of tissue coagulation.
Ultrasound in Medicine & Biology 06/1999; 25(4):609-22. · 2.29 Impact Factor
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ABSTRACT: Direct-coupled (DC) and catheter-cooled (CC) ultrasound applicator configurations were evaluated for high-temperature ultrasound interstitial thermal therapy (USITT) using computer simulations, acoustic beam measurements, and in vivo temperature measurements. The DC devices consist of 2.2-mm diameter tubular ultrasound transducers encapsulated within a thin biocompatible plastic coating, which can be inserted directly into the tissue. The CC devices incorporate 1.5-mm diameter tubular transducers, which are inserted within 2.2to 2.4-mm diameter plastic implant catheters and require an integrated water-cooling scheme. Simulated transient temperature profiles and cumulative thermal dose distributions indicate that each of these applicator configurations can produce target temperatures greater than 50 degrees C and corresponding thermal doses greater than 300 to 600 equivalent minutes at 43 degrees C (EM(43 degrees C)) within 5 min at a radial depth of 1 to 1.5 cm in moderately perfused tissues. Theoretical investigations of air-cooling implemented within DC applicators demonstrated a significant enhancement of thermal penetration compared with non-cooled DC applicators, thus approaching performance attainable with CC devices. Temperature distributions achieved with DC and CC applicators in vivo were in agreement with theoretical calculations and further demonstrate that the devices are practical, sufficient power output levels can be obtained, and the angular heating profiles can be shaped or directed to protect non-targeted critical normal tissues. This preliminary study demonstrates that these interstitial ultrasound applicators have potential to provide controlled thermal coagulation and necrosis of small target regions and deserve further investigation and development for possible implementation in the treatment of benign and cancerous lesions in sites such as prostate, liver, and brain.
IEEE Transactions on Ultrasonics Ferroelectrics and Frequency Control 02/1999; 46(5):1218-28. · 1.69 Impact Factor
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ABSTRACT: Multiple applicator implant strategies were evaluated for the ability to control temperature distributions within a volume of tissue. MRI-compatible multi-element ultrasound applicators were fabricated in directional and non-directional configurations using cylindrical piezo-ceramic transducers. These applicators can be inserted into 13 or 14 gage catheters integrated with water-cooling. Thermal lesion formation studies were performed in an in vitro setup using fresh beef muscle, as well as in in vivo experiments in pig thigh muscle, and canine prostate and liver. Temperature distributions were monitored using multiple-junction thermocouple probes. Results from these studies indicated the ability to conform thermal lesions to targeted tissue while protecting surrounding tissue from thermal damage. Thermal lesions confined to a 4 cm diameter treatment volume were produced in vitro within 4 minutes of heating with 10 W applied electrical power delivered to 6 equally spaced transducers. Three and 4 cm diameter conformal thermal lesions were produced in vivo using 3-6 directional and/or non-directional applicators in peripheral or clustered configurations. Peak temperatures measured in the center of the in vivo target volumes exceeded 85°C during heating, while temperatures outside the target volume remained below 45°C. It is anticipated that such treatment strategies may prove beneficial for the management of localized tumors in the prostate, uterus, liver or brain
Ultrasonics Symposium, 1999. Proceedings. 1999 IEEE; 02/1999
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ABSTRACT: The feasibility of using air-cooling to improve the thermal penetration of direct-coupled interstitial ultrasound (US) applicators was investigated using biothermal simulations, bench experiments, phantom testing, and in vivo thermal dosimetry. Two applicator configurations using tubular US transducers were constructed and tested. The first design, intended for simultaneous thermobrachy-therapy, utilizes a 2.5 mm OD transducer with a central lumen to accommodate a radiation source from remote afterloaders. The second applicator consists of a 2.2 mm OD transducer designed for coagulative thermal therapy. Both designs provide cooling of the inner transducer surface by the counterflow of chilled air or CO2 gas through the annulus of the enclosed applicator. The average convective heat transfer (ha) associated with each applicator was determined empirically from curve-fits of radial steady-state temperatures measured in a tissue-mimicking phantom. High levels of convective heat transfer (ha > 500 W m-2 degrees C-1) were demonstrated in both designs at relatively low flow rates (< 5 L min-1). Transient and steady-state radial heating profiles were also measured in vivo (pig thigh muscle) with and without cooling. The therapeutic radius for hyperthermia (41-45 degrees C) was extended from 5-6 mm (without cooling) to 11-19 mm with air-cooling (4.8 L min-1, airflow 10 degrees C), effectively doubling and tripling the thermal penetration in vivo. Similar improvements were demonstrated at higher temperatures with the thermal coagulation applicator. Biothermal simulations, which modeled the physical, thermal, and acoustic parameters of the air-cooled applicator and surrounding tissue, were also used to investigate potential improvements in heating patterns. The simulated radial heating profiles with transducer cooling demonstrated significantly enhanced thermal penetration over the experimental range of convective transfer, and also agreed with in vivo results. These theoretical and experimental results clearly show air-cooling controls the transducer surface temperature, significantly increases thermal penetration, and produces a greater treatment volume for direct-coupled US applicators in hyperthermia and thermal coagulation.
Medical Physics 01/1999; 25(12):2400-9. · 2.83 Impact Factor
