Multisectored interstitial ultrasound applicators for dynamic angular control of thermal therapy

Dynamic angular control of thermal ablation and hyperthermia therapy with current interstitial heating technology is limited in capability, and often relies upon nonadjustable angular power deposition patterns and/or mechanical manipulation of the heating device. The objective of this study was to investigate the potential of multisectored tubular interstitial ultrasound devices to provide control of the angular heating distribution without device manipulation. Multisectored tubular transducers with independent sector power control were incorporated into modified versions of internally cooled (1.9 mm OD) and catheter-cooled (2.4 mm OD) interstitial ultrasound applicators in this work. The heating capabilities of these multisectored devices were evaluated by measurements of acoustic output properties, measurements of thermal lesions produced in ex vivo tissue samples, biothermal simulations of thermal ablation and hyperthermia treatments, and MR temperature imaging of ex vivo and in vivo experiments. Acoustic beam measurements of each applicator type displayed a 35 degrees -40 degrees acoustic dead zone between each independent sector, with negligible mechanical or electrical coupling. Thermal lesions produced in ex vivo liver tissue with one, two, or three sectors activated ranged from 13-18 mm in radius with contiguous zones of coagulation between active sectors. The simulations demonstrated the degree of angular control possible by using variable power levels applied to each sector, variable duration of applied constant power to individual sectors, respectively, or a multipoint temperature controller to vary the power applied to each sector. Despite the acoustic dead zone between sectors, the simulations also showed that the variance from the maximum lesion radius with three elements activated is within 4%-13% for tissue perfusions from 1-10 kg m(-3) s(-1). Simulations of hyperthermia with maximum tissue temperatures of 45 degrees C and 48 degrees C displayed radial penetration up to 2 cm of the 40 degrees C steady-state contour. Thermal characterizations of trisectored applicators in ex vivo and in vivo muscle, using real-time MR thermal imaging, reinforced angular controllability and negligible radial variance of the heating pattern from the applicators, demonstrated effective heating penetration, and displayed MR compatibility. The multisectored interstitial ultrasound applicators developed in this study demonstrated a significant degree of dynamic angular control of a heating pattern without device manipulation, while maintaining heat penetration consistent with previously reported results from other interstitial ultrasound applicators.     

Theoretical model of internally cooled interstitial ultrasound applicators for thermal therapy

Interstitial ultrasound applicators for high-temperature thermal therapy are currently being developed for treating cancerous and benign disease. Internally cooled, direct-coupled (ICDC) applicators, composed of a segmented array of cylindrical ultrasound transducers, have demonstrated capabilities of producing controllable and conformal heating distributions along the applicator length and angular orientation. In this study, 2D transient acoustic and biothermal models of ICDC applicators were developed using a mixed implicit and explicit finite difference solution with variable node spacing in cylindrical coordinates for enhanced speed, stability and accuracy. The model incorporates dynamic behaviour of acoustic parameters and blood perfusion as a function of temperature and thermal dose. Acoustic intensity distributions were modelled as a composite of measured and theoretical intensity distributions. The shape and time evolution of temperature contours and thermal lesions for 90 degrees, 200 and 360 degrees angularly directional applicators and multi-transducer applicators were modelled for heating durations between 1 and 5 min. Model parameters were selected to match previously reported ex vivo and in vivo studies of 2.2 mm diameter ICDC devices in thigh muscle and liver (15-30 W cm(-2) applied power density, 0.5-5 min treatment times, 2.8-3.6 cm diameter thermal lesions). The temperatures and lethal thermal dose (600 EM43 degrees C) contours calculated using the models were in excellent agreement with temperatures and thermal lesion dimensions (visible coagulation) determined experimentally. The differences between maximum radial depths of coagulation calculated using the r-z and r-theta models were small, less than approximately 2 mm for 10-15 mm lesions. There was a strong correlation between the calculated 50 degrees C contour and the radial, angular and axial lesion dimensions obtained for 3-5 min heating protocols. The models developed in this study have significant application in design studies and potential future use in treatment planning of ICDC interstitial ultrasound thermal therapy.     

Evaluation of multielement catheter-cooled interstitial ultrasound applicators for high-temperature thermal therapy

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.     

Transurethral ultrasound applicators with dynamic multi-sector control for prostate thermal therapy: in vivo evaluation under MR guidance

The purpose of this study was to explore the feasibility and performance of a multi-sectored tubular array transurethral ultrasound applicator for prostate thermal therapy, with potential to provide dynamic angular and length control of heating under MR guidance without mechanical movement of the applicator. Test configurations were fabricated, incorporating a linear array of two multi-sectored tubular transducers (7.8-8.4 MHz, 3 mm OD, 6 mm length), with three 120 degrees independent active sectors per tube. A flexible delivery catheter facilitated water cooling (100 ml min(-1)) within an expandable urethral balloon (35 mm long x 10 mm diameter). An integrated positioning hub allows for rotating and translating the transducer assembly within the urethral balloon for final targeting prior to therapy delivery. Rotational beam plots indicate approximately 90 degrees-100 degrees acoustic output patterns from each 120 degrees transducer sector, negligible coupling between sectors, and acoustic efficiencies between 41% and 53%. Experiments were performed within in vivo canine prostate (n = 3), with real-time MR temperature monitoring in either the axial or coronal planes to facilitate control of the heating profiles and provide thermal dosimetry for performance assessment. Gross inspection of serial sections of treated prostate, exposed to TTC (triphenyl tetrazolium chloride) tissue viability stain, allowed for direct assessment of the extent of thermal coagulation. These devices created large contiguous thermal lesions (defined by 52 degrees C maximum temperature, t43 = 240 min thermal dose contours, and TTC tissue sections) that extended radially from the applicator toward the border of the prostate (approximately15 mm) during a short power application (approximately 8-16 W per active sector, 8-15 min), with approximately 200 degrees or 360 degrees sector coagulation demonstrated depending upon the activation scheme. Analysis of transient temperature profiles indicated progression of lethal temperature and thermal dose contours initially centered on each sector that coalesced within approximately 5 min to produce uniform and contiguous zones of thermal destruction between sectors, with smooth outer boundaries and continued radial propagation in time. The dimension of the coagulation zone along the applicator was well-defined by positioning and active array length. Although not as precise as rotating planar and curvilinear devices currently under development for MR-guided procedures, advantages of these multi-sectored transurethral applicators include a flexible delivery catheter and that mechanical manipulation of the device using rotational motors is not required during therapy. This multi-sectored tubular array transurethral ultrasound technology has demonstrated potential for relatively fast and reasonably conformal targeting of prostate volumes suitable for the minimally invasive treatment of BPH and cancer under MR guidance, with further development warranted.     

Transurethral ultrasound applicators with directional heating patterns for prostate thermal therapy: in vivo evaluation using magnetic resonance thermometry

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.     

Theoretical comparison of two interstitial ultrasound applicators designed to induce cylindrical zones of tissue ablation

Although interstitial techniques are invasive, they are still the first-line therapeutic modalities for certain types of tumour. They are mainly relevant to tumours that are either inoperable or located so deep that access is complicated. Of the various types of radiation that can be delivered by the interstitial route, ultrasound is the most suitable for deep heating. The study compares the efficacy of two types of applicator with respect to their ability to induce cylindrical zones of coagulation necrosis. The transducer of the first applicator is tubular, whereas the second is plane and can rotate around its axis. Both have an external diameter of 4 mm, are fitted with surface cooling systems and operate at 10.7 MHz and 14 W.cm-2. Comparison involves mathematical modelling of ablated tissue in the targeted area by resolving the bioheat transfer equation (BHTE) using an algorithm based on finite differences. The BHTE gives a temperature value from which the thermal dose can be determined. It is shown that tissue ablation by tubular transducers is slow, and, in consequence, perfusion disturbs the heating pattern: in vivo, irradiation with a tubular transducer lasting 1081 s would be required to ablate a tissue mass with a radius of 8 mm. The corresponding period using a rotating plane transducer with 20 firing angles is only 618 s. The mean exposure time of each shot lasts 31 +/- 7 s. Therefore perfusion would have much less impact in the case of therapy administered using a plane transducer than that using a tubular one.     

Theoretical comparison of two interstitial ultrasound applicators designed to induce cylindrical zones of tissue ablation

Although interstitial techniques are invasive, they are still the first-line therapeutic modalities for certain types of tumour. They are mainly relevant to tumours that are either inoperable or located so deep that access is complicated. Of the various types of radiation that can be delivered by the interstitial route, ultrasound is the most suitable for deep heating. The study compares the efficacy of two types of applicator with respect to their ability to induce cylindrical zones of coagulation necrosis. The transducer of the first applicator is tubular, whereas the second is plane and can rotate around its axis. Both have an external diameter of 4mm, are fitted with surface cooling systems and operate at 10.7 MHz and 14W.cm⁻². Comparison involves mathematical modelling of ablated tissue in the targeted area by resolving the bioheat transfer equation (BHTE) using an algorithm based on finite differences. The BHTE gives a temperature value from which the thermal dose can be determined. It is shown that tissue ablation by tubular transducers is slow, and, in consequence, perfusion disturbs the heating pattern: in vivo, irradiation with a tubular transducer lasting 1081 s would be required to ablate a tissue mass with a radius of 8mm. The corresponding period using a rotating plane transducer with 20 firing angles is only 618s. The mean exposure time of each shot lasts 31±7s. Therefore perfusion would have much less impact in the case of therapy administered using a plane transducer than that using a tubular one.     

Feasibility of linear arrays for interstitial ultrasound thermal therapy

The feasibility of linear array transducers for interstitial ultrasound thermal therapy was evaluated. Theoretical acoustic power distributions were used to calculate spatial heating patterns using the bioheat transfer equation. The spatial heating patterns of linear array and single element planar rectangular transducers were compared. Scanned heating with both transducer geometries produced asymmetric heating volumes; however, a more uniform radial temperature profile with a sharper margin was achieved with linear arrays. Single element transducers produced excessive heating near the probe surface. Homogeneous blood flow is predicted to reduce the mean temperature within the heated region, with little effect on the spatial pattern.     

Highly directional transurethral ultrasound applicators with rotational control for MRI-guided prostatic thermal therapy

Transurethral ultrasound applicators with highly directional energy deposition and rotational control were investigated for precise treatment of benign prostatic hyperplasia (BPH) and adenocarcinoma of the prostate (CaP). Two types of catheter-based applicators were fabricated, using either 90 degrees sectored tubular (3.5 mm OD x 10 mm) or planar transducers (3.5 mm x 10 mm). They were constructed to be MRI compatible, minimally invasive and allow for manual rotation of the transducer array within a 10 mm cooling balloon. In vivo evaluations of the applicators were performed in canine prostates (n = 3) using MRI guidance (0.5 T interventional magnet). MR temperature imaging (MRTI) utilizing the proton resonance frequency shift method was used to acquire multiple-slice temperature overlays in real time for monitoring and guiding the thermal treatments. Post-treatment T1-weighted contrast-enhanced imaging and triphenyl tetrazolium chloride stained tissue sections were used to define regions of tissue coagulation. Single sonications with the 90 degrees tubular applicator (9-15 W, 12 min, 8 MHz) produced coagulated zones covering an 80 degrees wedge of the prostate extending from 1-2 mm outside the urethra to the outer boundary of the gland (16 mm radial coagulation). Single sonications with the planar applicator (15-20 W, 10 min, approximately 8 MHz) generated thermal lesions of approximately 30 degrees extending to the prostate boundary. Multiple sequential sonications (sweeping) of a planar applicator (12 W with eight rotations of 30 degrees each) demonstrated controllable coagulation of a 270 degrees contiguous section of the prostate extending to the capsule boundary. The feasibility of using highly directional transurethral ultrasound applicators with rotational capabilities to selectively coagulate regions of the prostate while monitoring and controlling the treatments with MRTI was demonstrated in this study.     

Method for MRI-guided conformal thermal therapy of prostate with planar transurethral ultrasound heating applicators

A method for conformal prostate thermal therapy using transurethral ultrasound heating applicators incorporating planar transducers is described. The capability to shape heating patterns to the geometry of the prostate gland from a single element in a multi-element heating applicator was evaluated using Bioheat transfer modelling. Eleven prostate geometries were obtained from patients who underwent MR imaging of the prostate gland prior to radical prostatectomy. Results indicate that ultrasound heating applicators incorporating multi-frequency planar transducers (4 x 20 mm, f = 4.7 MHz, 9.7 MHz) are capable of shaping thermal damage patterns to the geometry of individual prostates. A temperature feedback control algorithm has been developed to control the frequency, rotation rate and applied power level from transurethral heating applicators based on measurements of the boundary temperature during heating. The discrepancy between the thermal damage boundary and the target boundary was less than 5 mm, and the transition distance between coagulation and normal tissue was less than 1 cm. Treatment times for large prostate volumes were less than 50 min, and perfusion did not have significant impact on the control algorithm. Rectal cooling will play an important role in reducing undesired heating near the rectal wall. Experimental validation of the simulations in a tissue-mimicking gel phantom demonstrated good agreement between the predicted and generated patterns of thermal damage.     

Optimizing the shape of ultrasound transducers for interstitial thermal ablation

Heat deposition by interstitial routes, especially with ultrasound-based instruments, is becoming a valuable therapeutic option for the treatments of sites, which are difficult to access from outside of the body. The active part of most interstitial ultrasound applicators described in the literature is logically tubular to induce cylindrical volumes of coagulation necrosis. Because the pressure generated by such tubular transducers falls off rapidly with radial distance, we previously proposed using a rotating plane transducer. For a plane wave, the pressure fall-off is only due to attenuation, which makes deeper lesions and shorter treatment times possible. This work represents an advance in the development of ultrasound applicators designed for interstitial applications. This new applicator used a rotating slightly focused transducer. A brief theoretical analysis resulted in the choice of a long focal distance of 22 mm to obtain a nearly constant pressure all along the therapeutic depth. To experimentally validate this focal distance, pressure measurements were made in a tissue mimicking liquid phantom and the results were compared with those obtained with a plane transducer. In vitro experiments showed that necrosis could be induced at a depth of 15 mm. In the same conditions, the greatest depth attained with a plane transducer was only 10 mm. Because each individual lesion is narrower, more lesions and more time are required to necrose a cylindrical volume. The main advantage of this new type of applicator is that it can be used to induce necrosis at a greater depth without varying either the frequency, the intensity or the transducer cooling efficiency.     

MRI-guided interstitial ultrasound thermal therapy of the prostate: a feasibility study in the canine model

The feasibility of MRI-guided interstitial ultrasound thermal therapy of the prostate was evaluated in an in vivo canine prostate model. MRI compatible, multielement interstitial ultrasound applicators were developed using 1.5 mm diameter cylindrical piezoceramic transducers (7 to 8 MHz) sectored to provide 180 degrees of angular directional heating. Two in vivo experiments were performed in canine prostate. The first using two interstitial ultrasound applicators, the second using three ultrasound applicators in conjunction with rectal and urethral cooling. In both experiments, the applicators were inserted transperineally into the prostate with the energy directed ventrally, away from the rectum. Electrical power levels of 5-17 W per element (approximately 1.6-5.4 W acoustic output power) were applied for heating periods of 18 and 48 min. Phase-sensitive gradient-echo MR imaging was used to monitor the thermal treatment in real-time on a 0.5 T interventional MRI system. Contrast-enhanced T1-weighted images and vital-stained serial tissue sections were obtained to assess thermal damage and correlate to real-time thermal contour plots and calculated thermal doses. Results from these studies indicated a large volume of ablated (nonstained) tissue within the prostate, extending 1.2 to 2.0 cm from the applicators to the periphery of the gland, with the dorsal margin of coagulation well-defined by the applicator placement and directionality. The shape of the lesions correlated well to the hypointense regions visible in the contrast-enhanced T1-weighted images, and were also in good agreement with the contours of the 52 degrees C threshold temperature and t43 > 240 min. This study demonstrates the feasibility of using directional interstitial ultrasound in conjunction with MRI thermal imaging to monitor and possibly control thermal coagulation within a targeted tissue volume while potentially protecting surrounding tissue, such as rectum, from thermal damage.     

Feasibility of using interstitial ultrasound for intradiscal thermal therapy: a study in human cadaver lumbar discs

Application of heat in the spine using resistive wire heating devices is currently being used clinically for minimally invasive treatment of discogenic low back pain. In this study, interstitial ultrasound was evaluated for the potential to heat intradiscal tissue more precisely by directing energy towards the posterior annular wall while avoiding vertebral bodies. Two single-element directional applicator design configurations were tested: a 1.5 mm OD direct-coupled (DC) applicator which can be implanted directly within the disc, and a catheter-cooled (CC) applicator which is inserted in a 2.4 mm OD catheter with integrated water cooling and implanted within the disc. The transducers were sectored to produce 90 degrees spatial heating patterns for directional control. Both applicator configurations were evaluated in four human cadaver lumbar disc motion segments. Two heating protocols were employed in this study in which the temperature measured 5 mm away from the applicator was controlled to either T=52 degrees C, or T>70 degrees C for the treatment period. These temperatures (thermal doses) are representative of those required for thermal necrosis of in-growing nociceptor nerve fibres and disc cellularity alone, or with coagulation and restructuring of annular collagen in the high-temperature case. Steady-state temperature maps, and thermal doses (t43) were used to assess the thermal treatments. Results from these studies demonstrated the capability of controlling temperature distributions within selected regions of the disc and annular wall using interstitial ultrasound, with minimal vertebral end-plate heating. While directional heating was demonstrated with both applicator designs, the CC configuration had greater directional heating capabilities and offered better temperature control than the DC configuration, particularly during the high-temperature protocol. Further, ultrasound energy was capable of penetrating within the highly attenuating disc tissue to produce more extensive radial thermal penetration, lower maximum intradiscal temperature, and shorter treatment times than can be achieved with current clinical intradiscal heating technology. Thus, interstitial ultrasound offers potential as a more precise and faster heating modality for the clinical management of low back pain.     

Heating patterns of microwave applicators in inhomogeneous arm and thigh phantoms.

Heating rate (HR) patterns in cylindrical structures were studied with inhomogeneous limb phantoms. These phantoms, arm and thigh models consisting of fat, bone, and muscle material, were heated with Clini-Therm L, M, and MS applicators at 915 MHz. The thigh model is 18 cm in diameter with 2.65-cm-thick fat on the outside and a 4-cm-diam bone in the center. The arm model is 9 cm in diameter with 1.35-cm-thick fat and 2-cm-diam bone. All models are 29 cm long with phantom muscle in the space between fat and bone and were heated with their long axes parallel or perpendicular to the E field. HR patterns in the transverse and longitudinal planes were obtained thermographically. A large water bolus, with the water channels parallel to the E field, was used in every case. In the thigh model, maximum heating was in the muscle for both L and M applicators when the E field was parallel to the long axis. When it was perpendicular, the maximum heating occurred in the fat layer. However, the peak HR in the fat remained about the same for both E field orientations. For the small applicator, heating was mostly in the fat, and the two field orientations did not cause much difference in the heating pattern. In the arm model, the maximum heating of all three applicators occurred mostly in the muscle for both E-field orientations. However, the maximum HR was reduced by a factor of 2 to 3 when the E field was perpendicular rather than parallel to the long axis.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of applicator diameter on lesion size from high temperature interstitial ultrasound thermal therapy

High temperature ultrasound thermal therapy using interstitial and external approaches is becoming increasingly acceptable as a minimally invasive clinical treatment for cancerous and benign disease. The diameter of an interstitial applicator can influence its clinical practicality and effectiveness as well as application site. The purpose of this study was to determine whether the use of larger ultrasound transducers and the inherent increase in applicator size could be justified by potentially producing larger lesion diameters. Four applicator configurations and sizes were studied using ex vivo tissue experiments in liver and beef and using acoustic and biothermal simulations. Catheter-cooled and internally cooled applicators with outer diameters between 2.2 and 4.0 mm produced 3.5 to 5.0 cm diameter lesions in ex vivo liver and 3.0 to 3.5 cm lesions in ex vivo beef muscle with 20-40 W/cm applied for 10 min. Larger applicators produced lesions with radial penetration depths superior to their smaller counterparts at power levels in the 20-40 W/cm range. The higher cooling rates along the outer surface of the larger diameter applicators due to their greater surface area was a dominant factor in increasing lesion size. The higher cooling rates pushed the maximum temperature farther from the applicator surface and reduced the formation of high acoustic attenuation tissue zones. Applicator configuration and frequency (6.7-8.2 MHz) had less influence on lesion size than diameter in the ranges studied. Acoustic and biothermal simulations matched the experimental data well and were applied to model these applicators within sites of clinical interest such as prostate, uterine fibroid, brain, and normal liver. Lesions of 3.9 to 4.7 cm diameter were predicted for moderately perfused tissues such as prostate and fibroid and 2.8 to 3.2 cm for highly perfused tissues such as normal liver. In sites such as uterine fibroid where larger applicators placed using an endoscopic approach could be tolerated, treatment volume increases of 37% were predicted for an applicator diameter increase from 2.4 to 4.0 mm.     

Helical antenna arrays for interstitial microwave thermal therapy for prostate cancer: tissue phantom testing and simulations for treatment

Interstitial microwave therapy is an experimental treatment for prostate cancer. The objective of this work was to measure the power deposition (specific absorption rate, SAR) patterns of helical microwave antennae both individually and in array patterns that would be useful for clinical treatment protocols. Commercial helical antenna 3D SAR patterns were measured in muscle equivalent phantoms using a thermographic technique. Two array patterns were tested: a 'square' and a 'crescent' array, both surrounding the urethra. To assess the feasibility of pre-treatment planning, the measured SAR patterns were input to a treatment planning computer simulation program based on a series of trans-rectal ultrasound images from a prostate cancer patient. The simulation solved the Pennes linear bioheat heat transfer equation in prostate tissue, with the aim of achieving a target of 55 degrees C at the prostate periphery while not allowing normal surrounding tissues (bladder, urethra, rectum) to rise above 42 C. These criteria could not be met with the square array but they could be met with the crescent array, provided that the prostate was first dissected away from the rectum. This can be done with a procedure such as 'hydrodissection', where sterile saline is injected to separate the prostate and rectum. The results of these SAR measurements and heat transfer simulations indicate that arrays of helical antennae could be used for safe and effective thermal therapy for prostate cancer.     

水冷式微波天线热凝固离体猪肝的实验研究

微波辐射器的热场特性对肿瘤热疗有重要的实用意义,它直接决定了治疗肿瘤的尺寸和大小.本文通过水冷式微波天线在离体猪肝热消融的实验,研究其治疗区域的形态、温度分布等信息,将有助于临床参考.结果表明,水冷式微波天线沿径向方向的温度衰减快,能量主要集中在天线半径1cm以内;天线轴向上的温度分布由于有冷却水是非对称的,从而影响了凝固区的形状;增加加热时间会在一定程度上增加凝固范围.     

The efficiency of clinical microwave applicators measured by a calorimetric method

When inducing localized hyperthermia for superficial cancer therapy with microwaves there has often been question about the total power output from the applicator. Although specific absorption rates and thermograms are used to obtain localized power distributions and heating patterns, these provide, at best, only an approximation of the total power applied to tissues or phantoms. In this paper a calorimetric technique for obtaining total microwave output power from applicators is described. An experimental apparatus was constructed and it was found to be accurate to approximately +/- 5 W. The power output from four clinical microwave applicators as a function of applied electric power was measured and the efficiency was found to be 40% in average. Along with enhancing quality assurance, the areas of hyperthermia research which may benefit the most from this calorimetric technique are computer modeling and patient treatment planning.     

Direct thermal dose control of constrained focused ultrasound treatments: phantom and in vivo evaluation

The first treatment control system that explicitly and automatically balances the efficacy and safety goals of noninvasive thermal therapies is described, and its performance is evaluated in phantoms and in vivo using ultrasound heating with a fixed, focused transducer. The treatment efficacy is quantified in terms of thermal dose delivered to the target. The developed feedback thermal dose controller has a cascade structure with the main nonlinear dose controller continuously generating the reference temperature trajectory for the secondary, constrained, model predictive temperature controller. The control system ensures thermal safety of the normal tissue by automatically complying with user-specified constraints on the maximum allowable normal tissue temperatures. To reflect hardware limitations and to prevent cavitation, constraints on the maximum transducer power can also be imposed. It is shown that the developed controller can be used to achieve the minimum-time delivery of the desired thermal dose to the target without violating safety constraints, which is a novel and clinically desirable feature. The developed controller is model based, and requires patient- and site-specific models for its operation. These models were obtained during pre-treatment identification experiments. In our implementation, predictive models, internally used by the automatic treatment controller, are dynamically updated each time new temperature measurements become available. The adaptability of internal models safeguards against adverse effects of modelling errors, and ensures robust performance of the control system in the presence of a priori unknown treatment disturbances. The successful validation with two experimental models of considerably different thermal and ultrasound properties suggests the applicability of the developed treatment control system to different anatomical sites.