The purpose of this study was to determine the feasibility of using a transurethral ultrasound applicator in combination with implantable ultrasound applicators for inducing thermal coagulation and necrosis of localized cancer lesions or BPH within the prostate gland. The concept being evaluated is the potential to treat target zones in the anterior and lateral portions of the prostate with the transurethral applicator, while simultaneously treating regions of extracapsular extension and zones in the posterior prostate with the directive implantable applicators in combination with a rectal cooling bolus. Biothermal computer simulations, acoustic characterizations, and in vivo thermal dosimetry experiments were used to evaluate the performance of each applicator type and combinations thereof. The preliminary results of this investigation demonstrate that implantable ultrasound applicators, in combination with a transurethral ultrasound applicator, have the potential to provide thermal coagulation and necrosis of small or large regions within the prostate gland, while sparing thermally sensitive rectal tissue.

Direct-coupled and catheter-cooled interstitial ultrasound applicators have been evaluated for thermal necrosis of small, localized tumors. Emphasis of the design criteria has been on directionality of power deposition and the corresponding tissue heating. Ultrasound applicators have been fabricated using piezoceramic tubes operating at approximately 7 MHz. The applicators have full 360 degree(s) active acoustic zones, or are sectored to provide different angular heating patterns. The applicators were characterized through acoustic power output measurements, beam profile distributions in water, thermal distribution measurements in an in vitro perfused kidney model, and in vivo thermal dosimetry in porcine thigh muscle. Bench tests demonstrated that high power output levels could be sustained in both the direct-coupled and catheter-cooled devices without degradation of the ultrasound transducer. The angular power depositions obtained in water were closely correlated to the resultant temperature distributions measured both in the in vitro kidney and in vivo experiments, thus demonstrating the ability to shape the beam profiles for controlled, directional ablation of tissue.

We discuss potential of ultrasound waveguide applicator arrays for interstitial heating of brain tissue. First we describe specific absorption rate and show the importance of attenuation term at higher frequencies for cylindrical applicators. Ultrasound propagation characteristic to the applicator results in a shear component in the tissue. The component is significant over a small distance from the applicator. We obtain 3D temperature distribution using finite element analysis simulations for four-applicator array. The simulations show that the array is capable of heating the tissue to 56 degree(s)C. This high temperature leads to an undesired effect of heat toxicity in the adjacent tissue. The following simulations demonstrate how to optimize energy deposition, especially for asymmetries in boundary temperature. We investigate effects of effective thermal conductivity and antenna's length on the temperature pattern. We find that the array can be used for thermotherapy in clinically relevant volumes with the transducers operated at higher frequencies.

A new catheter-based cryosurgery system is proving its potential as a valuable tool for electrophysiologic (EP) applications inside the heart. The long, narrow, flexible catheter evaporates room-temperature liquid refrigerant within its tip to produce localized cryogenic temperatures. The catheter provides a `less invasive' percutaneous approach to the inner walls of the heart to confirm and ablate arrhythmogenic sites with cryosurgical benefits-- preservation of tissue integrity, absence of thrombus formation. The system's primary engineering challenges are safe refrigerant handling and catheter temperature performance. Surgical results in the animal model demonstrate the system has sufficient cooling power and temperature range to alter EP response, both temporarily and permanently. With tip temperatures between -20 degree(s)C and -35 degree(s)C at the atrioventricular (AV) junction, reversible conduction block is produced in the AV node with minimal damage to heart structures. Taking the tip below -50 degree(s)C creates permanent block in the AV node through formation of a necrotic lesion. Summing these results leads to the conclusion that the technology has the potential to identify arrhythmogenic sites without damage, to verify the site, and to ablate it--within the same procedure, without moving the catheter.

The purpose of this article is to review the physical construction and power deposition characteristics of interstitial microwave antennas that may be used for highly localized heating of tissue at depth in the human body. Several different antenna designs are described and matched with potential clinical applications that range from moderate temperature Hyperthermia therapy to tissue- necrosing Thermal Ablation therapy. Typical clinical procedures are outlined for thermal treatment of target sites such as brain, prostate, heart, and gynecologic region tissues. Associated methods of implanting the antennas and coupling microwave energy into the surrounding tissue are also described, including the use of single or multi-chamber stiff, flexible or inflatable balloon type catheters, with or without circulating air or water cooling. With numerous references to the primary literature, this material should provide a framework for analyzing potential new applications for interstitial microwave antennas, as derived from the physical capabilities and limitations of the available hardware and techniques.

A microwave applicator was developed and tested in a rabbit model, with the goal of developing a system to sterilize a human female through a transvaginal-transcervical- transuterine retrograde technique. The clinical procedure wold create an occluding lesion in the isthmic portion of the human fallopian tube in an out-patient procedure. The microwave applicator consisted of a flexible coaxial cable from which the inner conductor was extended to form a resonant monopole antenna. The coaxial cable and monopole were placed within a sealed teflon catheter of 3 mm diameter. A second parallel catheter of 1 mm diameter was secured to the first to provide guidance for a microwave- immune thermometry probe. Following laparotomy exposure, the applicator was placed with a transvaginal-transcervical retrograde technique in each uterine horn in succession. The temperature was elevated to 65 degree(s)C for 5 minutes. Thirty days following treatment, there was marked constriction and discoloration of the treated site as well as significant architectural effacement of the tissue composing the uterine wall. In some cases, the uterine lumen was completely occluded. Future experiments will assess the tissue response to smaller thermal doses.

In the past several years, there has been an increasing interest in the use of advanced energy modalities such as microwave for ablation of more complex cardiac arrhythmia including atrial flutter, atrial fibrillation and ventricular tachycardia. It has been demonstrated that microwave energy can produce larger and deeper lesions than radio-frequency (RF) energy and therefore clinicians worldwide are interested in using microwave ablation. Thus, the development of efficient microwave generators, catheters and applicators that are well adapted for cardiac tissue ablation is essential. This paper pertains to the physical properties of tissue at microwave frequencies, the theoretical background of microwave ablation and the initial clinical results obtained with the Fidus's microwave ablation system.

Argon gas-enhanced electrosurgery has recently been introduced for its potential beneficial effects on hemostasis during electrical cutting. In this study, the influence of argon gas on electrosurgery on tissue was investigated. A standard electrosurgery unit was used extended with a gas unit and accommodated handset, which enabled a flow of argon blown along the electrode in contact with tissue. The temperature distribution was visualized in polyacrylamide gel using a color-Schlieren technique. Bovine tissue was used to evaluate the macroscopic effect of the lesions. The electrode was moved over the tissue surface with different settings for speed, gas flow, gas-outlet positioning and depth of the electrode in the tissue. During cutting, coagulation was significantly increased using argon gas; coagulation on both sides of the track ranging from 1.0 mm without argon flow up to 4.5 mm with argon flow could be obtained. Changing the gas flow from laminar to affected neither the coagulation nor the cutting. The extent of the coagulation depended on the combination of power and distance of the gas-outlet to the tissue. The coagulation depth beyond the bottom of the tracks was not influenced by argon and remained less than 1 mm. Argon gas-enhanced electrode surgery is especially effective when just touching the tissue thus obtaining a superficial coagulation (and hemostasis) of the surrounding tissue.

The Cavitron Ultrasonic Surgical Aspirator (CUSA) is being used, especially in neuro- and liver surgery, to resect selectively soft and hard tissue in favor of elastic tissues like blood vessels, enabling the removal of tumors with minimal loss of blood. In this study the phenomena associated with CUSA were visualized to expand the understanding of the mechanism of action of the CUSA. Real- time high-speed imaging techniques were applied to capture cavitation phenomena during application of the CUSA under physiological settings: in water, at tissue surfaces and inside artificial tissue. Close-up photography using a 1 microsecond(s) flashlight showed the expanding and imploding cavitation bubbles around the rim of the ultrasonic vibrating hollow tip. Shock waves generated by imploding cavitation bubbles were observed using Schlieren techniques with a temporal resolution of 10 ns and synchronized with the duty cycle of the vibrating tip. In addition, thermal effects associated with friction between the vibrating tip and tissue were visualized sing a thermal imaging technique. The CUSA mechanism has proven to be a combined effect of cavitation induced fragmentation, mechanical cutting and thermal deterioration of tissue depending on the irrigation/aspiration flow, intermittent vibration regime and degree of tissue contact. The impact of the shock waves observed is undetermined yet. These real-time imaging techniques will contribute to expand the understanding of the working mechanism of CUSA and to show the characteristics of probe designs and influence of driving frequency.

Radiofrequency (RF) tumor ablation has been demonstrated as a reliable method for creating thermally induced coagulation necrosis using either a percutaneous approach with image- guidance or direct surgical application of thin electrodes into treated tissues. Early clinical trials with this technology have studied the treatment of hepatic, cerebral, and bony malignancies. The extent of coagulation necrosis induced with conventional monopolar radiofrequency electrodes is dependent on overall energy deposition, the duration of RF application, and RF electrode tip length and gauge. This article will discuss these technical considerations with the goal of defining optimal parameters for RF ablation. Strategies to further increase induced coagulation necrosis including: multiprobe and bipolar arrays, and internally-cooled RF electrodes, with or without pulsed-RF or cluster technique will be presented. The development and laboratory results for many of these radiofrequency techniques, initial clinical results, and potential biophysical limitations to RF induced coagulation, such as perfusion mediated tissue cooling (vascular flow) will likewise be discussed.

Quantitative and qualitative histopathologic techniques were used to compare the distribution, severity and depths of acute thermal lesions formed by in vivo placement of three different intracavitary thermal balloon instruments in the uteri of 19 women scheduled for hysterectomy. Thermal damage reflected by (1) Nitro Blue Tetrazolium stains separating `living' from `dead' tissues, (2) red zone formation and the (3) presence of a clear zone observed in histologic slides extended into the myometrium. One hysterectomy specimen removed 4 days after treatment showed superficial slough of the endometrium but solid, coagulation necrosis of the deeper endometrium and adjacent myometrium. The treatment effect and success of intracavitary thermal coagulation may be related to a delicate balance of complete irradiation of endometrium versus fibrous stricture and intracavitary adhesions of the uterus.

A radio frequency tissue welding system has been developed for occlusion of vessels during surgery. The system is designed to replace commonly used mechanical clip and suture ligation techniques. Other energy based ligation techniques are limited to use on small structures (<EQ 2 mm) due to slow heating, unreliable sealing, and charring/sticking to the forceps. The system consists of forceps and an RF electrosurgery generator, both of which are specifically designed for optimal tissue sealing. The method combines optimal pressure delivery to the tissue and energy delivery consisting of a high heat cycle, a low heat cycle and a cooling cycle. The generator output is also voltage limited and delivers high current in order to remodel the collagen in approximately 5 seconds with no sticking or charring. The vessel sealing system was compared to other energy based ligation techniques including ultrasonic sealing and other bipolar systems. The pressure required to burst the vessel was used for comparison. Average burst pressures on 3 - 7 mm arteries were 126 +/- 154 mmHg, 607 +/- 314 mmHg, and 913 +/- 304 mmHg for ultrasonic, standard bipolar, and vessel sealing, respectively. Histologic evaluation showed vessel wall fusion and minimal thermal damage to adjacent tissues for the vessel sealing system.

Stents are often inserted into internal orifices to treat blockage due to tumor ingrowth. Stents are favored due to their minimally invasive nature, possible avoidance of a surgical procedure, and their ability to palliate surgically non-resectable disease. Because of rapid tumor growth however, a treatment means to prevent overgrowth through the stent and resultant blockage is required. To further this goal, experiments were performed in which a stent was placed in tissue and heated with radiofrequency (RF) energy to coagulate a cylinder of tissue, thereby eradicating viable tissue in the proximity of the stent. Temperatures were measured at the central stent surface and edges over time during a 5 - 10 minute heating in phantom and in fresh tissue. In addition, a finite element model was used to simulate the electric field and temperature distribution. Blood flow was also introduced in the model by evaluating RF application to stents to determine effectiveness of the energy applications. Changing perfusion and tissue electrical conductivity as a function of temperature was applied as the tissue was heated to 100 degree(s)C. Results from the electric field model will be shown as well as the thermal distribution over time from the simulations. Lastly, results from the damage integral will be discussed.

A new electrosurgical ball electrode (SilverBullet^TM) has been developed for applying radiofrequency (RF) energy to fuse biological and other materials to tissue surfaces. Specifically, the electrode was developed for use in conjunction with the Fusion Medical Technologies, Inc. gelatin patch (RapiSeal^TM) for use in pulmonary surgery to seal air leaks, and in solid abdominal organ surgeries to provide hemostatic tamponade. The new electrode allows for the application of RF energy in contact mode without the problems of the electrode sticking to the gelatin patch or the underlying tissue. Designed for use with commercially available electrosurgical handpieces, the electrode consists of a stainless steel connector that fits into the hand- piece, and an electrode assembly made from silver that includes a shank region, and a tip extension extending distally from the shank region. The distal tip of the tip extension is rounded and has a length of about 10 mm. The uniqueness of this electrode is the shank region which has a cross sectional area that is larger than the tip extension. The shank region acts as a heat sink to draw away heat from the tip extension while the tip extension itself remains sufficiently small to access desired target sites and display the desired energy transfer properties. In addition to the physical design, the use of silver as the core element provides a material with high electrical and thermal conductivities. The bulk of the electrode is appropriately insulated.

Image guidance is one of the major challenges common to all minimally invasive procedures including biopsy, thermal ablation, endoscopy, and laparoscopy. This is essential for (1) identifying the target lesion, (2) planning the minimally invasive approach, and (3) monitoring the therapy as it progresses. MRI is an ideal imaging modality for this purpose, providing high soft tissue contrast and multiplanar imaging, capability with no ionizing radiation. An interventional/surgical MRI suite has been developed at Brigham and Women's Hospital which provides multiplanar imaging guidance during surgery, biopsy, and thermal ablation procedures. The 0.5T MRI system (General Electric Signa SP) features open vertical access, allowing intraoperative imaging to be performed. An integrated navigational system permits near real-time control of imaging planes, and provides interactive guidance for positioning various diagnostic and therapeutic probes. MR imaging can also be used to monitor cryotherapy as well as high temperature thermal ablation procedures sing RF, laser, microwave, or focused ultrasound. Design features of the interventional MRI system will be discussed, and techniques will be described for interactive image acquisition and tracking of interventional instruments. Applications for interactive and near-real-time imaging will be presented as well as examples of specific procedures performed using MRI guidance.

A laboratory scale multi-illumination microwave imaging system has been successfully demonstrated for the reconstruction of biologically relevant materials and has also been shown to be sensitive to thermally induced electrical property changes. Several challenges are currently being addressed in an effort to bring the system to the clinic for use in non-invasive thermometry. These include: (1) increasing the size of the imaging region, (2) reducing image artifacts due to the presence of multiple antennas during illumination, and (3) developing a solid illumination chamber to replace the saline bath tank. The first two are intimately related. As the imaging system size is increased to clinically useful dimensions, we must inevitably reduce the lossiness of the surrounding medium which will subsequently increase the antenna-antenna interactions. This is being addressed both in terms of the design of the individual antennas and in terms of introducing compensating techniques into the numerical model. Investigating these areas will provide insight into the ultimate capability of this imaging modality. Additionally, a solid material is being developed with electrical properties close to that of saline. Working in such a lossy medium has significant advantages in terms of the antenna performance and the reduction of unwanted multi- path signals propagating into and out of the desired imaging plane. With these tasks completed, quantification of the system's ability to recover electrical property distributions and thermal profiles based on difference imaging techniques will be investigated for both phantom and in vitro experiments.

The feasibility of real-time non-invasive spatio-temporal temperature estimation from pulse-echo diagnostic ultrasound data has been previously demonstrated in stationary phantoms. The method is based on first estimating the axial shifts of the RF-echo data due to local changes in the speed of sound and thermal expansion in the propagating medium, and then differentiating these estimates along axial direction to obtain the temperature rise map. In a clinical setup, however, translation, rotation and deformation affect the estimates. In this paper we introduce an algorithm to compensate for tissue translation and uniform deformation along the axial and lateral directions of the ultrasound imaging plane. This is achieved by separating the components of the time-shift map due to temperature rise (a local effect, occurring within the vicinity of the heated region) from the component due to translation and deformation (effect observed over a larger region). A rubber phantom experiment was designed where high intensity focused ultrasound was used to generate localized heating while motion was applied to the phantom and/or imaging transducer. Temperature profiles were successfully estimated while the phantom was translated by 30 mm and axially deformed by 13%.

Electrical properties of tissues in the 10 KHz to 10 MHz range are known to be temperature sensitive making the monitoring and assessment of thermal insult delivered for therapeutic purposes possible through imaging schemes which spatially resolve these changes. We have been developing electrical impedance imaging technology from both the hardware data acquisition and software image reconstruction perspectives in order to realize the capability of spectroscopically examining the electrical property response of tissues undergoing hyperthermia therapy. Results from simulations, in vitro phantom experiments and in vivo studies including in human patients are presented. Specifically, a new prototype multi-frequency data acquisition system which is functional to 1 MHz in both voltage and current modes is described. In addition, recent advances in image reconstruction methods which include the enhancement techniques of total variation minimization, dual meshing and spatial filtering are discussed. It is also clear that the electrical impedance spectrum of tissue has the potential to monitor other types of treatment-induced injury. Preliminary in vivo electrical impedance measurements in a rat leg model suggest that the tissue damage from radiation therapy can be tracked with this technique. Both dose and time-dependent responses have been observed in the electrical impedance data when compared to measurements recorded in an untreated control. Correlations with histological examination have also been performed and indicate that electrical impedance spectroscopy may provide unique information regarding tissue functional status and cellular morphology. Representative results from these studies are reported.

Ablation of the endometrium has become a viable treatment for dysfunctional bleeding of the uterus in women. Surgical applications of thermal ablation utilized a rolling electrode to ablate the inner uterine lining, but required practiced surgical skills and made it difficult to assess subsurface damage. Recently, various energy systems have been applied to the endometrium such as lasers, microwaves, RF electrodes, hot water balloons, and cryotherapy. A finite element model is presented to compare a multi-electrode, multiplexed RF device with a balloon containing hot fluid. The temperature fields in the uterine wall are plotted over time for various blood flow values. Assumptions of constant electrical conductivity are compared to temperature- dependent electrical conductivity. Temperatures are shown to be a maximum of about 10 - 20 degree(s)C higher when varying electrical conductivity is used. Results are also shown for cases with a 2 mm blood vessel in the field and how each device adjusts its operation to compensate for this heat sink. Damage integral results will be shown according to the time and temperature of the treatments.

Sealing and fusion of vessels by electrosurgical current is strongly influenced by the inhomogeneous architecture of the tissue constituents, particularly in the large arteries. Inhomogeneities in electrical properties of the constituents, specifically smooth muscle, collagen and elastin, lead to sharp spatial gradients in volumetric power deposition which results in uneven heating. The mechanical properties of the various tissue constituents are also of considerable importance. Vessel collagen and elastin distribution varies from vessel to vessel, species to species in the same artery, and point to point in the same vessel of the same animal or person. We present histologic evidence of vascular constituent variations, measurements of germane tissue electrical properties and numerical model studies of their effect on local heating rates and temperature rise in geometrically realistic finite difference vessel models. Comparisons between predicted and measured damage boundaries showed favorable agreement.

The current status and the future needs of image-guided ultrasound phased array systems for noninvasive surgery are addressed. Preliminary results from an integrated image- guided therapeutic phased array system for noninvasive surgical applications currently being developed at our laboratory are shown. The therapeutic array utilizes piezo- composite transducer technology and operates (therapeutically) at 1 and 2 MHz. It consists of 64 elements on a spherical shell with a geometric center at 100 mm from its apex. The array was shown to be capable of producing well defined thermal lesions in tissue media at depths from 40 to 60 mm and to scan therapeutic foci up to +/- 15 mm from its geometric center. Image guidance is provided by a modified diagnostic ultrasound scanner which, in addition to providing standard B-scan images of the target region, provides real-time images of the temperature rise due to the therapeutic beam. The temperature information is obtained using a correlation based algorithm for echo displacement estimation, which can be directly related to local variation in tissue temperature due to the therapeutic beam. A complete description of the combined imaging/therapy system is given. Furthermore, illustrative examples of noninvasive real-time image-guided tissue ablation, temperature estimation, and temperature control are presented and discussed.

Objectives: Compare the muscular parietal heating during high intensity focused ultrasound (HIFU) at different acoustic power levels. Methods: 3 series of 100 acoustic pulses of same energy but variable power were applied in vivo on the muscular lumbar and abdominal walls in 4 pigs. The target volume was located 47 to 94 mm behind the skin. Two thermocouples were inserted into the muscular wall facing the focal point 10 - 15 m and 25 - 30 mm from the skin. The position of the treatment head was the same for the 3 series of acoustic pulses. The temperatures in the muscular wall were recorded during the HIFU treatment. Conclusion: During extracorporeal HIFU treatments, the muscular parietal heating is variable among individuals and increases with the acoustic power.

Knowledge of the spatial distribution of intensity loss from an ultrasonic beam is critical to predicting lesion formation in focused ultrasound surgery. To date most models have used linear propagation models to predict the intensity profiles needed to compute the temporally varying temperature distributions. These can be used to compute thermal dose contours that can in turn be used to predict the extent of thermal damage. However, these simulations fail to adequately describe the abnormal lesion formation behavior observed for in vitro experiments in cases where the transducer drive levels are varied over a wide range. For these experiments, the extent of thermal damage has been observed to move significantly closer to the transducer with increasing transducer drive levels than would be predicted using linear propagation models. The simulations described herein, utilize the KZK (Khokhlov-Zabolotskaya-Kuznetsov) nonlinear propagation model with the parabolic approximation for highly focused ultrasound waves, to demonstrate that the positions of the peak intensity and the lesion do indeed move closer to the transducer. This illustrates that for accurate modeling of heating during FUS, nonlinear effects must be considered.

Introduction: The efficacy of extracorporeal High Intensity Focused Ultrasound (HIFU) on bladder wall has been demonstrated. However, the treatment is still slow, needing about 15 min to treat 1 cm². Objectives: Demonstrate the feasibility of HIFU with electronic scanning and reduce the during of HIFU treatments. Conclusions: Necrotic lesions on bladder posterior wall can be obtained with HIFU treatments using electronic scanning of the focal point. Average treatment duration with electronic scanning is reduced to 3 min 30 sec/cm².

Aim: This study investigates whether it is possible to occlude blood flow in vivo using high intensity focused ultrasound surgery (FUS). Such an effect could be used in the non-invasive treatment of fetal dysfunctions. Conclusion: Our ability to curtail blood flow using FUS allows the possibility of non-invasively treating feto-fetal transfusion syndrome by occluding the placental shunt vessels responsible for the vascular imbalance in twins sharing a placenta. This would have advantages over currently available interventional treatments (surgery or intrauterine lasers), which have significant related mortality and morbidity.

Introduction: High Intensity Focused Ultrasound (HIFU) is a minimally invasive therapy which can produce tissues necrosis in depth by a local thermal effect. Objectives: Evaluate the effect of extracorporeal HIFU with electronic scanning in the treatment of pig kidney. Conclusion: Extracorporeal HIFU with electronic scanning can produce tissue necrosis into pig's kidneys. This treatment could be used in human as a non invasive therapy of small kidney's tumors less than 30 cm³ of volume.

A prototype extra-corporeal focused ultrasound surgery device has been built and tested extensively in model systems both in vivo and ex vivo. A phase 1, normal tissue toxicity trial is now underway. Patients with soft tissue tumors lying 4 - 12 cm below the skin surface are being treated using a dose escalation technique. An in situ `ablative intensity' (AI) has been established from preclinical studies. This was found to be 1500 Wcm^-2 for up to 3 seconds at 1.7 MHz. Groups of three patients are being treated at doses rising from 25% AI through 50%, 63%, 80%, 100% to 125% AI for 1 second. Finally, 2 and 3 second exposure will be used for the two highest doses. Patients receive no sedation or anaesthetic. Target sites include all soft tissue tumors, especially those of the prostate, kidney and liver. Patients are examined for skin erythema, and are asked to complete pain and symptom questionnaires prior to treatment, immediately after treatment, one week and one month later. Patients receive a diagnostic ultrasound scan before and immediately after exposure, and after 1 week. Those at the highest dose levels are also offered a magnetic resonance scan.

Laser-induced interstitial thermotherapy has proven to be an effective method for the treatment of different types of tumors. Until now the attainable coagulation volume was limited by the maximum applicable energy. The limiting factor was the high tissue temperature around the applicator which may have caused applicator damage. Consequently an internally cooled catheter system has been developed in order to reduce the temperature of the applicator surface and to allow for the application of higher laser powers. The optimal treatment parameters for the Nd:YAG laser were determined on the basis of in vitro studies with porcine tissue. Following these experimental studies, 127 patients with liver metastases were treated with the cooled system. The applicator position and the resulting tissue damage were verified using a MRI on-line monitoring system applying a FLASH-2D sequence. The optimal in vivo treatment parameters were found to be 25 watts for an exposure time of 20 minutes, resulting in coagulated volumes of up to 20 cm³. The experimental and clinical results have proven that the combination of a scattering laser applicator with an internally flushed catheter enables a significant increase in the coagulation volume.

In addition to the laser, microwave or other energy sources, interstitial thermotherapy with radio-frequency current (RFITT) in bipolar technique has already been shown in vitro to be a safe and an economical alternative energy source with a comparable operating performance. The therapeutical application efficiency of these bipolar RF-needle applicators was evaluated using 3 different types of probes: standard, flushed and high performance cooled RF-probes (3 mm). These can be used to create large coagulation volumes in tissue such as for the palliative treatment of liver metastases or the therapy of the benign prostate hyperplasia. It was shown that the achievable lesion size resulting from the cooled RF-probes could be increased by a factor of three compared to a standard bipolar probe. With these bipolar power RF-applicators, coagulation dimensions of 5 cm length and 4 cm diameter with a power input of 40 watt could be achieved within 20 minutes. No carbonization and electrode tissue adherence was observed. Investigations in vitro with adapted RFITT-probes, using paramagnetic materials such as titanium alloys and high performance plastic, have shown that monitoring under MRI (Siemens Magnetom, 1.5 Tesla) allows visualization of the development of the spatial temperature distribution in tissue using an intermittent diagnostic and therapeutical application. This is no loss in performance compared to continuous applications. A ratio of 1:4 (15 s Thermal Flash MRI, 60 s RF-energy) has shown to be feasible. A computer simulation of the temperature and damage distribution during a bipolar RFITT application has been developed. The simulation works on-line with a RF-generator and measures the output power continuously. The electric power density (heat generating term) and the damage distribution is displayed graphically in real time.

A new combined Laser and Ultrasound Surgical Therapy (LUST) device for an endoscopically suitable coagulation and tissue fragmentation based on the transmission of laser radiation and ultrasound via flexible silica glass fibers was developed at the LMTB. The ultrasound tissue interaction is based on the well-known CUSA-technology, which enables the surgeon to cut various types of tissue with different degrees of effectiveness. This selective cutting performance is a very useful feature, e.g. for a brain tumor extirpation, where it must be guaranteed that vessels and nerves are not affected while ensuring a fast reduction of the tumor mass. Application fields are in oncology, neurosurgery and angioplasty. The laser radiation can be used for tissue coagulation purposes and homeostasis. With a fiber based LUST-system working at a resonance frequency of 30 kHz, using a laser-vibrometer, velocity amplitudes of up to 20 m/s could be detected at the distal end which corresponds to an elongation of more than 100 micrometers . The investigations have shown that the velocity amplitude, next to suction, frequency and cross section of the active fiber tip, has the greatest impact on the fragmentation rate. With a suction setting of 5 W, the following tissue fragmentation rates could be achieved with a 1.3 mm² fiber cross section and a tip amplitude velocity of 12 m/s: brain tissue 50 mg/s, liver 4,5 mg/s and kidney 4 mg/s. Laser radiation up to 25 watt was sufficient to coagulate soft tissue. This new approach in developing an application system for the therapeutical use of laser radiation and ultrasound via optical waveguides offers new possibilities in minimally invasive surgery, providing a complete new working sphere for the surgeon. The flexible opto-acoustic waveguide (400 - 1700 micrometers ) can be bent making areas accessible which were inaccessible before. The surgeon can use the laser radiation for tissue coagulation or cutting and the ultrasound for tissue fragmentation and tissue reduction without changing the instrumentation.