X-Ray- and catheter-guided intracardial laser photocoagulation of arrhythmogenic substrates

Cardiac arrhythmias resistant to drug treatment and correlated to an arrhythmogenic anatomic structure can be treated in several cases by surgical intervention. A further method of treatment is the percutaneous, ECG mapping-guided catheter ablation with either direct current (DC) or radio frequency (RF) ablation of localized arrhythmogenic foci. These methods overcome some complications inherent to open surgery to the heart; DC ablation, however, often induces further arrhythmias or ventricular or auricular fibrillation, while RF ablation shows only little success. In the method presented here disturbing electric effects are avoided by laser photocoagulation via a transcatheter quartz fibre. An electrode/laser catheter recently developed simultaneously allows for a mapping-guided localization of arrhythmogenic substrates and their percutaneous transluminal and intracardial deactivation. An anchoring mechanism at the catheter's tip keeps it in place during the treatment. This mechanism also prevents contact of the quartz fibre to the endomyocardium in order to avoid destruction of the fibre tip due to local overheating of the myocardium. A Nd-YAG laser (wavelength 1.064m) delivers pulses of 5–10 s and a power of 10–15 W at the tip of a quartz fibre with a core diameter of 400m. The corresponding parameter or irradiation of the myocardium are power densities of 700–1500 W cm^–2 and energy densities of 8–15 kJ cm^–2. In dog hearts lesions 2–10 mm in diameter and up to 11 mm in depth were obtained. Simultaneously registered electrocardiograms show that primarily induced arrhythmias lasted only for about 3 min but could not again be observed during the 2–3 months follow-up. Crater formation or perforation of the myocardium always could be avoided by an ECG-controlled laser irradiation. This indicates the safety and efficacy of the laser method for intracardial intervention in arrhythmogenic substrates. Clinical tests have been started recently.     

Transcatheter laser photocoagulation for treatment of cardiac arrhythmias

Cardiac arrhythmias can be treated by endocardial laser photocoagulation of arrhythmogenic foci. Transcatheter continuous wave Nd-YAG laser (1064 nm) photocoagulation was studied for impairment of atrioventricular (AV) conduction. First, studies on the dose-effect relation revealed that in vitro applications to ventricular endocardium produce dose-dependent coagulation necrosis. However, energy absorption varies in tissue specimens from different experimental animals and humans and may be dependent on differences in surfaces ultrastructure. Ten times more energy is required for human endocardial specimens than for canine endocardium.The long-term in vivo experiment demonstrates that the chronic laser-induced myocardial lesion size is proportional to the applied energy, exhibit no arrhythmogenic effects and may thus be suitable for the treatment of cardiac arrhythmias. Transcatheter laser application using a combined electrode-laser catheter reliably achieved a slowing of atrioventricular AV conduction (AV modification) or a complete AV block.Thus, the transcatheter application of Nd-YAG laser energy at the AV node may be useful for treatment of tachyarrhythmias in man.     

Neodymium: YAG contact laser photocoagulation of the in vivo canine epicardium: Dosimetry,effects of various lasing modes,and histology

Neodymium:YAG laser photocoagulation (LPC) is an investigational technique for the treatment of drug-resistant postinfarction ventricular tachycardia that requires cardiopulmonary bypass and open heart surgery. The purpose of this experimental study was to evaluate the feasibility of contact epicardial LPC to reach subepicardial, deep myocardial, and subendocardial regions without ventriculotomy and to analyze the dose-response relationships of this new technology. In 24 open-chest, closed heart dogs, a total of 219 epicardial laser lesions were generated under sterile conditions. Laser source was a continuous wave 1064 nm Nd:YAG; the delivery system consisted of a 400 μ bare quartz fiber fed into an 8F electrode catheter in direct contact with the epicardial surface. Laser variables were power at 10–15–20 W; lasing time, 6–12–18–24 s; lasing mode, continuous and pulse-pause; fiber mode, optical fiber tip inside and extending out from the electrode catheter. Hearts were harvested at 1 week. Histologic measurements included lesion surface diameter, internal diameter, and depth. Results: Significant linear correlation was found between the total delivered energy and all measured lesion parameters. Transmural lesions were found at energy levels of 270–480 J. Maximum depth measured up to 1.68 cm; at high energies, it was limited by myocardial thickness only. At constant energy levels, power and lasing time did not independently affect lesion dimensions; lasing mode and fiber mode had statistically significant but minor effects. Laser lesions were homogeneous and sharply demarcated without epicardial crater or endocardial thrombus formation. Endocardial surface cooling by circulating blood and the presence of epicardial vessels resulted in modifications in lesion morphology. In 29 out of 74 grossly transmural lesions, there was a 0.082 ± 0.127 cm thin surviving subendocardium. Forty-five coagulation lesions were truly transmural with no surviving myocardial strands. In conclusion, contact epicardial LPC can result in deep, transmural myocardial coagulation lesions. Lesion dimensions can be adequately controlled by total delivered energy. Closed heart epicardial Nd:YAG laser photocoagulation could become a new, less invasive surgical technique for the treatment of drug-resistant ventricular tachycardia. © 1993 Wiley-Liss, Inc.     

Low power interstitial Nd-YAG laser photocoagulation in normal rabbit brain

The safe, effective, clinical application of interstitial laser irradiation to destroy brain tumour tissue requires a knowledge of the relation of the extent of laser-induced (thermal) necrosis to the delivered laser power and total energy, and to time post-irradiation. We have conducted experiments to determine these relationships in normal rabbit brain. Irradiation by a Nd-YAG laser (1064 nm), at powers of 0.5–3.0 W and exposures of 200–1333 s produced well-defined necrotic lesions whose size increased with both the power and the total energy delivered. Lesions of 6 mm diameter made by 0.75 W for 1000s were well tolerated by animals allowed to recover from anaesthesia following irradiation. The diameter of the lesion was greatest at 48 h after irradiation. Following evolution of a characteristic healing response to necrosis in brain, the residual damage at 4 weeks was no greater in volume than that of the acute lesion. The results suggest that low power interstitial Nd-YAG laser photocoagulation in brain can be reliably and safely effected.     

Catheter-directed neodymium:YAG laser injury of the left ventricle for arrhythmia ablation: dosimetry and hemodynamic, hematologic, and electrophysiologic effects

The study evaluated catheter-directed laser injury of the left ventricular myocardium in a canine model as a potential method for treatment of resistant ventricular tachycardia. The relationship between laser energy delivered, the laser delivery characteristics, and the lesion size was evaluated both in vitro and in vivo. The effect of laser injury on several hematologic and hemodynamic parameters was studied. Programmed electrical stimulation studies were performed to assess the proarrhythmia potential of laser injury of the myocardium. The results showed the depth and diameter of myocardial injury to be directly related to total laser energy applied. Both depth and diameter of injury were increased by multiple applications of laser energy to the same myocardial location. One-half to full thickness left ventricular injury was produced without perforation or embolization. Endocardial vaporization occurred only with laser powers higher than necessary to produce full-thickness injury. There was no sustained effect on the hemodynamic or hematologic variables studied, nor were proarrhythmia effects identified. The laser injury produced in these studies was of smaller magnitude than might be necessary for clinical arrhythmia ablation, and the lack of sustained hemodynamic effects and pro-arrhythmia potential needs to be verified in subsequent studies with larger laser lesions. The results support continued evaluation of catheter-directed laser injury of the myocardium as a potential method for elimination of life-threatening ventricular arrhythmias.     

Interstitial thermotherapy for liver tumours: Studies of different fibres and radiation characteristics

Interstitial thermal laser destruction is an attractive modality for treatment of primary and metastatic liver tumours. Using a bare fibre the volume of tissue destroyed is limited and repeated therapy is necessary. This is time consuming and it is not easy to be certain that the entire lesion is treated. We have investigated the use of a modified laser fibre. The tip of this fibre is provided with a cap made of quartz glass emitting the laser radiation laterally and circumferentially. The necrosis had an ellipsoid shape with a size amounting to 2.1±0.63 cm on 1.6±0.58 cm using 3 W power with an exposure of 600s. These lesions were much larger than the necrosis induced by bare laser fibre photocoagulation which, at most, amounts to 1.2±0.46 cm using 1.5W power for 600s. These sizes were very suitable for using the method clinically. However, the lesions were not always reproducible due to damage of the fibre during long time exposure. We conclude that further research is necessary to increase the reliability of the fibre before clinical application for therapy of liver metastasis.     

Interstitial laser thermotherapy: Comparison between bare fibre and sapphire probe

A sapphire probe and a bare fibre were compared with respect to temperature control and distribution and light fluence in interstitial laser thermotherapy. Experiments were performed in processed liver using an Nd-YAG laser and output power levels of 1–4 W. The temperature was controlled at a distance of 10 mm using a feedback circuit with an automatic thermometry system and thermistor probes. With the sapphire probe, carbonization was rare at power levels of 1–2 W but was observed in half of the experiments at 3 W and in all experiments at 4 W. Using the bare fibre, carbonization was seen in almost all experiments. Absence of carbonization was associated with a moderate decrease in the penetration of light and excellent control of the temperature, whereas carbonization led to rapid impairment of light penetration and temperature control. In addition, the temperature gradient was smaller with the sapphire probe than with the bare fibre or when carbonization was absent. It is concluded that a diffuser tip, such as the sapphire probe, may be preferable to the bare fibre for interstitial laser thermotherapy because it gives a smaller temperature gradient and helps to avoid carbonization which results in preserved light penetration and improved temperature control.     

Computer simulated light distributions in myocardial tissues at the Nd-YAG wavelength of 1064 nm

Continuous wave Nd-YAG myocardial laser photocoagulation of ventricular tachycardia at the 1064 nm wavelength is a new investigational procedure in the treatment of arrhythmias. Computer simulated light diffusion patterns, generated by the Monte Carlo technique, are used to illustrate the calculated light distribution in myocardial tissues such as normal, and photocoagulated (canine) myocardium, epicardial fat on myocardium, and aneurysm for the optical properties at the Neodymium-YAG wavelength of 1064 nm. Calculated total reflection from myocardial tissue increased from 16% to 28% as a result of photocoagulation in the Monte Carlo light distribution simulations. The calculated total transmission dropped from 0.8% to 0.1% as a result of coagulation. Transmission reduced from 5% to 0.8% due to an increase in absorption from 0.04 mm⁻¹ to 0.1 mm⁻¹, based on capillary blood absorption. Adding an epicardial fat layer to normal myocardium total reflection and backscatter increased from 16% to 44%. Computer simulations for normal and pathological myocardium showed a wide diversity in light propagation characteristics, indicating the importance of including all myocardial tissues in light dosimetry analyses.     

Effects of super-pulsed Nd-YAG laser on arterial tissue: comparison with continuous wave

The effects of a super-pulsed Nd-YAG laser at 1.32 μm wavelength on normal or atherosclerotic human arterial tissue were evaluated and compared with those obtained with continuous wave. One joule per pulse was delivered through a 0.2 mm optical fibre with a pulse width of 10 ms at 10 Hz (super-pulse), or 10 W (10 J) were delivered at continuous wave in saline or blood. Ten joules were delivered with super-pulse or continuous wave for each tissue specimen. The aortic specimens were lased either by continuous wave or super-pulse. At super-pulse mode, ablation efficiency (mm³ J⁻¹) was 0.0149±0.0044 for normal tissue in saline, 0.0148±0.0043 for atheroma in saline, 0.0138±0.0062 for normal tissue in blood, and 0.0146±0.0049 for atheroma in blood. There was no significant difference between the groups. At continuous wave mode, ablation efficiency was 0.0507±0.0299 for atheroma in blood (p<0.001 vs super-pulse). However, extensive charring was observed with continuous wave lasing (41% with continuous against 14% with pulsed mode,p<0.001). Heavily calcified plaques were also ablated at 1.5 J per pulse and 15 W (continuous wave), resulting in extensive charring with continuous wave (77% vs 18% with super-pulse,p<0.01). In conclusion, at super-pulse mode, 1.32 μm Nd-YAG laser has neither the selectivity for atheroma nor influence of blood, thermal injury induced by super-pulse is less than that induced by continuous wave (cw), calcified plaques can be ablated by super-pulse, and super-pulsed Nd-YAG laser angioplasty is safer to use than continuous wave.     

Assessment of a new device for laser angioplasty

Experimental work has shown that a transparent laser device delivering pulsed energy to an artery results in a smaller area of surrounding damage than does an opaque device with a continuous wave laser. The combination of a transparent ball-tipped device with a pulsed Nd-YAG laser has been investigated. The system delivers pulses of 100 microseconds at a rate of 10 Hz and average energy of 0.5 J per pulse with an energy loss of 5-10% between the output at the laser rail and the fibre tip. The dose/response was measured and showed that on normal aorta under saline the device produces craters with a depth of 5 microns/J and 1.5 mm radius. There is a 100% increase in dose response with diseased aorta and a 50% increase when exposure is carried out under blood. The effect of a varying angle of incidence upon the arterial wall has been measured. Angulation of the device at 10 degrees from the perpendicular reduces the crater depth to 50%, as compared with a 50% reduction at 60 degrees using a bare fibre. As estimated with a thermal camera in air, the device heats up to a maximum of 50 degrees C during a 50 J exposure, compared to 110 degrees C after 5 J for the sapphire device. Artificial circulation experiments were carried out using diseased femoral vessels occluded by a ligature. The new system recanalised 100% of occlusion in straight vessels, and 40% of occlusions in curved vessels at a radius of 2.5 cm.(ABSTRACT TRUNCATED AT 250 WORDS)     

In vivo temperature measurement during transcatheter endomyocardial Nd-YAG laser irradiation in dogs

Intramural temperatures were monitored during catheter-directed endomyocardial non-contact laser irradiation by means of an array of thermocouples inserted epicardially. Via a novel electrode-laser catheter, a total of 30 Nd-YAG laser pulses, 1064 nm, 25 W/15 s, 20 W/30 s, 15 W/45 s,n = 10 each, were aimed at various endocardial sites in the ventricles of four dogs. Temperatures increased gradually after the onset of the laser pulses. Maximum values were measured in the central area of the irradiated spots: 100 ± 15 ‡C at 25 W, 78± 23 ‡C at 20 W and 80 ± 13 ‡C at 15 W. Application of 300 J of laser energy induced significantly higher temperatures at 25 W (p< 0.02) than at 20 or 15 W (20vs 15 W: p> 0.05). Initial temperature rise and lesion formation was accompanied by dwindling of local electrical potentials. There was no crater formation. Transcatheter non-contact Nd-YAG laser coagulation of healthy ventricular myocardium can be performed in a safe and controllable manner at power settings up to 20 W and laser energies up to 675 J.     

Interstitial laser photocoagulation: Evaluation of a 1320 nm Nd-YAG and an 805 nm diode laser: the significance of charring and the value of precharring the fibre tip

Interstitial laser photocoagulation (ILP) is a new percutaneous technique of thermal destruction (necrosis) of deep-seated tumours, using low power laser energy. Our purpose was to investigate: (i) the effects of different laser wavelengths on the extent of thermal damage produced; and (ii) the role of charring around the fibre tip during ILP. Forty-five normal Wistar rats (250–300 g) had ILP to their liver (exposed at laparotomy) by inserting a 400 μm optical fibre into the liver, and activating the laser at 1, 2 or 3W. This was performed at three laser wavelengths (1064 nm Nd-YAG, 1320 nm Nd-YAG, 805 nm diode) using a clean plane-cleaved fibre, and at two wavelengths (1064 nm and 1320 nm Nd-YAG) using a fibre with pre-charring at its tip. The 805 nm and 1320 nm laser wavelengths produced significantly greater necrosis than the 1064 nm, using a clean fibre tip (mean diameters at 2 W were 21.7 mm, 18.3 mm, 8 mm respectively). Pre-charring the fibre significantly increased the necrotic lesion size at 1064 nm (mean diameter at 2 W was 14.7 mm). Using more strongly absorbed wavelengths (805 nm and 1320 nm) and pre-charring the fibre tip give greater thermal damage during ILP, contrary to previously held views that the optimal wavelength for ILP was 1064 nm in the absence of charring.     

Cooled-tip diode laser catheter for improved catheter ablation of ventricular tachycardia.

Catheter ablation for the treatment of arrhythmias has evolved dramatically over the past two decades. Researchers have investigated alternative energy sources and catheter constructs to improve the efficacy and safety of catheter ablation. This study tested the hypothesis that a new prototype cooled-tip laser catheter used with a low-power diode laser would improve catheter ablation of ventricular tachycardias. Four mongrel dogs underwent a median sternotomy. The cooled-tip laser catheter was advanced into the left ventricle via the left carotid artery and positioned adjacent to the endocardium. Laser powers of 3 and 4 W were delivered at four exposure times to select areas of the endocardium. During application of laser energy, room-temperature saline was circulated through the catheter. At necropsy the hearts were examined and fixed in formalin for histologic examination. Gross examination of the endocardial surfaces showed no indication of crater or thrombus formation. Cross-section of the lesions revealed sharply demarcated, circular-shaped areas of coagulative necrosis extending into the mid-myocardium. Areas of coagulative necrosis were identified within the myocardium extending into the mid-myocardium and occasionally the subepicardium. A sharp line of demarcation was observed between the lesions and the surrounding normal myocardium. The results of this study showed that we could use surface cooling during slow laser heating to create large subsurface lesions with characteristics appropriate for treatment of ventricular tachycardia and little to no surface damage. We believe our catheter system addresses many of the previous issues with laser-based approaches.     

Temperature along the surface of modified fiber tips for Nd:YAG laser angioplasty

For laser angioplasty probes, the thermal properties of the probes will primarily determine their mechanism of action. We examined the absorption, temperature increase, and probe degradation of transparent contact probes (hemispherical contact probe and ball-shaped fibers) and metal laser probes coupled to a continuous-wave Nd-YAG laser. Temperature was recorded by means of thermocouples and the measurements were corrected for direct light absorption by the thermocouple. During 15 W, 1 s exposure, the peak temperature rise of the hemispherical contact probe in contact with tissue dropped from approximately 1,000 degrees C at the front end to below 45 degrees C (95% drop) at the lateral side. In contrast, during continuous exposure the peak temperature rise of metal laser probes in contact with tissue dropped from 560 degrees C at the front end to near 400 degrees C (30% drop) at the 5.5 mm proximal rear end. During exposure in blood or tissue, the transparent contact probes became contaminated. Their absorption increased from 5 to 33% and the probe deteriorated. Repeated use of metal laser probes in blood resulted in a higher temperature at the rear than at the front end due to backburing of the fiber. Owing to the large temperature drop along the surface of transparent contact probes, the area of thermal destruction is limited to the tissue in front of the probe, whereas along the entire surface of metal laser probes the tissue will be affected. The large difference between these temperature distributions should be respected during clinical application of the transparent contact probe and the metal laser probe.     

A new transluminal angioplasty for the treatment of arteriosclerosis obliterans

A Peripheral Simpson Atherectomy Catheter (PSAC), which permits transluminal excision and removal of diseased intima, has been tested experimentally and clinically. The device consists of a tubular housing and a cutting mechanism to excise the intima protruding into the housing. Experimentally, vascular clay model was used to estimate its efficacy. Multiple passages improved lumen size and inner smoothness, and there was some difficulty to treat curved models. Clinically, 9 lesions in 7 patients of iliofemoral atheromatous disease were treated with PSAC. Recanalization was achieved successfully except for two lesions, one a soft atheroma and the other a curved vessel. No angiographic dissections or acute arterial occlusions occurred. Despite the procedure was complicated, it seemed safe and effective. In order to improve this method, an angioplastic catheter which had a laser probe in its housing was newly designed. Nd-YAG laser was irradiated with this catheter to the atherosclerotic lesions which were induced by a high cholesterol diet in rabbit. It left smoother surface and less thermal injury than usual irradiation with contact laser probe, but it needed more energy. These results showed that PSAC can be an alternative transluminal angioplasty and the new designed laser catheter is useful, but more experiments are required to establish these procedures.     

Mechanism of CW Nd:YAG laser recanalization with modified fiber tips: influence of temperature and axial force on tissue penetration in vitro

Modified fiber tips are used for laser angioplasty of totally occluded peripheral arteries. It has not been established, however, to what extent the mechanism of action of various laser probes is optical, thermal, or mechanical. We examined transparant contact probes (hemispherical contact probes and ball-shaped fibers) and metal laser probes, coupled to a continuous-wave Nd-YAG laser. By using homogeneous thick porcine fatty tissue samples submerged in blood plasma, tissue penetration was measured in relation to the temperature of the probe and the axial force exerted on the tissue. By using 15 W, 1 s laser pulses, the surface of transparent contact probes had to be first contaminated by carbonized tissue particles to achieve tissue penetration. Penetration increased from 1 to 10 mm per pulse when axial force increased from 20 to 100 g. Metal probes had to be sufficiently insulated from the liquid environment by water vapour entrapped in a denatured protein layer to exceed the threshold temperature of 225 degrees C for tissue penetration. When axial force increased from 20 to 80 g at 10 W continuous exposure, the velocity of tissue penetration increased in the range from 1 to 4 mm/s. Tissue penetration by modified fiber tips is attributed to both remodeling and vaporization of tissue. With transparent contact probes, tissue is heated partly by direct light absorption and partly by a hot probe surface. Axially directed force is necessary to displace lateral non-ablated tissue and to overcome mechanical resistance. We conclude that mechanical dilation due to axial catherization force (Dotter effect) contributes substantially to tissue penetration by transparent contact probes.     

The contact neodymium-yttrium aluminum garnet laser. A new approach to arthroscopic laser surgery

Arthroscopic treatment of meniscal lesions has been modified as technological advances have occurred. However, alternatives to conventional arthroscopic cutting tools, including electrocautery and CO2 lasers, have thus far met with limited success. The recent development of a sapphire tip has enabled the use of the neodymium-yttrium aluminum garnet (Nd-YAG) laser in a contact mode in a saline medium. This study compares the biology of the Nd-YAG laser to that of electrocautery and scalpel techniques with respect to its effects on articular cartilage and the meniscus. The contact Nd-YAG laser has advantages over both scalpel and electrocautery with regard to its effects on articular cartilage. It also has significant biologic advantages over electrocautery for meniscal lesions. Although in its infancy in the clinical setting, the contact Nd-YAG laser represents the possible beginning of a new era for application of laser energy in arthroscopy.     

Comparison of fibre tips for percutaneous interstitial laser photocoagulation

This paper seeks to optimize the parameters of interstitial laser photocoagulation, and clarify controversies regarding the effect of three types of fibre tips used in percutaneous application on the size of thermal lesions produced. Bare-cut, pre-charred and conical diffused tips of 600 μm silica core fibre were compared using a diode laser (805 nm) applied in vitro using fresh porcine livers. Continuous wave laser radiation at 805 nm was applied at 1.0, 1.5, 2.0 and 2.5 W for 5, 10 and 15 min. The sizes of the resultant lesion, cavity and char were measured and analysed using the method of analysis of variance for statistical significance. It is concluded that the fibre tips do not significantly affect the lesion size (p> 0.05). Conical fibre tips produced less char when applied for 10 min or more at 2–2.5 W. Laser power and irradiation time affect the lesion size significantly (p< 0.05), and the size of the lesion produced depends on both laser power and irradiation time, not merely on the total laser energy applied.     

The bactericidal activity of pulsed Nd-YAG laser radiation in vitro

Pulsed Nd-YAG laser irradiation of bacteria has been suggested as a possible means of treating contaminated intra-oral sites although relatively few studies have been conducted. In this investigation, the antimicrobial activity of a pulsed Nd-YAG laser was assessed in vitro for a range of oral bacteria using several pulse energies and exposure durations. Pure cultures of each organism were lased in saline suspensions followed by standard colony counting techniques for test and control samples. Microbial inhibition was found to be organism-dependent and varied with energy dose and pulse energy. For all nine test species 120-mJ laser pulses proved more efficient than 80-mJ pulses, with 99.9% kills compared with around 90% kills after exposure to 1800 pulses. These killing activity levels compare favourably with those achieved with other lasers in vitro.     

Bare fibre low power Nd-YAG laser interstitial hyperthermia. Comparison between diffuser tip and non-modified tip. An in vitro study

Interstitial low power Nd-YAG laser hyperthermia is a new method with potential for the future treatment of localized neoplasms. A laser fibre is inserted into the tumour. Energy is applied and the tumour is destroyed by heat and phototoxicity. Since most tumours are spherical in shape, the optimal design of the laser fibre tip would be one which caused the light to spread uniformly in all directions. We describe a cone shaped forsted modification of the bare fibre proper (‘the diffuser tip’), that produces coagulation lesions of spherical shape. In vitro experiments show that the lesions were highly reproducible with respect to size and shape, and an excellent doseresponse relationship was observed. The largest lesion produced was achieved with 4 W for 1800 s and measured 44 mm in diameter. By comparison with lesions produced with the non-modified fibre (‘the endfire fibre’) it is concluded that the diffuser tip is superior to the endfire tip, since the latter produces cylinder shaped lesions. Based on these observations it seems that the diffuser tip will be advantageous, e.g. in the treatment of liver metastases.