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Background and Purpose:The therapeutic application of noninvasive tissue ablation by high-intensity focused ultrasound (HIFU) requires precise physical definition of the focal size and determination of control parameters. The objective of this study was to measure the extent of ex-vivoporcine kidney tissue ablation at variable generator parameters and to identify parameters to control lesion size.Materials and Methods:The ultrasound waves generated by a cylindrical piezoceramic element (1.04 MHz) were focused at a depth of 100 mm using a parabolic reflector (diameter 100 mm). A needle hydrophone was used to measure the field distribution of the sound pressure. The morphology and extent of tissue necrosis were examined at generator powers of up to 400 W (Pel) and single pulse durations of as long as 8 seconds.Results:The two-dimensional field distribution resulted in an approximately ellipsoidal focus of 32 × 4 mm (–6 dB). A sharp demarcation between coagulation necrosis and intact tissue was observed. Lesion size was controlled by both the variation of generator power and the pulse duration. At a constant pulse duration of 2 seconds, a generator power of 100 W remained below the threshold doses for inducing a reproducible lesion. An increase in power to as high as 400 W induced lesions with average dimensions of as much as 11.2 × 3 mm. At constant total energy (generator power × pulse duration), lesion size increased at higher generator power.Conclusions:This ultrasound generator can induce defined and reproducible necrosis in ex-vivokidney tissue. Lesion size can be controlled by adjusting the generator power and pulse duration. Generator power, in particular, turned out to be a suitable control parameter for obtaining a lesion of a defined size.

Background and Purpose: Endoscopic examinations of stone-forming kidneys show a coincidence of plaques and microliths on the surface of and within papillary epithelial tissue. These calcifications are thought to be precursors of calcium oxalate urolithiasis. We hypothesized that minimally invasive endoscopic laser ablation of microliths and necrotic cell layers enables epithelial regeneration and prevents recurrent urolithiasis. The aim of this study was to determine the most suitable laser type and dose intensity for selective superficial cell ablation.Materials and Methods: Conventional Nd:YAG (1-40 W) or Ho:YAG (0.5-3 J/single impulse) lasers were used endoscopically on an ex vivoblood-perfused porcine kidney model. Defined doses were applied to the papillary surface in the contact and noncontact modes for 10 to 30 seconds. Papillae were excised after treatment and histopathologically analyzed in continuous sections. Lesions were microscopically assessed with the aid of a Leica Quantimed computer program.Results: Depending on the time and dose, vaporization by the Nd:YAG laser caused large tissue defects and coagulation necrosis at energy levels over 5 W (contact and noncontact mode). Lower energy levels with tissue contact produced only superficial cell defects (<20 cell layers) but more extensive coagulation necrosis, whereas no histologic effects were observed at the same energy level without contact. In contrast, independent of delivered energy but dependent on time, Ho:YAG laser application caused pure tissue loss without relevant coagulation necrosis. The generation of small lesions (6-10 cell layers) without tissue contact was possible at energy levels under 2 J.Conclusions: Selective superficial papillary cell ablation is possible. Low-energy Nd:YAG treatment in the contact mode and Ho:YAG treatment in the noncontact mode led to superficial vaporization with no (Ho:YAG) or minimum (Nd:YAG) coagulation defects.

Background and Purpose: Interstitial laser application is successful for clinical tissue ablation in various organs. One great drawback of this method is the high costs caused by the vulnerability of the probes. Our objective was to investigate whether the prevention of carbonization deposits on laser probes averts reduced transmission after the application of high laser energy.Materials and Methods: Interstitial laser probes were used to pierce an in vitrokidney model, and 9900 J of energy (Nd:YAG laser) was applied. The transmission of the laser probe was determined and compared with that of a reference probe.Results: The application of laser energy, up to a total of 49,500 J, did not reduce transmission. The deviation of the transmission comparing working probes and the reference probe was between 2% and 7%. The application of high energy (40 W) over a longer period (> 20 seconds) led to carbonization and consequent distinct reduction of transmission.Conclusions: The durability of laser probes can be prolonged by preventing carbonization. The economic effect achieved is the repeated use of the probes at less expense.