Thermal Tissue Damage Research Articles

Thermal effects of laser-osteotomy on bone: Mathematical computation using maple

In recent years, interest in medical application of lasers especially as a surgical alternative is considerably increasing due to their distinct advantages such as non-contact intervention, bacteriostasis, less traumatization, minimal invasiveness, decreased bleeding and less heat damage. The present study aimed to evaluate the temperature changes and the consequent released thermal stress in cortical bone caused by an Erbium:yttrium aluminum garnet (Er:YAG) laser (Fideliss 320A, Fotona Inc., Deggingen, Germany) during osteotomy, using mathematical computation by means of Maple software, version 9.5 (Maplesoft, a division of Waterloo Maple Inc., Canada). The results obtained here were compared with the experimental measurements using Er:YAG laser in the osteotomy clinics. A bone slab with thickness of 1 mm was simulated in Maple software. Then, an Er:YAG laser emitting 100 μs pulses at a wavelength of 2940 nm were modeled. Two different clinical settings of the Er:YAG laser with 200 mJ and 400 mJ energies, both with 100 μs exposure and 500 μs silence were studied. To investigate the temperature distribution in the cortical bone, the time-dependent heat conduction equations were defined and solved in the Maple software. Finally, by defining the heat distribution function in the Maple, thermal stress in the bone was investigated. Results of the computations showed that, on the bone irradiated area (center of the bone surface) the maximum temperature rise was 0.8°C and 1.6°C, for 200 mJ and 400 mJ Er:YAG laser exposure, respectively. The temperature rise reached to its minimum at radial distances of 1.2 cm from the point of irradiated area for 200 mJ laser while it was 1.5 cm for 400 mJ laser. For 200 mJ laser the maximum derived radial (σ rr ), axial (σ zz ) and azimuthally (σ θθ ) stress components were 0.20, 0.16 and 0.08 MPa, respectively. While, for 400 mJ laser the maximum derived σ rr , σ zz and σ θθ stress components were 0.39, 0.31 and 0.16 MPa, respectively. These results confirm that use of 100 μs Er:YAG laser pulses with 500 μs silence at 200 and 400 mJ energies minimizes thermal tissue damage for the laser osteotomies, without continued water cooling (irrigation) on the exposed area.

MRI-Induced Tissue Heating at Metallic Sutures (Cerclages)

Magnetic resonance imaging (MRI) has become an important diagnostic tool with an ongoing dynamic development towards application of increasing static magnetic flux densities and consequently, exposures to electromagnetic fields (EMF) of increasing radio frequencies (RF). This raises particular concern metallic implants could lead to excess tissue heating and consequently, to thermal tissue damage. In thorax surgery the intersected sternum is reconnected by metallic sutures (cerclages). To investigate whether patients with such implants can be accepted for MRI and whether there may be limitations with regard to static magnetic fields, by numerical anatomical and thermal modelling MRI induced tissue heating was assessed for magnetic flux densities 1.5 T, 3 T, 4 T and 7 T. Results show that overall tissue temperature increased with increasing RF EMF frequency. However, even for setting MRI exposure parameters at maximum permissible level partial body heating remained marginally affected and even at local level the additional contribution of the presence of the metallic cerclage remained below 1°C. This allows concluding that from a heating point of view metallic sutures as used to fix the sternum after thorax surgery are no contraindication for MRI with static magnetic flux densities up to 7 T.

Finite element analysis of thermal and acoustic processes during laser tattoo removal

Q-switched laser therapy is commonly used for the removal of tattoos. However, despite ever increasing demand for this intervention, a better understanding of the mechanisms that result in pigment reduction is required in order to optimise outcomes and reduce the number of treatment episodes. A finite element analysis computer simulation was developed to model the fragmentation response of ink granules during irradiation of a professional black tattoo using a Q-switched Nd:YAG laser. Thermal and acoustic mechanisms were considered, allowing the optimal laser settings to be predicted throughout the course of treatment. Changes in the thermal properties of the ink during heating were taken into account to improve the reliability of the results obtained. The simulated results are in close agreement with clinical observations. Thermal fragmentation was shown to be the dominant mechanism in pigment reduction when using a 6 nanoseconds pulse at 1,064 nm. In order to provide maximum clearance whilst maintaining acceptable levels of tissue thermal damage, later treatments were shown to benefit from higher fluence levels than initial treatments. Larger spot diameters were also preferable throughout the course of treatment. The results from the simulation build upon previous work carried out in the field, applying ink thermal coefficients which vary with temperature for the first time. These results compliment clinical knowledge, suggesting that a proactive increase in fluence during a course of treatments is likely to improve the response to laser therapy.

Effect of Locally Applied Electricity On Clot Formation and Hemostasis.

AbstractAbstract 2220 Background:Electricity has historically been used in medical applications such as defibrillators, cauterization, and electrosurgery. Additionally, Kalghatgi et al. [1] demonstrate that high voltage electrical fields (∼30 kilo volts) activate platelets and induce coagulation. However, the exact effect of applied electrical current on clot formation is unknown. We show that upon direct application of electricity (voltages between 5 – 40 volts), platelets are activated immediately and clots rapidly form without excessive heating. This newfound application to induce blood clot formation may enable a new and novel class of therapeutics to achieve hemostasis at sites of bleeding. As newer hemostatic agents are currently derived from animal or human products, which carry risk of blood borne infections and immune dysregulation, a clear need exists for novel therapies to achieve hemostasis. Method:Our experimental setup consisted of a silicone polydimethylsiloxane (PDMS) chamber with embedded metallic wires (Figure 1A). The metallic leads were connected to an Agilent E3649A variable power supply and a constant voltage was applied to the chamber for 1 minute. A Fluke 179 multimeter monitored the temperature with a thermocouple lead inserted into the liquid in the chamber. Experiments were conducted using whole human blood, platelet rich plasma (PRP), platelet poor plasma (PPP), and isolated, washed platelets. Additionally, fluorescently labeled fibrinogen (Alexa Fluor 488) was added to the blood product, 5%v/v, to assess fibrin formation. Experiment:Initial experiments characterized the electrical characteristics of the different components of blood pertinent for clot formation to assess the potential safety concerns. Voltages between 5 – 40 volts (V) result in currents below 0.1 amps (A) and temperatures between 20 – 50°C. Nominally, current values of 0.1A and greater are considered deadly [2], and thermal tissue damage caused by temperatures below 45°C are considered reversible [2], therefore the majority of our work focuses on voltages less than 30V. Figure 1B shows the timeline of fibrin network formation for a control whole blood sample versus a whole blood sample exposed to 30V for 60 seconds. At 30V and immediately after electrical stimulus, platelet aggregation begins to form. At 120 seconds, fibrin polymerization initiates and showed complete coverage at 180 seconds, as well as numerous clusters of platelet aggregates. In the absence of electrical stimulus, no fibrin polymerization or platelet aggregation was detected until greater than 240 seconds and full network coverage was complete by 420 second. Platelet aggregration was more pronouced with electrical stimulus, as compared to the control case. Various voltages were tested with the mean time to complete the fibrin network formation in stimulated and unstimulated whole blood was 170 seconds vs. 320 seconds, respectively (n = 3) indicating a 53% increase in fibrin formation and platelet aggregation. Additional experiments were conducted on anti-coagulated PPP, PRP, and isolated, washed platelets showing no evidence of fibrin polymerization. This suggests that all the components in blood are necessary to create the fibrin scaffold when exposed to electricity. Continued work will focus on unraveling the underlying mechanisms of how electrical stimulation affects platelet aggregation and coagulation. Conclusion:Our results suggest that direct electrical stimulation promotes clot formation and could potentially lead to a new category of hemostatic therapies that are free from the infectious risks and immune effects that encumber current human or animal-derived agents. With the addition of electrical stimulus, fibrin networks form on average 53% faster than control conditions. We anticipate this concept of applying electricity to different processes in the blood will have significant implications for experimental and clinical hematology. [Display omitted] Disclosures:No relevant conflicts of interest to declare.

Comparison of lateral thermal damage of the human peritoneum using monopolar diathermy, Harmonic scalpel and LigaSure

New hemostatic technologies are often employed in open and laparoscopic surgery to reduce duration of surgery and complications. Monopolar diathermy, Harmonic scalpel and LigaSure are routinely used in open and laparoscopic surgery for tissue cutting and hemostasis. We compared lateral thermal damage following in vivo application of 3 commonly used instruments. We used monopolar diathermy, Harmonic scalpel and LigaSure to coagulate and divide the peritoneum of patients who underwent median laparotomy. After anesthesia, median supraumbilical laparotomy was performed, and the peritoneum of each patient was coagulated using different devices. Using light microscopy and morphometric imaging analysis, the width of tissue lateral thermal damage was measured from the point of the peritoneal incision. We included 100 patients in our study. After a peritoneal incision, the mean lateral thermal damage of monopolar diathermy, Harmonic scalpel (output power 3), Harmonic scalpel (output power 5) and LigaSure were 215.79 μm, 90.42 μm, 127.48 μm and 144.18 μm, respectively. The degree of lateral thermal spread varied by instrument type, power setting and application time. LigaSure and Harmonic scalpel were the safest and most efficient methods of tissue coagulation. Monopolar diathermy resulted in the greatest degree of thermal damage in tissues.

The influence of water/air cooling on collateral tissue damage using a diode laser with an innovative pulse design (micropulsed mode)—an in vitro study

Since the diode laser is a good compromise for the daily use in dental offices, finding usage in numerous dental indications (e.g., surgery, periodontics, and endodontics), the minimization of the collateral damage in laser surgery is important to improve the therapeutical outcome. The aim of this study was to investigate the effect of water/air cooling on the collateral thermal soft tissue damage of 980-nm diode laser incisions. A total of 36 mechanically executed laser cuts in pork liver were made with a 980-nm diode laser in micropulsed mode with three different settings of water/air cooling and examined by histological assessment to determine the area and size of carbonization, necrosis, and reversible tissue damage as well as incision depth and width. In our study, clearly the incision depth increased significantly under water/air cooling (270.9 versus 502.3 μm-test group 3) without significant changes of incision width. In test group 2, the total area of damage was significantly smaller than in the control group (in this group, the incision depth increases by 65 %). In test group 3, the total area of damage was significantly higher (incision depth increased by 85 %), but the bigger part of it represented a reversible tissue alteration leaving the amount of irreversible damage almost the same as in the control group. This first pilot study clearly shows that water/air cooling in vitro has an effect on collateral tissue damage. Further studies will have to verify, if the reduced collateral damage we have proved in this study can lead to accelerated wound healing. Reduction of collateral thermal damage after diode laser incisions is clinically relevant for promoted wound healing.

The comparison of thermal tissue injuries caused by ultrasonic scalpel and electrocautery use in rabbit tongue tissue

The aim of this study compares to the increase in tissue temperature and the thermal histological effects of ultrasonic scalpel, bipolar and unipolar electrosurgery incisions in the tongue tissue of rabbits. This study evaluates the histopathological changes related to thermal change and the maximum temperature values in the peripheral tissue brought about by the incisions carried out by the three methods in a comparative way. To assess thermal tissue damage induced by the three instruments, maximum tissue temperatures were measured during the surgical procedure and tongue tissue samples were examined histopathologically following the surgery. The mean maximum temperature values of the groups were 93.93±2.76 Cº for the unipolar electrocautery group, whereas 85.07±5.95 Cº for the bipolar electrocautery group, and 108.23±7.64 Cº for the ultrasonic scalpel group. There was a statistically significant relationship between the increase in maximum temperature values and the separation among tissue layers, edema, congestion, necrosis, hemorrhage, destruction in blood vessel walls and fibrin accumulation, and between the existence of fibrin thrombus and tissue damage depth (p<0.05). It was concluded that the bipolar electrocautery use gives way to less temperature increase in the tissues and less thermal tissue damage in comparison to the other methods.

Numerical simulation of endovenous laser treatment of the incompetent great saphenous vein with external air cooling

Endovenous laser treatment (ELT) has been proposed as an alternative in the treatment of reflux of the great saphenous vein. Before the procedure, peri-saphenous subcutaneous tumescent saline solution infiltration is usually performed. However, diffusion of this tumescent fluid is rapidly observed and can potentially reduce the efficacy as a heat sink. External skin cooling with cold air was proposed as an alternative solution. The objective of this study is to compare endovenous laser treatment without and with air cooling by realistic numerical simulations. An optical-thermal damage model was formulated and implemented using finite element modeling. The general model simulated light distribution using the diffusion approximation of the transport theory, temperature rise using the bioheat equation, and laser-induced injury using the Arrhenius damage model. Parameters, used in clinical procedures, were considered: power, 15 W; pulse duration, 1 s; fiber pull back, 3-mm increments every second; cold air applied in continuous mode during ELT; and no tumescent anesthesia. Simulations were performed for vein locations at 5, 10, and 15 mm in depth, with and without air cooling. For a vein located at 15 mm in depth, no significant difference was observed with and without cooling. For a vein located at 10 mm in depth, surface temperature increase up to 45 °C is observed without cooling. For a vein located at 5 mm, without cooling, temperature increase leads to irreversible damage of dermis and epidermis. Conversely, with air cooling, surface temperature reaches a maximum of 38 °C in accordance with recordings performed on patients. ELT of the incompetent great saphenous vein with external air cooling system is a promising therapy technique. Use of cold air on the skin continuously flowing in the area of laser shot decreased significantly the heat extent and the thermal damage in the perivenous tissues and the skin.

Design of Laser Treatment Protocols for Bacterial Disinfection in Root Canals Using Theoretical Modeling and MicroCT Imaging

Theoretical simulations of temperature elevations in root dentin are performed to evaluate, how heating protocols affect the efficacy of using erbium, chromium; yttrium, scandium, gallium, garnet (Er,Cr;YSGG) pulsed lasers for bacterial disinfection during root canal treatments. The theoretical models are generated based on microcomputer tomography (microCT) scans of extracted human teeth. Heat transfer simulations are performed using the Pennes bioheat equation to determine temperature distributions in tooth roots and surrounding tissue during 500 mW pulsed Er,Cr;YSGG laser irradiation on the root canal for eradicating bacteria. The study not only determines the heat penetration within the deep dentin but also assesses potential thermal damage to the surrounding tissues. Thermal damage is assumed to occur when the tissue is subject to a temperature above at least 47 °C for a minimum duration of 10 s. Treatment protocols are identified for three representative tooth root sizes that are capable of maintaining elevated temperatures in deep dentin necessary to eradicate bacteria, while minimizing potential for collateral thermal tissue damage at the outer root surfaces. We believe that the study not only provides realistic laser heating protocols for various tooth root geometries but also demonstrates utility of theoretical simulations for designing individualized treatments in the future.

O-030 Laser thermotherapy of malignant brain lesions using MRI for needle guidance and real-time temperature mapping

PurposeMinimally invasive techniques for treatment of malignant brain lesions when surgically unresectable continue to emerge. We describe neurointerventional surgery with MR-guided laser-induced interstitial thermotherapy of glioblastomas & metastases using intra-operative...

TH‐C‐217BCD‐01: Best in Physics (Imaging) ‐ Evaluation of Apoptosis and Proliferation in Non‐Thermal Pulsed HIFU Treated Mouse Prostate Tumors

Purpose: The underlying mechanism of non‐thermal effects of pulsed high intensity focused ultrasound (pFUS) on normal and tumor tissues is not well understood. Our recent studies showed significant prostate cancer cell killing in vitro and significant prostate tumor growth delay in vivo after pFUS treatment. We hypothesized that these effects of non‐thermal pFUS are due to an increase in cell apoptosis and decrease in cell proliferation. Therefore, the purpose of this study was to analyze the biomarkers of apoptosis and proliferation in prostate tumors after pFUS exposures. Methods: An orthotopic prostate tumor model was established in nude mice. Prostate tumors were sonicated with MR guided pFUS (1MHz, 5W acoustic power, 5Hz frequency; 0.1duty cycle) for 60 sec for each sonication with temperature &lt;42°C (InSightec ExAblate 2000 with a 1.5T GE MR scanner). Untreated tumors were used as controls. The mice were sacrificed at predetermined times up to 7 days following therapy. Tumors were processed for light microscopic examination with H&amp;E staining and immunohistochemical staining for caspase 3 (a marker of apoptosis) and Ki67 (aproliferation marker) expression. Results: Light microscopy revealed the absence of thermal damage and acute destruction of tumor tissues exposed to pFUS. The microvessel walls in the tissues remained intact. There was no change in the extent of hemorrhage upon pFUS treatment over time. The apoptotic index, defined as a percentage of apoptotic cells per total number of cells, peaked at 24 hours after FUS treatment relative to control. There was no dramatic difference in the proliferation indices between different time points. Conclusions: Our results suggested that non‐thermal pulsed pFUS induced caspase 3‐mediated apoptosis and did not produce any thermal damage in the prostate tumor tissues. We are performing additional studies to evaluate blood vessel density and cellular DNA damage upon pFUS treatment.

63 Breast Lesion Excision System for Diagnosis of Suspicious Non-palpable Breast Lesions: Does Thermal Tissue Damage Affect Diagnosis and Outcome?

63 Breast Lesion Excision System for Diagnosis of Suspicious Non-palpable Breast Lesions: Does Thermal Tissue Damage Affect Diagnosis and Outcome?

Reduction of thermal tissue damage caused by bipolar radiofrequency-induced thermofusion

Reduction of thermal tissue damage caused by bipolar radiofrequency-induced thermofusion

Stereotaxie und Robotik zur Ablation – Spielzeug oder Werkzeug?

CLINICAL AND METHODOLOGICAL ISSUES: Ablative technologies allow local curative tumor treatment by thermal tissue damage. An important prerequisite is the coverage of all tumor cells. Tumor size is the most important limiting factor. The drawbacks of conventional computed tomography/ultrasound/magnetic resonance imaging (CT/US/MRI) guided radiofrequency ablation (RFA) are the absence of planning software, imprecise probe placement, imprecise control of probe placement and the ablation zone as well as the lack of reliability and reproducibility. Stereotactic and robot-assisted systems allow planning of multiple probe positions based on CT/MRI and positron emission tomography (PET) planning data. The probes can be precisely placed according to the coordinates of the image datasets. The 1 and 3 year survival rates after stereotactic RFA (SRFA) of cholangiocellular carcinoma were 91% and 70% respectively and the median overall survival was 60 months. After SRFA of 189 colorectal liver metastases in 63 patients there was no significant difference in local recurrence rates between tumors < 3cm (17.7%), 3-5cm (11.1%) and > 5cm (17.4%). The median overall survival was 33.2 months and the 1, 3, and 5 year overall survival rates after SRFA in patients with resectable colorectal cancer were 92%, 66% and 48%, respectively. In our opinion the excellent and, to a large extent user-independent results justify the increased efforts in time and costs especially for the treatment of patients with large and irregular tumors. Stereotaxy and robotics are valuable tools for effective tumor ablation especially of large tumors and are likely to gain in importance in the next few years.

Single-sided and small-scaled grasping of delicate tissues: Effectiveness of indirect heat-induced attachment and detachment

Introduction: Indirect heat-induced attachment and detachment (iHIAD) is a promising concept for gripping delicate tissues in microsurgery. However, the optimal settings of iHIAD are unknown. This study evaluates the effects of the instrument heating properties and initial contact force on the adhesion force, detachment success and thermal damage. Material and methods: An instrument prototype was developed to test attachment and detachment for different combinations of generated energy (3.5–20.0 mJ) and pulse length (0.25–2.50 ms). The tissues tested on were kidney and eye from the pig. Thermal tissue damage was estimated with a histological analysis. Results: The adhesion force Fa depended strongly on the amount of generated energy (Fa = 0.03–2.5 mN) and contact force (Fa = 0.25–1.0 mN). Pulse length played a minor role. Detachment success (0–100%) was determined by generated energy (3–16 mJ). Histological analysis showed minimal damage. Conclusion: Adhesion forces increased with increasing contact forces. iHIAD proved sensitive to heating characteristics. Detachment success increased with generated energy. Thermal damage was minimal. iHIAD creates a potential to build better performing tissue manipulators.

Management of heat in laser tissue welding using NIR cover window material

Laser tissue welding (LTW) is a novel method of surgical wound closure by the use of laser radiation to induce fusion of the biological tissues. Molecular dynamics associated with LTW is a result of thermal and non-thermal mechanisms. This research focuses exclusively on better heat management to reduce thermal damage of tissues in LTW using a near infrared laser radiation. An infrared continuous-wave (CW) laser radiation at 1,450 nm wavelength corresponding to the absorption band from combination vibrational modes of water is used to weld together ex vivo porcine aorta. In these studies we measured the optimal laser power and scan speed, for better tensile strength of the weld and lesser tissue dehydration. Significant amount of water loss from the welded tissue results in cellular death and tissue buckling. Various thermally conductive optical cover windows were used as heat sinks to reduce thermal effects during LTW for the dissipation of the heat. The optimal use of the method prevents tissue buckling and minimizes the water loss. Diamond, sapphire, BK7, fused silica, and IR quartz transparent optical cover windows were tested. The data from this study suggests that IR-quartz as the material with optimal thermal conductivity is ideal for laser welding of the porcine aorta.

Mathematical models of laser-induced tissue thermal damage

Laser sources are under increasing study for in vivo tumour ablation. Photo-thermal ablation in tissues varies tremendously in governing physical phenomena, depending on wavelength, owing to wide variation in the optical properties of tissues, specifically the dominant chromophore and degree and type of scattering. Once converted into local tissue heating, however, the governing thermodynamic principles remain the same. Observed irreversible thermal alterations range from substantial structural disruption due to steam evolution in high temperature short-term activations to low temperature rise, longer-term initiation of the complex protein cascades that result in apoptosis and/or necroptosis. The usual mathematical model in hyperthermia studies, the thermal isoeffect dose, arising from the relative reaction rate formulation, is not an effective description of the higher temperature effects because multiple processes occur in parallel. The Arrhenius formulation based on the theory of absolute reaction rates is much more useful and descriptive in laser heating since the multiple thermodynamically independent processes may be studied separately.

Urinary Complications in Rectal Cancer Patients are Related to the Dissection Tool

To assess the incidence and risk of urinary complications after anterior rectal cancer resection with regard to the surgical device used for total mesorectal excision (TME). During the years 2004-2009 we operated 374 rectal cancer patients with TME and the intent of autonomic nerves sparing intent. Seventeen patients underwent mesorectal dissection with ultrasound scalpel (US). They were compared to the control series of 35 cases selected from the patients for whom electrocautery was used. Selection was done in the manner to eliminate any other significant differences between groups. Intraoperative complications, postoperative mortality, anastomotic leakage and infectious complications did not occur. Urinary bladder disturbances developed in US group in 1 patient (6%) while in 12 patients (34%) in EC group (p<0.05). In US group the character of complication was transient stress incontinence with symptoms being significantly reduced during six postoperative months. In EC group two patients had dysuria, two nycturia, one had both. Stress incontinence occurred in six patients, complete incontinence requiring catheterization in one. When compared to EC, TME with US is related to lower risk of urinary complications and facilitates autonomic nerve preservation due to minimized thermal lateral tissue damage.

Reduction of collateral thermal impact of diode laser irradiation on soft tissue due to modified application parameters

The aim of this study was to investigate the effect of different working modes (pulsed and micropulsed) and power settings of a standardized 980-nm diode laser on collateral thermal soft-tissue damage. A total of 108 bovine liver samples were cut with a diode laser at various settings in pulsed and micropulsed mode and histologically assessed to determine the area and depth of carbonization, necrosis and reversible tissue damage, as well as incision depth and width. Incision depth and width and the area and depth of carbonization, necrosis and reversible damage were correlated strongly with cutting speed. The area and depth of reversible damage were correlated with average power. The micropulsed mode produced a smaller zone of carbonization and necrosis and a smaller incision width. Setting the laser parameters in accordance with the absorption characteristics of the tissue reduced collateral thermal tissue damage while maintaining an acceptable cutting ability. Reducing collateral thermal damage from diode laser incisions is clinically relevant for promoting wound healing.

Acoustic radiation force applied to the treatment of lower pole kidney stones

Lower pole kidney stones have lower rates of clearance after shock wave lithotripsy or ureteroscopy compared to other stone locations. Residual stone fragments, post treatment, often lead to additional morbidity and secondary surgery. We describe the use of acoustic radiation force created by transcutaneous focused ultrasound to manipulate the location of stone fragments within the collecting system in order to facilitate their passage. Artificial and human stones were placed in the lower pole of kidneys of a porcine animal model. An open architecture, software based diagnostic ultrasound system and scanhead were modified to produce roughly 100 μs pulses of up to 16 MPa peak positive pressure in water. Stone motion was observed in real-time with simultaneous imaging through the same scanhead and with fluoroscopy. All stones were seen to move. Stone velocities were on the order of 1 cm/s. Stone displacement distance was up to 3 cm, and operators could generally control the direction of stone movement. No evidence of thermal necrosis or mechanical damage of renal tissue was observed. Thus acoustic radiation force can be used to facilitate lower pole stone fragment clearance. [Work supported by NIH DK43881, DK086371, DK092197, and NSBRI through NASA NCC 9-58.]