Subsurface tissue lesions created using an Nd:YAG laser and a sapphire contact cooling probe

Feasibility of Monitoring Tissue Properties During Microcirculation Disorder Using a Compact Fiber-Based Probe With Sapphire Tip

One of the urgent tasks of modern medicine is to detect microcirculation disorder during surgery to avoid possible consequences like tissue hypoxia, ischemia, and necrosis. To address this issue, in this article, we propose a compact probe with sapphire tip and optical sensing based on the principle of spatially resolved diffuse reflectance analysis. It allows for intraoperative measurement of tissue effective attenuation coefficient and its alteration during the changes of tissue condition, caused by microcirculation disorder. The results of experimental studies using (1) a tissue-mimicking phantom based on lipid emulsion and hemoglobin and (2) a model of hindlimb ischemia performed in a rat demonstrated the ability to detect rapid changes of tissue attenuation confirming the feasibility of the probe to sense the stressful exposure. Due to a compact design of the probe, it could be useful for rather wide surgical operations and diagnostic purposes as an auxiliary instrument.

Computer simulations of thermal tissue remodeling during transvaginal and transurethral laser treatment of female stress urinary incontinence

BACKGROUND AND OBJECTIVES

A non-surgical method is being developed for treating female stress urinary incontinence by laser thermal remodeling of subsurface tissues with applied surface tissue cooling. Computer simulations of light transport, heat transfer, and thermal damage in tissue were performed, comparing transvaginal and transurethral approaches.

STUDY DESIGN/MATERIALS AND METHODS

Monte Carlo (MC) simulations provided spatial distributions of absorbed photons in the tissue layers (vaginal wall, endopelvic fascia, and urethral wall). Optical properties (n,μa ,μs ,g) were assigned to each tissue at λ = 1064 nm. A 5-mm-diameter laser beam and incident power of 5 W for 15 seconds was used, based on previous experiments. MC output was converted into absorbed energy, serving as input for finite element heat transfer simulations of tissue temperatures over time. Convective heat transfer was simulated with contact probe cooling temperature set at 0°C. Variables used for thermal simulations (κ,c,ρ) were assigned to each tissue layer. MATLAB code was used for Arrhenius integral thermal damage calculations. A temperature matrix was constructed from ANSYS output, and finite sum was incorporated to approximate Arrhenius integral calculations. Tissue damage properties (Ea ,A) were used to compute Arrhenius sums.

RESULTS

For the transvaginal approach, 37% of energy was absorbed in the endopelvic fascia target layer with 0.8% deposited beyond it. Peak temperature was 71°C, the treatment zone was 0.8-mm-diameter, and 2.4 mm of the 2.7-mm-thick vaginal wall was preserved. For transurethral approach, 18% energy was absorbed in endopelvic fascia with 0.3% deposited beyond the layer. Peak temperature was 80°C, treatment zone was 2.0-mm-diameter, and 0.6 mm of 2.4-mm-thick urethral wall was preserved.

CONCLUSIONS

Computer simulations suggest that transvaginal approach is more feasible than transurethral approach. Lasers Surg. Med. 49:198-205, 2017. © 2016 Wiley Periodicals, Inc.

Diffusing, Side-Firing, and Radial Delivery Laser Balloon Catheters for Creating Subsurface Thermal Lesions in Tissue

Infrared lasers have been used in combination with applied cooling methods to preserve superficial skin layers during cosmetic surgery. Similarly, combined laser irradiation and tissue cooling may also allow development of minimally invasive laser therapies beyond dermatology. This study compares diffusing, side-firing, and radial delivery laser balloon catheter designs for creation of subsurface lesions in tissue, ex vivo, using a near-IR laser and applied contact cooling. An Ytterbium fiber laser with 1075 nm wavelength delivered energy through custom built 18 Fr (6-mm-OD) balloon catheters incorporating either 10-mm-long diffusing fiber tip, 90 degree side-firing fiber, or radial delivery cone mirror, through a central lumen. A chilled solution was flowed through a separate lumen into 9-mm-diameter balloon to keep probe cooled at 7°C. Porcine liver tissue samples were used as preliminary tissue model for immediate observation of thermal lesion creation. The diffusing fiber produced subsurface thermal lesions measuring 49.3 ± 10.0 mm² and preserved 0.8 ± 0.1 mm of surface tissue. The side-firing fiber produced subsurface thermal lesions of 2.4 ± 0.9 mm² diameter and preserved 0.5 ± 0.1 mm of surface tissue. The radial delivery probe assembly failed to produce subsurface thermal lesions, presumably due to the small effective spot diameter at the tissue surface, which limited optical penetration depth. Optimal laser power and irradiation time measured 15 W and 100 s for diffusing fiber and 1.4 W and 20 s, for side-firing fiber, respectively. Diffusing and side-firing laser balloon catheter designs provided subsurface thermal lesions in tissue. However, the divergent laser beam in both designs limited the ability to preserve a thicker layer of tissue surface. Further optimization of laser and cooling parameters may be necessary to preserve thicker surface tissue layers.

Laser Treatment of Female Stress Urinary Incontinence: Optical, Thermal, and Tissue Damage Simulations

Treatment of female stress urinary incontinence (SUI) by laser thermal remodeling of subsurface tissues is studied. Light transport, heat transfer, and thermal damage simulations were performed for transvaginal and transurethral methods. Monte Carlo (MC) provided absorbed photon distributions in tissue layers (vaginal wall, endopelvic fascia, urethral wall). Optical properties (n,μ_a,μ_s,g) were assigned to each tissue at λ=1064 nm. A 5-mm-diameter laser beam and power of 5 W for 15 s was used, based on previous experiments. MC output was converted into absorbed energy, serving as input for ANSYS finite element heat transfer simulations of tissue temperatures over time. Convective heat transfer was simulated with contact cooling probe set at 0 °C. Thermal properties (κ,c,ρ) were assigned to each tissue layer. MATLAB code was used for Arrhenius integral thermal damage calculations. A temperature matrix was constructed from ANSYS output, and finite sum was incorporated to approximate Arrhenius integral calculations. Tissue damage properties (E_a,A) were used to compute Arrhenius sums. For the transvaginal approach, 37% of energy was absorbed in endopelvic fascia layer with 0.8% deposited beyond it. Peak temperature was 71°C, treatment zone was 0.8-mm-diameter, and almost all of 2.7-mm-thick vaginal wall was preserved. For transurethral approach, 18% energy was absorbed in endopelvic fascia with 0.3% deposited beyond it. Peak temperature was 80°C, treatment zone was 2.0-mm-diameter, and only 0.6 mm of 2.4-mm-thick urethral wall was preserved. A transvaginal approach is more feasible than transurethral approach for laser treatment of SUI.

Comparison of Four Lasers (λ = 650, 808, 980, and 1075 nm) for Noninvasive Creation of Deep Subsurface Lesions in Tissue

Lasers have been used in combination with applied cooling methods to preserve superficial skin layers (100's μm's) during cosmetic surgery. Preservation of a thicker tissue surface layer (millimeters) may also allow development of other noninvasive laser procedures. We are exploring noninvasive therapeutic laser applications in urology (e.g. laser vasectomy and laser treatment of female stress urinary incontinence), which require surface tissue preservation on the millimeter scale. In this preliminary study, four lasers were compared for noninvasive creation of deep subsurface thermal lesions. Laser energy from three diode lasers (650, 808, and 980 nm) and a Ytterbium fiber laser (1075 nm) was delivered through a custom built, side-firing, laser probe with integrated cooling. An alcohol-based solution at -5^°C was circulated through a flow cell, cooling a sapphire window, which in turn cooled the tissue surface. The probe was placed in contact with porcine liver tissue, ex vivo, kept hydrated in saline and maintained at ~ 35^°C. Incident laser power was 4.2 W, spot diameter was 5.3 mm, and treatment time was 60 s. The optimal laser wavelength tested for creation of deep subsurface thermal lesions during contact cooling of tissues was 1075 nm, which preserved a surface layer of ~ 2 mm. The Ytterbium fiber laser provides a compact, low maintenance, and high power alternative laser source to the Neodymium:YAG laser for noninvasive thermal therapy.

Noninvasive laser coagulation of the human vas deferens: optical and thermal simulations

BACKGROUND AND OBJECTIVES

Successful noninvasive laser coagulation of the canine vas deferens, in vivo, has been previously reported. However, there is a significant difference between the optical properties of canine and human skin. In this study, Monte Carlo (MC) simulations of light transport through tissue and heat transfer simulations are performed to determine the feasibility of noninvasive laser vasectomy in humans.

MATERIALS AND METHODS

A laser wavelength of 1,064 nm was chosen for deep optical penetration in tissue. MC simulations determined the spatial distribution of absorbed photons inside the tissue layers (epidermis, dermis, and vas). The results were convolved with a 3-mm-diameter laser beam, and then used as the spatial heat source for the heat transfer model. A laser pulse duration of 500 milliseconds, pulse rate of 1 Hz, and cryogen spray cooling were incident on the tissue for 60 seconds. Average laser power (5-9 W), cryogen pulse duration (60-100 milliseconds), cryogen cooling rate (0.5-1.0 Hz), and increase in optical transmission due to optical clearing (0-50%) were studied.

RESULTS

After application of an optical clearing agent (OCA) to increase skin transmission by 50%, an average laser power of 6 W, cryogen pulse duration of 60 milliseconds, and cryogen cooling rate of 1 Hz resulted in vas temperatures of approximately 58°C, sufficient for thermal coagulation, while 1 mm of the skin surface (epidermis and dermis) remained at a safe temperature of approximately 45°C.

CONCLUSIONS

MC and heat transfer simulations indicate that it is possible to noninvasively thermally coagulate the human vas deferens without adverse effects (e.g., scrotal skin burns), if an OCA is applied to the skin prior to the procedure.

Noninvasive laser vasectomy: preliminary ex vivo tissue studies

BACKGROUND AND OBJECTIVES

Male sterilization (vasectomy) is more successful, safer, less expensive, and easier to perform than female sterilization (tubal ligation). However, female sterilization is more popular, primarily due to male fear of vasectomy complications (incision, bleeding, infection, and scrotal pain). The development of a completely noninvasive vasectomy technique may eliminate these concerns.

MATERIALS AND METHODS

Ytterbium fiber laser radiation with a wavelength of 1,075 nm, average power of 11.7 W, 1-second pulse duration, 0.5 Hz pulse rate, and 3-mm-diameter spot was synchronized with cryogen cooling of the scrotal skin surface in canine tissue for a treatment time of 60 seconds.

RESULTS

Vas thermal lesion dimensions measured 2.0+/-0.3 mm diameter by 3.0+/-0.9 mm length, without evidence of skin damage. The coagulated vas bursting pressure measured 295+/-72 mm Hg, significantly higher than typical vas ejaculation pressures of 136+/- 29 mm Hg.

CONCLUSIONS

Noninvasive thermal coagulation and occlusion of the vas was produced in an ex vivo canine tissue model. However, chronic in vivo animal studies will be necessary to optimize the laser/cooling treatment parameters and confirm long-term vas occlusion with absence of sperm in the ejaculate, before clinical application.

Noninvasive thermal coagulation of deep subsurface tissue structures using a laser probe with integrated contact cooling

Cooling methods are used during cosmetic laser surgery to preserve a superficial layer of the skin surface. This study investigates contact cooling for sparing a deeper layer of the tissue surface during laser irradiation of subsurface tissues, with the goal of developing noninvasive laser therapy applications beyond cosmetic surgery. A laser probe was designed and tested for simultaneous laser irradiation and contact cooling of liver tissue, ex vivo. Gross and histologic examination was used to quantify thermal lesion dimensions. Liver lesions of 5.8-mm-diameter were created, while preserving the tissue surface to a depth of 1.5 mm. In vivo animal studies are planned to optimize the laser and cooling parameters for potential clinical applications.