Deep subsurface cavities in skin utilizing mechanical optical clearing and femtosecond laser ablation.

BACKGROUND AND OBJECTIVES: High precision subsurface ablation can be produced in transparent materials using femtosecond laser pulses and multiphoton absorption. Light scattering limits application of the same technique to most biological tissues. Previously, subsurface ablation was demonstrated at superficial depths (50-250 µm) in highly scattering tissues including murine skin and human sclera. We report application of mechanical optical clearing to produce deeper subsurface femtosecond ablation in rodent skin. Ability to target deeper structures in skin using subsurface ablation may allow novel clinical applications for dermatological laser surgery. STUDY DESIGN/MATERIALS AND METHODS: Operation of a prototype tissue optical clearing device (TOCD) was verified with white light photography in ex vivo rodent skin. A focused femtosecond beam transmitted through the TOCD and was scanned across rodent skin to produce subsurface ablation at increasing focal depths. Histological sections with H&E staining of the laser irradiated rodent skin were examined for subsurface ablation features following laser irradiation. RESULTS: Subsurface cavities were observed as deep as 1.7 mm below the skin surface in histological tissue sections. Diameter of subsurface cavities varied from tens of microns to over 100 µm. Subsurface cavities produced by scanning the focused femtosecond beam were contiguous and formed a continuous cut. Mechanical disruption of the overlying tissues was not observed. CONCLUSIONS: Mechanical optical clearing can be applied directly to in situ rodent skin and produces an optical clearing effect. High precision subsurface ablation can be produced at positions substantially deeper than previously demonstrated. Future studies may be targeted in in vivo human skin to investigate potential clinical applications of subsurface femtosecond ablation using mechanical optical clearing.

Femtosecond laser lithotripsy: feasibility and ablation mechanism.

Light emitted from a femtosecond laser is capable of plasma-induced ablation of various materials. We tested the feasibility of utilizing femtosecond-pulsed laser radiation (lambda=800 nm, 140 fs, 0.9 mJ/pulse) for ablation of urinary calculi. Ablation craters were observed in human calculi of greater than 90% calcium oxalate monohydrate (COM), cystine (CYST), or magnesium ammonium phosphate hexahydrate (MAPH). Largest crater volumes were achieved on CYST stones, among the most difficult stones to fragment using Holmium:YAG (Ho:YAG) lithotripsy. Diameter of debris was characterized using optical microscopy and found to be less than 20 microm, substantially smaller than that produced by long-pulsed Ho:YAG ablation. Stone retropulsion, monitored by a high-speed camera system with a spatial resolution of 15 microm, was negligible for stones with mass as small as 0.06 g. Peak shock wave pressures were less than 2 bars, measured by a polyvinylidene fluoride (PVDF) needle hydrophone. Ablation dynamics were visualized and characterized with pump-probe imaging and fast flash photography and correlated to shock wave pressures. Because femtosecond-pulsed laser ablates urinary calculi of soft and hard compositions, with micron-sized debris, negligible stone retropulsion, and small shock wave pressures, we conclude that the approach is a promising candidate technique for lithotripsy.

Plasma-mediated ablation: an optical tool for submicrometer surgery on neuronal and vascular systems.

Plasma-mediated ablation makes use of high energy laser pulses to ionize molecules within the first few femtoseconds of the pulse. This process leads to a submicrometer-sized bubble of plasma that can ablate tissue with negligible heat transfer and collateral damage to neighboring tissue. We review the physics of plasma-mediated ablation and its use as a tool to generate targeted insults at the subcellular level to neurons and blood vessels deep within nervous tissue. Illustrative examples from axon regeneration and microvascular research highlight the utility of this tool. We further discuss the use of ablation as an integral part of automated histology.

Time and frequency resolved XeCl laser-induced mechanical transients in otic capsule bone.

OBJECTIVE: This study identifies the presence of photoacoustic waves during excimer laser treatment of porcine otic capsule bone. BACKGROUND DATA: Pulsed ultraviolet lasers have been suggested for use in middle ear surgery due to their potential for fiberoptic delivery, decreased thermal trauma, and precise ablation characteristics. However, the short pulse width of excimer lasers (typically 10-150 ns) can create large thermoelastic stresses in the ablation specimen. MATERIALS AND METHODS: A XeCl (lambda = 308 nm, tau = 12 ns) excimer laser was used to ablate wafers of bone with energies of 90, 35, 13, 5, and 1.8 mJ/pulse. Custom high-frequency polyvinyldifluoride (PVDF) piezoelectric film transducers were fabricated and attached to the slices of bone. During ablation photoacoustic signals were amplified using a low-noise preamplifier and recorded on a digitizing oscilloscope. RESULTS: Photoacoustic waves were clearly identified. Stress wave amplitude increased with laser fluence. CONCLUSION: A laser fluence must be found that compromises between an increased ablation rate and increased stress wave amplitude. The acoustic power levels generated during ablation are below maximum exposure limits.

Ultrashort pulse laser ossicular ablation and stapedotomy in cadaveric bone.

BACKGROUND AND OBJECTIVE: The purpose of this study was to evaluate the ablation of ossicular tissue using a 1,053 nm Ti:Sapphire chirped pulse amplifier laser system configured to deliver ultrashort pulses of 350 femtoseconds (fs) (3.5x10(-13) seconds) in cadaver temporal bone. STUDY DESIGN/MATERIALS AND METHODS: Ablation of the formalin-fixed incus and stapes was performed using an ultrashort pulse laser (USPL) (0.4 mm beam diameter, pulse fluence of 2.0 J/cm2, and pulse repetition rate of 10 Hz). The ablation rate was measured using optical micrometry, and crater surface morphology examined using scanning electron microscopy. RESULTS: The laser produced precise bone ablation at a rate of 1.26 microm/pulse, with almost no evidence of thermal damage, and very little evidence of photomechanical injury. CONCLUSIONS: Ultrashort pulse lasers may provide a useful clinical tool for otologic and skull base surgery, where precise hard tissue ablation is required adjacent to critical structures.