PARAMETRIC STUDY OF TISSUE OPTICAL CLEARING BY LOCALIZED MECHANICAL COMPRESSION USING COMBINED FINITE ELEMENT AND MONTE CARLO SIMULATION

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.

Mechanical tissue optical clearing technique increases imaging resolution and contrast through ex vivo porcine skin

BACKGROUND AND OBJECTIVES

Mechanical tissue optical clearing permits light delivery deeper into turbid tissue, which may improve current optical diagnostics and laser-based therapeutic techniques. We investigated the effects of localized compression on brightfield imaging through ex vivo porcine skin by evaluating resolution and contrast of a target positioned beneath native, mechanically compressed, or chemically cleared specimens. We also evaluated the effects of indentation on dynamic tissue thickness and light transmission.

STUDY DESIGN/METHODS

A 5 mm diameter, hemispherically tipped, manual load transducer was used to compress specimens using 2-44 N for 60 seconds. Chemically cleared specimens were immersed for 1 hour in glycerol or dimethyl sulfoxide. A USAF 1951 resolution target was positioned beneath specimens and imaged using brightfield microscopy. Resolution and contrast of target features were analyzed. In separate experiments, a mechanical test instrument was used to compress and hold specimens at a final thickness while measuring applied load and light transmission.

RESULTS

Image intensity profiles showed that while uncompressed skin did not allow resolution of any target, localized compression allowed maximum resolution up to a line width of 173 ± 21 µm. Mechanical clearing achieved up to four times higher maximum resolution and 2-3 times higher contrast sensitivity than chemical immersion. Resolving capability was highly correlated with compressive tissue strain. Light transmission increased during tissue compression, but also increased while holding final thickness constant.

CONCLUSION

Localized compression is an effective technique for increasing resolution and contrast of target features through tissue and may improve light-based diagnostics. Thickness reduction and other mechanisms appear to contribute to this effect.

A study on tetrahedron-based inhomogeneous Monte Carlo optical simulation

Monte Carlo (MC) simulation is widely recognized as a gold standard in biophotonics for its high accuracy. Here we analyze several issues associated with tetrahedron-based optical Monte Carlo simulation in the context of TIM-OS, MMCM, MCML, and CUDAMCML in terms of accuracy and efficiency. Our results show that TIM-OS has significant better performance in the complex geometry cases and has comparable performance with CUDAMCML in the multi-layered tissue model.