Basic theoretical considerations of the optical and thermal transfer processes that govern the thermal damage induced in tissue by lasers are discussed. An approximate, predictive model and data are proposed for the purpose of selecting a laser that maximizes damage to cutaneous blood vessels and minimizes damage to the surrounding connective tissue and the overlying epidermis. The variables of wavelength, exposure duration, and incident energy density are modeled, and a flashlamp-pumped dye laser operating at or near the 577 nm absorption band of HbO2, with a pulse width (0.3 microsecond) less than the estimated, approximately 1 millisecond, thermal relaxation times for microvessels is chosen for experimental exposures of normal Caucasian skin. Highly specific laser-induced damage to blood vessels is demonstrated both clinically and histologically. This is in striking contrast to the previously reported widespread, diffuse necrosis caused by other lasers. The incident energy and preliminary observations of wavelength and temperature dependence for vascular damage thresholds are consistent with theoretical predictions. Whereas typically 20 joules/cm2 of argon laser irradiation (514 and 488 nm, approximately 100 msec) is required to induce widespread thermal damage, the pulsed dye laser requires only about 2 joules/cm2 to induce highly specific vascular damage. The potential usefulness of dye laser-induced selective vascular damage as a treatment modality for portwine stain hemangiomas and other vascular lesions is discussed. In addition to possible treatment applications, the dye laser or other sources meeting the requirements for producing such damage may also offer a useful experimental tool for inducing predictable damage to microvasculature. Histopathologic and clinical studies related to these possibilities are in progress.

Microvasculature can be selectively damaged using dye lasers: a basic theory and experimental evidence in human skin.

Thomas Jefferson National Accelerator Facility, Newport News, VirginiaBackground and Objectives: In theory, infrared vibra-tionalbandscouldbeusedforselectivephotothermolysisoflipid-rich tissues such as fat, sebaceous glands, or athero-sclerotic plaques.Study Design/Materials and Methods: Absorptionspectraofhumanfatweremeasured,identifyingpromisingbandsnear1,210and1,720nm.Photothermalexcitationofporcine fat and dermis were measured with a 3.5–5 mmthermal camera during exposure to the free electron laser(FEL) at Jefferson National Laboratory. Thermal damageto full-thickness samples exposed at 1,210 nm through acoldcontactwindow,wasassessedbynitrobluetetrazoliumchloride staining in situ and by light microscopy.Results: Photothermal excitation of fat was twice that ofdermis, at lipid absorption bands (1,210, 1,720 nm). At1,210 nm, a subcutaneous fat layer several mm thick wasdamaged by FEL exposure, without apparent injury tooverlying skin.Conclusion: Selective photothermal targeting of fattytissues is feasible using infrared lipid absorption bands.Potential clinical applications are suggested by this FELstudy. Lasers Surg. Med. 38:913–919, 2006. 2006 Wiley-Liss, Inc.Key words: dermis; fat; tissue opticsINTRODUCTIONSelective photothermolysis (SP) is a widely used treat-ment approach that preferentially heats tissue ‘‘targets,’’whichabsorbapulseofopticalradiationmorestronglythansurrounding tissues [1]. Both source and tissue factorsaffect SP. In general, the source wavelength region andbeam optics are chosen to produce preferential absorptionin the desired targets; and pulse duration or exposure timeis chosen to be nearly equal to the thermal relaxation timeof the desired target structures [2]. Cooling at the tissuesurface can be employed before, during, and/or after lightdelivery to prevent thermal injury of a superﬁcial tissuelayer [3]. Since its conception over 20 years ago, SP hasbeen developed clinically for treating microvascular skinmalformations [4], pigmented lesions [5], tattoos [6],pigmented hair [7], glaucoma [8], laryngeal lesions [9],and other conditions. In all of these applications, visible ornear-infrared lasers or intense pulsed light sources areusedtopumpelectronicabsorptiontransitionsofthetissue‘‘pigments’’ hemoglobin,oxyhemoglobin, melanin,ortattooinks.SPhasnot yet been developed using infrared vibrationalabsorptionbands.Thefar-infraredisknownasa‘molecularﬁngerprint’ spectral region because strong, narrow vibra-tionalmodesarecharacteristicofchemicalbondstructures.While in theory SP could be performed using far-infraredpulses, the utility of wavelengths greater than about2,500 nm is limited because strong absorption by waterprevents photons from penetrating more than about0.1 mm into tissue. In contrast, the near- and mid-infraredspectral region of about 900–2,800 nm includes the mosttissue-penetratingopticalwavelengths,speciﬁcallyaround1,200 nm.Inthisspectralregion,absorptionislowbecausethe photon quantum energy is less than electronic transi-tion energy in most organic molecules, and optical scatter-ing is weak [10]. In this region, there are multiple weakvibrationalabsorptionbands,consistingofovertonesofthefundamental modes responsible for strong far-infraredabsorption.In this study, we investigate the potential for SP of lipid-rich tissues, by pumping vibrational absorption bands.Lipids contain copious CH and CH

Selective photothermolysis of lipid-rich tissues: a free electron laser study.

A variable frequency magnetic induction heating system has been developed for localized hyperthermic treatment of deep-seated tumors via power deposition into arrays of 1-2 mm diameter ferromagnetic cylindrical implants. The frequency dependence of implant heating compared to that induced directly into tissues indicates use of frequencies below 4 MHz. Above 10 MHz, tissues are heated directly from the induced eddy currents. Thermographic analyses of temperature distributions induced in tissues equivalent phantom models and in exposed animal tissues in vivo have been performed for several implant materials and array configurations. Results of the thermal dosimetry show that the majority of tissue contained within multiimplant arrays can be heated such that the temperature rises to 55-70 percent of the implant temperature differential. The resulting tissue temperature distribution is dependent on local blood flow and array characteristics, but not significantly on tissue electrical properties. Operating the system at 1.9. MHz, we demonstrate that it is possible to raise an implanted volume to therapeutic temperatures safely, in less than 10 min, with little increase in temperature outside the array boundaries.

Magnetic Induction Heating of Ferromagnetic Implants for Inducing Localized Hyperthermia in Deep-Seated Tumors

A chronically implantable, wireless neural interface device will require integrating electronic circuitry with the interfacing microelectrodes in order to eliminate wired connections. Since the integrated circuit (IC) dissipates a certain amount of power, it will raise the temperature in surrounding tissues where it is implanted. In this paper, the thermal influence of the integrated 3-D Utah electrode array (UEA) device implanted in the brain was investigated by numerical simulation using finite element analysis (FEA) and by experimental measurement in vitro as well as in vivo. The numerically calculated and experimentally measured temperature increases due to the UEA implantation were in good agreement. The experimentally validated numerical model predicted that the temperature increases linearly with power dissipation through the UEA, with a slope of 0.029degC/mW over the power dissipation levels expected to be used. The influences of blood perfusion, brain metabolism, and UEA geometry on tissue heating were also investigated using the numerical model.

Thermal Impact of an Active 3-D Microelectrode Array Implanted in the Brain

Advances in laser, microwave, and similar technologies have led to recent developments of thermal treatments for disease and injury involving skin tissue. In spite of the widespread use of heating therapies in dermatology, they do not draw upon the detailed understanding of the biothermomechanics of behavior, for none exists to date, even though each behavioral facet is well established and understood. It is proposed that a detailed understanding of the coupled biological-mechanical response under thermal agitation will contribute to the design, characterization, and optimization of strategies for delivering better treatment. For a comprehensive understanding on the underlying mechanisms of thermomechanical behavior of skin tissue, recent progress on bioheat transfer, thermal damage, thermomechanics, and thermal pain should be systematically reviewed. This article focuses on the transfer of heat through skin tissue. Experimental study, theoretical analysis, and numerical modeling of skin thermal behavior are reviewed, with theoretical analysis carried out and closed-form solutions obtained for simple one-layer Fourier theory based model. Non-Fourier bioheat transfer models for skin tissue are discussed, and various skin cooling technologies summarized. Finally, the predictive capacity of various heat transfer models is demonstrated with selected case studies. DOI: 10.1115/1.3124646

T. A. Balasubramaniam

Papers

Thermal Conductivity and Thermal Diffusivity of Biomaterials: A Simultaneous Measurement Technique