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

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