A noncontact method to determine the thermal diffusivity of biomaterials is derived theoretically and demonstrated experimentally. Assuming that the initial three-dimensional temperature distribution in the biomaterial immediately following pulsed laser irradiation can be separated into lateral and longitudinal factors, a convolution equation is derived that relates a pair of infrared emission images to the lateral thermal point spread function. Pairs of infrared emission images recorded following pulsed laser irradiation are used in combination with a nonlinear least-squares algorithm to determine the lateral thermal point spread function and thus the thermal diffusivity of the biomaterial. Sequences of infrared emission images recorded following pulsed laser irradiation are used to determine thermal diffusivity (0.103±0.007 mm2s) of an in vitro type I hydrated collagen film. Application of the method to in vivo tissues is discussed.

Specifying the distribution of laser energy within a tissue is the first step toward understanding and capitalizing on a variety of laser-tissue interactions. Whether photothermal, photochemical, or photomechanical in nature, laser-tissue interactions begin with the absorption of photon energy. The spatial distribution of photon absorption specifies the required laser exposure to be delivered and the extent of subsequent therapeutic action. Using infrared tomography (IRT), the broad, long term objective of this research is the development of a three-dimensional tomographic reconstruction algorithm (TRA) as a means to determine the: (1) initial space-dependent temperature increase in subsurface chromophores [ΔT CHR(ξ,η,ζ,t equals 0)] immediately following pulsed laser exposure; and (2) depths and physical dimensions of discrete subsurface chromophores. Analysis of the recorded time sequence of infrared emission images [ΔM CHR(x,y,t)] by longitudinal inversion and lateral deconvolution algorithms provides a direct means to determine the depths and physical dimensions of subsurface chromophores. Although our research is being shared with workers in a variety of disciplines, and pertinent to many clinical applications involving laser-induced photothermal mechanisms, we are particularly interested in addressing the problems associated with determination of the initial space-dependent temperature increase in subsurface chromophores in human skin in general, and port wine stain (PWS) blood vessels in particular.

A solution method is proposed to the inverse problem of determining the unknown initial temperature distribution in a laser-exposed test material from measurements provided by infrared radiometry. A Fredholm integral equation of the first kind is derived that relates the temporal evolution of the infrared signal amplitude to the unknown initial temperature distribution in the exposed test material. The singular-value decomposition is used to demonstrate the severely ill-posed nature of the derived inverse problem. Three inversion methods are used to estimate solutions for the initial temperature distribution. A nonnegatively constrained conjugate-gradient algorithm using early termination is found superior to unconstrained inversion methods and is applied to image the depth of laser-heated chromophores in human skin.

We demonstrate a method for infrared tomographic imaging of the initial three-dimensional (3-D) temperature increase of in vivo blood vessels following pulsed laser exposure. Using a time-sequence of infrared images recorded by a fast focal plane array camera as input data, a reconstruction algorithm is used to compute the initial 3-D temperature increases in the chick chorioallantoic membrane and port wine stain in human skin. The noncontact tomographic imaging method is relevant to various laser therapies that rely on a photothermal mechanism for selective modification of tissue and may find diagnostic application to determine optimal laser dosimetry, position, and size of targeted subsurface chromophores.

Specifying the distribution of laser energy within a tissue is the first step towards understanding and capitalizing on a variety of laser-tissue interactions. Whether photothermal, photochemical or photomechanical in nature, laser-tissue interactions begin with the absorption of photon energy. The spatial distribution of photon absorption specifies the required laser exposure to be delivered and the extent of subsequent therapeutic action. Using infra-red tomography (IRT), the broad, long-term objective of this research is the development of a tomographic reconstruction algorithm as a means to determine: (1) the depths and physical dimensions of discrete subsurface port-wine-stain (PWS) blood vessels in human skin; and (2) the initial space-dependent temperature increase in PWS blood vessels immediately following pulsed laser exposure. In this report, preliminary studies are described which demonstrate the potential application of IRT in the clinical management of PWS patients.

In many port wine stain (PWS) patients, successful clearing is not achieved even after multiple laser treatments because of inadequate heat generation within the targeted blood vessels. Use of higher radiant exposures has been suggested to improve lesion clearing, but risk of epidermal injury due to nonspecific absorption by melanin increases. It has been demonstrated that cryogen spray cooling (CSC) can protect the epidermis from nonspecific thermal injury during laser treatment of PWS. Inasmuch as epidermal melanin concentration and blood vessel depth vary among patients, evaluation of internal skin temperatures in response to CSC is essential for further development and optimization of treatment parameters on an individual patient basis. We present internal temperature measurements in an epoxy resin phantom in response to CSC and use the results in conjunction with a mathematical model to predict the temperature distribution within human skin for various cooling parameters. Measurements on the epoxy resin phantom show that cryogen film temperature is well below the cryogen boiling point, but a poor thermal contact exists at the cryogen-phantom interface. Based on phantom measurements and model predictions, internal skin temperature reduction remains confined to the upper 400 μm for spurt durations as long as 200 ms. At the end of a 100 ms spurt, our results show a 31°C temperature reduction at the surface, 12°C at a depth of 100 μm, and 4°C at a depth of 200 μm in human skin. Analysis of estimated temperature distributions in response to CSC and temperature profiles obtained by pulsed photothermal radiometry indicates that a significant protective effect is achieved at the surface of laser irradiated PWS skin. Protection of the epidermal basal layer, however, poses a greater challenge when high radiant exposures are used.