Depth profiling of laser-heated chromophores in biological tissues by pulsed photothermal radiometry

Abstract

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.

Description and Analysis of Treatments for Port-wine Stain Birthmarks

“…Our group, along with other researchers, has developed optical instruments for noninvasive characterization of PWS skin, including photothermal imaging, [30][31][32] optical Doppler tomography, 33 and reflectance spectroscopy. 34,35 With these instruments, preoperative knowledge of skin characteristics such as epidermal thickness and melanin content; blood vessel depth, size, flow, and distribution; and skin optical properties can be achieved.…”

Thermal depth profiling of vascular lesions: automated regularization of reconstruction algorithms

“…A brief description of the three algorithms used in this study is as follows. (Milner et al 1995) refers to a minimization algorithm and is used in our application to minimize the norm of the residue R (equation (3)),…”

“…The 15 generated temperature profiles are shown in figures 1(B), (E) and (H). For each T profile, the associated radiometric signal ΔS(t) or simply S was computed (shown in figures 1 (C), (F) and (I)) using a well-described model (Milner et al 1995, Verkruysse et al 2005, described briefly in the following section. Finally, we created for each profile two additional signals by adding noise with corresponding signal-to-noise ratios (SNR) of 100 and 1000.…”

“…In simpler terms, the 'ill-posedness' means that an inversion algorithm produces several different spatial profiles as solutions to the measured temporal profiles. 'Regularization', a term describing the process of making an informed choice from the different spatial profiles is considered to be of critical importance in PPTR (Milner et al 1995, Li et al 2004a, Li et al 2004b, Verkruysse et al 2005. Many existing regularization techniques require some degree of skill by the user and are inherently subjective.…”

Three-dimensional thermographic imaging using a virtual wave concept

“…2 Active thermography involves heating or cooling of a sample to create a temperature gradient and observation of the heat diffusion process by measuring the infrared thermal signature of the sample using an infrared camera. To heat up the sample, various methods are used including a flash-lamp or laserheating, [3][4][5][6] ultrasonic waves, 7 or inductive heating. 8 Various temporal excitation methods, such as pulses in pulsed thermography, 9 or frequency-domain signals with a singlefrequency lock-in technique, 10 are applied to heat the sample.…”