Experimental verification of the effect of refractive index mismatch on the light fluence in a turbid medium

Fast multislice fluorescence molecular tomography using sparsity-inducing regularization

Fluorescence molecular tomography (FMT) is a rapidly growing imaging method that facilitates the recovery of small fluorescent targets within biological tissue. The major challenge facing the FMT reconstruction method is the ill-posed nature of the inverse problem. In order to overcome this problem, the acquisition of large FMT datasets and the utilization of a fast FMT reconstruction algorithm with sparsity regularization have been suggested recently. Therefore, the use of a joint L1/total-variation (TV) regularization as a means of solving the ill-posed FMT inverse problem is proposed. A comparative quantified analysis of regularization methods based on L1-norm and TV are performed using simulated datasets, and the results show that the fast composite splitting algorithm regularization method can ensure the accuracy and robustness of the FMT reconstruction. The feasibility of the proposed method is evaluated in an in vivo scenario for the subcutaneous implantation of a fluorescent-dye-filled capillary tube in a mouse, and also using hybrid FMT and x-ray computed tomography data. The results show that the proposed regularization overcomes the difficulties created by the ill-posed inverse problem.

Thermal impact of near-infrared laser in advanced noninvasive optical brain imaging

The propagation of laser light in human tissues is an important issue in functional optical imaging. We modeled the thermal effect of different laser powers with various spot sizes and different head tissue characteristics on neonatal and adult quasirealistic head models. The photothermal effect of near-infrared laser (800 nm) was investigated by numerical simulation using finite-element analysis. Our results demonstrate that the maximum temperature increase on the brain for laser irradiance between 0.127 (1 mW) and [Formula: see text] (100 mW) at a 1 mm spot size, ranged from 0.0025°C to 0.26°C and from 0.03°C to 2.85°C at depths of 15.9 and 4.9 mm in the adult and neonatal brain, respectively. Due to the shorter distance of the head layers from the neonatal head surface, the maximum temperature increase was higher in the neonatal brain than in the adult brain. Our results also show that, at constant power, spot size changes had a lesser heating effect on deeper tissues. While the constraints for safe laser irradiation to the brain are dictated by skin safety, these results can be useful to optimize laser parameters for a variety of laser applications in the brain. Moreover, combining simulation and adequate in vitro experiments could help to develop more effective optical imaging to avoid possible tissue damage.

Analytical reconstruction of the bioluminescent source with priors

Bioluminescence imaging has been a popular tool in small animal imaging. During the last decade, the efforts have focused on the development of tomographic systems. However, due to the difficulties in the nature of inverse source problem, multi-modal systems have been the center of attention for the last couple of years. These systems provide complementary information such that the difficulties of the inverse source problem could be overcome using the a priori information obtained. Motivated by these advances in multi-modal systems, this work presents a novel analytical reconstruction of the bioluminescent source. It is shown that if source strength is known a priori then source position could be calculated or vice versa, if source location is known a priori, source strength could be calculated as well as the photon fluence rate. The determination of the source location can be achieved by another imaging system such as X-ray computed tomography. Therefore, in bioluminescence tomography together with an imaging system can be utilized as a multi-modal system. In this work, conventional finite element based simulations are also performed and the numerical results are compared with the analytical ones. It turns out to be that the analytical results are in a good accordance with the numerical results.

Frequency domain photon migration in the delta- P1 approximation: analysis of ballistic, transport, and diffuse regimes

The standard diffusion approximation (SDA) to the Boltzmann transport equation (BTE) is commonly used to describe radiative transport for biomedical applications of frequency-domain diffuse optical imaging and spectroscopy. Unfortunately, the SDA is unable to provide accurate radiative transport predictions on spatial scales comparable to the transport mean free path and for media in which optical scattering is not dominant over absorption. Here, we develop and demonstrate the use of the delta- P1 approximation to provide improved radiative transport estimates in the frequency domain via the addition of a Dirac delta function to both radiance and phase function approximations. Specifically, we consider photon density wave propagation resulting from the illumination of an infinite turbid medium with an embedded, intensity-modulated, spherical light source. We examine the accuracy of the standard diffusion and delta- P1 approximations relative to Monte Carlo simulations that provide exact solutions to the BTE. This comparison establishes the superior accuracy of the delta- P1 approximation relative to the SDA that is most notable at distances less than 3 transport mean free paths from the source. In addition, we demonstrate that the differences in photon density wave propagation in a highly forward scattering medium (g1=0.95) vs an isotropically scattering medium (g1=0) provides a basis to define three spatial regimes where the light field is dominated by (a) unscattered/ballistic light, (b) minimally scattered light, and (c) diffusely scattered light. We examine the impact of optical properties, source modulation frequency, and numerical aperture of detection on the spatial extent and location of these regimes.

Techniques for delivery and monitoring of TOOKAD (WST09)-mediated photodynamic therapy of the prostate: Clinical experience and practicalities

Photodynamic therapy of solid organs requires sufficient PDT dose throughout the target tissue while minimizing the dose to proximal normal structures. This requires treatment planning for position and power of the multiple delivery channels, complemented by on-line monitoring during treatment of light delivery, drug concentration and oxygen levels. We describe our experience in implementing this approach in Phase I/II clinical trials of the Pd-bacteriophephorbide photosensitizer TOOKAD (WST09)-mediated PDT of recurrent prostate cancer following radiation failure. We present several techniques for delivery and monitoring of photodynamic therapy, including beam splitters for light delivery to multiple delivery fibers, multi-channel light dosimetry devices for monitoring the fluence rate in the prostate and surrounding organs, methods of measuring the tissue optical properties in situ, and optical spectroscopy for monitoring drug pharmacokinetics of TOOKAD in whole blood samples and in situ in the prostate. Since TOOKAD is a vascular-targeted agent, the design and implementation of the techniques are different than for cellular-targeted agents. Further development of these delivery and monitoring techniques will permit full on-line monitoring of the treatment that will enable real-time, patient-specific and optimized delivery of PDT.

Determination of refractive indices of porcine skin tissues and intralipid at eight wavelengths between 325 and 1557 nm

We constructed an automated reflectometry system for accurate measurement of coherent reflectance curves of turbid samples and analyzed the presence of coherent and diffuse reflection near the specular reflection angle. An existing method has been validated to determine the complex refractive indices of turbid samples on the basis of nonlinear regression of the coherent reflectance curves by Fresnel's equations. The complex refractive indices of fresh porcine skin epidermis and dermis tissues and Intralipid solutions were determined at eight wavelengths: 325, 442, 532, 633, 850, 1064, 1310, and 1557 nm.

Amending of fluorescence sensor signal localization in human skin by matching of the refractive index

Fluorescence diagnostic techniques are notable amongst many other optical methods because they offer high sensitivity and noninvasive measurement of tissue properties. However, a combination of multiple scattering and physical heterogeneity of biological tissues hampers interpretation of the fluorescence measurements. Analyses of the spatial distribution of endogenous and exogenous fluorophores excitation within tissues and their contribution to the detected signal localization are essential for many applications. We have developed a novel Monte Carlo technique that gives a graphical perception of how the excitation and fluorescence detected signal are localized in tissues. Our model takes into account the spatial distribution of fluorophores, the variation of concentrations and quantum yield. We demonstrate that matching the refractive indices of the ambient medium and topical skin layer improves spatial localization of the detected fluorescence signal within the tissues.

Influence of refractive index matching on the photon diffuse reflectance

Photon migration in a randomly inhomogeneous, highly scattering and absorbing semi-infinite medium with a plane boundary is considered by a Monte Carlo (MC) technique. The employed MC technique combines the statistical weight scheme and real photon paths simulation, allowing the exclusion of the energy conservation problem. The internal reflection of the scattered radiation on the medium interface is taken into account by allowing the trajectories of photon packets to be split into reflected and transmitted parts. The spatial photon sensitivity profile (SPSP), spatially resolved diffuse reflectance and angular and spatial photon detector weight distributions are considered in terms of Fresnel's reflection/refraction on the boundary of the medium. The effect of the refractive index match is predicted correctly by the MC method and by the diffusion approximation. The results demonstrate that matching of the refractive index of the medium significantly improves the contrast and spatial resolution of the spatial photon sensitivity profile (SPSP). The results of simulation of the spatially resolved diffuse reflectance agree well with the results predicted by the diffusion approximation and the experimental results reported earlier.