General-purpose computing on graphics processing units (GPGPU) is shown to dramatically increase the speed of Monte Carlo simulations of photon migration. In a standard simulation of time-resolved photon migration in a semi-infinite geometry, the proposed methodology executed on a low-cost graphics processing unit (GPU) is a factor 1000 faster than simulation performed on a single standard processor. In addition, we address important technical aspects of GPU-based simulations of photon migration. The technique is expected to become a standard method in Monte Carlo simulations of photon migration.
A novel scheme for fully scalable White Monte Carlo (WMC) has been developed and is used as a forward solver in the evaluation of experimental time-resolved spectroscopy. Previously reported scaling problems are avoided by storing detection events individually, turning spatial and temporal binning into post-simulation activities. The approach is suitable for modeling of both interstitial and noninvasive settings (i.e., infinite and semi-infinite geometries). Motivated by an interest in in vivo optical properties of human prostate tissue, we utilize WMC to explore the low albedo regime of time-domain photon migration—a regime where the diffusion approximation of radiative transport theory breaks down, leading to the risk of overestimating both reduced scattering (µ) and absorption (µa). Experimental work supports our findings and establishes the advantages of Monte Carlo–based evaluation.
We demonstrate a novel non-intrusive technique based on tunable diode laser absorption spectroscopy to investigate
human maxillary sinuses in vivo. The technique relies on the fact that free gases have much sharper
absorption features (typical a few GHz) than the surrounding tissue. Molecular oxygen was detected at 760 nm.
Volunteers have been investigated by injecting near-infrared light fibre-optically in contact with the palate inside
the mouth. The multiply scattered light was detected externally by a handheld probe on and around the cheek
bone. A significant signal difference in oxygen imprint was observed when comparing volunteers with widely
different anamnesis regarding maxillary sinus status. Control measurements through the hand and through the
cheek below the cheekbone were also performed to investigate any possible oxygen offset in the setup. These
provided a consistently non-detectable signal level. The passages between the nasal cavity and the maxillary
sinuses were also non-intrusively optically studied, to the best of our knowledge for the first time. These measurements
provide information on the channel conductivity which may prove useful in facial sinus diagnostics.
The results suggest that a clinical trial together with an ear-nose-throat (ENT) clinic should be carried out to
investigate the clinical use of the new technique.
The development of photodynamic therapy into a modality for treatment of prostate cancer calls for reliable optical dosimetry. We employ, for the first time, interstitial time-resolved spectroscopy to determine in vivo optical properties of human prostate tissue. Nine patients are included in the study, and measurements are conducted prior to primary brachytherapy treatment of prostate cancer. Intrasubject variability is examined by measuring across three tissue volumes within each prostate. The time-resolved instrumentation proves its usefulness by producing good signal levels in all measurements. We are able to present consistent values on reduced scattering coefficients (µ), absorption coefficients (µa), and effective attenuation (µeff) at the wavelengths 660, 786, and 916 nm. At 660 nm, µ is found to be 9±2 cm?1, and µa is 0.5±0.1 cm?1. Derived values of µeff are in the range of 3 to 4 cm?1 at 660 nm, a result in good agreement with previously published steady state data. Total hemoglobin concentration (THC) and oxygen saturation are spectroscopically determined using derived absorption coefficients. Derived THC values are fairly variable (215±65 µM), while derived values of oxygen saturation are gathered around 75% (76±4%). Intrasubject variations in derived parameters correlate (qualitatively) with the heterogeneity exhibited in acquired ultrasound images.