Alex Vitkin

Three-dimensional optical phantom and its application in photodynamic therapy

A technique to manufacture a stable, reproducible three‐dimensional optical phantom is presented. This phantom reproduces the tissues optical properties as well as the geometry and, to some extent, the mechanical properties of the organ concerned. Easy to make and to handle, this phantom is a useful tool for numerous medical applications involving light interaction with biological tissues.

Monte Carlo study of pathlength distribution of polarized light in turbid media

Photon pathlength distributions as a function of the number of scattering events in cylindrical turbid samples are studied using a polarization-sensitive Monte Carlo model with linearly polarized light input. Sample scattering causes extensive depolarization, yielding a photon field comprised of polarized and depolarized sub-populations. It is found that the pathlength of polarization-preserving photons is distributed within a defined spatial range with strong angular dependence. This pathlength, averaged over the range, is 2-3X smaller than the one averaged over the widely-spread range of all (polarized + depolarized) collected photons. It is also demonstrated that changes in optical properties of the media affect the pathlength distributions.

Optothermal transfer simulation in laser-irradiated human dentin.

Laser technology has been studied as a potential replacement to the conventional dental drill. However, to prevent pulpal cell damage, information related to the safety parameters using high-power lasers in oral mineralized tissues is needed. In this study, the heat distribution profiles at the surface and subsurface regions of human dentine samples irradiated with a Nd:YAG laser were simulated using Crank-Nicolsons finite difference method for different laser energies and pulse durations. Heat distribution throughout the dentin layer, from the external dentin surface to the pulp chamber wall, were calculated in each case, to investigate the details of pulsed laser-hard dental tissue interactions. The results showed that the final temperature at the pulp chamber wall and at the dentin surface are strongly dependent on the pulse duration, exposure time, and the energy contained in each pulse.

Frequency domain photoacoustic correlation (radar) imaging: a novel methodology for non-invasive imaging of biological tissues

We report the development of a novel frequency-domain biomedical photoacoustic (PA) system that utilizes a continuous-wave laser source with a custom intensity modulation pattern for spatially-resolved imaging of biological tissues. The feasibility of using relatively long duration and low optical power laser sources for spatially-resolved PA imaging is presented. We demonstrate that B-mode PA imaging can be performed using an ultrasonic phased array coupled with multi-channel correlation processing and a frequency-domain beamforming algorithm. Application of the frequency-domain PA correlation methodology is shown using tissue-like phantoms with embedded optical contrast, tissue ex-vivo samples and a small animal model in-vivo.

POLARIZED LIGHT and the asymmetry of life

The study of polarization properties of light forms a fascinating chapter in the history of science, generating key developments in physics, chemistry, and biology. Vitkin highlights the history and physics of polarized light, and discusses its relation to the intriguing asymmetric nature of molecules from living tissues.

Accurate viscosity measurements of flowing aqueous glucose solutions with suspended scatterers using a dynamic light scattering approach with optical coherence tomography.

The viscosity of turbid colloidal glucose solutions has been accurately determined from spectral domain optical coherence tomography (OCT) M-mode measurements and our recently developed OCT dynamic light scattering model. Results for various glucose concentrations, flow speeds, and flow angles are reported. The relative combined standard uncertainty uc(η) on the viscosity measurements was ±1% for the no-flow case and ±5% for the flow cases, a significant improvement in measurement robustness over previously published reports. The available literature data for the viscosity of pure water and our measurements differ by 1% (stagnant case) and 1.5% (flow cases), demonstrating good accuracy; similar agreement is seen across the measured glucose concentration range when compared to interpolated literature values. The developed technique may contribute toward eventual noninvasive glucose measurements in medicine.

SU-FF-T-370: Real-Time Point-Based in Vivo Dosimetry Using Radiochromic Materials and Remote Optical Fiber System

Purpose: Accurate in‐situ dosimetry continues to be challenging in both radiotherapy and imaging applications. Radiochromic media (GafChromic MD‐55 and EBT films) have been recently developed for application across a wide range of applied total doses,dose rates, and energies. These materials demonstrate a fast response component that allows consideration for use in a novel point‐based in vivodosimetrysystem using remote, passive optical fiber readout. Method and Materials: Changes in optical density corresponding to the main absorbance peaks and overall absorbance spectra of the two media have been remotely detected during irradiation within Solid Water™ phantoms, which were equipped with optical fibers for real‐time readout. The interrogation light source provided a spot size 650 μm in diameter, simulating the intended point‐based use, and the transmitted spectrum was recorded by a spectrophotometer. The films were exposed to a wide range of doses under conditions varying in temperature, dose rate and beam energy typical of clinical use. Results: The spectral change with time was observed to have two components. The fast response component occurred during exposure to ionizing radiation, and was easily detected in real‐time. Because of the passive readout with these media, the frequency of measurements and statistical noise for any dose and dose rate was controlled by modifying the power of the light source and integration time on the spectrophotometer. The slow response component continued post‐irradiation, as reported previously by others, having a small effect on real‐time measurements. Conclusion: Radiochromic formulations can be used for dosimetry in real‐time during application of ionizing radiation. Their performance with respect to variations in dose rate, temperature and energy of beam were also promising, with some limitations that are being investigated further. Acknowledgements: Supported by NCIC 016646, NIH/NIA R21/R33 AG19381, Scace Prostate Cancer Research award, and the Fidani Center for Radiation Physics.

Po-Thur Eve General-28: Characterization and Real-Time Measurements of Optical Density with GafChromic EBT Film

To establish whether GafChromic EBT medium is suitable for real‐time fibre‐based in vivodosimetry, we are investigating whether dose‐rate and temperature fluctuations introduce extraneous uncertainties to change in optical density (ΔOD) measurements. 1 cm × 1cm pieces of film were irradiated with a 6 MV beam within a 30 cm × 30 cm × 10 cm Solid Water phantom fitted with optical fibres for real‐time measurements, using a 10 cm × 10 cm field at SAD, 100 cm SAD, and 1.5 cm depth. Each exposure delivered a dose of 5, 10, 25, 50, 100, 250, 500 or 1000 cGy at a dose‐rate of 14, 32, 65, 130, 260 or 520 cGy/min. Change in OD obtained immediately at the end of each exposure were compared. The percent uncertainty in ΔOD for a given dose was calculated by using all the values obtained, regardless of the dose‐rate used. The percent uncertainty ranged between 1.8% and 3.6%, with an average of 2.8%. The expected variation in ΔOD within a single sheet of GafChromic EBT film for a given dose is 1.5% (two standard deviations). This suggests that there is an increase in uncertainty of real‐time ΔOD measurements when variations in dose‐rate are introduced for the doses in 5–1000 cGy range, and dose‐rates between 14 and 520 cGy/min. The average increase in uncertainty is 4%.

SU‐FF‐T‐121: Characterization and Real‐Time Measurements of Optical Density with GafChromic EBT Film

Purpose: Over the last few decades, various groups have investigated optical fiber‐based dosimeters for in vivomeasurements. Recently, two radiochromic films have also been considered for real‐time in vivo point‐based optical dosimetry, and both GafChromic MD‐55 and GafChromic EBT films faired well in several criteria. However, GafChromic MD‐55 was shown to have the change in optical density depend both on dose rate and on temperature, as measured immediately at the end of a given exposure. To continue with the search for a suitable medium for real‐time fiber‐based in vivodosimetry, GafChromic EBT film is also being investigated for dose rate and temperature effects. Method and Materials: 1 cm × 1cm pieces of film were irradiated with a 6 MV beam within a 30 cm × 30 cm × 10 cm Solid Water phantom fitted with optical fibers for real‐time measurements, using a 10 cm × 10 cm field at SAD, 100 cm SAD, and 1.5 cm depth. Each exposure delivered a dose in 5 to 1000 cGy range at a dose rate in 14 to 520 cGy/min range. Changes in OD obtained immediately at the end of each exposure were compared. The percent uncertainty in ΔOD for a given dose was calculated by using all the values obtained, regardless of the dose‐rate used. Results: The percent uncertainty ranged between 1.8% and 3.6%, with an average of 2.8%. The expected variation in ΔOD within a single sheet of GafChromic EBT film for a given dose is 1.5% (two standard deviations). The average increase in uncertainty is 4%. Conclusion: An increase in uncertainty of real‐time ΔOD measurements is observed when variations in dose‐rate are introduced for the doses in 5 to 1000 cGy range, and dose‐rates between 14 and 520 cGy/min.

SU‐FF‐T‐138: Comparison of Change in Optical Density Between Three Radiochromic Films Due to 100 CGy Dose‐To‐Water Delivered by X‐Rays in the 75 KVp to 18 MV Range

Purpose: Over the last few decades, optical fiber‐based dosimeters for in vivomeasurements have been investigated. One of the attractive features of optical dosimetry is the potential for improved water‐equivalency, compared to methods that use electrical signals. The materials considered thus far have been scintillating and optically stimulated luminescent sensors,doped and scintillating optical fibers, and radiochromic films. Several of the radiochromic films, including GafChromic MD‐55 and HS, have shown a lower response for a given dose when low‐energy (orthovoltage) x‐ray beams were used, compared to a Co‐60 source. This study continues the investigation of suitability of another radiochromic film, GafChromic EBT, for real‐time in vivo point‐based dosimetry by considering its response to a given dose delivered by beams varying in nominal energy. Method and Materials: 1 cm × 1cm pieces of MD‐55, HS and EBT film were irradiated within a 30 cm × 30 cm × 10 cm Solid Water phantom fitted with optical fibers for real‐time measurements. Each exposure delivered a dose of 100 cGy, using x‐ray beams ranging in energy from 75 kVp to 18 MV. Changes in optical density (ΔOD) were compared for the three types of film across all beam energies used. Results: Both MD‐55 and HS films showed a decrease of approximately 35% in ΔOD for a 75 kVp beam compared to a Co‐60 beam. Decrease in response for 225 kVp beam was less pronounced (approximately 15%). Both HS and MD‐55 exhibited this trend, confirming previous reports and validating our technique. The EBT film showed no decrease in response through the orthovoltage range. Conclusion: Due to a near‐constant response of EBT film for a given dose delivered using 75 kVp to 18 MV x‐ray beams, EBT medium is more suitable for in vivomeasurements than MD‐55 and HS films.