Biomedical subjects

I Alex Vitkin

Publications and source records attributed to I Alex Vitkin.

17 recordsLinked to original sources

PubMed2007

Wide dynamic range detection of bidirectional flow in Doppler optical coherence tomography using a two-dimensional Kasai estimator.

We demonstrate extended axial flow velocity detection range in a time-domain Doppler optical coherence tomography (DOCT) system using a modified Kasai velocity estimator with computations in both the axial and transverse directions. For a DOCT system with an 8 kHz rapid-scanning optical delay line, bidirectional flow experiments showed a maximum detectable speed of >56 cm/s using the axial Kasai estimator without the occurrence of aliasing, while the transverse Kasai estimator preserved the approximately 7 microm/s minimum detectable velocity to slow flow. By using a combination of transverse Kasai and axial Kasai estimators, the velocity detection dynamic range was over 100 dB. Through a fiber-optic endoscopic catheter, in vivoM-mode transesophageal imaging of the pulsatile blood flow in rat aorta was demonstrated, for what is for the first time to our knowledge, with measured peak systolic blood flow velocity of >1 m/s, while maintaining good sensitivity to detect aortic wall motion at <2 mm/s, using this 2D Kasai technique.

Algorithms↗

PubMed2007

Ex vivo imaging of chronic total occlusions using forward-looking optical coherence tomography.

BACKGROUND AND OBJECTIVES: Percutaneous coronary interventions (PCI) of chronic total occlusions (CTOs) of arteries are more challenging lesions to treat with angioplasty and stenting than stenotic vessels due primarily to the difficulty in guiding the wire across the lesion. Angiography alone is unable to differentiate between the occluded lumen and the vessel wall and to characterize the content of the occlusion. New technologies to aid in interventional guidance are therefore highly desirable. We sought to evaluate tissue characterization in arterial (CTOs) by imaging ex vivo peripheral arterial samples with optical coherence tomography (OCT). STUDY DESIGN/MATERIALS AND METHODS: Ex vivo arterial samples were obtained from patients undergoing peripheral limb amputation. Samples were imaged in an enface orientation using an OCT system, enabling sequential acquisition of longitudinal images and volumetric reconstruction of cross-sectional views of the occluded arteries. Histology was performed for comparison. RESULTS: OCT imaging reliably differentiated between the occluded lumen and the underlying arterial wall in peripheral CTOs. OCT correctly identified tissue composition within the CTO, such as the presence of collagen and calcium and was also able to identify intraluminal microchannels. CONCLUSIONS: OCT imaging of CTO anatomy and tissue characteristics may potentially lead to substantial improvements in PCI interventions by providing novel guiding capabilities.

Angiography↗

PubMed2006

Doppler optical coherence tomography with a micro-electro-mechanical membrane mirror for high-speed dynamic focus tracking.

An elliptical microelectromechanical system (MEMS) membrane mirror is electrostatically actuated to dynamically adjust the optical beam focus and track the axial scanning of the coherence gate in a Doppler optical coherence tomography (DOCT) system at 8 kHz. The MEMS mirror is designed to maintain a constant numerical aperture of approximately 0.13 and a spot size of approximately 6.7 microm over an imaging depth of 1mm in water, which improves imaging performance in resolving microspheres in gel samples and Doppler shift estimation precision in a flow phantom. The mirror's small size (1.4 mm x 1 mm) will allow integration with endoscopic MEMS-DOCT for in vivo applications.

Computer Systems↗

PubMed2006

Information content of point radiance measurements in turbid media: implications for interstitial optical property quantification.

Motivated by a recent report by Dickey et al. [Phys. Med. Biol. 46, 2359 (2001)], who demonstrated optical property retrieval by using relative radiance measurements at a single position, we investigate the uniqueness of relative radiance measurements for quantifying the optical properties of turbid media by studying the solutions of the diffusion and P3 approximations of the Boltzmann transfer equation for a point source. Using the P3 approximation, we investigate the potential of radiance measurements for optical property recovery by examining the optical property response surface for point radiance information. We further derive first-order similarity relations for relative point radiance measurements and use these expressions to examine analytically the effects of noise on optical property retrieval over a wide range of optical properties typical of biological tissue. Finally, optimal experimental configurations are studied and explicit conditions for uniqueness derived that suggest potential strategies for improving optical property recovery. It is expected that point radiance measurements will prove valuable for both on-line treatment planning of minimally invasive laser therapies and optical characterization of tissues.

Algorithms↗

PubMed2005

Laser photothermoacoustic heterodyned lock-in depth profilometry in turbid tissue phantoms.

Frequency-domain correlation and spectral analysis photothermoacoustic (FD-PTA) imaging is a promising new technique, which is being developed to detect tumor masses in turbid biological tissue. Unlike conventional biomedical photoacoustics which uses time-of-flight acoustic information induced by a pulsed laser to indicate the tumor size and location, in this research, a new FD-PTA instrument featuring frequency sweep (chirp) and heterodyne modulation and lock-in detection of a continuous-wave laser source at wavelength is constructed and tested for its depth profilometric capabilities with regard to turbid media imaging. Owing to the linear relationship between the depth of acoustic signal generation and the delay time of signal arrival to the transducer, information specific to a particular depth can be associated with a particular frequency in the chirp signal. Scanning laser-fluence modulation frequencies with a linear frequency sweep method preserves the depth-to-delay time linearity and recovers FD-PTA signals from a range of depths. Combining with the depth information carried by the back-propagated acoustic chirp signal at each scanning position, one could rapidly generate subsurface three-dimensional images of the scanning area at optimal signal-to-noise ratios and low laser fluences, a combination of tasks that is difficult or impossible by use of pulsed photoacoustic detection. In this paper, results of PTA scans performed on tissue mimicking control phantoms with various optical, acoustical, and geometrical properties are presented. A mathematical model is developed to study the laser-induced photothermoacoustic waves in turbid media. The model includes both the scattering and absorption properties of the turbid medium. A good agreement is obtained between the experimental and numerical results. It is concluded that frequency domain photothermoacoustics using a linear frequency sweep method and heterodyne lock-in detection has the potential to be a reliable tool for biomedical depth-profilometric imaging.

Acoustics↗

PubMed2005

Interstitial Doppler optical coherence tomography.

Doppler optical coherence tomography (OCT) can image tissue structure and blood flow at micrometer-scale resolution but has limited imaging depth. We report a novel, linear-scanning, needle-based Doppler OCT system using angle-polished gradient-index or ball-lensed fibers. A prototype system with a 19-guage (diameter of approximately 0.9 mm) echogenic needle is constructed and demonstrates in vivo imaging of bidirectional blood flow in rat leg and abdominal cavity. To our knowledge, this is the first demonstration of Doppler OCT through a needle probe in interstitial applications to visualize deeply situated microcirculation.

Animals↗

PubMed2005

Optical and acoustic properties at 1064 nm of polyvinyl chloride-plastisol for use as a tissue phantom in biomedical optoacoustics.

A novel optoacoustic phantom made of polyvinyl chloride-plastisol (PVCP) for optoacoustic studies is described. The optical and acoustic properties of PVCP were measured. Titanium dioxide (TiO2) powder and black plastic colour (BPC) were used to introduce scattering and absorption, respectively, in the phantoms. The optical absorption coefficient (mua) at 1064 nm was determined using an optoacoustic method, while diffuse reflectance measurements were used to obtain the optical reduced scattering coefficient (mu's). These optical properties were calculated to be mua = (12.818 +/- 0.001)ABPC cm(-1) and mu's = (2.6 +/- 0.2)S(TiO2) + (1.4 +/- 0.1) cm(-1), where ABPC is the BPC per cent volume concentration, and S(TiO2) is the TiO2 volume concentration (mg mL(-1)). The speed of sound in PVCP was measured to be (1.40 +/- 0.02) x 10(3) m s(-1) using the pulse echo transmit receive method, with an acoustic attenuation of (0.56 +/- 1.01) f(1.51+/-0.06)MHz (dB cm(-1)) in the frequency range of 0.61-1.25 MHz, and a density, calculated by measuring the displacement of water, of 1.00 +/- 0.04 g cm(-3). The speed of sound and density of PVCP are similar to tissue, and together with the user-adjustable optical properties, make this material well suited for developing tissue-equivalent phantoms for biomedical optoacoustics.

Acoustics↗

PubMed2005

Characterization of measurement artefacts in fluoroptic temperature sensors: implications for laser thermal therapy at 810 nm.

BACKGROUND AND OBJECTIVES: Fluoroptic sensors are used to measure interstitial temperatures but their utility for monitoring laser interstitial thermal therapy (LITT) is unclear because these sensors exhibit a measurement artefact when exposed to the near-infrared (NIR) treatment light. This study investigates the cause of the artefact to determine whether fluoroptic sensors can provide reliable temperature measurements during LITT. STUDY DESIGN/MATERIALS AND METHODS: The temperature rise measured by a fluoroptic sensor irradiated in non-absorbing media (air and water) was considered an artefact. Temperature rise was measured as a function of distance from a laser source. Two different sensor designs and several laser powers were investigated. A relationship between fluence rate and measurement artefact in water was determined and coupled with a numerical simulation of LITT in liver to estimate the error in temperature measurements made by fluoroptic sensors in tissue in proximity to the laser source. The effect of ambient light on the performance of sensors capped with a transparent material ("clear-capped sensors") was also investigated. RESULTS: The temperature rise recorded in air by both clear- and black-capped fluoroptic sensors decreased with distance from a laser source in a manner similar to fluence rate. Sensor cap material, laser power, and the thermal properties of the surrounding medium affected the magnitude of the artefact. Numerical simulations indicated that the accuracy of a clear-capped fluoroptic sensor used to monitor a typical LITT treatment in liver is > 1 degrees C provided the sensor is further than approximately 3 mm from the source. It was also shown that clear-capped fluoroptic sensors are affected by ambient light. CONCLUSIONS: The measurement artefact experienced by both black-capped and clear-capped fluoroptic sensors irradiated by NIR light scales with fluence rate and is due to direct absorption of the laser light, which results in sensor self-heating. Clear-capped fluoroptic sensors can be used to accurately monitor LITT in tissue but should be shielded from ambient light.

Air↗

PubMed2005

Endoscopic Doppler optical coherence tomography in the human GI tract: initial experience.

BACKGROUND: Expanding the current endoscopic optical coherence tomography (OCT) system with Doppler capability may augment this novel high-resolution cross-sectional imaging technique with functional blood flow information. The aim of this feasibility study was to assess the clinical feasibility of an endoscopic Doppler OCT (EDOCT) system in the human GI tract. METHODS: During routine endoscopy, 22 patients were imaged by using a prototype EDOCT system, which provided color-Doppler and velocity-variance images of mucosal and submucosal blood flow at one frame per second, simultaneously with high-spatial-resolution (10-25 mum) images of tissue microstructure. The images were acquired from normal GI tract and pathologic tissues. OBSERVATIONS: Subsurface microstructure and microcirculation images of normal and pathologic GI tissues, including Barrett's esophagus, esophageal varices, portal hypertensive gastropathy, gastric antral vascular ectasia, gastric lymphoma, and duodenal adenocarcinoma, were obtained from 72 individual sites in vivo. Differences in vessel diameter, distribution, density, and blood-flow velocity were observed among the GI tissue pathologies imaged. CONCLUSIONS: To our knowledge, this is the first study to demonstrate the feasibility of EDOCT imaging in the human GI tract during routine endoscopy procedures. EDOCT may detect the different microcirculation patterns exhibited by normal and diseased tissues, which may be useful for diagnostic imaging and treatment monitoring.

Adult↗

PubMed2005

Suitability of radiochromic medium for real-time optical measurements of ionizing radiation dose.

A system, consisting of a novel optical fiber-based readout configuration and model-based method, has been developed to test suitability of a certain radiochromic medium for real-time measurements of ionizing radiation dose. Using this system with the radiochromic film allowed dose measurements to be performed during, and immediately after, exposure. The rates of change in OD before, during, and after exposure differ, and the change in OD during exposure was found to be proportional to applied dose in the tested range of 0-4 Gy. Estimating applied dose within an average error of less than 5% did not require a waiting time of 24-48 h as generally recommended with this radiochromic film. The errors can be further reduced by performing a calibration for each individual dosimeter setup instead of relying on batch calibration. Measurements of change in OD were found to be independent of dose-rate in the 95-570 cGy/min range for applied dose of 1 Gy or less. Some error was introduced due to dose-rate variation for doses of 2 Gy and above. The major limiting factor in utilizing this radiation sensitive medium for real-time in vivo dosimetry is the strong dependence on temperature in the clinically relevant range of 20-38 degrees C.

Acetylene↗

PubMed2005

Characterization and real-time optical measurements of the ionizing radiation dose response for a new radiochromic medium.

A new radiochromic film, GafChromic EBT, was investigated for use in a real-time radiation dosimetry system. It was found to be approximately eight times more sensitive to ionizing radiation dose, exhibited less postexposure development and achieved stable readout faster than one of its predecessors, GafChromic MD-55. A clear distinction in change in optical density between exposure and postexposure was observed, but the measurements obtained during exposure were not linear with time or dose. This could not be explained by a shift in wavelength of maximum change in absorbance, as it was stable at approximately 636 nm during the entire exposure range (up to 9.52 Gy). Increasing the spectral window of interest over which calculations were performed did little to correct the nonlinearity. The radiochromic film exhibited small dose rate dependence in real-time measurements, with an increase in standard deviation of change in optical density measurements from 0.9% to 1.8% over a sixfold variation in dose rate. Overall, GafChromic EBT has increased sensitivity and decreased postexposure darkening, and this bodes well for its potential role as a radiation dosimeter, including real-time applications.

Computer Systems↗

PubMed2004

Micromachined array tip for multifocus fiber-based optical coherence tomography.

High-resolution optical coherence tomography demands a large detector bandwidth and a high numerical aperture for real-time imaging, which is difficult to achieve over a large imaging depth. To resolve these conflicting requirements we propose a novel multifocus fiber-based optical coherence tomography system with a micromachined array tip. We demonstrate the fabrication of a prototype four-channel tip that maintains a 9-14-microm spot diameter with more than 500 microm of imaging depth. Images of a resolution target and a human tooth were obtained with this tip by use of a four-channel cascaded Michelson fiber-optic interferometer, scanned simultaneously at 8 kHz with geometric power distribution across the four channels.

Equipment Design↗

PubMed2004

Radiance-based monitoring of the extent of tissue coagulation during laser interstitial thermal therapy.

Optical monitoring relates the dynamic changes in measured light intensity to the extent of treatment-induced coagulation that occurs during laser interstitial thermal therapy. We utilized a two-region Monte Carlo simulation to elucidate the nature of the changes in interstitial radiance and fluence that result from the formation of a volume of thermal coagulation surrounding a cylindrical emitter. Using simulation results, we demonstrate that radiance sensors are more sensitive than traditional fluence sensors to coagulation-induced scattering changes. Radiance measurements take advantage of directional detection angles that are more receptive to the onset and passing of the coagulation boundary. We performed experiments with albumen phantoms to demonstrate the practicality of the radiance method for monitoring interstitial laser thermal therapy.

Albumins↗

PubMed2004

Development of a laser photothermoacoustic frequency-swept system for subsurface imaging: theory and experiment.

In conventional biomedical photoacoustic imaging systems, a pulsed laser is used to generate time-of-flight acoustic information of the subsurface features. This paper reports the theoretical and experimental development of a new frequency-domain (FD) photo-thermo-acoustic (PTA) principle featuring frequency sweep (chirp) and heterodyne modulation and lock-in detection of a continuous-wave laser source at 1064 nm wavelength. PTA imaging is a promising new technique which is being developed to detect tumor masses in turbid biological tissue. Owing to the linear relationship between the depth of acoustic signal generation and the delay time of signal arrival to the transducer, information specific to a particular depth can be associated with a particular frequency in the chirp signal. Scanning laser modulation with a linear frequency sweep method preserves the depth-to-delay time linearity and recovers FD-PTA signals from a range of depths. Preliminary results performed on rubber samples and solid tissue phantoms indicate that the FD-PTA technique has the potential to be a reliable tool for biomedical depth-profilometric imaging.

Acoustics↗

PubMed2003

Models and measurements of light intensity changes during laser interstitial thermal therapy: implications for optical monitoring of the coagulation boundary location.

We have developed a multi-region spherical Monte Carlo (MC) model to simulate the dynamic changes in light intensity measured during laser interstitial thermal therapy (LITT). Model predictions were validated experimentally in tissue-simulating albumen phantoms with well-characterized optical properties that vary dynamically with LITT in a way similar to tissue. For long treatments (2.5 W, approximately 1800 s), the transient light intensity changes demonstrated better qualitative agreement with a three-region MC model (with an inner layer of fully coagulated optical properties, a middle layer of partially coagulated properties and an outer region of native properties); for short treatments (4 W, approximately 240 s), better qualitative agreement was seen with a two-region MC model (with an inner layer of fully coagulated properties and outer region of native properties). These differences were attributed to differences in coagulation formation during low- and high-powered heating regimes, respectively. At the end of heating, a three-region coagulation zone was observed for both heating schemes. Quantitatively, final light intensity changes at the end of heating were compared with changes predicted by both two- and three-region MC for the same experimentally measured coagulation size and found to agree within approximately 30% for both models. The developed MC model helps lend insight into the nature of thermal coagulation events occurring for low and high power LITT irradiation schemes.

Dose-Response Relationship, Radiation↗

PubMed2002

Optical rotation and linear and circular depolarization rates in diffusively scattered light from chiral, racemic, and achiral turbid media.

The polarization properties of light scattered in a lateral direction from turbid media were studied. Polarization modulation and synchronous detection were used to measure, and Mueller calculus to model and derive, the degrees of surviving linear and circular polarization and the optical rotation induced by turbid samples. Polystyrene microspheres were used as scatterers in water solutions containing dissolved chiral, racemic, and achiral molecules. The preservation of circular polarization was found to exceed the linear polarization preservation for all samples examined. The optical rotation induced increased with the chiral molecule concentration only, whereas both linear and circular polarizations increased with an increase in the concentrations of chiral, racemic, and achiral molecules. This latter effect was shown to stem solely from the refractive index matching mechanism induced by the solute molecules, independent of their chiral nature.

Biophysical Phenomena↗

PubMedDate not supplied

Angular measurements of light scattered by turbid chiral media using linear Stokes polarimeter.

The effects of turbid chiral media on light polarization are studied in different directions around the scattering samples using a refined linear Stokes polarimeter, which simplifies the signal analysis, and allows for the detailed investigations of scattered light. Because no moving parts are involved in a measurement at a specific detection direction, the determination accuracy of polarization states is increased. The results show that light depolarization increases with both turbidity and detection angle for low and moderately turbid samples; however, the angular dependence decreases with increasing turbidity. When the turbidity is increased to approximately 100 cm(-1), the depolarization becomes higher in the forward than in the backward direction. Polarization sensitive Monte Carlo simulations are used to verify some experimental observations. The results also demonstrate that surviving linear polarization fraction and overall intensity are more sensitive to the increase of glucose concentration in backward than in the forward direction in highly turbid media, indicating that backward geometry may be preferable for potential glucose detection in a biomedical context. Comparison measurements with optically inactive glycerol suggest that the refractive index matching effect, and not the chiral nature of the solute, dominates the observed optical rotation engendered by glucose in highly turbid media.

Equipment Design↗