Intramyocardial oxygen transport by quantitative diffuse reflectance spectroscopy in calves

Real-time optical spectroscopic monitoring of nonirrigated lesion progression within atrial and ventricular tissues

Despite considerable advances in guidance of radiofrequency ablation (RFA) therapy for the treatment of cardiac arrhythmias, success rates have been hampered by a lack of tools for precise intraoperative evaluation of lesion extent. Near-infrared spectroscopic (NIRS) techniques are sensitive to tissue structural and biomolecular properties, characteristics that are directly altered by radiofrequency (RF) treatment. In this work, a combined NIRS-RFA catheter is developed for real-time monitoring of tissue reflectance during RF energy delivery. An algorithm is proposed for processing NIR spectra to approximate nonirrigated lesion depth in both atrial and ventricular tissues. The probe optical geometry was designed to bias measurement influence toward absorption enabling enhanced sensitivity to changes in tissue composition. A set of parameters termed "lesion optical indices" are defined encapsulating spectral differences between ablated and unablated tissue. Utilizing these features, a model for real-time tissue spectra classification and lesion size estimation is presented. Experimental validation conducted within freshly excised porcine cardiac specimens showed strong concordance between algorithm estimates and post-hoc tissue assessment.

Oxygen saturation, red blood cell tissue fraction and speed resolved perfusion - A new optical method for microcirculatory assessment

We have developed a new fiber-optic system that combines diffuse reflectance spectroscopy (DRS) and laser Doppler Flowmetry (LDF) for a multi-modal assessment of the microcirculation. Quantitative data is achieved with an inverse Monte Carlo algorithm based on an individually adaptive skin model. The output parameters are calculated from the model and given in absolute units: hemoglobin oxygen saturation (%), red blood cell (RBC) tissue fraction (%), and the speed resolved RBC perfusion separated into three speed regions; 0-1mm/s, 1-10mm/s and above 10mm/s (% mm/s). The aim was to explore microcirculatory parameters using the new optical method, integrating DRS and LDF in a joint skin model, during local heating of the dorsal foot and venous and arterial occlusion of the forearm in 23 healthy subjects (age 20-28years). There were differences in the three speed regions in regard to blood flow changes due to local heating, where perfusion for high speeds increased the most. There was also a high correlation between changes in oxygenation and changes in perfusion for higher speeds. Oxygen saturation at baseline was 44% on foot, increasing to 83% at plateau after heating. The larger increase in perfusion for higher speeds than for lower speeds together with the oxygenation increase during thermal provocation, shows a local thermoregulatory blood flow in presumably arteriolar dermal vessels. In conclusion, there are improved possibilities to assess microcirculation using integrated DRS and LDF in a joint skin model by enabling both oxygenation and speed resolved blood flow assessment simultaneously and in the same skin site. Output parameters in absolute units may also yield new insights about the microcirculatory system.

Inverse Monte Carlo for estimation of scattering and absorption in liquid optical phantoms

A spectroscopic probe with multiple detecting fibers was used for quantifying absorption and scattering in liquid optical phantoms. The phantoms were mixtures of Intralipid and red and blue food dyes. Intensity calibration for the detecting fibers was undertaken using either a microsphere suspension (absolute calibration) or a uniform detector illumination (relative calibration between detectors). Two different scattering phase functions were used in an inverse Monte Carlo algorithm. Data were evaluated for residual spectra (systematic deviations and magnitude) and accuracy in estimation of scattering and absorption. Spectral fitting was improved by allowing for a 10% intensity relaxation in the optimization algorithm. For a multi-detector setup, non-systematic residual spectrum was only found using the more complex Gegenbauer-kernel phase function. However, the choice of phase function did not influence the accuracy in the estimation of absorption and scattering. Similar estimation accuracy as in the multi-detector setup was also obtained using either two relative calibrated detectors or one absolute calibrated detector at a fiber separation of 0.46 mm.

Inverse Monte Carlo method in a multilayered tissue model for diffuse reflectance spectroscopy

Model based data analysis of diffuse reflectance spectroscopy data enables the estimation of optical and structural tissue parameters. The aim of this study was to present an inverse Monte Carlo method based on spectra from two source-detector distances (0.4 and 1.2 mm), using a multilayered tissue model. The tissue model variables include geometrical properties, light scattering properties, tissue chromophores such as melanin and hemoglobin, oxygen saturation and average vessel diameter. The method utilizes a small set of presimulated Monte Carlo data for combinations of different levels of epidermal thickness and tissue scattering. The path length distributions in the different layers are stored and the effect of the other parameters is added in the post-processing. The accuracy of the method was evaluated using Monte Carlo simulations of tissue-like models containing discrete blood vessels, evaluating blood tissue fraction and oxygenation. It was also compared to a homogeneous model. The multilayer model performed better than the homogeneous model and all tissue parameters significantly improved spectral fitting. Recorded in vivo spectra were fitted well at both distances, which we previously found was not possible with a homogeneous model. No absolute intensity calibration is needed and the algorithm is fast enough for real-time processing.