Matti Kinnunen

Effect of the size and shape of a red blood cell on elastic light scattering properties at the single-cell level

We demonstrate the use of a double-beam optical tweezers system to stabilize red blood cell (RBC) orientation in the optical tweezers during measurements of elastic light scattering from the trapped cells in an angle range of 5-30 degrees. Another laser (He-Ne) was used to illuminate the cell and elastic light scattering distribution from the single cell was measured with a goniometer and a photomultiplier tube. Moreover, CCD camera images of RBCs with and without laser illumination are presented as complementary information. Light scattering from a RBC was measured in different fixed orientations. Light scattering from cells was also measured when the length of the cell was changed in two different orientations. Light scattering measurements from spherical and crenate RBCs are described and the results are compared with other cell orientations. Analysis shows that the measured elastic light scattering distributions reveal changes in the RBC’s orientation and shape. The effect of stretching on the changes in scattering is larger in the case of face-on incidence of He-Ne laser light than in rim-on incidence. The scattering patterns from RBCs in different orientations as well as from a spherical RBC were compared with numerical results found in literature. Good correlation was found.

Effect of glucose on photoacoustic signals at the wavelengths of 1064 and 532 nm in pig blood and intralipid

Non-invasive glucose monitoring is one of the most active areas in biomedical research. Various techniques have been developed over the years to meet the clinical requirements of non-invasive monitoring in the physiologically relevant glucose concentration range, but without a breakthrough. This paper used the pulsed photoacoustic (PA) technique to study glucose-induced changes in pig whole blood and 1% Intralipid™ using an Nd : YAG laser with wavelengths of 1064 and 532 nm as the optical energy source. Scattering properties of the sample significantly affect the laser-induced pressure waves. Glucose was found to affect both the scattering and the absorption properties of the samples. The results showed an increase of 11.4%/500 mg dl−1 added glucose in the peak-to-peak value of the PA signal in blood at 1064 nm, whereas the corresponding increase was only 1.35%/500 mg dl−1 in 1% Intralipid™. At a wavelength of 532 nm, the glucose increased the peak-to-peak value of the PA signal by 6.0%/500 mg dl−1 added glucose in blood. On the whole, the pulsed PA technique proved to be an efficient tool for the study of glucose-induced changes in blood and tissue phantoms in vitro.

Measurements of fundamental properties of homogeneous tissue phantoms

Abstract. We present the optical measurement techniques used in human skin phantom studies. Their accuracy and the sources of errors in microscopic parameters’ estimation of the produced phantoms are described. We have produced optical phantoms for the purpose of simulating human skin tissue at the wavelength of 930 nm. Optical coherence tomography was used to measure the thickness and surface roughness and to detect the internal inhomogeneities. A more detailed study of phantom surface roughness was carried out with the optical profilometer. Reflectance, transmittance, and collimated transmittance of phantoms were measured using an integrating-sphere spectrometer setup. The scattering and absorption coefficients were calculated with the inverse adding-doubling method. The reduced scattering coefficient at 930 nm was found to be 1.57±0.14  mm−1 and the absorption was 0.22±0.03  mm−1. The retrieved optical properties of phantoms are in agreement with the data found in the literature for real human tissues.

Skin phantoms with realistic vessel structure for OCT measurements

We present here a novel phantom for optical coherence tomography (OCT) made of polyvinyl chloride-plastisol (PVCP). The optical properties of PVCP were estimated by the Mie theory and deduced from OCT measurements. Titanium dioxide (TiO2) powder and black plastic colour (light-absorbing plastic ink) were used to introduce scattering to the phantom and create capillary structure, respectively.

Light Propagation in NIR Spectroscopy of the Human Brain

In study of the brain, oxygenation changes in the cerebral cortex are of great interest, since the concentrations of oxyhaemoglobin and deoxyhaemoglobin change due to coupling of hemodynamics to cortical neural activity. In order to non-invasively monitor oxygenation in the cerebral cortex by near-infrared spectroscopy (NIRS), light should penetrate into brain tissue to a depth of approximately 1-2 cm. Many studies show that by increasing the source-detector distance, illuminating light penetrates deeper into brain tissue. Using tissue-mimicking phantom measurements, forehead in vivo measurements, and Monte Carlo (MC) simulations, this paper estimates light propagation in the brain and the minimum source-detector distance to allow sensing of the cerebral cortex. We present optical sensing of a pulsating aqueous intralipid suspension in a vessel located at different depths within a multilayered phantom of the human forehead. Experimental results are compared with the MC simulations accounting for the optical properties of the phantom. The thickness and morphology of the different tissue layers were obtained from an anatomical magnetic resonance image of a test subjects head. Results from these three methods correlate with each other and show that the brain cortex can be sensed with optical methods based on NIRS.

Imaging of subchondral bone by optical coherence tomography upon optical clearing of articular cartilage

Optical clearing is an effective method to reduce light scattering of biological tissues that provides significant enhancement of light penetration into the biological tissues making non-invasive diagnosis more feasible. In current report Optical Coherence Tomography (OCT) in conjunction with optical clearing is applied for assessment of deep cartilage layers and cartilage-bone interface. The solution of Iohexol in water has been used as an optical clearing agent. The cartilage-bone boundary becomes visible after 15 min of optical clearing that enabling non-invasive estimation of its roughness: Sa = 10 ± 1 µm. The results show that for 0.9 mm thick cartilage optical clearing is stopped after 50 min with an increase of refractive index from 1.386 ± 0.008 to 1.510 ± 0.009. Current approach enables more reliable detection of arthroscopically inaccessible regions, including cartilage-bone boundary and subchondral bone, and potentially improves accuracy of the osteoarthritis diagnosis.

Glucose sensing in aqueous Intralipid suspension with an optical coherence tomography system: experiment and Monte Carlo simulation

Peculiarities of light transport in IntralipidTM solutions and the effect of glucose on light scattering properties of the solution at two different IntralipidTM concentrations were studied with optical coherence tomography (OCT) technique in vitro. An open air OCT system using a superluminescent light source with center wavelength = 830 nm was used. 5% IntralipidTM solutions were used to simulate a biological tissue (skin) in our experiment. Glucose concentrations at the physiologically relevant level were added to IntralipidTM solutions. Increasing IntralipidTM concentration increases the scattering coefficient of the media meanwhile increasing glucose concentration increases the refractive index of the media and reduces the scattering coefficient of the media. The experimental data were compared to Monte Carlo simulations. We also made the simulations for 2% IntralipidTM solution. The results indicate that glucose added to 2 and 5% IntralipidTM solutions changes their scattering properties, which is manifested by a decrease in the slope of the OCT signal. This finding shows the ways of using OCT for sensing glucose and monitoring the alterations of its content in biotissues. Some discrepancies between measurements and simulations were found, which need further investigation.

Measurements of glucose content in scattering media with time-of-flight technique: comparison with Monte Carlo simulations

Scattering effect of the media can be seen as a pulse broadening and time delay of the pulse maximum relatively to the initial pulse. The purpose of this paper is to study the applicability of the time-of-flight measurement technique for glucose detection in 2% Intralipid solutions in vitro. Glucose samples with concentrations of 100, 200, 300, 500, 1000, 2000, 4000, and 8000 mg/dl are studied. Laser pulses with = 906 nm and FWHM λ = 30 ps are used in the experiments to investigate scattering properties of Intralipid . 1 - 5% suspensions are used to simulate scattering properties of different skin layers in the NIR spectrum region. Measurements are conducted with a slab cuvette, with 300-μm step index type fibers, and with 100-μm gradient index type fibers. Light propagation in the aqueous solutions is also studied by the Monte Carlo simulation. The simulations and the measurement results seem to correlate quite well for Intralipid suspensions. A clear correlation of pulse parameters as a function of Intralipid concentration was found. Slight changes of time delays of pulse maxima and the pulse broadening as a function of glucose concentration were revealed. Gradient index type fibers are found to be better choice for sensing glucose than the step index type fibers.

Optical properties of plasmon-resonant bare and silica-coated nanostars used for cell imaging

Abstract. We synthesized and characterized gold nanostars and their silica-coated derivatives with 7- to 50-nm shell thicknesses as contrast agents for optical imaging. The scattering and absorption coefficients of the nanoparticles (NPs) were estimated by means of collimated transmittance and diffuse reflectance/transmittance analyses. The contrasting properties of the nanostructures were studied in optical coherence tomography glass capillary imaging. The silica-coated nanostars with the thickest shell have higher scattering ability in comparison with bare nanostars. Viability assays confirmed weak in vitro toxicity of nanostructures at up to ∼200-μg/mL concentrations. We showed real-time visualization of nanostars in both agarose and cultured cells by analyzing the backscattering signal using a conventional laser confocal microscope. The signal intensity detected from the silica-coated NPs was almost 1.5 times higher in comparison with bare nanostars. To the best of our knowledge, this is the first time that conventional laser confocal microscopy was applied in combined scattering and transmitted light modes to detect the backscattered signal of gold nanostars, which is useful for direct monitoring of the uptake, translocation, and accumulation of NPs in living cells.

Optical clearing at cellular level

Abstract. Strong light scattering in tissues and blood reduces the usability of many optical techniques. By reducing scattering, optical clearing enables deeper light penetration and improves resolution in several optical imaging applications. We demonstrate the usage of optical tweezers and elastic light scattering to study optical clearing [one of the major mechanisms—matching of refractive indices (RIs)] at the single particle and cell level. We used polystyrene spheres and human red blood cells (RBCs) as samples and glycerol or glucose water solutions as clearing agents. Optical tweezers kept single microspheres and RBCs in place during the measurement of light scattering patterns. The results show that optical clearing reduces the scattering cross section and increases g. Glucose also decreased light scattering from a RBC. Optical clearing affected the anisotropy factor g of 23.25-μm polystyrene spheres, increasing it by 0.5% for an RI change of 2.2% (20% glycerol) and 0.3% for an RI change of 1.1% (13% glucose).