Moritz Friebel

Optical Properties of Circulating Human Blood in the Wavelength Range 400–2500 nm

Knowledge about the optical properties μa,μs, and g of human blood plays an important role for many diagnostic and therapeutic applications in laser medicine and medical diagnostics. They strongly depend on physiological parameters such as oxygen saturation, osmolarity, flow conditions, haematocrit, etc. The integrating sphere technique and inverse Monte Carlo simulations were applied to measure μa,μs, and g of circulating human blood. At 633 nm the optical properties of human blood with a haematocrit of 10% and an oxygen saturation of 98% were found to be 0.210±0.002 mm-1 for μa,77.3±0.5 mm-1 for μs, and 0.994±0.001 for the g factor. An increase of the haematocrit up to 50% lead to a linear increase of absorption and reduced scattering. Variations in osmolarity and wall shear rate led to changes of all three parameters while variations in the oxygen saturation only led to a significant change of the absorption coefficient. A spectrum of all three parameters was measured in the wavelength range 400-2500 nm for oxygenated and deoxygenated blood, showing that blood absorption followed the absorption behavior of haemoglobin and water. The scattering coefficient decreased for wavelengths above 500 nm with approximately λ-1.7; the g factor was higher than 0.9 over the whole wavelength range.

Determination of optical properties of human blood in the spectral range 250 to 1100 nm using Monte Carlo simulations with hematocrit-dependent effective scattering phase functions.

The absorption coefficient mu(a), scattering coefficient mu(s), and anisotropy factor g of diluted and undiluted human blood (hematocrit 0.84 and 42.1%) are determined under flow conditions in the wavelength range 250 to 1100 nm, covering the absorption bands of hemoglobin. These values are obtained by high precision integrating sphere measurements in combination with an optimized inverse Monte Carlo simulation (IMCS). With a new algorithm, appropriate effective phase functions could be evaluated for both blood concentrations using the IMCS. The best results are obtained using the Reynolds-McCormick phase function with the variation factor alpha = 1.2 for hematocrit 0.84%, and alpha = 1.7 for hematocrit 42.1%. The obtained data are compared with the parameters given by the Mie theory. The use of IMCS in combination with selected appropriate effective phase functions make it possible to take into account the nonspherical shape of erythrocytes, the phenomenon of coupled absorption and scattering, and multiple scattering and interference phenomena. It is therefore possible for the first time to obtain reasonable results for the optical behavior of human blood, even at high hematocrit and in high hemoglobin absorption areas. Moreover, the limitations of the Mie theory describing the optical properties of blood can be shown.

Model function to calculate the refractive index of native hemoglobin in the wavelength range of 250-1100 nm dependent on concentration

The real part of the complex refractive index of oxygenated native hemoglobin solutions dependent on concentration was determined in the wavelength range 250 to 1100 nm by Fresnel reflectance measurements. The hemoglobin solution was produced by physical hemolysis of human erythrocytes followed by ultracentrifugation and filtration. A model function is presented for calculating the refractive index of hemoglobin solutions depending on concentration in the wavelength range 250 to 1100 nm.

Optical properties of platelets and blood plasma and their influence on the optical behavior of whole blood in the visible to near infrared wavelength range

The optical parameters absorption coefficient, scattering coefficient, and the anisotropy factor of platelets (PLTs) suspended in plasma and cell-free blood plasma are determined by measuring the diffuse reflectance, total and diffuse transmission, and subsequently by inverse Monte Carlo simulation. Furthermore, the optical behavior of PLTs and red blood cells suspended in plasma are compared with those suspended in saline solution. Cell-free plasma shows a higher scattering coefficient and anisotropy factor than expected for Rayleigh scattering by plasma proteins. The scattering coefficient of PLTs increases linearly with the PLT concentration. The existence of physiological concentrations of leukocytes has no measurable influence on the absorption and scattering properties of whole blood. In summary, red blood cells predominate over the other blood components by two to three orders of magnitude with regard to absorption and effective scattering. However, substituting saline solution for plasma leads to a significant increase in the effective scattering coefficient and therefore should be taken into consideration.

Determination of the complex refractive index of highly concentrated hemoglobin solutions using transmittance and reflectance measurements

The complex refractive index of highly concentrated hemoglobin solutions as they appear in red blood cells are determined in the wavelength range of 250 to 1100 nm using transmittance and Fresnel reflectance measurements. The determined real parts of the refractive indices are on average 0.02 units higher than the values found in the literature. The wavelength dependence of the measured data in the UV region differs from the calculated data using the Kramers-Kronig relation.

Empirical model functions to calculate hematocrit-dependent optical properties of human blood

The absorption coefficient, scattering coefficient, and effective scattering phase function of human red blood cells (RBCs) in saline solution were determined for eight different hematocrits (Hcts) between 0.84% and 42.1% in the wavelength range of 250-1100 nm using integrating sphere measurements and inverse Monte Carlo simulation. To allow for biological variability, averaged optical parameters were determined under flow conditions for ten different human blood samples. Based on this standard blood, empirical model functions are presented for the calculation of Hct-dependent optical properties for the RBCs. Changes in the optical properties when saline solution is replaced by blood plasma as the suspension medium were also investigated.

Influence of osmolarity on the optical properties of human erythrocytes

Plasma osmolarity influences the volume and shape of red blood cells (RBCs). The volume change is inversely related to the hemoglobin concentration and as a consequence to the complex refractive index within the cell. These morphological changes can be linked to changes in the optical behavior of the cells. The optical parameters, absorption coefficient μa, scattering coefficient μs, and effective scattering phase function of red blood cells are investigated in dependence on osmolarity in the spectral range from 250 to 1100 nm. Integrating sphere measurements of light transmittance and reflectance in combination with inverse Monte-Carlo simulations are carried out for osmolarities from 225 to 400 mosmol/L. Osmolarity changes have a significant influence on the optical parameters, which can in part be explained by changes in the complex refractive index, cell shape, and cell volume. Spherical forms of RBCs induced by low osmolarity show reduced scattering effects compared to the normal RBC biconcave disk shape. Spinocytes, which are crenated erythrocytes induced by high osmolarity, show the highest scattering effects. Even only a 10% change in osmolarity has a drastic influence on the optical parameters, which appears to be of the same order as for 10% hematocrit and oxygen saturation changes.

Optical properties of circulating human blood

We investigated the optical properties (mu) a, (mu) s, and g of human blood under flow conditions using integrating sphere measurements and inverse Monte-Carlo-simulations. The experiments were conducted at 633 nm with regard to the influence of the most important physiological and biochemical blood parameters. In addition, a spectrum of all three parameters was measured in the wavelength range 400 to 2500 nm for oxygenated and deoxygenated blood.

A novel device for the non – invasive measurement of free hemoglobin in blood bags / Messgerät zur zerstörungsfreien Messung von freiem Hämoglobin in Blutkonserven

Abstract The production of red blood cell concentrates from human donors is a very expensive procedure and human resources are in short supply. Under perfect storage conditions at a temperature of 2 – 6 °C, a blood bag must be used within 35–49 days (in Germany). Visual inspection of the bag for apparent hemolysis by a blood bank physician is a crucial but subjective quality control assessment. Since an interruption of the cold chain cannot be definitely ruled out, bags are often disposed of prematurely for safety reasons. There is currently no method of testing a closed blood bag with respect to hemolysis for its suitability to be used in a transfusion. The proposed optical measuring device is a hemoglobin sensor which determines the free hemoglobin in standard erythrocyte concentrates without opening the bag. The optical measurements are done on the flexible tube connected to the main bag. The optical measurements were evaluated using standard hemoglobin solutions with an accuracy of 0.005 g/dL. These investigations show that in the future each blood bag can be tested non-invasively for its content of free hemoglobin. This will contribute to decreasing the wastage rate of red blood cell concentrates. Die Gewinnung von humanen Erythrozytenkonzentraten zur Transfusion ist ein sehr aufwendiges und teures Verfahren und die Verfügbarkeit ist begrenzt. Unter idealen Lagerbedingungen bei Temperaturen von 4°C beträgt die Haltbarkeit der Konserven max. 35–49 Tage. Die Qualitätsprüfung der Blutkonserve erfolgt, neben Qualitätskontrollen an Stichproben, allein durch Sichtkontrolle eines Facharztes. Da eine Unterbrechung der Kühlkette in vielen Fällen nicht ausgeschlossen werden kann, werden diese Konserven oft aus Sicherheitsgründen vorzeitig verworfen. Derzeit ist keine Methode bekannt, die eine Überprüfung einer geschlossenen Blutkonserve auf deren Eignung zur Transfusion durchführt. Der hier beschriebene optische Sensor dient der Qualitätssicherung von Erythrozytenkonzentraten am ungeöffneten, nicht modifizierten Standard-Blutbeutel. Die Messung wird an dem mit dem Beutel verbundenen Schlauch nach Sedimentation der Erythrozyten durchgeführt. Die optische Messung wurde an Standard-Hemoglobin-Lösungen mit einer Genauigkeit von 0,005g/dL validiert. Diese Untersuchungen zeigen, dass eine solche Qualitätssicherung in Zukunft an jeder Blutkonserve möglich ist, sodass nur solche Konserven verworfen werden müssen, die den Qualitätsansprüchen nicht mehr genügen.

Determination of oxygen saturation and hematocrit of flowing human blood using two different spectrally resolving sensors / Bestimmung von Sauerstoffsättigung und Hämatokrit an fließendem Blut mit zwei verschiedenen spektralen Sensoren

Abstract The blood parameters oxygen saturation and hematocrit were determined by two different spectral sensors using reflectance spectra from 550 to 900 nm and partial transmission spectra centered at 660 nm. The spectra were analyzed by the method of partial least squares. One sensor consists of a miniature integrating sphere, while the other was fiber-guided. The results show that the geometry of the sensors and different blood flows do not influence the spectral analysis significantly. Independent of the sensor geometry, both hematocrit and oxygen saturation could be determined with an absolute predicted root mean square error of less than 3%. Furthermore, the analysis showed that hematocrit prediction requires eight wavelength regions and oxygen saturation prediction requires four wavelength regions using reflectance spectroscopy. This implies that if the measurement is restricted to reflectance, a spectrometer is indispensable for determining both blood parameters. Hematocrit determination could be improved using reflectance measurements in combination with transmission.