Diffusion model of the optical absorbance of whole blood

Abstract

Photon-diffusion theory has had limited success in modeling the optical transmittance of whole blood. Therefore we have developed a new photon-diffusion model of the optical absorbance of blood. The model has benefited from experiments designed to test its fundamental assumptions, and it has been compared extensively with transmittance data from whole blood. The model is consistent with both experimental and theoretical notions. Furthermore, when all parameters associated with a given optical geometry are known, the model needs no variational parameters to predict the absolute transmittance of whole blood. However, even if the exact value of the incident light intensity is unknown (which is the case in many situations), only a single additive constant is required to scale experiment to theory. Finally, the model is shown to be useful for simulating scattering effects and for delineating the relative contributions of the diffuse transmittance and the collimated transmittance to the total optical density of whole blood. Applications of the model include oximetry and measurements of the arteriovenous oxygen difference in whole, undiluted blood.

T-matrix computations of light scattering by red blood cells

“…Computations on light propagation in blood are often performed in order to be able to correlate the results with those obtained from measurements [28,31,32]. Detailed information on biochemical and morphological properties of blood which are important to determine, both in vivo and in vitro [5,6,8,13,25,32,35], can be deduced and followed by relating measured to theoretical parameters.…”

“…Detailed information on biochemical and morphological properties of blood which are important to determine, both in vivo and in vitro [5,6,8,13,25,32,35], can be deduced and followed by relating measured to theoretical parameters. The choice of theoretical model and approximations must thus be made in close relation to physical reality.…”

“…The choice of theoretical model and approximations must thus be made in close relation to physical reality. Measurements have proven that Mie theory can successfully be applied, when studying the size of randomly oriented red blood cells in a suspension [32]. The various shapes of the red blood cells are more difficult to examine, although several experimental studies [4,19,27] have measured the influence of cell shape and alignment on optical parameters.…”

“…Despite its general character, the macroscopic average absorption parameter reveals several relevant, physiological properties of blood, such as the degree of oxygen saturation, concentration of haemoglobin and other light absorbing solutes that are characteristic of blood and thus relevant to the state of the patient's health. Measurements of optical absorption properties of blood are often performed by recording either reflected, [5,20,28,30] or transmitted [6,13,32] probing light from, or through a blood sample, or blood perfused tissue. A theoretical multiple scattering model, for example the diffusion approximation of the transport equation [28,30], inverse adding-doubling computations (IAD) [26] or Monte Carlo simulations [8], is then applied in order to relate the measured quantities (transmittance and/or reflectance) to the optical absorption parameter of blood.…”

“…However, there is a growing interest in the scattering properties of blood, partly since a correct compensation for light scattering improves the accuracy of the absorption measurements, but mainly as the scattering properties themselves provide interesting information on morphological properties of the blood cells [4,14,15,25,26,32,35]. Equivalent with the absorption properties of blood, the macroscopic average scattering properties can be determined by applying the previously mentioned theoretical multiple scattering models.…”