Acquiring photoacoustic signature of hematocrit variation from plexus layer of in-silico human skin phantom

Abstract

Optical penetration inside human skin is constrained by the wavelength dependent scattering and absorption losses by tissue microstructure and chromophores. This computational study investigates whether the signature of hematocrit variation from plexus i.e., the first skin layer having very small blood volume percentage distributed in capillary vessels, is retained by the detected photoacoustic response. The in-silico skin phantom is irradiated by a light source equivalent to a small footprint and low power (below 5 W) continuous wave LASER diode. As the low fluence can be compensated by exploiting strong absorption by targeted chromophores (hemoglobin molecules), an irradiation of wavelength 405 nm has been used to generate detectable pressure from capillary blood vessels of plexus. Optical energy deposition inside the tissue has been modelled using Monte Carlo technique and the pressure wave is computed using k-wave. It is found that with the increase in hematocrit from 10% to 50%, photoacoustic amplitude monotonically increases and gets almost doubled. The increment is about 30% in the range of hematocrit of physiological interest (from 30% to 50%). The variation follows a quadratic relationship for the entire hematocrit range. This photoacoustic signature of hematocrit variation has further been validated against minimum detectable pressure (800 Pa). This numerical model is expected to be an important basis to realize the idea of low cost small footprint in-vivo photoacoustic hematocrit measurement device.

Determining total hemoglobin in blood irrespective of hemolysis status using a high-power LED-based low-cost photoacoustic system

“…al. [26], also presented a similar trend of PA amplitude with increasing PCV, under 405 nm irradiation. The PA amplitude in fig.…”

“…The PA amplitude in fig. 4 leans towards saturation for higher values of PCV, due to the saturation in optical energy deposition caused by the very high optical absorption coefficient of Hb [26]. The present study is advocating the idea that blood seems to be a continuous media to the PA system designed with shorter wavelength (380 nm -480 nm), whereas the same appears as an ensemble of discrete particles (RBCs) when the PA system operates at NIR wavelengths.…”

Observing temporal variation in hemolysis through photoacoustics with a low cost LASER diode based system

“…The linear dependence between the PA response and the amount of lysis can be rationalized with the help of the following observations: (i) confinement of Hb molecule enables each RBC to act as a cellular PA generator 38 , 39 , (ii) since the scattering coefficient of RBC suspension is small in the NIR range, the number of internal reflections of photons inside the optically dense RBC increases the probability of absorption of a photon by the RBC. As a result, RBCs upon NIR excitation generate a significantly higher amplitude of PA signal than the same amount of encapsulated Hb while mixed in the same volume of plasma or any similar isotonic buffer 34 and (iii) the PA amplitude varies almost linearly with the number density of RBC in the hematocrit range from 0 to 20% 34 , 40 .The amount of hemolysis is directly proportional to the number of RBCs destroyed i.e., the fall in the number density of PA sources which in turn decreases the amplitude of the PA signal linearly according to observation (iii).…”