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. 2014 Sep 1;105(9):094103. doi: 10.1063/1.4894635

FIG. 2.

FIG. 2.

(a) Schematic showing that volumetric fraction of erythrocytes (hematocrit or Ht) significantly varies through the entire vascular network, decreasing from systemic blood vessels down to capillaries. (b) Experiments with whole and diluted blood show that the temperature dependent optoacoustic response is scaled proportionally to hematocrit. (c) However, when normalized at 37 °C, it becomes invariant as the curves representing whole and diluted blood coincide. Curves labeled Ht 40 represent whole porcine blood. Curves with reduced Ht indicate that the blood sample was diluted, for example, Ht 20 represents a sample with dilution fraction of ½. Dashed-dotted line marks zero optoacoustic response. Yellow dashed line on the panel (c) shows the universal blood calibration curve OA¯=4ΔTmax(T1T0)3(TT0)(TT1)+TT0T1T0 with parameters: T1 = 37 °C—fixed normalization temperature, where normalized optoacoustic intensity OA¯=1; T0 = −12.8 ± 0.5 °C—temperature of zero optoacoustic response estimated directly from the image (N = 8 animal samples); and ΔTmax = 10.8 ± 0.2 °C—maximum nonlinear temperature deviation in the temperature range [T0 T1]. If ΔTmax = 0, the calibration is linear. For comparison, normalized Grüneisen parameter of water has maximum nonlinear temperature deviation ΔTmax = 1.9 °C in the range of 4–37 °C. For more details on ΔTmax, see supplementary material.45