Quantitative diffuse optical spectroscopy for noninvasive measurements of the malaria pigment hemozoin

Abstract

Hemozoin (Hz) is a crystal by-product of hemoglobin consumption by malaria parasites. There are currently no in vivo deep tissue sensing methods that can quantify Hz presence noninvasively, which would be advantageous for malaria research and treatment. In this work, we describe the broadband near-infrared optical characterization of synthetic Hz in static and dynamic tissue-simulating phantoms. Using hybrid frequency domain and continuous-wave near-infrared spectroscopy, we quantified the broadband optical absorption and scattering spectra of Hz and identified the presence of Hz at a minimum tissue-equivalent concentration of 0.014 µg/mL in static lipid emulsion phantoms simulating human adipose. We then constructed a whole blood-containing tissue-simulating phantom and demonstrated the detection of Hz at physiologically-relevant tissue oxygen saturations ranging from 70-90%. Our results suggest that quantitative diffuse optical spectroscopy may be useful for detecting deep tissue Hz in vivo.

Fig. 1.

Schematic of the FD/CW-DOS multi-distance system used in this study.

Fig. 2.

Representative self-calibrated multi-distance FD-DOS data and the accompanying model fit at 660 nm. The medium is an Intralipid phantom with 1 µg/mL Hz. The modulation frequency cutoff (202 MHz) was chosen in combination with the maximum SDS (18 mm) to ensure all data were above the noise floor. Each line represents a comparison of the measurements at two source-detector separations: ratio of amplitude (a) and difference in phase (b). The markers represent every fifth frequency measured.

Fig. 3.

Representative self-calibrated multi-distance broadband CW reflectance data and model fit, with ${\mu }_S^{\prime }$ constrained by the frequency domain power law fit. The medium is an Intralipid phantom with 1 µg/mL Hz.

Fig. 4.

(a) Absorption and reduced scattering spectrum obtained with quantitative broadband DOS, of a bovine blood phantom at 80% saturation. Scattering was estimated via a power-law fit to the FD-DOS coefficients. Broadband reflectance was calibrated with the reconstructed broadband system response from a multi-distance fit. (b) Individual chromophore spectra

Fig. 5.

(a) Absorption spectra for increasing concentrations of Hz. (b) Measured molar absorption spectrum for Hz, overlaid with attenuation spectrum of Hz in DI water. The shaded uncertainty region represents the interquartile range of the normalized difference between absorption spectra and their fits.

Fig. 6.

Measured absorption of Hz at 643 nm with increasing concentration.

Fig. 7.

Measured concentrations of Hz in Intralipid phantoms, using the absorption spectrum in Fig. 5(b). Error bars represent +/- 2 standard error of the chromophore concentration estimated from the linear regression of the Hz absorption spectrum to the measured absorption spectrum.

Fig. 8.

${\mathrm{HbO}}_2$, HHb, and THb measurements versus DO, and the reconstructed ${\mathrm{HbO}}_2$ dissociation curves for each experiment. Closed circles represent the experiment containing Hz, while open circles represent the control.

Fig. 9.

Measured Hz concentration as a function of measured oxygen saturation between 70 and 90%.