Broadband (550-1350 nm) diffuse optical characterization of thyroid chromophores

Abstract

Thyroid plays an important role in the endocrine system of the human body. Its characterization by diffuse optics can open new path ways in the non-invasive diagnosis of thyroid pathologies. Yet, the absorption spectra of tyrosine and thyroglobulin-key tissue constituents specific to the thyroid organ-in the visible to near infrared range are not fully available. Here, we present the optical characterization of tyrosine (powder), thyroglobulin (granular form) and iodine (aqueous solution) using a time domain broadband diffuse optical spectrometer in the 550-1350 nm range. Various systematic errors caused by physics of photo migration and sample inherent properties were effectively suppressed by means of advanced time domain diffuse optical methods. A brief comparison with various other known tissue constituents is presented, which reveals key spectral regions for the quantification of the thyroid absorbers in an in vivo scenario.

Figure 1

Absorption and reduced scattering spectra of (a) thyroglobulin, with high absorption in the visible region, and (b) tyrosine, with multiple peaks over the measured window; and absorption spectrum of (c) iodine, rapidly decreasing upon increasing wavelength reaches asymptotically zero around 800 nm. (d) Anatomy of the thyroid along with images of thyroglobulin and tyrosine samples are shown in insets (top right corner).

Figure 2

Comparison of absolution absorption spectrum with various distortion effects: (a) absorption features distorted by high scattering of tyrosine (red triangles), (b) fluorescence distorting the spectrum of thyroglobulin in the visible region (red circles), (c) Effect of finite laser bandwidth at sharp absorption change of tyrosine (1145 nm peak, red circles), (d) overestimation of absorption at small sample sizes (sample width: 3.2 mm, 4.8 mm, 6.4 mm), only spectra above 10 mm sample width match with each other.

Figure 3

Comparison of thyroglobulin, tyrosine and iodine spectra with typical tissue constituent spectra (oxy- and deoxy-hemoglobin, lipid, water, collagen): (a) tissue constituent spectra with absorption values relevant to thyroid tissues, (b) second order derivative spectra of various tissue constituents and the inset reveals some unique features of tyrosine spectrum.

Figure 4

Optical layout of time resolved diffuse optical instrumentation. The two detectors effectively cover the broadband range (500–1350 nm).