In vivo water state measurements in breast cancer using broadband diffuse optical spectroscopy

Abstract

Structural changes in water molecules are related to physiological, anatomical and pathological properties of tissues. Near infrared (NIR) optical absorption methods are sensitive to water; however, detailed characterization of water in thick tissues is difficult to achieve because subtle spectral shifts can be obscured by multiple light scattering. In the NIR, a water absorption peak is observed around 975 nm. The precise NIR peak's shape and position are highly sensitive to water molecular disposition. We introduce a bound water index (BWI) that quantifies shifts observed in tissue water absorption spectra measured by broadband diffuse optical spectroscopy (DOS). DOS quantitatively measures light absorption and scattering spectra and therefore reveals bound water spectral shifts. BWI as a water state index was validated by comparing broadband DOS to magnetic resonance spectroscopy, diffusion-weighted MRI and conductivity in bound water tissue phantoms. Non-invasive DOS measurements of malignant and normal breast tissues performed in 18 subjects showed a significantly higher fraction of free water in malignant tissues (p < 0.0001) compared to normal tissues. BWI of breast cancer tissues inversely correlated with Nottingham-Bloom-Richardson histopathology scores. These results highlight broadband DOS sensitivity to molecular disposition of water and demonstrate the potential of BWI as a non-invasive in vivo index that correlates with tissue pathology.

Figure 1

(a) In vivo tissue absorption spectrum (solid line) from normal breast tissue. (b) Tissue water spectrum after subtracting other tissue components’ spectra (solid line). (c) Normalized tissue water spectrum at 935-998nm (solid line). The pure water spectrum at 36°C is shown in each panel (a, b and c, dashed lines) for comparison.

Figure 2

BWI of bound water phantoms (linear fit, R=0.98, p=0.02). Error bars represent the differences between three measurements on the phantoms.

Figure 3

T2 relaxation time/rate and conductivity of the phantoms. (a) T2 relaxation times vs. gelatin concentration (R²=0.996, T2 values were measured one time per phantom). (b) Conductivity vs. gelatin concentrations (R=0.95, measured at 100kHz). (c) R2 vs. BWI (R=0.96) (d) Conductivity vs. BWI (R=0.99) Error bars represent the differences between two measurements on the phantoms (most of them are smaller than the symbol size).

Figure 4

BWI vs. ADC of water of bound water phantoms (R=−0.97). Error bars represent the differences between three measurements on the phantoms.

Figure 5

Line scanned Tissue Optical Index (TOI = ctH₂O × ctHHb/ctLipid) and Bound Water Index (BWI) of normal (blue squares) and malignant breast tissues (red triangles). TOI is higher and BWI is lower in malignant tissues with respect to normal tissues. Three continuous peak points of the malignant tissues are highlighted.

Figure 6

Box plots of BWI of malignant (1.96±0.3) and normal breast tissues (2.77±0.47) for 18 subjects. Tumor and normal tissues were differentiated with statistical significance with p<0.0001 (Wilcoxon ranked-sum test).

Figure 7

BWI vs. bulk water concentration of normal (blue squares) and malignant (red triangles) breast tissues. Both bulk water concentrations and BWI values were acquired from the same spatial locations. Normal and malignant tissue groups were discriminated by two separate linear fittings with slope_normal = −0.07±0.02, and slope_malignant= −0.01.

Figure 8

BWI vs. Nottingham-Bloom-Richardson score. The average and standard deviation of BWI values of breast cancer tissue samples from 18 patients are depicted with black squares and error bars. The linear fit had R= −0.96.