Rationale and objectives

Near-infrared (NIR) diffuse optical spectroscopy and imaging may enhance existing technologies for breast cancer screening, diagnosis, and treatment. NIR techniques are based on sensitive, quantitative measurements of functional contrast between healthy and diseased tissue. In this study, the authors quantified the origins of this contrast in healthy breasts.

Materials and methods

A seven-wavelength frequency-domain photon migration probe was used to perform noninvasive NIR measurements in the breasts of 28 healthy women, both pre- and postmenopausal, aged 18-64 years. A diffusive model of light transport quantified oxygenated and deoxygenated hemoglobin, water, and lipid by their absorption signatures. Changes in the measured light-scattering spectra were quantified by means of a "scatter power" parameter.

Results

Substantial quantitative differences were observed in both absorption and scattering spectra of breast as a function of subject age. These physiologic changes were consistent with long-term hormone-dependent transformations that occur in breast. Instrument response was not adversely affected by subject age or menopausal status.

Conclusion

These measurements provide new insight into endogenous optical absorption and scattering contrast mechanisms and have important implications for the development of optical mammography. NIR spectroscopy yields quantitative functional information that cannot be obtained with other noninvasive radiologic techniques.

Near infrared (NIR) diffuse optical spectroscopy and imaging may enhance existing technologies for breast cancer screening, diagnosis, and treatment. NIR spectroscopy yields quantitative functional information that cannot be obtained with other non-invasive radiological techniques. In this study we focused upon the origins of this contrast in healthy breast, especially from water and lipids. Non-invasive NIR measurements were performed on the breasts of 30 healthy women using a seven-wavelength frequency-domain photon migration probe. Subjects included pre- and post-menopausal women between the ages of 18 and 64. A diffusive model of light transport quantified oxygenated and deoxygenated hemoglobin, water, and lipid by their absorption signatures. Changes in the measured light-scattering spectra were quantified by a "scatter power" parameter. Substantial quantitative differences were observed in both absorption and scattering spectra of breast as a function of age. These physiological changes were consistent with long-term hormone-dependent transformations that occur in breast. Immanent response was not adversely affected by subject age or menopausal status. The impact of neglecting water and lipids upon optical measurements in the breast is to artificially ascribe extra absorption. The effect can be significant, causing 20-30% errors in hemoglobin concentration and 5% in hemoglobin saturation. These errors cold dull the inherent contrast between normal and tumor tissue and thus affect optical mammography.

A technique for measuring broadband near-infrared absoprtion spectra of turbid media is presented using a combination of frequency-domain (FD) and steady-state (SS) reflectance methods. Most of the wavelength coverage is provided by a white-light SS measurement, while the FD data are acquired at a few selected wavelengths. Coefficients of absorption (μa) and reduced scattering (μs') derived from the FD data are used to intensity-calibrate the SS measurements and to estimate μs'at all wavelengths in the spectral window of interest. After these steps are performed, μa can be determined by comparing the SS reflectance values to the predictions of diffusion theory, wavelength by wavelength. We present an application of this method to breast tumor characterization. A case study of a fibroadenoma is shown, where different absorption spectra were found between the normal and the tumor sides.