Near-infrared (NIR) optical spectroscopy characterizes breast tissue hormonal and age status.

The coming of age of tissue spectroscopy is heralded by the article from the Tromberg laboratory on quantitative breast tissue spectroscopy by Cerussi et al (1), which appears in this issue of Academic Radiology. In contrast to other noninvasive tissue monitoring and imaging capabilities, near-infrared (NIR) tissue spectroscopy affords indeed what would be expected of it: quantitation of optical parameters for normal and abnormal tissues. This article takes up in detail the quantitation of normal tissues, while others have performed necessary, less-extensive studies on the similar types of tissues (2,3). The introduction and acceptance of quantitative tissue optical studies by the radiology community were severely confounded by “diaphanography” of the 1930s (4), in which shadow images failed to show adequate specificity because of the low contrast ratio due to nonspecific background absorption. Only recently has multiwavelength imaging in the time and frequency domain provided adequate spectroscopic and fluorometric resolution of the intrinsic and extrinsic signals from human in vivo breast tissues (2,3). A particular example of the progress of quantitative spectroscopy in human breast tissue is afforded by the article of Cerussi and colleagues (1). They show how the three components of blood—oxygenated hemoglobin, deoxygenated hemoglobin, and total—together with water, in fact can be adequately and reproducibly measured in human subjects in various physiologic states with sufficient sensitivity to identify aging and hormonal changes. While nuclear MR imaging, positron emission tomography (PET), and ultrasonography (US) provide separate morphologic and biochemical parameters, the NIR optical method is outstanding for its simplicity, economy, and, as shown by this article, high specificity. The quantitative study of biochemical and structural parameters of photon migration in tissues is a relatively young area of research; measured in terms of MR imaging development, it would scarcely bridge a third of the time required for the clinical application of phosphorus-31 spectroscopy to the point where quantitative values were obtained on the hedgehog brain (5) or the exercising arm (6,7). Phosphocreatine is the principal biochemical signal of tissue nuclear MR, and the accumulation of fluorodeoxyglucose is the principal metabolic signal of PET. The optical method relies on the clear and prominent signals of oxyhemoglobin and deoxyhemoglobin, as measured in the NIR window where hemoglobin itself absorbs the dominant and proper amount of light for accurate measurements. Water and lipid show small but important absorbances, as well. Quantitative NIR optical studies cited by Cerussi et al (1) and others give the physiologists delight: the fractional saturation of tissue hemoglobin with oxygen and the important quantity, blood concentration, a major difference from arterial pulse oximetry. The optical measures occur where oxygen extraction by the tissue is actually occurring in the arteriolar-capillary venular beds, as opposed to the inactive arterial and venous portions of the circulatory system. The blood parameters are highly responsive to the age and hormonal state of the tissue and thus acquire immediate clinical importance as a new measure of evaluating vascular disease. NIR tissue spectroscopy can rightly be compared with nuclear MR in the 1980s, when MR spectroscopy was producing unrivaled information on tissue bioenergetics, and imaging was fighting the battle of resolution with less possibilities for physiologic importance. The technological develAcad Radiol 2001; 8:209–210