## 1 Introduction  
Near-infrared spectroscopy (NIRS), in the range of 650-1000 nm, is widely used to noninvasively measure the concentration of light absorbing substances such as hemoglobin, water, and lipids in living tissues. In breast cancer studies, NIRS has been used to monitor tumor response to neoadjuvant chemotherapy (NAC), as hemoglobin, water, and lipid proportions reflect microvasculature, cellular metabolism, angiogenesis, edema, hypoxia, and necrosis [1; 2; 3; 4; 5; 6; 7]. These parameters are significantly altered in tumor growth and regression. The validity of NIRS systems has generally been verified using tissue simulating phantoms such as intralipid-based aqueous phantoms [8; 9] and resin-based hard phantoms [10], while phantoms with various lipid and water contents have been proposed in previous studies [11; 12; 13; 14]. [PERSON] et al. and [PERSON] et al. used such phantoms to compare lipid and water measurements between a diffuse optical spectroscopy (DOS) system and magnetic resonance imaging (MRI), which was defined as the \"gold standard\" technique [11; 14]. In those studies, the authors confirmed that MRI and DOS measurements of water/lipid ratios in phantoms were almost identical. Studies using breast phantoms [13] and simulations [15] have also been conducted, but the structure of the living body is more complicated. Several studies have reported on the relationship between DOS parameters and breast density on MRI and their link with breast cancer risk [3; 16; 17]. Although previous studies have compared DOS with other modalities such as MRI, there have been no reports comparing DOS with other modalities in the direct measurement of lipid and water signals in the human breast. Although we previously compared lipid and water measurements of the contents of a phantom between MRI and time-domain diffuse optical spectroscopy (TD-DOS) [14], comparison of these parameters in living humans is difficult, because the measurement postures required in the two modalities differ: MRI measurement is conducted with the subject in a prone (downward facing) position, while the subject is generally in a supine (upward facing) or standing position for the DOS measurement. Computed tomography (CT) is a cross-sectional, high-resolution, three-dimensional diagnostic imaging modality that generally uses single-energy polychromatic X-rays. Its recently increased clinical utility is primarily attributed to significantly increased scan speed as a synergic effect of increased gantry rotation speed and increased longitudinal detector coverage, as well as the development of various radiation-lowering techniques to improve patient risk/benefit ratios [18]. In many cases, chest CT scans are performed as the first choice of investigation to stage breast cancers in our hospital. Although CT has an inherent limitation in terms of soft tissue differentiation because the pixel value or CT number depends entirely on the linear attenuation coefficient, which has considerable overlap between different body materials, dual-energy CT (DECT) can improve material differentiation by using two different X-ray energy spectra. Applications of DECT can largely be divided into exploration of material-nonspecific and material-specific energy-dependent information. Both types of evaluations can be qualitative or quantitative. The former includes virtual monoenergetic imaging, effective atomic mapping, and electron density mapping. The latter includes material decomposition, material labeling, and material highlighting [18]. In this study, we evaluated the quantitative correlations between in vivo TD-DOS and DECT signals to examine the validity of TD-DOS. The CT examination was performed in the same supine position as the TD-DOS measurement was to facilitate comparisons between the two modalities. Lipid and water contents were measured on TD-DOS by assessing tissue absorption due to the vibrational overtones of the lipid C-H bonds (930 nm) and the water O-H bond (978 nm) [11].