Water/Fat Separation MR Improves Optical Breast Imaging MR Water/Fat Separation Improves Optical Breast Imaging

Background Optical imaging may be added to MR mammography to provide absolute concentration images of oxygenated and deoxygenated hemoglobin, water, and other endogenous and exogenous molecular agents. This is beneficial because hemoglobin and tissue oxygenation have been shown in several studies to have high specificity to breast cancer [1]. This combined optical/MR instrument has been shown to improve upon the spatial resolution limitations of optical imaging [2]. However, insufficient spectral sampling, either due to the need to achieve higher temporal resolution, or due to the non-linear response of optical detectors, can introduce significant errors in tissue quantification. In particular, oxyhemoglobin (HbO) and water have similar spectral features below 850nm, as shown in Figure 1, which leads to cross-talk. For optical imaging in breast tissue, this issue is particularly important, as the most sensitive photodetectors (photomultipler tubes) have extremely low photocathode sensitivity to longer wavelengths where the water absorption peak is more pronounced. In this study, MRI water/fat separation was used to provide water quantification to reduce crosstalk between oxyhemoglobin and water. This is shown to greatly improve hemodynamic quantification when photomultiplier tubes are used, and when spectral sampling is sparse. Methods Optical images were obtained by measuring the near infrared light attenuation through an optical breast phantom with 15 photodetectors from 16 source positions. Six wavelengths of light were sequentially collected. Water/fat separated images were calculated using the IDEAL MR sequence [3]. A model-based image reconstruction was used to fit tissue properties to a model for light propagation in breast tissue. The data/model misfit, δ, is minimized along with controls for system noise by minimizing: