not-yet-known not-yet-known not-yet-known unknown 1 Optical filter-based Raman system We used a lab-made optical filter-based Raman system, as previously described26. The Raman system integrates a continuous-wave 1064 nm laser to illuminate the samples, a handheld probe and an optical filter system for selecting specific wavelength bands, and a conditioner which employs a phase-sensitive technique detection to amplify and isolate the component of the signal at a specific reference frequency. Laser beam is modulated at a 5600 Hz frequency. The light reflected from the liver specimen is collected, guided back into the system, split into two distinct wavelength ranges (reflectance and Raman) and measured by detectors for further analysis. We calibrated the Raman system using MRI-calibrated duck fat-agar phantoms with the fat content of 0%-70% before each series of measurements (an example calibration curve is shown in Figure S1 ). The Raman and reflectance measurements followed the methods outlined in Ref. [26] in ambient room light26. The Raman and reflectance light intensities were converted to voltage changes by InGaAs photodiodes and a transimpedance amplifier for direct readout. 1 Optical filter-based Raman system