Srt 99 100

Fiber-optic point measurements were acquired with a measurement setup [shown in Fig. 2(b)] including a broadband light source, a spectrometer for the visual wavelength region (Andor Technology DU420A-BRDD), and a spectrometer for the near-infrared region (Andor Technology, DU492A-1.7). A fiber-optic probe was attached to the measurement setup to measure diffuse reflectance spectra between 400 and 1600 nm [Fig. 2(d)].¹⁸ (link)^,19 (link) The distance between the illuminating and collecting fiber at the tip of the probe was 1 mm. Before measuring, the setup was calibrated by acquiring a white reference measurement of Spectralon^® (SRT-99-100, Labsphere, Inc., Northern Sutton, New Hampshire) in a calibration cap.
To acquire a DRS measurement, the probe was brought in contact with the tissue. During the probe measurements, a custom-made grid with holes [consisting of two pieces and shown in Fig. 2(c)] was placed on top of the tissue to gently fixate the tissue in the cassette and allow correlation between the measurement locations and H&E section afterward. The grid was 3-D printed and transparent. In addition, the grid stabilized the probe while ensuring contact between the probe and the tissue and restraining it from moving during measurements.

Hyperspectral images were obtained with a push-broom HSI setup (VLNIR CL-350-N17E, Specim, Spectral Imaging Ltd., Finland) that captures light in the near-infrared ( $\sim 900$ to 1700 nm, 256 wavelength bands, 5 nm increments) with an InGaAs sensor ( $320\times 256\text{\hspace{0.17em}\hspace{0.17em}pixels}$ ). The setup [shown in Fig. 2(a)] and measurement calibration are described in a prior publication.¹⁷ (link) In short, the tissue is placed under the camera and imaged line-by-line through moving the imaged scene. Thereby a three-dimensional (3-D) hypercube is obtained that contains multiple two-dimensional (2-D) images of the tissue at different wavelengths, i.e., each pixel in the 2-D image contains a full diffuse reflectance spectrum [Fig. 2(b)]. Raw hyperspectral data obtained from the tissue were normalized to a diffuse reflectance percentage relative to Spectralon^® (SRT-99-100, Labsphere, Inc., Northern Sutton, New Hampshire).