First experience with spatial frequency domain imaging and red-light excitation of protoporphyrin IX fluorescence during tumor resection.

Fluorescence-guided surgery (FGS) enhances intraoperative visualization of tumors to maximize safe resection, and quantitative fluorescence imaging (qFI) of protoporphyrin IX (PpIX) has provided additional information for guidance during intracranial tumor surgery. Previous developments in fluorescence quantification have demonstrated that the depth of fluorescence signals can be estimated given known optical properties in a lab setting, and now with the work described here that these optical properties can be determined in vivo in human brain tissue in the operating room (OR) during tumor resection procedures. More specifically, we report the first depth estimation of subsurface tumor intraoperatively, achieved with the combination of spatial frequency domain imaging (SFDI) for optical property measurement and red-light excitation of PpIX. We modified a commercial surgical microscope (Zeiss) with a digital light processing module (DLI Austin, TX) to modulate light from a xenon arc lamp to illuminate the field. White-light excitation and a liquid crystal tunable filter (LCTF Verispec) were used to measure diffuse reflectance at discrete wavelengths of 670 nm and 710 nm on a sCMOS camera. An illumination-side filter wheel allowed excitation of PpIX fluorescence at 405 nm and 635 nm, and the LCTF measured fluorescence emissions at 670 nm and 710 nm. Data acquisition and processing generated wide-field images of the depth of PpIX fluorescence within 1 minute in the OR. The ability of the clinical microscope to perform optical property mapping with SFDI and convert these wide-field estimates into images of the depth of fluorescence was tested in tissue simulating phantoms and in vivo during a craniotomy for brain tumor resection. Results indicate that wide-field optical property estimates with SFDI can be combined with depth sensing algorithms to produce maps of the depth of PpIX when exposed to red-light in the OR.