  Complete surgical resection of the tumor for Head and neck squamous cell
carcinoma (HNSCC) remains challenging, given the devastating side effects of
aggressive surgery and the anatomic proximity to vital structures. To address
the clinical challenges, we introduce a wide-field, label-free imaging tool
that can assist surgeons delineate tumor margins real-time. We assume that
autofluorescence lifetime is a natural indicator of the health level of
tissues, and ratio-metric measurement of the emission-decay state to the
emission-peak state of excited fluorophores will enable rapid lifetime mapping
of tissues. Here, we describe the principle, instrumentation, characterization
of the imager and the intraoperative imaging of resected tissues from 13
patients undergoing head and neck cancer resection. 20 x 20 mm2 imaging takes 2
second/frame with a working distance of 50 mm, and characterization shows that
the spatial resolution reached 70 {\mu}m and the least distinguishable
fluorescence lifetime difference is 0.14 ns. Tissue imaging and
Hematoxylin-Eosin stain slides comparison reveals its capability of delineating
cancerous boundaries with submillimeter accuracy and a sensitivity of 91.86%
and specificity of 84.38%.
