Depth penetration and molar sensitivity for near-infrared fluorescence-enhanced optical imaging

Non-invasive, in vivo imaging modalities are valuable diagnostic indicators of tissue abnormalities, disease, metabolic changes, and other cellular anomalies that occur beneath the skin´s surface. Frequency domain photon migration imaging (FDPM) is a maturing optical tool that is based upon the propagation of near infrared (NIR) radiation through tissue and scattering media. Fluorescence-enhanced FDPM exploits a NIR light source for the excitation of a fluorescent contrast agent for detection. The research presented by this contribution seeks to further develop fluorescence-enhanced FDPM for cancer screening via sentinel lymph node mapping. Sentinel lymph node mapping involves the localization and resection for biopsy of the sentinel node to determine cancer spread. Investigating the propagation of NIR fluorescence deep within tissue-like scattering media will provide evidence to support the sensitivity of NIR imaging for sentinel lymph node localization, particularly when the sentinel lymph node is located up to 4 cm below the skin surface. The work presented herein provides a systematic examination of FDPM detection of signals originating from deeply embedded fluorescent inclusions within a tissue-mimicking phantom. Two dimensional multipixel images of embedded fluorescent targets are examined to determine if detection of fluorescence is possible over a reflectance geometry provided using an intensified charge coupled device camera system (ICCD). Data collected show fluorescence from targets containing 1 mM, 0.01 mM and 1 nM can be located when originating from 1 to 7-cm below the imaged surface, and as few as 0.01 femtomoles of ICG can be detected by the ICCD system, with proper choice of rejection filters.