The goal of this work is to highlight those unique aspects of contrast-enhanced diagnostic optical imaging (OI) that
favor a broad clinical utilization of this emerging diagnostic technique and to illustrate certain identified challenges
opposing the enthusiastic clinical welcome for the OI method. We consider the single most appealing feature of OI to be
its much-touted exquisite sensitivity for the detection of near-infrared fluorescing (NIRF) probes, a sensitivity
supporting the development of disease- and molecule-specific NIR diagnostic probes, akin to nuclear imaging but
without the ionizing radiation and with superior spatial resolution. But a qualitative OI diagnostic examination, merely
defining the presence or absence of NIRF signal, may not be sufficient. The signal must be measurable. A quantitative
OI examination, capable of accurately assaying the tissue concentration of the fluorescing probe and changes in that
probe concentration related to disease progression or treatment would be extremely valuable. We discuss here at least
three challenges to quantitative diagnostic OI, a non-linear relationship between probe concentration and signal
intensity, background signal in the form of tissue auto-fluorescence, and the requirement to define precise location and
depth of the signal origin from within the subject.
Benign and malignant mammary tumors were induced in rats using a potent carcinogen, N-ethyl-N-nitrosurea (ENU). Induced tumors were examined under near-infrared (NIR) fluorescence imaging (excitation wavelength 670 to 730 nm, detection wavelength 750 and 800 nm) to search for a difference in the photophysical properties of the tumors reflecting their pathologic status. Three benign and eight malignant tumors were examined optically and pathologically. The non-enhanced optical images showed a significantly lower (P<0.05) spontaneous fluorescent signal in the benign tumors than in their malignant counterparts. The precise chemical origin for the observed differences in tumor autofluorescence remains undetermined. It can be hypothesized that the reported high concentration of porphyrins, NIR-fluorescing compounds, in the malignant lesions, could account for the observed increased autofluorescence.