Imaging and detection of long-lived fluorescence probes in presence of highly emissive and scattering background.

Optical biomedical imaging and diagnostics is a rapidly growing field that provides both structural and functional information with uses ranging from fundamental to practical clinical applications. Nevertheless, imaging/visualizing fluorescence objects with high spatial resolution in a highly scattering and emissive biological medium continues to be a significant challenge. A fundamental limiting factor for imaging technologies is the signal-to-background ratio (SBR). For a long time to improve the SBR, we tried to improve the brightness of fluorescence probes. Many novel fluorophores with improved brightness (almost reaching the theoretical limit), redshifted emission, highly improved photostability, and biocompatibility greatly helped advance fluorescence detection and imaging. However, autofluorescence, scattering of excitation light, and Raman scattering remain fundamental limiting problems that drastically limit detection sensitivity. Similarly, significant efforts were focused on reducing the background. High-quality sample purification eliminates the majority of autofluorescence background and in a limited confocal volume allows detection to reach the ultimate sensitivity to a single molecule. However, detection and imaging in physiological conditions does not allow for any sample (cells or tissue) purification, forcing us to face a fundamental limitation. A significant improvement in limiting background can be achieved when fluorophores with a long fluorescence lifetime are used, and time-gated detection is applied. However, all long-lived fluorophores present low brightness, limiting the potential improvement. We recently proposed to utilize multipulse excitation (burst of pulses) to enhance the relative signal of long-lived fluorophores and significantly improve the SBR. Herein, we present results obtained with multipulse excitation and compare them with standard single-pulse excitation. Subtraction of images obtained with a single pulse from those obtained with pulse burst (differential image) highly limits background and instrumental noise resulting in more specific/sensitive detection and allows to achieve greater imaging depth in highly scattering media, including skin and tissue.