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Metabolic FLIM (fluorescence lifetime imaging) is used to image bioenergetic status in cells and tissue. Whereas an attribution of the fluorescence lifetime of coenzymes as an indicator for cell metabolism is mainly accepted, it is debated whether this is valid for the redox state of cells. In this regard, an innovative algorithm using the lifetime characteristics of nicotinamide adenine dinucleotide (phosphate) (NAD(P)H) and flavin adenine dinucleotide (FAD) to calculate the fluorescence lifetime induced redox ratio (FLIRR) has been reported so far. We extended the FLIRR approach and present new results, which includes FLIM data of the various enzymes, such as NAD(P)H, FAD, as well as flavin mononucleotide (FMN). Our algorithm uses a two-exponential fitting procedure for the NAD(P)H autofluorescence and a three-exponential fit of the flavin signal. By extending the FLIRR approach, we introduced FLIRR1 as protein-bound NAD(P)H related to protein-bound FAD, FLIRR2 as protein-bound NAD(P)H related to free (unbound) FAD and FLIRR3 as protein-bound NAD(P)H related to protein-bound FMN. We compared the significance of extended FLIRR to the metabolic index, defined as the ratio of protein-bound NAD(P)H to free NAD(P)H. The statistically significant difference for tumor and normal cells was found to be highest for FLIRR1.

Fluorescence lifetime imaging microcopy (FLIM) is successfully used to image the intracellular fluorescent coenzymes NAD(P)H and FAD[+]. The redox state of these coenzymes is a parameter which helps to reveal the metabolic status of living cells and tissues. However, metabolic reactions are strongly dependent on the intracellular oxygen level. One promising optical method to monitor oxygen in biomedical samples is phosphorescence lifetime imaging microscopy (PLIM). PLIM is based on oxygen-dependent quenching of the phosphorescence of so-called \"phosphors\". In this way, PLIM enables measurement of the oxygen partial pressure (pO[2]) within living cells. This review describes the FLIM and PLIM approaches used in biomedical research, drawing particular attention to the techniques of simultaneous FLIM and PLIM, which provide correlative imaging of both the fluorescence lifetime of metabolic coenzymes and pO[2]-sensitive phosphorescence lifetime.

This monograph focuses on modern femtosecond laser microscopes for two photon imaging and nanoprocessing, on laser tweezers for cell micromanipulation as well as on fluorescence lifetime imaging (FLIM) in Life Sciences. The book starts with an introduction by Dr. Wolfgang Kaiser, pioneer of nonlinear optics and ends with the chapter on clinical multiphoton tomography, the novel high resolution imaging technique. It includes a foreword by the nonlinear microscopy expert Dr. Colin Sheppard. Contents Part I: Basics Brief history of fluorescence lifetime imaging The long journey to the laser and its use for nonlinear optics Advanced TCSPC-FLIM techniques Ultrafast lasers in biophotonics Part II: Modern nonlinear microscopy of live cells STED microscopy: exploring fluorescence lifetime gradients for super-resolution at reduced illumination intensities Principles and applications of temporal-focusing wide-field two-photon microscopy FLIM-FRET microscopy TCSPC FLIM and PLIM for metabolic imaging and oxygen sensing Laser tweezers are sources of two-photon effects Metabolic shifts in cell proliferation and differentiation Femtosecond laser nanoprocessing Cryomultiphoton imaging Part III: Nonlinear tissue imaging Multiphoton Tomography (MPT) Clinical multimodal CARS imaging In vivo multiphoton microscopy of human skin Two-photon microscopy and fluorescence lifetime imaging of the cornea Multiscale correlative imaging of the brain Revealing interaction of dyes and nanomaterials by multiphoton imaging Multiphoton FLIM in cosmetic clinical research Multiphoton microscopy and fluorescence lifetime imaging for resection guidance in malignant glioma surgery Non-invasive single-photon and multi-photon imaging of stem cells and cancer cells in mouse models Bedside assessment of multiphoton tomography