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Fluorescence imaging is widely used in biological and medical science, and more recently has application to in-vivo tumor detection. The ability to generate image contrast on the basis of luminescence decay time, as well as intensity, opens up new contrast mechanisms and improves image quantization by compensating for most quenching processes. Decay-time imaging using modulated excitation and homodyne detection is a sensitive and versatile technique, which presently has application to fluorescence microscopy, analytical fluorescence spectroscopy, and sensor technology. Apart from fluorescence applications, the technology potentially is valuable for noninvasive time- and phase-resolved spectroscopy of tissues. Two approaches to implement frequency domain imaging are discussed. One of these uses a custom-built modulated intensified CCD system, while the other uses a modified imaging single-photon detector (IPD) in conjunction with a multichannel photon correlator. The CCD system is best suited to microscopy and nanosecond measurements, while the IPD has the potential for much higher time resolution with extremely high sensitivity. Time- and frequency-domain methods for biological and medical imaging are now the subject of considerable research effort worldwide. Up to the present time most studies have been aimed at establishing the principles involved, rather than producing robust devices of the sophistication expected for routine medical applications. In particular, measurements have been made by scanning a single detector across the region of interest--building up an image point-by-point. Scanning methods of imaging have their virtues, and have been widely implemented in biological microscopy. However, the imaging-light emerging from a scattering sample, single-point detection is highly inefficient since information from the sample is necessarily dispersed over an area even when the excitation or illumination is locally concentrated. In this paper, the alternative of using spatially resolving area detectors for time- and frequency-domain imaging is discussed and an attempt is made to identify the advantages and present limitations of the technology.
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