Time-gated fluorescence imaging for the diagnosis of tumors in a murine model

Hyperspectral imaging and spectral unmixing for improving whole-body fluorescence cryo-imaging

Whole-animal fluorescence cryo-imaging is an established technique that enables visualization of the biodistribution of labeled drugs, contrast agents, functional reporters and cells in detail. However, many tissues produce endogenous autofluorescence, which can confound interpretation of the cryo-imaging volumes. We describe a multi-channel, hyperspectral cryo-imaging system that acquires densely-sampled spectra at each pixel in the 3-dimensional stack. This information enables the use of spectral unmixing to isolate the fluorophore-of-interest from autofluorescence and/or other fluorescent reporters. In phantoms and a glioma xenograft model, we show that the approach improves detection limits, increases tumor contrast, and can dramatically alter image interpretation.

Contrast improvement in indocyanine green fluorescence sensing in thick tissue using a time-gating method

Indocyanine green-based fluorescence imaging techniques are very powerful in clinical applications, but the imaging is restricted to the signal from the near-surface region of tissue. Here, we focus on the method to discriminate the fluorescence signal from the background using a time-domain gating technique. The contrast of the fluorescence image from a fluorescence object at more than 1 cm depth in a meat phantom could be enhanced about 4-5 times relative to the continuous wave method if the time-gate range was properly selected. Further, a Monte Carlo simulation with a simple background model indicates that a shorter source and detector distance is more effective to improve the contrast. The simple time-gating method will enable a highly sensitive fluorescence detection in thick tissue.

Gated Luminescence Imaging of Silicon Nanoparticles

The luminescence lifetime of nanocrystalline silicon is typically on the order of microseconds, significantly longer than the nanosecond lifetimes exhibited by fluorescent molecules naturally present in cells and tissues. Time-gated imaging, where the image is acquired at a time after termination of an excitation pulse, allows discrimination of a silicon nanoparticle probe from these endogenous signals. Because of the microsecond time scale for silicon emission, time-gated imaging is relatively simple to implement for this biocompatible and nontoxic probe. Here a time-gated system with ∼10 ns resolution is described, using an intensified CCD camera and pulsed LED or laser excitation sources. The method is demonstrated by tracking the fate of mesoporous silicon nanoparticles containing the tumor-targeting peptide iRGD, administered by retro-orbital injection into live mice. Imaging of such systemically administered nanoparticles in vivo is particularly challenging because of the low concentration of probe in the targeted tissues and relatively high background signals from tissue autofluorescence. Contrast improvements of >100-fold (relative to steady-state imaging) is demonstrated in the targeted tissues.

In vivo time-gated fluorescence imaging with biodegradable luminescent porous silicon nanoparticles

Fluorescence imaging is one of the most versatile and widely used visualization methods in biomedical research. However, tissue autofluorescence is a major obstacle confounding interpretation of in vivo fluorescence images. The unusually long emission lifetime (5-13 μs) of photoluminescent porous silicon nanoparticles can allow the time-gated imaging of tissues in vivo, completely eliminating shorter-lived (< 10 ns) emission signals from organic chromophores or tissue autofluorescence.Here, using a conventional animal imaging system not optimized for such long-lived excited states, we demonstrate improvement of signal to background contrast ratio by > 50-fold in vitro and by > 20-fold in vivo when imaging porous silicon nanoparticles. Time-gated imaging of porous silicon nanoparticles accumulated in a human ovarian cancer xenograft following intravenous injection is demonstrated in a live mouse. The potential for multiplexing of images in the time domain by using separate porous silicon nanoparticles engineered with different excited state lifetimes is discussed.

A multimode optical imaging system for preclinical applications in vivo: technology development, multiscale imaging, and chemotherapy assessment

PURPOSE

Several established optical imaging approaches have been applied, usually in isolation, to preclinical studies; however, truly useful in vivo imaging may require a simultaneous combination of imaging modalities to examine dynamic characteristics of cells and tissues. We developed a new multimode optical imaging system designed to be application-versatile, yielding high sensitivity, and specificity molecular imaging.

PROCEDURES

We integrated several optical imaging technologies, including fluorescence intensity, spectral, lifetime, intravital confocal, two-photon excitation, and bioluminescence, into a single system that enables functional multiscale imaging in animal models.

RESULTS

The approach offers a comprehensive imaging platform for kinetic, quantitative, and environmental analysis of highly relevant information, with micro-to-macroscopic resolution. Applied to small animals in vivo, this provides superior monitoring of processes of interest, represented here by chemo-/nanoconstruct therapy assessment.

CONCLUSIONS

This new system is versatile and can be optimized for various applications, of which cancer detection and targeted treatment are emphasized here.

Multimode Optical Imaging for Translational Chemotherapy: In Vivo Tumor Detection and Delineation by Targeted Gallium Corroles

We report the feasibility of tumor detection and delineation in vivo using multimode optical imaging of targeted gallium corrole (HerGa). HerGa is highly effective for targeted HER2+ tumor elimination in vivo, and it emits intense fluorescence. These unique characteristics of HerGa prompted us to investigate the potential of HerGa for tumor detection and delineation, by performing multimode optical imaging ex vivo and in vivo; the imaging modes included fluorescence intensity, spectral (including ratiometric), lifetime, and two-photon excited fluorescence, using our custom-built imaging system. While fluorescence intensity imaging provided information about tumor targeting capacity and tumor retention of HerGa, ratiometric spectral imaging offered more quantitative and specific information about HerGa location and accumulation. Most importantly, the fluorescence lifetime imaging of HerGa allowed us to discriminate between tumor and non-tumor regions by fluorescence lifetime differences. Finally, two-photon excited fluorescence images provided highly resolved and thus topologically detailed information around the tumor regions where HerGa accumulates. Taken together, the results shown in this report suggest the feasibility of tumor detection and delineation by multimode optical imaging of HerGa, and fluorescent chemotherapy agents in general. Specifically, the multimode optical imaging can offer complementary and even synergetic information simultaneously in the tumor detection and delineation by HerGa, thus enhancing contrast.

Optical imaging in cancer research: basic principles, tumor detection, and therapeutic monitoring

Accurate and rapid detection of diseases is of great importance for assessing the molecular basis of pathogenesis, preventing the onset of complications, and implementing a tailored therapeutic regimen. The ability of optical imaging to transcend wide spatial imaging scales ranging from cells to organ systems has rejuvenated interest in using this technology for medical imaging. Moreover, optical imaging has at its disposal diverse contrast mechanisms for distinguishing normal from pathologic processes and tissues. To accommodate these signaling strategies, an array of imaging techniques has been developed. Importantly, light absorption, and emission methods, as well as hybrid optical imaging approaches are amenable to both small animal and human studies. Typically, complex methods are needed to extract quantitative data from deep tissues. This review focuses on the development of optical imaging platforms, image processing techniques, and molecular probes, as well as their applications in cancer diagnosis, staging, and monitoring therapeutic response.

Predicting in vivo fluorescence lifetime behavior of near-infrared fluorescent contrast agents using in vitro measurements

Fluorescence lifetime (FLT) information is complementary to intensity measurement and can be used to improve signal-to-background contrast and provide environment sensing capability. In this study, we evaluate the FLTs of eight near-infrared fluorescent molecular probes in vitro in various solvent mediums and in vivo to establish the correlation between the in vitro and in vivo results. Compared with other mediums, two exponential fittings of the fluorescence decays of dyes dissolved in aqueous albumin solutions accurately predict the range of FLTs observed in vivo. We further demonstrate that the diffusion of a near-infrared (NIR) reporter from a dye-loaded gel can be detected by FLT change in mice as a model of controlled drug release. The mean FLT of the NIR probe increases as the dye diffuses from the highly polar gel interior to the more lipophilic tissue environment. The two-point analysis demonstrates an efficient in vitro method for screening new NIR fluorescent reporters for use as FLT probes in vivo, thereby minimizing the use of animals for FLT screening studies.

In Vivo Resolution of Multiexponential Decays of Multiple Near-Infrared Molecular Probes by Fluorescence Lifetime-Gated Whole-Body Time-Resolved Diffuse Optical Imaging

The biodistribution of two near-infrared fluorescent agents was assessed in vivo by time-resolved diffuse optical imaging. Bacteriochlorophyll a (BC) and cypate-glysine-arginine-aspartic acid-serine-proline-lysine-OH (Cyp-GRD) were administered separately or combined to mice with subcutaneous xenografts of human breast adenocarcinoma and slow-release estradiol pellets for improved tumor growth. The same excitation (780 nm) and emission (830 nm) wavelengths were used to image the distinct fluorescence lifetime distribution of the fluorescent molecular probes in the mouse cancer model. Fluorescence intensity and lifetime maps were reconstructed after raster-scanning whole-body regions of interest by time-correlated single-photon counting. Each captured temporal point-spread function (TPSF) was deconvolved using both a single and a multiexponental decay model to best determine the measured fluorescence lifetimes. The relative signal from each fluorophore was estimated for any region of interest included in the scanned area. Deconvolution of the individual TPSFs from whole-body fluorescence intensity scans provided corresponding lifetime images for comparing individual component biodistribution. In vivo fluorescence lifetimes were determined to be 0.8 ns (Cyp-GRD) and 2 ns (BC). This study demonstrates that the relative biodistribution of individual fluorophores with similar spectral characteristics can be compartmentalized by using the time-domain fluorescence lifetime gating method.

Determination of the modulation transfer function for a time-gated fluorescence imaging system

The use of fluorescence for cancer detection is currently under investigation. Presently, steady-state fluorescence detection methods are in use, but have limitations due to poor contrast between the fluorescence of the tumor and background autofluorescence. Improved contrast can be obtained with time-resolved techniques because of the differing lifetimes between autofluorescence and exogenous photosensitizers that selectively accumulate within tumor tissue. An imaging system is constructed using a fast-gated (200-ps) charge-coupled device (CCD) camera and a pulsed 635-nm laser diode. To characterize the ability of the system to transfer object contrast to an image, the modulation transfer function (MTF) of the system is acquired by employing an extended knife-edge technique. A knife-edge target is assembled by drilling a rectangular well into a block of polymethyl methacrylate (PMMA). The imaging system records images of the photosensitizer, chloroaluminum phthalocyanine tetrasulfonate (AlPcTS), within the well. AlPcTS was chosen to test the system because of its strong absorption of 635-nm, high fluorescence yield, and relatively long fluorescence lifetime (approximately 7.5 ns). The results show that the system is capable of resolving 10(-4) M AlPcTS fluorescence as small as 1 mm. The findings of this study contribute to the development of a time-gated imaging system using fluorescence lifetimes.

Simple time-domain optical method for estimating the depth and concentration of a fluorescent inclusion in a turbid medium

A simple time-domain optical method for estimating the depth and concentration of fluorescent inclusions in turbid media is described. We demonstrate direct depth estimation of a localized fluorescent object from the temporal position of the temporal point-spread function maximum. The depth estimation permits recovery of the fluorophore concentration, both of which are essential quantities for optical molecular imaging studies. Since the maximum is independent of the fluorophore concentration, excitation laser power, detector gain, and other system-dependent factors, this method ensures a robust and efficient approach.

The use of chloroaluminium phthalocyanine tetrasulfonate (AlPcTS) for time-delayed fluorescence imaging

Phthalocyanine derivatives are currently under investigation for use in photodynamic therapy, which is a promising cancer treatment. These materials, which display preferential uptake in cancerous cells, also exhibit high fluorescence yields and can be used for tumour detection. Problems with steady-state fluorescence techniques such as excitation scatter and background autofluorescence can be eliminated by using time-resolved imaging techniques without the need for filters. A tissue phantom was assembled to test a constructed time-gated imaging system by drilling 36 wells of varying diameter and depth (10 mm to 1 mm) into a block of polymethyl methacrylate (PMMA). The system was used to record images of chloroaluminium phthalocyanine tetrasulfonate (AlPcTS) at differing concentrations in neat aqueous solvent and cell suspensions within the wells. A mixture of Intralipid (to mimic tissue scatter) and Evan's blue (to mimic tissue absorption) of depths ranging from 1 mm to 10 mm was placed on top of the PMMA block. The ensuing images were analysed using signal-to-noise ratios and contrast-detail curves. The results indicate that the time-gated imaging system can prevent background excitation scatter from distorting the fluorescence signal from a longer-lived photosensitizer without the need for filters.

Quantitative Comparison of the Sensitivity of Detection of Fluorescent and Bioluminescent Reporters in Animal Models

Bioluminescent and fluorescent reporters are finding increased use in optical molecular imaging in small animals. In the work presented here, issues related to the sensitivity of in vivo detection are examined for standard reporters. A high-sensitivity imaging system that can detect steady-state emission from both bioluminescent and fluorescent reporters is described. The instrument is absolutely calibrated so that animal images can be analyzed in physical units of radiance allowing more quantitative comparisons to be performed. Background emission from mouse tissue, called autoluminescence and autofluorescence, is measured and found to be an important limitation to detection sensitivity of reporters. Measurements of dual-labeled (bioluminescent/fluorescent) reporter systems, including PC-3M-luc/DsRed2-1 and HeLa-luc/PKH26, are shown. The results indicate that although fluorescent signals are generally brighter than bioluminescent signals, the very low autoluminescent levels usually results in superior signal to background ratios for bioluminescent imaging, particularly compared with fluorescent imaging in the green to red part of the spectrum. Fluorescence detection sensitivity improves in the far-red to near-infrared, provided the animals are fed a low-chlorophyll diet to reduce autofluorescence in the intestinal region. The use of blue-shifted excitation filters is explored as a method to subtract out tissue autofluorescence and improve the sensitivity of fluorescent imaging.

Sensitivity and depth penetration of continuous wave versus frequency-domain photon migration near-infrared fluorescence contrast-enhanced imaging

The development of near-infrared fluorescent contrast agents and imaging techniques depends on the deep penetration of excitation light through several centimeters of tissue and the sensitive collection of the re-emitted fluorescence. In this contribution, the sensitivity and depth penetration of various fluorescence-enhanced imaging studies is surveyed and compared with current studies using continuous wave (CW) and frequency-domain photon migration (FDPM) measurements with planar wave illumination of modulated excitation light at 100 MHz and area collection of reemitted fluorescent light using a previously developed modulated intensified charge-coupled device camera system. Fluorescence was generated from nanomolar to micromolar solutions of indocyanine green (ICG) in a 100 microL volume submerged at 1-4 cm depths in a 1% Liposyn solution to mimic tissue scattering properties. Enhanced depth penetration and sensitivity are achieved with optimal filter rejection of excitation light, and FDPM rejection of background light is not achieved using CW methods. We show the ability to detect as few as 100 fmol of ICG from area illumination of 785 nm light (5.5 mW/cm2) and FDPM area collection of 830 nm fluorescent light generated from 3 cm below the phantom surface. The lowered noise floor of FDPM measurements enables greater sensitivity and penetration depth than comparable CW measurements.

Preliminary evaluation of two fluorescence imaging methods for the detection and the delineation of basal cell carcinomas of the skin

BACKGROUND AND OBJECTIVE

Fluorescence techniques can provide powerful noninvasive means for medical diagnosis, based on the detection of either endogenous or exogenous fluorophores. The fluorescence of delta-aminolevulinic acid (ALA)-induced protoporphyrin IX (PpIX) has already shown promise for the diagnosis of tumors. The aim of the study was to investigate the localization of skin tumors after the topical application of ALA, by detecting the PpIX fluorescence either in the spectral or in the time domain. STUDY DESIGN/MATERIALS N AND METHODS: Two fluorescence imaging systems were used to identify basal cell carcinomas of the skin in humans, after topical application of 20% ALA ointment. Both systems rely on the comparison between the exogenous and the endogenous fluorescence, performed either in the spectral domain or in the time domain. The first system works by using three images acquired through different spectral filters, whereas the second one measures the spatial map of the average fluorescence lifetime of the sample.

RESULTS

A clear demarcation of skin malignancies was successfully performed in vivo noninvasively with both fluorescence imaging systems.

CONCLUSION

The two complementary approaches considered in the present study show promise for skin tumor detection and delineation based on specific fluorescence features.

Two-Dimensional Visualization of Fluorescence Lifetimes by use of a Picosecond Laser and a Streak Camera

Two-dimensional distributions of the effective lifetime of the fluorescence emission induced by short-pulsed laser radiation are obtained from two-dimensional images recorded with a streak camera and a charge-coupled device by means of a separation algorithm method (SAM). In theory, the best response with respect to noise is obtained for lifetimes corresponding to a range of pixels of 5-50 in the CCD, that is, 5-50 ps at the fastest streak speed. In experiments the SAM is compared with pure time-resolved measurements, and it is used for two-dimensional lifetime evaluation. The laser-pulse duration is 25 ps, and the lower limit of the lifetime resolution as used in the experiments is estimated to be 200-250 ps. The results demonstrate the possibility of performing pattern recognition independently of the relative distribution of emission intensity between regions of different fluorescence lifetimes. The technique is demonstrated for static objects but can in principle be extended to nonstationary objects if two detectors are used.

Physiological and pathological factors of human breast disease that can influence optical diagnosis

The 'normal' human female breast is a very complex organ that changes considerably during development, pregnancy and menopause. In addition, it is an excretory organ that, during lactation, discharges various metabolites and certain drugs that can be optically active. Optical diagnosis of breast cancers requires detection of differential concentrations of 1) various absorbers and scatterers or 2) native or exogenous fluorophores to distinguish cancers from surrounding 'normal' and benign breast tissues. The differential concentrations are due to the biology of the cancer cells and the host responses to the cancer growth. For most patients, the cancer will be intermixed with a complex 3-dimensional array of 'normal' breast tissue and benign breast lesions. This complexity will challenge the optical biopsy investigator but, with the recent advances in our understanding of light transport, optical diagnostic techniques and devices can be developed to complement and supplement current breast cancer screening techniques.

Tumour visualization in a murine model by time-delayed fluorescence of sulphonated aluminium phthalocyanine

Mice bearing the MS-2 fibrosarcoma were administered 0.25, 0.5 or 1 mg kg(-1) body weight (b.w.) of sulphonated aluminium phthalocyanine (AlS(2)Pc) (with average degree of sulphonation of 2.1), and time-gated fluorescence images were acquired up to 6 h after the injection. Different excitation wavelengths (610, 650 and 670 nm) were tested. Red light excitation and 3 ns delayed detection allow one to minimize natural fluorescence and scattered laser light, respectively. The best conditions for tumour detection are reached under either 650 or 670 nm Excitation, 2-4 h after the administration of either 0.5 or 1 mg kg(-1) b.w. of AlS(2)Pc. In these situations, the average fluorescence contrast between tumour area and surrounding healthy tissue is > 2, providing a clear identification of the pathological region. However, tumour localization is possible even after the injection of 0.25 mg kg(-1) b.w. of sensitizer. In conclusion, under low power excitation (< 100MuW cm(-2)), the technique allows real time detection of an intradermal tumour with good contrast.

Fluorescence lifetime imaging of experimental tumors in hematoporphyrin derivative-sensitized mice

Tumor detection has been carried out in mice sensitized with hematoporphyrin derivative (HpD) by measuring the spatial distribution of the fluorescence lifetime of the exogenous compound. This result has been achieved using a time-gated video camera and a suitable mathematical processing that led to the so-called "lifetime images." Extensive experimental tests have been performed on mice bearing the MS-2 fibrosarcoma or the L1210 leukemia. Lifetime images of mice show that the fluorescence decay of HpD is appreciably slower in the tumor than in healthy tissues nearby, allowing a reliable detection of the neoplasia. The lengthening of the lifetime in tumors depends little on the drug dose, which in our experiments could be lowered down to 0.1 mg/kg body weight, still allowing a definite tumor detection. In order to ascertain the results achieved with the imaging apparatus, high-resolution spectroscopy, based on a time-correlated single photon counting system, has also been performed to measure the fluorescence lifetime of the drug inside the tumor and outside. The outcomes obtained with two techniques are in good agreement.

In vivo fluorescence imaging for tissue diagnostics

Non-invasive fluorescence imaging has the potential to provide in vivo diagnostic information for many clinical specialties. Techniques have been developed over the years for simple ocular observations following UV excitation to sophisticated spectroscopic imaging using advanced equipment. Much of the impetus for research on fluorescence imaging for tissue diagnostics has come from parallel developments in photodynamic therapy of malignant lesions with fluorescent photosensitizers. However, the fluorescence of endogenous molecules (tissue autofluorescence) also plays an important role in most applications. In this paper, the possibilities of imaging tissues using fluorescence spectroscopy as a mean of tissue characterization are discussed. The various imaging techniques for extracting diagnostic information suggested in the literature are reviewed. The development of exogenous fluorophores for this purpose is also presented. Finally, the present status of clinical evaluation and future directions are discussed.

Imaging of fluorescent yield and lifetime from multiply scattered light reemitted from random media

The feasibility of employing fluorescent contrast agents to perform optical imaging in tissues and other scattering media has been examined through computational studies. Fluorescence lifetime and yield can give crucial information about local metabolite concentrations or environmental conditions within tissues. This information can be employed toward disease detection, diagnosis, and treatment if noninvasively quantitated from reemitted optical signals. However, the problem of inverse image reconstruction of fluorescence yield and lifetime is complicated because of the highly scattering nature of the tissue. Here a light propagation model employing the diffusion equation is used to account for the scattering of both the excitation and fluorescent light. Simulated measurements of frequency-domain parameters of fluorescent modulated ac amplitude and phase lag are used as inputs to an inverse image-reconstruction algorithm, which employs the diffusion model to predict frequency-domain measurements resulting from a modulated input at the phantom periphery. In the inverse image-reconstruction algorithm, a Newton-Raphson technique combined with a Marquardt algorithm is employed to converge on the fluorescent properties within the medium. The successful reconstruction of both the fluorescence yield and lifetime in the case of a heterogeneous fluorophore distribution within a scattering medium has been demonstrated without a priori information or without the necessity of obtaining absence images.

Quantitative optical spectroscopy for tissue diagnosis

The interaction of light within tissue has been used to recognize disease since the mid-1800s. The recent developments of small light sources, detectors, and fiber optic probes provide opportunities to quantitatively measure these interactions, which yield information for diagnosis at the biochemical, structural, or (patho)physiological level within intact tissues. However, because of the strong scattering properties of tissues, the reemitted optical signal is often influenced by changes in biochemistry (as detected by these spectroscopic approaches) and by physiological and pathophysiological changes in tissue scattering. One challenge of biomedical optics is to uncouple the signals influenced by biochemistry, which themselves provide specificity for identifying diseased states, from those influenced by tissue scattering, which are typically unspecific to a pathology. In this review, we describe optical interactions pursued for biomedical applications (fluorescence, fluorescence lifetime, phosphorescence, and Raman from cells, cultures, and tissues) and then provide a descriptive framework for light interaction based upon tissue absorption and scattering properties. Finally, we review important endogenous and exogenous biological chromophores and describe current work to employ these signals for detection and diagnosis of disease.

Fluorescence-lifetime determination in tissues or other scattering media from measurement of excitation and emission kinetics

Measurements of nanosecond and subnanosecond fluorescence lifetimes are restricted to dilute, nonscattering systems since excitation and emission photon times of flight significantly affect measured fluorescent decay kinetics. We provide the theoretical rationale for frequency-domain measurements of phase-shift and amplitude demodulation made at excitation and emission wavelengths for direct determination of lifetimes in tissues and other scattering media. We confirm our analytical expressions using standard laser dyes such as 3,3'-diethylthiatricarbocyanine iodide, IR- 125, and IR- 140 in polystyrene suspensions with similar scattering properties as tissues. Our results have significant implication for lifetime-based spectroscopy in tissues and other scattering media.

delta-Aminolevulinic acid induced fluorescence in tumour-bearing mice

The potential of protoporphyrin IX fluorescence induced by the systemic administration of delta-aminolevulinic acid (ALA) for the detection of tumours was tested in three different murine models (MS-2 fibrosarcoma, L1210 leukaemia, and Lewis lung carcinoma). Time-gated fluorescence images were acquired up to 4 h after the intraperitoneal injection of ALA (200 mg (kg body mass (BM))-1). For comparison images were acquired also after the administration of 25 mg (kg BM)-1 of haematoporphyrin derivative. The latter drug was characterized by better localization in the tumour area, leading to higher fluorescence contrast between neoplastic mass and surrounding healthy tissue, and consequently was preferable for tumour diagnosis in all the models under study.

Study of porphyrin fluorescence in tissue samples of tumour-bearing mice

A time-gated fluorescence-imaging technique was applied to study the distribution of sensitizer in porphyrin-treated tumour-bearing mice. The animals were administered with either haematoporphyrin derivative (HpD) or Photofrin and sacrificed 4 or 12 h later. Fluorescence images were acquired from tumour, skin, muscle, fat, brain, lymph node, bowel and bone of both treated and untreated mice. The results obtained with HpD and Photofrin are similar. In images acquired 30 ns after excitation a bright exogenous fluorescence allows clear detection of the tumour. Nevertheless, the images show that porphyrins localize with different concentrations in all the examined tissues except the brain. Moreover, an appreciable long-living endogenous emission was observed in the bone.

Medical diagnostic system based on simultaneous multispectral fluorescence imaging

A multicolor fluorescence imaging system applied to medical diagnostics is described. The system presented simultaneously records four fluorescence images in different wavelength bands, permitting low-resolution spectroscopy imaging. An arithmetic function image of the four spectral images is constructed by a pixel-to-pixel calculation and is presented on a monitor in false-color coding. A sensitive detector is required for minimizing the excitation energy necessary to obtain an image and thus avoid side effects on the investigated tissue. Characteristics of the system of importance for the detector sensitivity as well as image quality are discussed. A high degree of suppression of ambient background light is reached with this system by the use of a pulsed laser as an excitation source together with gated detection. Examples of fluorescence images from tumors on the hind legs and in the brain of rats injected with Photofrin are given.