Hematoporphyrin-mediated fluorescence reflectance imaging: application to early tumor detection in vivo in small animals

Trans-illumination fluorescence imaging of deep-seated tumors in small animals

Background:

Fluorescence diffuse tomography (FDT) is the most accurate technique for the imaging of labeled tumors in the small animal body. However, the procedure for reconstruction of the spatial distribution of the fluorophore requires a high signal-to-noise ratio due to the ill-condition of the inverse problem. Therefore, the FDT technique is ineffective for imaging tumors of small size or with dim fluorophores because of the low intensity of their fluorescence compared with the high level of tissue autofluorescence. In these cases, the size and position of a marked tumor in the animal body can be estimated from two-dimensional fluorescence images obtained using trans- or epi-illumination techniques.

Material and methods:

A versatile system for small animal fluorescence imaging which combines planar epi- and trans-illumination geometries of the light source and of the fluorescence receiver was created and tested. For epi-illumination imaging, light-emitting diode sources were used to provide homogeneous and stable illumination of the experimental animal, in combination with a cooled CCD camera which covers the entire illuminated area. For trans-illumination imaging, mechanical raster-scanning devices modulated at a low frequency were used for the laser source, together with a cooled photomultiplier tube, which provided outstanding sensitivity.

Results:

Monitoring the orthotopic tumor growth in animal bodies has demonstrated the efficacy of trans-illumination imaging in comparison with the epi-illumination technique. The results obtained also showed that the effective use of the trans-illumination technique requires Born normalization of the fluorescence signal and the exclusion of lateral illumination by surrounding the animal with additional light absorption material using light-absorption pads on both sides of the body.

In vivo study of photosensitizer pharmacokinetics by fluorescence transillumination imaging

The possibility of in vivo investigation of the pharmacokinetics of photosensitizers by means of fluorescence transillumination imaging is demonstrated. An animal is scanned in the transilluminative configuration by a single source and detector pair. Transillumination is chosen as an alternative approach to reflection imaging. In comparison with the traditional back-reflection technique, transillumination is preferable for photosensitizer detection due to its higher sensitivity to deep-seated fluorophores. The experiments are performed on transplantable mouse cervical carcinomas using three drugs: photosens, alasens, and fotoditazin. For quantitative evaluation of the photosensitizer concentration in tumor tissue the fluorescence signal is calibrated using tissue phantoms. We show that the kinetics of photosensitizer tumor uptake obtained by transillumination imaging in vivo agree with data of standard ex vivo methods. The described approach enables rapid and cost-effective study of newly developed photosensitizers in small animals.