Digital multimodal imaging for biomarker detection

1st April 2013

Darlene Wood reports on the use of charged-coupled device imaging systems in life science research imaging applications.

Researchers traditionally have used charged-coupled device (CCD) imaging systems to record and quantitate the presence and/or amount of biomolecules in flat-field assay formats such as microtiter plates, electrophoresis gels, and membranes, including southern, northern and western blots.

Probes for nucleic acids or proteins of interest are labelled with luminescent, colourimetric, fluorescent, and radioactive tags, which allow them to be specifically detected when exposed to an appropriate imaging system.

While in vitro assay formats have traditionally been used to assess and monitor nucleic acids and proteins, the availability of transformed cells, transgenic animals, and near-IR probes has enabled the use of in vivo assay models such as laboratory mice.

These models permit monitoring of the changes in molecular activity of specific cells inside living organisms ­ such as specific enzyme production in tumour cells when treated with drugs ­ long before the morphological changes, such as changes in tumour size, can be detected. New amultimodal' digital imaging systems, particularly those utilising cooled CCD cameras, multi-wavelength illumination, and x-ray imaging capability are being used for detection and monitoring of biomarkers using in vivo animal models with increasing frequency.

The KODAK Image Station Multimodal Imaging System, for example, is a cooled, CCD-based imaging system that can detect and image luminescence, multi-wavelength fluorescence, colourimetric, and radio-isotopic labels, in both in vitro and in vivo assay formats. In addition, an optional X-Ray imaging module allows digital radiology of small animals. The system's cooled CCD and a parfocal 10xzoom lens captures n-bit image files with high resolution and accurate quantitative accuracy. Large pixel size and very low noise allow high-sensitivity detection over a wide linear dynamic range.

An external illumination module and a series of narrow-pass filters allow for specific excitation of fluorochromes from 385nm to 755nm, and an emission filter wheel allows for specific detection from 465nm to 830nm.

The selectable illumination capability significantly improves sensitivity for most fluorescence labels used for nucleic acid and protein detection. In addition, it enables imaging of multi-fluorochrome assays, allowing standard single fluorochrome molecular assays to be multiplexed. The ability to select and apply longer excitation wavelengths (green to near-IR) improves the penetration of light into tissue, enabling whole body, optical in vivo molecular imaging research using the same imaging system.

For radio-isotopic imaging, the KODAK Image Station system uses phosphor technology in a cassette format to convert beta particles emitted by the label (such as 32P, 125I, 99mTc, 111In) to photons, which are detected by the system's cooled CCD camera.

Combined with proprietary optical filters, the patented phosphor screen technology enables efficient conversion of the radio-isotopic signal from the sample into photons that can be captured by a highly sensitive camera.

The optional Image Station X-Ray Imaging Module provides a shielded, integrated x-ray source to provide high quality digital x-ray imaging capability. Using this system, one can serially image a luminescence, fluorescence, or isotopic biomarker in vivo, capture a digital radiograph and precisely co-register the optical molecular imaging modalities with the anatomical radiograph for improved anatomical localisation of molecular biomarkers.

KODAK's 1D Image Analysis software is supplied with the Image Station system and provides control of the imaging process. Options for single/multiple capture, time lapse, and progressive interval/ increment enable accurate control for simple and complex exposure routines. Real-time preview, and a Predict Exposure mode make it easy to determine exposure parameters based on desired grayscale range. Image processing options feature lens correction, illumination correction, and anti-warping algorithms to ensure data integrity.

Darlene Wood is Product Manager, Eastman Kodak Company, Scientific Imaging Systems, New Haven, CT, USA.





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