28 April 2011

Watching cells with less artefacts

Biology-motivated biosciences quite often necessitate cell imaging in 3D and 4D, which means 3D cell observation over time. Optical coherence tomography (OCT) has been proposed as alternative to conventional cellular imaging modalities which suffer from opaque substrates. OCT captures micrometer-resolution, 3D images from within optical scattering media such as biological tissue using an interferometric technique, typically employing near-infrared light. Yet, difficulties have to be faced when using standard signal processing routines for image formation because of the high dynamic range of OCT signals from cell samples. Visualization of cell migration during chemotaxis using spectral domain OCT, for example, requires non-standard processing techniques. Stripe artefacts and camera noise floor present in OCT data prevent detailed computer-assisted reconstruction and quantification of cell locomotion. Furthermore, imaging artefacts lead to unreliable results in automated texture based cell analysis.

Wolfgang Drexler and his colleagues from Medical University Vienna (Austria), Vienna University of Technology, and Cardiff University (UK) now characterized three pronounced artefacts that become visible when imaging sample structures with high dynamic range such as cultured cells: time-varying fixed pattern noise; stripe artefacts generated by background estimation using tomogram averaging; and image modulations due to spectral shaping. They evaluated techniques to minimize these artefacts using an 800 nm optical coherence microscope.

The scientists were able to show that median based background elimination procedure greatly reduces transversal artefacts prominent in cell imaging using reflective substrates such as agar or nitrocellulose filters. High dynamic range of images unveils increased noise floor caused by camera specific time-varying fixed-pattern noise. This artefact could be greatly reduced through application of a spread domain filter while minimally influencing morphology in final images. In cell-culture substrates with reflective interfaces, modulation artefacts from spectral shaping can be avoided by estimation of the effective spectral envelope from analytic signals. Specifically choosing a Kaiser window for envelope shaping results in high sidelobe suppression while retaining high axial resolution.

Effect of artefact reduction was shown exemplarily on two cell cultures, i.e. Dictyostelium on nitrocellulose substrate, and retinal ganglion cells (RGC-5) cultured on a glass coverslip. Retinal imaging also profits from the proposed processing techniques.

J. Biophotonics 4(5), 355-367 (2011)

DOI: 10.1002/jbio.201000109

Round Robin Experiment

Raman spectroscopy has already proved its effectiveness in many cases for medical diagnostics such as for cancer, cardiovascular diseases and infections. However, there are no standards in the different working groups, e.g. for sample preparation, implementation of the Raman experiments, spectra pre-treatment, data evaluation, etc.In a round robin experiment, the required groundwork will take place in order to define standardised Raman measurement methods, which will be fundamental for establishing Raman spectroscopy for clinical diagnostic procedures.

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