Methodology article

Characterisation and correction of signal fluctuations in successive acquisitions of microarray images

Annie Glatigny¹ , Hervé Delacroix^1,2 , Thomas Tang^1,3 , Nicolas François^1,2,4 , Lawrence Aggerbeck¹ and Marie-Hélène Mucchielli-Giorgi^1,3

¹  Centre de Génétique Moléculaire, CNRS FRE3144, F-91198 Gif-sur-Yvette, France

²  Université Paris-Sud 11, F-91405 Orsay, France

³  Université Pierre et Marie Curie – Paris 6, F-75005 Paris, France

⁴  TELECOM & Management SudParis, 9 rue Charles Fourier, F-91011 Evry, France

author email corresponding author email

BMC Bioinformatics 2009, 10:98doi:10.1186/1471-2105-10-98

Published:	30 March 2009

Abstract

Background

There are many sources of variation in dual labelled microarray experiments, including data acquisition and image processing. The final interpretation of experiments strongly relies on the accuracy of the measurement of the signal intensity. For low intensity spots in particular, accurately estimating gene expression variations remains a challenge as signal measurement is, in this case, highly subject to fluctuations.

Results

To evaluate the fluctuations in the fluorescence intensities of spots, we used series of successive scans, at the same settings, of whole genome arrays. We measured the decrease in fluorescence and we evaluated the influence of different parameters (PMT gain, resolution and chemistry of the slide) on the signal variability, at the level of the array as a whole and by intensity interval. Moreover, we assessed the effect of averaging scans on the fluctuations. We found that the extent of photo-bleaching was low and we established that 1) the fluorescence fluctuation is linked to the resolution e.g. it depends on the number of pixels in the spot 2) the fluorescence fluctuation increases as the scanner voltage increases and, moreover, is higher for the red as opposed to the green fluorescence which can introduce bias in the analysis 3) the signal variability is linked to the intensity level, it is higher for low intensities 4) the heterogeneity of the spots and the variability of the signal and the intensity ratios decrease when two or three scans are averaged.

Conclusion

Protocols consisting of two scans, one at low and one at high PMT gains, or multiple scans (ten scans) can introduce bias or be difficult to implement. We found that averaging two, or at most three, acquisitions of microarrays scanned at moderate photomultiplier settings (PMT gain) is sufficient to significantly improve the accuracy (quality) of the data and particularly those for spots having low intensities and we propose this as a general approach. For averaging and precise image alignment at sub-pixel levels we have made a program freely available on our web-site http://bioinfome.cgm.cnrs-gif.fr webcite to facilitate implementation of this approach.