Rappels technologiques, biais méthodologiques possibles et conditions de mesure

Resume

De nombreux facteurs peuvent invalider les résultats d’une analyse du mouvement des spermatozoïdes par les systèmes de microvidéographie assistée par ordinateur: une standardisation de l’analyse est donc indispensable. Des notions de base de la technologie de ces systèmes, une bonne connaissance de la physiologie spermatique et une rigueur dans toutes les étapes de l’analyse sont des pré-requis essentiels pour garantir une analyse fiable car ces systèmes ne sont pas des automates “prêt à l’emploi”.

Abstract

Computer-aided sperm analysis (CASA) was developed in the last decade to overcome the problems related to the inherent subjectivity of manual semen analysis. A typical CASA system includes a video camera, a microscope equiped with phase optics and a warming stage, a microcomputer with hardware and software dedicated to motion analysis, a monitor and a printer. The image of the sperm cells under the microscope is transformed into discrete pixels producing a voltage proportional to the intensity of the light as light strikes the camera’s CCD array. In standard video technology, the first field (set of rows; odd lines) is scanned in one sixtieth of a second and remains illuminated, then the alternate set of rows (even lines) is scanned in the next one sixtieth of a second to display the complete video frame which requires one thirthieth of a second. The first step called digitization is the encoding of video pixels as numbers following a gray level scale between white and black. From this process the detection of the sperm heads in the field is performed and repeated on a field by field basis. The centroïd coordinates of the sperm heads in the field are calculated and sequentially transmitted to the hard disk. A path finder algorithm connects the centroïds through time allowing the reconstruction of trajectories and the calculation of various parameters. Many factors modulating CASA measurements can invalidate the results of analysis. Consequently, a standardization of CASA is necessary. Various guidelines for accurate analysis are proposed.

In summary, basic knowledge on sperm physiology, CASA technology and rigorous standardized conditions of analysis are required for providing reliable and accurate results because CASA systems are not ready-to-use “robots”. CASA is useful for routine semen analysis and essential for the measurement of sperm kinematics for clinical as well as for research purposes.