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Comparaison of the performances of two optical arrangements for laser Doppler flowmetry (LDF)

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The aim of the project is to compare the performances of two optical arrangements for laser Doppler flowmetry (LDF) of the subfoveal choroid: one collecting the scattered light at the site of illumination and the other excluding it but detecting around it.

The LDF technique is based on the Doppler effect where light scattered from moving red blood cells (RBCs) is shifted in frequency by an amount proportional to the velocity of the particles. This scattered light is optically mixed with the light reflected by the tissues and electronically detected by an avalanche photodiode.

The point laser source, the illuminated point at the foveola and the fibre tips plane (detection) are conjugated. The subject is asked to look at a faint point, so that this light is focussed on his foveola. The light scattered by the RBCs in the choriocapillaris is collected by the optical system and detected by an APD either through a single fibre of (direct-detection) or through a fibre bundle excluding the centre (indirect-detection).

The signal is then amplified, digitalized and analysed by a computer with a PCMCIA DAQ card controlled by a LabVIEW program. The signal is sampled at 240 kHz with a resolution of 16 bits. About 15 times per second a Fourier analysis (over a frame of 214 samples) of the digital signal is done. The shot-noise level is estimated from the signal between 30 and 40kHz and is then subtracted from the power spectrum P(f). The zero, first and second moment Mi of the noise-free power spectral density are calculated. From these moments, the following blood flow parameters are derived: volume (ChBVol) from M0, velocity (ChBVel) from M1, choroidal blood flow (ChBF= ChBVol x ChBVel) and also ChB2nd from M2.

Results

Detecting backscattered light around the illumination point (indirect) gives a higher signal and has a better sensitivity than detecting light at the site of illumination (direct).

Due to multiple scattering, indirect-ChBvel is, as expected, larger than direct-ChBvel.

In the population that participated to the study, age and gender are not discriminated by blood flow parameters.