A field-programmable gate array based system for high frame rate laser Doppler blood flow imaging.

This paper presents a general embedded processing system implemented in a field-programmable gate array providing high frame rate and high accuracy for a laser Doppler blood flow imaging system. The proposed system can achieve a basic frame rate of flow images at 1 frame/second for 256 x 256 images with 1024 fast Fourier transform (FFT) points used in the processing algorithm. Mixed fixed-floating point calculations are utilized to achieve high accuracy but with a reasonable resource usage. The implementation has a root mean square deviation of the relative difference in flow values below 0.1% when compared with a double-precision floating point implementation. The system can contain from one or more processing units to obtain the required frame rate and accuracy. The performance of the system is significantly higher than other methods reported to date. Furthermore, a dedicated field-programmable gate array (FPGA) board has been designed to test the proposed processing system. The board is linked with a laser line scanning system, which uses a 64 x 1 photodetector array. Test results with various operating parameters show that the performance of the new system is better, in terms of noise and imaging speed, than has been previously achieved.

Non-invasive laser Doppler perfusion measurements of large tissue volumes and human skeletal muscle blood RMS velocity.

This study proposes the implementation of an algorithm allowing one to derive absolute blood root-mean-square (RMS) velocity values from laser Doppler perfusion meter (LDP) data. The algorithm is based on the quasi-elastic light scattering theory and holds for multiple scattering. While standard LDP measurements are normally applicable to a small region of interest (approximately 1 mm2), the present method allows the analysis of both small and large tissue volumes with small and large interoptode spacings (e.g., 1.5 cm). The applicability and the limits of the method are demonstrated with measurements on human skeletal muscle using a custom-built near-infrared LDP meter. Human brachioradialis muscle RMS velocity values of 9.99 +/- 0.01 and 5.58 +/- 0.03 mm s(-10 at 1.5 cm and of 5.18 +/- 0.01 and 2.54 +/- 0.09 mm s(-1) at 2 cm were found when the arm was (a) at rest and (b) occluded, respectively. At very large optode spacings or very high moving particle densities, the theory developed here would need to be amended to take into account second-order effects.

Laser Doppler flowmetry--peripheral microcirculation during cessation of cerebral and cardiocirculatory function.

Laser Doppler flowmetry (LDF) is a noninvasive method to study the microcirculation. We studied the feasibility of the clinical use of LDF in the intensive care unit by continuously recording different LDF parameters during the cessation of cerebral and cardiocirculatory function. The LDF-Conc (concentration of red blood cells) reflected biological phenomena correlated with the heart rate and microvibration that could not be detected with conventional means (systemic blood pressure, peripheral oxygen saturation).

Laser Doppler velocimetry: the problem of fibre movement artefact.

The problem of fibre movement artefact in laser Doppler velocimetry recordings of skin blood flow has been investigated. Beat frequency spectra were obtained for a stationary target, unheated skin, heated skin (43.5 degrees C), with and without blood flow occluded for various induced amounts of fibre movement. Possible methods for either eliminating, or obtaining an indicator of, fibre movement artefact were considered. The solution proposed is to incorporate additional fibres terminating within the transducer to permit an index of fibre movement to be obtained that is independent of blood flow.

Laser Doppler measurements of blood flow in skin tissue.

Measurements of blood flow in skin tissue have been made using a low power helium neon laser, with a pair of multi-modal plastic optic fibres to transmit the laser radiation to the skin, one similar fibre to collect the scattered laser radiation, and photodiode detection. Before transmission to the skin surface one of the laser beams is frequency shifted by reflection from a moving corner cube, enabling the experimenter to distinguish between low frequency blood flow dependent signals having a Doppler shift origin, and those low frequency signals generated by other sources of intensity fluctuation such as fibre movement, number fluctuations and laser amplitude modulation.