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Laser Doppler flow monitor
| Details |
Inventors: Adrian, Ronald J.; Borgos, John A.;
Assignee: TSI Research Associates Limited Partnership (Minneapolis, MN)
Primary Examiner:
Assistant Examiner:
Attorney, Agent or Firm:
A method and apparatus for measuring the velocity of moving particles such as red blood cells in a tissue sample is disclosed, characterized by digital processing techniques and autocorrelation. The moving particles are illuminated to produce a spread spectrum optical signal resulting from the Doppler shift occurring when photons are scattered by the moving particles. A spread spectrum electrical signal corresponding with the optical signal and containing spectral and noise components is generated from the optical signal. The electrical signal is filtered to produce the bandpass and DC signals which are subsequently converted to digital form. A first autocorrelation function is calculated from the bandpass signal and a noise autocorrelation function is determined in accordance with the DC signal level. The first and noise autocorrelation functions are compared to produce an autocorrelation function free of a noise component. From the autocorrelation function, the mean frequency of the electrical signal is linearly calculated, the mean frequency corresponding with the average velocity of the moving particles. |
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DETAILED DESCRIPTION As shown in the drawing, an optical system 2 is provided for illuminating a tissue sample 4 containing a plurality of red blood cells whose velocity is to be measured.
The optical system includes a laser source 6 connected with an optical transducer 8 via a bidirectional fiber optic bundle 10.
The optical transducer 8 is arranged adjacent the sample and transmits laser energy to the tissue and receive an optical signal therefrom.
More particularly, when the tissue sample is illuminated with coherent light, some of the light penetrates the tissue, is randomly scattered by both stationary tissue elements and moving red blood cells, and emerges from the tissue sample.
A portion of this light is received by the transducer and delivered as an optical signal to a photodetector 12 such as a photodiode by the fiber optic bundle 10.
The optical signal received by the photodetector has a broadened spectrum resulting from the Doppler shifting that occurs when photons are scattered by moving particles.
The photodetector converts the optical signal into an electrical signal having the same spectral shape centered around zero frequency.
The width of this spectrum is proportional to the average speed of the moving red blood cells.
See Bonner, R.
and Nossal, R.
, "Model for Laser Doppler Measurements of Blood Flow in Tissue", Applied Optics, Vol.
20, No.
12, June 15, 1981, pages 2097-2107.
The electrical signal produced by the photodetector includes both spectral components resulting from the Doppler effect as set forth above and undesirable noise components.
The noise represents shot noise and amplifier noise, both of which are uncorrelated with the spectral components.
Accordingly, a bandpass filter 14 is connected with an output of the photodetector.
The bandpass filter removes unwanted noise from the electrical signal at both high and low frequencies.
For measuring blood perfusion in tissue, the bandpass is preferably between 30 and 20,000 Hz.
A low-pass filter 16 is also connected with an output of the photodetector
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