Laser Doppler flow monitor

• US 4,596,254 A
• Filed: 12/18/1984
• Issued: 06/24/1986
• Est. Priority Date: 12/18/1984
• Status: Expired due to Term

• Current Assignee: Optical Sensors Incorporated
• Original Assignee: TSI Research Associates Limited Partnership
• Inventors: Borgos, John A., Adrian, Ronald J.
• Primary Examiner(s): Howell, Kyle L.
• Assistant Examiner(s): Hanley, John C.
• Application Number:
  US06/682,986
• Time in Patent Office: 553 Days
• Field of Search: 128/666, 128/663, 128/691, 356/28
• US Class Current: 600/479
• CPC Class Codes: A61B 5/0261 using optical means, e.g. infrared light

Abstract

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.

Claims

1. Apparatus for measuring the velocity of particles moving in a media comprising:

(a) means for illuminating the particles to produce a spread spectrum optical signal resulting from the Doppler shift occurring when photons are scattered by the moving particles;

(b) photodetector means connected with said optical illumination means for producing a spread spectrum electrical signal corresponding with said optical signal, said electrical signal containing spectral and noise components;

(c) first filter means connected with said photodetector means for filtering noise from said electrical signal at high and low frequencies;

(d) second filter means connected with said photodetector means for producing a DC signal proportional to the total optical signal received by said photodetector means; and

(e) signal processing means connected with said first and second filter means for calculating the mean frequency of said spread spectrum electrical signal, the mean frequency corresponding with the average velocity of the moving particles, said processing means including

(1) first and second analog-to-digital converter means connected with said first and second filter means, respectively, for converting said electrical signal from the first filter means and said DC signal to digital signals;

(2) first correlation means connected with said first converter means for calculating a first autocorrelation function from said digital signal from the first filter means;

(3) means connected with said second converter means for determining a noise autocorrelation function from said digital DC signal;

(4) means connected with said first correlation means and said noise autocorrelation function determining means for comparing said noise autocorrelation function with said first autocorrelation function and for producing an autocorrelation function free of a noise component; and

(5) linear calculation means connected with said autocorrelation comparison means for calculating the mean frequency from the autocorrelation function.

2. Apparatus as defined in claim 1, wherein said illuminating means comprises a laser source connected to an optical fiber.

3. Apparatus as defined in claim 2, wherein said first correlation means comprises means for calculating a single-clipped autocorrelation function.

5. Apparatus as defined in claim 1, wherein said illuminating means comprises a laser source connected to an optical fiber.

6. Apparatus as defined in claim 2, wherein said first correlation means comprises means for calculating a single-clipped autocorrelation function.

4. Apparatus for measuring the velocity of moving blood cells in a tissue sample, comprising

(a) means for illuminating the blood cells to produce a spread spectrum optical signal resulting from the Doppler shift occurring when photons are scattered by the moving blood cells;

(b) photodetector means connected with said optical illumination means for producing a spread spectrum electrical signal corresponding with said optical signal, said electrical signal containing spectral and noise components;

(c) bandpass filter means connected with said photodetector means for filtering noise from said electrical signal at high and low frequencies;

(d) low-pass filter means connected with said photodetector means for producing a DC signal proportional to the total optical signal received by said photodetector means; and

(e) signal processing means connected with said bandpass and low-pass filter means for calculating the mean frequency of said spread spectrum electrical signal, the mean frequency corresponding with the average velocity of the moving blood cells, said processing means including

(1) first and second analog-to-digital converter means connected with said bandpass and low-pass filter means, respectively, for converting said bandpass signal and said DC signal to digital signals;

(2) first correlation means connected with said first converter means for calculating a first autocorrelation function from said digital bandpass signal;

(3) means connected with said second converter means for determining a noise autocorrelation function from said digital DC signal;

(4) means connected with said first correlation means and said noise autocorrelation function determining means for comparing said noise autocorrelation function with said first autocorrelation function to produce an autocorrelation function free of a noise component; and

(5) liner calculation means connected with said autocorrelation comparison means for calculating the mean frequency from the autocorrelation function.

7. A method for measuring the velocity of particles moving in a media, comprising the steps of

(a) illuminating the particles to produce a spread spectrum optical signal resulting from the Doppler shift occurring when photons are scattered by the moving particles;

(b) generating a spread spectrum electrical signal corresponding with said optical signal and containing spectral and noise components;

(c) filtering noise from said electrical signal at high and low frequencies to produce a bandpass signal and filtering AC components from said electrical signal to produce a DC signal proportional to said optical signal;

(d) converting said bandpass and DC signals to digital signals;

(e) calculating a first autocorrelation function from said digital bandpass signal and a noise autocorrelation function from said digital DC signal;

(f) comparing said first and noise autocorrelation functions to produce an autocorrelation function free of a noise component; and

(g) calculating the mean frequency from the autocorrelation function, the mean frequency corresponding with the average velocity of the moving particles.

8. A method for measuring the velocity of moving blood cells in a tissue sample, comprising the steps of

(a) illuminating the blood cells to produce a spread spectrum optical signal resulting from the Doppler shift occurring when photons are scattered by the moving blood cells;

(b) generating a spread spectrum electrical signal corresponding with said optical signal and containing spectral and noise components;

(c) filtering noise from said electrical signal at high and low frequencies to produce a bandpass signal;

(d) filtering AC components from said electrical signal to produce a DC signal proportional to said optical signal;

(e) converting said bandpass signal and said DC signal to digital signals;

(f) calculating a first autocorrelation function from said digital bandpass signal;

(g) calculating a noise autocorrelation function from said digital DC signal;

(h) comparing said first and noise autocorrelation functions to produce an autocorrelation function free of a noise component; and

(i) calculating the mean frequency from the autocorrelation function, the mean frequency corresponding with the average velocity of the moving blood cells.