Surgery – Diagnostic testing – Measuring or detecting nonradioactive constituent of body...
Reexamination Certificate
1999-07-09
2001-01-09
Winakur, Eric F. (Department: 3736)
Surgery
Diagnostic testing
Measuring or detecting nonradioactive constituent of body...
C600S504000
Reexamination Certificate
active
06173197
ABSTRACT:
The present invention relates to an apparatus for measuring microvascular blood flow.
Blood flow in the small blood vessels of the skin performs an essential role in the regulation of the metabolic, hemodynamic and thermal state of an individual and the condition of the microcirculation over both long and short time periods can reflect the general state of health. The degree of blood perfusion in the cutaneous microvascular structure often provides a good indicator of peripheral vascular disease and reduction of blood flow in the microcirculatory blood vessels can often be attributed to cutaneous vascularisation disorders; so there are many situations in routine clinical medicine where measurement of the blood flow is important.
The microcirculation, its responses to stimuli, and its response to therapeutic regimes, were not open to routine continuous assessment and investigation until the introduction of the laser Doppler technique in the 1970's and subsequent developments in the 1980's.
The technique depends on the Doppler principle whereby laser light (which must be highly monochromatic and hence have a long coherence length) which is incident on tissue (typically the skin surface), is scattered by moving red blood cells and undergoes frequency broadening. The frequency broadened laser light, together with laser light scattered from static tissue, is photo detected and the resulting photo current processed to provide a signal which correlates with blood flow.
Laser light can be directed to the tissue surface either via an optic fibre or as a light beam. For “fibre optic” monitors the optic fibre terminates in an optic probe which can be attached to the tissue surface. One or more light collecting fibres also terminate in the probe head and these fibres transmit a proportion of the scattered light to a photo detector and the signal processing electronics. Normal fibre separations in the probe head are a few tenths of a millimeter so consequently blood flow is measured in a tissue volume of typically 1 mm
3
or smaller.
When a larger volume of tissue is stimulated to vasodilate or vasoconstrict, or where for example a healing process results in increased blood flow, the measured blood flow changes in the small tissue volume is generally taken to be representative of the larger volume.
For laser beam monitors single point measurements can be made by directing the beam to the desired point on the surface. By scanning the beam in a raster fashion a series of measurements can also be made, and by colour coding the flow measurements a colour image of blood flow distribution over the scanned surface can be displayed on a computer monitor screen.
Single point measurements give a high temporal resolution (40 Hz data rates are typical) enabling rapid blood flow changes to be recorded, whereas the laser Doppler imager can provide spatial information and has the ability to average blood flow measurements over large areas. Fibre optic systems can measure at tissue sites not easily accessible to a laser beam. For example measurements in brain tissue, mouth, gut, colon, muscle and bone.
Perfusion measurements using single and multiple channel fibre optic laser Doppler monitors have been made on practically all tissues and applied in most branches of medicine and physiology. The technique and its application has been described in numerous publications. A representative selection of these are included in “Laser-Doppler Blood Flowmetry”, ed. A. P. Shepherd and P.Å. Oberg, Kluwer Academic Publishers 1990 and also “Laser Doppler”, ed. G. V. Belcaro, U. Hoffmann, A. Bollinger and A. N. Nicolaides, Med-Orion Publishing Co. 1994.
The basic principles of measuring blood flow using coherent radiation and the Doppler effect were first described by C. Johnson in U.S. Pat. No. 3,511,227 patented May 12, 1970 entitled “Measurement of Blood Flow using Coherent Radiation and Doppler Effect”.
The application of these principles to measurements in the microcirculation was described by M. D. Stern in “Nature”, Vol 254, 56, March 1975, “In vivo evaluation of microcirculation by coherent light scattering”; M. D. Stern et al, 1977 “Continuous measurement of tissue blood flow by laser-Doppler spectroscopy”, Am J. Physiol 232: H441-H448; and subsequent in U.S. Pat. No. 4,109,647, Aug. 29, 1978 “Method of and apparatus for Measurement of Blood Flow using Coherent Light”.
An apparatus using fibre optics to transmit the laser light to tissue site and collect scattered light using one or more optic fibres was described by Holloway, G. A. and D. W. Watkins, 1977, “Laser Doppler measurement of cutaneous blood flow”, J. Invest. Dermatology 69: 306-309 and D. W. Watkins and G. A Holloway, 1978, “An instrument to measure cutaneous blood flow using the Doppler shift of laser light”, IEEE Trans Biomed Eng BME-25: 28-33. Extensions to theory and investigation of experimental models were made by R. Bonner and R. Nossal June 1981, Vol 20 No. 12, Applied Optics, “Model for laser Doppler measurements of blood flow in tissue”. They showed that the first moment of the power spectral density of the photo current produced by the heterodyne mixing of Doppler shifted and unshifted laser light scattered from the microvasculature could be used as a measurement of perfusion. This parameter is commonly referred to as “Flux”. They described the photon characteristics both in terms of auto correlation functions and spectral properties and used photo correlation techniques for their experimental investigations.
A perfusion monitor based on the application of auto correlation techniques is described by R. J. Adrian and J. A. Burgos “Laser Doppler flow monitor”, U.S. Pat. No. 4,596,254, Jun. 24, 1986.
In the present investigation we have used mainly digital signal processing but have chosen to use the technique of Fast Fourier Transformation, implemented with large scale digital signal processor (DSP) ICs, for the Flux calculations. This enables the high data rates necessary for real time graphical display.
The algorithms we have implemented have the important advantage that noise due to fibre movements, a major problem in existing laser Doppler fiber optic instruments, is generally reduced to insignificant levels. Using FFT processing with post DSP systems has additional advantages in that processing algorithms can be changed without a corresponding change in hardware. For example, the processing bandwidth for the Doppler shifts can be changed, measurements at different bandwidths can be done simultaneously; different frequency weighting in the “flux” calculation can be used to provide a means of easily differentiating fast from slow blood flows and hence provide a means of depth discrimination.
Reducing the use of analogue circuits to a minimum has the added advantages of greater reliability, reduced size and weight, and reduced manufacturing and servicing costs.
The present invention provides an apparatus for measuring blood in tissue comprising:
a monochromatic light source;
means for irradiating a section of the tissue with the monochromatic light from the light source;
means for collecting light scattered from the irradiated section;
means for photodetecting the collected scattered light;
means for processing the electrical output signals from the photodetector;
means for calculating the power spectrum of the photocurrents generated in the detection of laser light scattered from static tissue and Doppler broadened laser light scattered from moving blood cells;
means for calculating and recording the average Doppler frequency shift;
means for calculating and recording the blood concentration;
means for measuring and recording the intensity of the detected scattered light;
means for calculating and recording the blood perfusion (flux);
means for filtering movement artefact noise;
means for displaying the blood perfusion measured parameters.
REFERENCES:
patent: 4590948 (1986-05-01), Nilsson
patent: 4596254 (1986-06-01), Adrian et al.
patent: 4862894 (1989-09-01), Fujii
patent: 5598841 (1997-02-01), Taniji et al.
patent: W
Boggett David
Huang Xiabing
Lipsitz Barry R.
Moor Instruments Limited
Winakur Eric F.
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