Surgery – Diagnostic testing – Measuring or detecting nonradioactive constituent of body...
Reexamination Certificate
1999-10-25
2001-07-10
Lateef, Marvin M. (Department: 3737)
Surgery
Diagnostic testing
Measuring or detecting nonradioactive constituent of body...
C600S322000, C600S328000, C600S553000, C600S473000, C600S475000, C600S476000, C604S020000
Reexamination Certificate
active
06259936
ABSTRACT:
The present invention relates to an apparatus for the measurement and imaging of particle movement and flow in fluids, particularly for the measurement and imaging of blood flow in the small superficial blood vessels of body tissue.
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 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 photodetected and the resulting photo current processed to provide a signal which correlates with blood flow.
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 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 subsequently in U.S. Pat. No. 4,109,647.
For some clinical applications, such as plastic surgery and wound healing, point measurements using optic probes attached to the skin are severely limited and this has prevented widespread application in these areas. Three reasons for this are: point to point variation (spatial variability) requiring several readings to give reliable measurement, contact between the probe and the tissue surface, and interference from fibre movements which degrade the measurements.
These problems have been mainly overcome by the development of laser Doppler scanners which map perfusion over an area of tissue, typically 100 cm
2
and in some cases over 1000 cm
2
, using a scanning laser beam and one or more photodetectors. EP-A-0282210 describes an apparatus for monitoring blood stream in the skin surface which employs a linear sensor comprising a plurality of light receiving elements to receive the laser light reflected by the skin surface, memory means for storing the output signals from the light receiving elements and calculating means for processing these signals to derive information about the blood stream. The blood stream velocity or distribution information may thereby be calculated and displayed. WO90/11044 describes a method of determination of blood flow and an apparatus for use therein which involves projecting a beam of laser light to move over a surface beneath which blood flow in a vessel or vascular bed is to be determined, collecting the reflected and scattered light, measuring a spectrum of frequencies in the collected light and determining from differences in the frequencies the blood flow beneath the surface under examination. WO91/06244 describes a system which includes means for directing a laser beam onto a body part to be examined and for guided movement of the laser beam through a series of measurement points over the body part in accordance with a predetermined scanning pattern. The laser beam is halted at each measurement point for a given time interval. These devices have found many research applications and have generated considerable clinical interest.
A serious problem which affects the production of images using these prior art laser Doppler imagers arises when the surface of the target area is highly reflective. Normally, the illumination of the surface of tissue with a beam of laser light will result in light being scattered from the surface and from the tissue below the surface.
Generally, the light is diffusely scattered so that only a very small fraction of the incident light would be detected by a photodetector “viewing” the surface, typically at a distance from the surface of several tens of centimeters. When the surface of the target area is highly reflective, for instance skin which is wet, oily or greasy or which is covered by a transparent or translucent dressing or the surface of an organ exposed during open surgery, a large fraction of the incident light is specularly reflected. If this specularly reflected light is photodetected a high photo current will be generated by the photodetector, e.g., photodiode, resulting in signal saturation of the current-to-voltage converter or current amplifier used and the consequent loss or impairment of image information.
Conditions necessary for photodetection of this specularly reflected light depend on the orientation of the reflecting surface relative to the incident beam and the position of the photodetector so that as the laser beam is scanned over the surface the condition of high detected light intensity and loss of image information occurs over a range of beam angles. In general the larger the effective photodetector, which can for example be increased by the use of a lens, the larger the area of image affected. Both the flux and light intensity (photo image) are affected.
The present invention seeks to reduce significantly or even eliminate the effects of specular reflection thus enabling measurements of intensity and blood flow (flux) to be made when one photodetector is saturated.
The present invention provides an apparatus for imaging blood perfusion in tissue which comprises:
a monochromatic laser light source;
means for irradiating a section of the surface of the tissue with monochromatic light from the light source;
means for collecting light scattered from the irradiated section;
means for scanning a beam of the monochromatic laser light over the tissue surface in a predetermined pattern;
means for scanning the beam at substantially constant speed across the tissue;
means for determining and recording the position of the beam at any point of the scan;
means for halting the beam at a predetermined position or positions during the scan;
two or more photodetectors positioned to collect light diffusely scattered from the tissue surface and so positioned to ensure that specularly reflected light is detected by either no detector or by only one detector at a given time;
means for processing the electrical output signals from the photodetectors;
means for registering saturation of a photodetector and eliminating the saturated photodetector signal from the signal
processing ensuring that only photodetector signals which are unsaturated are processed; and
means for recording and displaying the blood perfusion measurements.
Compared to the prior art laser Doppler imagers the apparatus of the present invention is able to provide images of blood perfusion in tissue when the tissue has a highly reflective surface.
The apparatus of the present invention includes a monochromatic laser light source for generating the laser beam used for irradiating a section
Boggett David
Huang Xiabing
Lateef Marvin M.
Lin Jeoyuh
Lipsitz Barry R.
Moor Instruments Limited
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