Surgery – Diagnostic testing – Detecting nuclear – electromagnetic – or ultrasonic radiation
Reexamination Certificate
1999-01-19
2001-07-17
Casler, Brian L. (Department: 3737)
Surgery
Diagnostic testing
Detecting nuclear, electromagnetic, or ultrasonic radiation
C600S473000, C600S476000, C356S039000
Reexamination Certificate
active
06263227
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 photocurrent 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 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.
The present invention seeks to significantly reduce imaging times, in some circumstances to sub second times, and provide means to record video and blood perfusion images of the same tissue site simultaneously.
In the present invention provision can be made in the apparatus to switch between two or more monochromatic laser light sources of different wavelengths to enable laser Doppler blood flow measurements to be made sequentially at the different wavelengths.
The differences between measurements at different wavelengths provides information on flows at different depths below the tissue surface and information on tissue and blood absorption of light for the different wavelengths.
In the present invention provision can be made in the apparatus to irradiate the tissue surface simultaneously with monochromatic laser light of different wavelengths and simultaneous detection of light scattered from the tissue at the two or more laser light wavelengths using a similar number of image sensors and suitable optical filters.
The magnitude of a blood perfusion measurement, commonly termed ‘flux’, which is proportional to the product of average red blood cell speed and red blood cell concentration in an element of the volume of tissue sampled, is dependent amongst other factors on the imager/tissue surface distance.
Provision can be made in the apparatus to measure the distance using either a CCD camera video image or an intensity photo image recorded using the image sensor and compensate in the image processing for the distance dependence.
The present invention provides an apparatus for measuring and imaging blood perfusion in tissue comprising
a monochromatic light source;
means for shaping the laser light beam;
means for irradiating a section of the surface of the tissue with the laser light beam;
means for collecting light scattered from the irradiated section;
an image sensor comprising a plurality of photodetectors, each photodetector of the sensor being able to receive collected light from a predetermined sub area of the section of the tissue surface and produce a corresponding electrical output signal linearly related to the detected instantaneous laser light intensity;
means for processing the electrical output signals from the plurality of photodetectors to produce processed output signals comprising;
measurements of 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;
the calculated average Doppler frequency shift for each sub area from which scattered light is detected;
the calculated blood volume concentration for each sub area from which scattered light is detected;
the calculated blood perfusion for each sub area from which scattered light is detected;
measurements of the intensity of the detected scattered light for each predetermined sub area;
means for producing an image of the blood perfusion in the tissue section irradiated from the process output signals; and
an image display means.
Compared with the systems disclosed in WO90/11044 and WO91/06244 the present invention provides fast tissue blood perfusion imaging using an image sensor which is either a linear or two dimensional photodetector array for detecting the Doppler shifted and Doppler unshifted scattered laser light, and signal processing done with the aid of large scale digital signal processor integrated circuits processing the multi-channel laser Doppler signals in paral
Boggett David
Huang Xiabing
Casler Brian L.
Lipsitz Barry R.
McAllister Douglas M.
Moor Instruments Limited
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