Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Reexamination Certificate
2000-04-25
2002-06-11
Le, Que T. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
C250S556000, C250S227140, C356S041000
Reexamination Certificate
active
06403944
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to apparatus for use in non-invasive in vivo monitoring of physiological substances such as blood and the like.
2. Discussion of the Art
One particular, but not exclusive, application of the present invention is in the monitoring of blood glucose, for example in the management of diabetes mellitus. It is accepted that the management of diabetes can be much improved by routine monitoring of blood glucose concentration and clinicians suggest that monitoring as often as four times per day is desirable.
The monitoring technique currently available for use by patients involves using a spring loaded lancet to stab the finger to obtain a blood sample which is transferred to a glucose test strip. The concentration is derived either by reading the test strip with a reflectance meter or by visual comparison of colour change against a colour scale. Many diabetics find the testing onerous as the technique is painful, inconvenient, messy, potentially embarrassing and offers a site for the transmittance and acceptance of infection.
Techniques have also been developed for non invasive measurement using transmittance or reflectance spectroscopy. However the required instruments are expensive and it is difficult to obtain accurate and repeatable measurements.
There are also known various types of in vivo chemical sensors. These rely on implanting minimally invasive sensors under the skin surface, but such sensors have poor long term reproducibility and bio-compatibility problems.
There is therefore a need for improved means for routine monitoring of blood glucose in a manner which is simple and straightforward to use.
The present invention makes use of photoacoustic techniques. The fundamentals of photoacoustic techniques are well known per se. A pulse of light, typically laser light, is applied to a substance containing an analyte of interest in solution or dispersion, the wavelength of the applied light being chosen to interact with the analyte. Absorption of the light energy by the analyte gives rise to microscopic localised heating which generates an acoustic wave which can be detected by an acoustic sensor. These techniques have been used to measure physiological parameters in vitro.
U.S. Pat. Nos. 5,348,002 and 5,348,003 (Caro) propose the use of photoacoustics in combination with photoabsorption for the measurement of blood components in vivo. However, the arrangement proposed by Caro has not been demonstrated as a workable system and may suffer from interference to a degree which would preclude useful acoustic signals, and since they would also suffer from interference and resonance effects from hard structures such as bone.
It has also been proposed by Poulet and Chambron in
Medical and Biological Engineering and Computing
, Nov. 1985, Page 585 to use a photoacoustic spectrometer in a cell arrangement to measure characteristics of cutaneous tissue, but the apparatus described would not be suitable for measuring blood analytes.
Published European Patent Application 0282234A1 (Dowling) proposes the use of photoacoustic spectroscopy for the measurement of blood analytes such as blood glucose. This disclosure however does not show or suggest any means which would permit the required degree of coupling to body tissues for use in vivo.
SUMMARY OF THE INVENTION
Accordingly, the present invention provides a sensor head for use in photoacoustic in vivo measurement, comprising a housing shaped to engage a selected body part, light transmission means terminating in said housing so as to transmit light energy form a light source to enter the body part along a beam axis, and acoustic transducer means mounted in the housing to receive acoustic waves generated by photoacoustic interaction within the body part, the acoustic transducer means being disposed in the housing to receive said acoustic wave in a direction of high acoustic energy.
The expression “direction of high acoustic energy” is used herein to denote a direction other than the forward direction of the light beam. Preferably, the transducer means is disposed so as to intercept acoustic energy propagating at right angles to the optical beam axis, or at an angle to the optical beam axis which may be down to about 20°, typically about 45°.
An exact measure of the angle of high acoustic energy can be worked out but is dependent upon the specific geometry of the light source, the properties of the tissue and the absorption coefficient of the tissue. One model for understanding the propagation of the acoustic energy in any homogenous media was developed by Huyghens and is called the principle of superposition. In this model each volume element that is illuminated by the light generates an acoustic pressure wave that radiates outward in a spherical manor. The magnitude of the pressure wave at each volume element depends on the intensity of the optical beam at that location, the absorption coefficient of the material at that location, the wavelength of light and on several other physical properties of the material such as the speed of sound and the specific heat. The signal measured at the detector is just the superposition of all pressure waves from all points that are illuminated by the source light. An analytical solution for the pressure wave has been worked out for a few cases in aqueous material. The analytical case that best matches the in-vivo measurements is that of a cylindrical optical beam propagating in a weekly absorbing material. In this case the direction of highest acoustic energy is perpendicular to the optical axis. The base detector location is with the plane of the detector perpendicular to the acoustic energy, or parallel to the optical axis. This is because the acoustic detector has the highest sensitivity when the acoustic energy strikes the detector perpendicular to the plane of the detector. This analytical model is not completely accurate for the in-vivo measurement case because of scattering of the tissue and because the tissue absorbs more than the model predicts. These differences indicate that a different position for the detector will be optimal. A detailed numeric model is required to determine the best detector location and is dependent upon the beam properties (focused to a point, colligated, etc.), body site (finger, earlobe, arm etc.) and wavelength. One skilled in the art can readily develop an appropriate mode. However, suitable locations for a detector will generally be at an angle to the optical axis. Angles between 40 and 90 degrees should be suitable.
In one preferred arrangement, the acoustic transducer means is arranged parallel to the optical beam axis. This arrangement is particularly suitable for use where the selected body part is the distal portion of a finger, in which case the housing may include a generally half-cylindrical depression in which the finger may be placed with the light transmission means aimed at the end of the finger.
Preferably, the acoustic transducer means comprises a piezoelectric transducer which most preferably is of a semi-cylindrical shape. This transducer may be provided with a backing of lead or other dense material, and the backing may have a rear surface shaped to minimise internal acoustic reflection.
Alternative transducer means include a capacitor-type detector, which is preferably small and disk-shaped; an integrated semiconductor pressure sensor; and an optical pressure sensor, for example based on an optical fibre.
In an alternative arrangement, the plane of the transducer may be arranged to be perpendicular to the optical axis to detect the acoustic wave which is propagating in a direction opposite to the direction of the light beam. For example, the acoustic transducer means may be part-spherical with an aperture to allow access for the light beam. This may be particularly suitable for engagement with a body part other than the finger, for example the back of the arm.
The generation of a surface acoustic wave is an inherent aspect of the in vivo pulsed photoacoustic generation in ti
Lindberg John Matthew
MacKenzie Hugh Alexander
Abbott Laboratories
Le Que T.
Weinstein David L.
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