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  Laser diode optical transducer assembly for non-invasive spectrophotometric blood oxygenationUSPTO Application #: 20080017800Title: Laser diode optical transducer assembly for non-invasive spectrophotometric blood oxygenation Abstract: An infrared spectrometric monitoring transducer includes a shielded cable, a fiber optic light guide, a transducer housing, a light source, and at least one light detector. The light source is mounted in the transducer housing and is operable to produce a light signal along a plurality of discrete wavelengths. The at least one light detector is mounted in the housing and is operable to receive light energy corresponding to the light signal. The at least one light detector is mounted in the transducer housing spaced a distance apart from the light source along a straight line extending the light source and the at least one light detector. The shielded cable and the fiber optic light guide are disposed adjacent the light source and the transducer has an “L” shape formed by the shielded cable and the fiber optic light guide extending outwardly from the transducer housing in a direction substantially perpendicular to the line between the extending the light source and the at least one light detector. (end of abstract)
Agent: O'shea, Getz & Kosakowski, P.C. - Springfield, MA, US Inventor: Paul Benni USPTO Applicaton #: 20080017800 - Class: 250339070 (USPTO) Related Patent Categories: Radiant Energy, Invisible Radiant Energy Responsive Electric Signalling, Infrared Responsive, With Selection Of Plural Discrete Wavelengths Or Bands, With Radiation Source, Including Spectrometer Or Spectrophotometer The Patent Description & Claims data below is from USPTO Patent Application 20080017800. Brief Patent Description - Full Patent Description - Patent Application Claims
[0001] This application is a continuation of U.S. patent application Ser. No. 11/384,615 filed Mar. 20, 2006, which is a continuation of U.S. patent application Ser. No. 09/951,221 filed Sep. 14, 2001, which is a continuation in part of U.S. patent application Ser. No. 09/434,142 filed Nov. 4, 1999, which claimed the benefit of the filing dates of U.S. Provisional Patent Application No. 60/123,849, filed Mar. 12, 1999; and 60/151,319 filed Aug. 30, 1999. TECHNICAL FIELD [0003] This invention relates to an improvement in a non-invasive near-infrared spectrophotometric (NIRS) optical transducer assembly, and a method of using the same. More particularly, this invention relates to a practical NIRS optical transducer assembly which can be reusable, is safe to use, and which can provide an accurately sized and consistent laser light field on a subject's skin. BACKGROUND ART [0004] Near-infrared spectroscopy (NIRS) is an optical spectrophotometric method of continually monitoring tissue oxygenation. The NIRS method is based on the principle that light in the near-infrared range (700 to 1,000 nm) can pass easily through skin, bone and other tissues but, within these wavelengths, hemoglobin has specific absorption spectra, dependent upon its oxidation state, i.e., oxygenated-hemoglobin (HbO.sub.2); and deoxygenated-hemoglobin (Hb). By using light sources that transmit near-infrared light at specific different wavelengths, and measuring changes in transmitted or reflected light attenuation, oxygenation concentration changes of HbO.sub.2 and Hb can be monitored. [0005] Total hemoglobin is the summation of the two states of hemoglobin (Total Hb=HbO.sub.2+Hb), and is proportional to relative blood volume changes, provided that the hematocrit or hemoglobin concentration of the blood is unchanged. The most valuable aspect of NIRS is that it allows one to continually monitor cerebral oxygenation levels in an adult or neonate, especially in diseased conditions, in which oxygenation levels in the brain can be compromised, leading to brain damage or death. [0006] It is known that near-infrared light passes through the skin and the skull of a neonate readily, and is absorbed by certain biological molecules in the brain. Near-infrared spectroscopy (NIRS) detects oxygenation changes in biological tissue (brain, muscle, or other organs) mainly at the micro circulation level (capillaries, arterioles, and venuoles) based on different absorption characteristics of the chromophores oxyhemoglobin (HbO.sub.2) and deoxyhemoglobin (Hb) in the near-infrared spectrum (700-1,000 nm). Average tissue penetration is 2-3 cm with sub-second time resolution. [0007] Another spectrophotometric method, called pulse oximetry, determines arterial oxygen saturation (SaO.sub.2) of peripheral tissue (i.e. finger, ear, nose) by monitoring pulsatile optical attenuation changes of detected light induced by pulsatile arterial blood volume changes in the arteriolar vascular system. The method of pulse oximetry requires pulsatile blood volume changes in order to make a measurement. Since venous blood is not pulsatile, pulse oximetry cannot provide any information about venous blood. Conversely, NIRS does not require pulsatile blood volume to calculate parameters of clinical value. [0008] Relative changes of the concentrations of HbO.sub.2 and Hb can be quantified by using the modified Beer-Lambert Law, which takes into account the optical attenuation in a highly scattering medium like biological tissue. The modified Beer-Lambert Law can be expressed as: A=-log(I/I.sub.0).sub.L=(a.sub.L.times.C.times.d.times.B)+G (Equation 1); [0009] wherein A is the optical attenuation in tissue at wavelength L (units: optical density OD); I.sub.0 is the incident light intensity (units: Wlcm.sub.2); I is the detected light intensity; (L is the wavelength-dependent absorption coefficient of the chromophore (units: OD.times.cm.sup.-1.times..mu.M.sup.-1); C is the concentration of chromophore (units: .mu.M); d is the light source-to-detector distance (units: cm); B is the light scattering differential path length factor (unitless); and G is a factor relating to tissue geometry and scattering of light (units: OD). [0010] Absolute measurement of chromophore concentration is very difficult because G is unknown. However, over a reasonable measuring period of several hours to days, G remains constant, allowing for the measurement of relative changes of chromophore from a zero reference baseline. Thus, if time t.sub.2 is an arbitrary time after the start of the optical measurement at t.sub.1 (baseline), differential attenuation ( A) can be calculated, canceling out the variables G and I.sub.0, providing that they remain constant. The objective is to determine changes in chromophore concentration [.DELTA.C.dbd.C(t.sub.2)-C(t.sub.1)] from A derived from the equation: .DELTA.A=-log(I.sub.2/I.sub.1).sub.L=a.sub.L=.DELTA.C.times.d.times.B (Equation 2); [0011] NIRS algorithms that are designed to calculate the relative changes of more than one chromophore use the multivariate form of Equation 2. To distinguish between, and to compute relative changes in, oxyhemoglobin (.DELTA.HbO.sub.2) and in deoxyhemoglobin (.DELTA.Hb), a minimum of two different wavelengths, preferably from narrow spectral bandwidth light sources, like laser diodes, are preferred. The units of .DELTA.HbO.sub.2 and .DELTA.Hb are in (moles per liter of tissue (.mu.M) which is determined from a dimensional analysis of Equation 1. [0012] It would be desirable to have a reusable NIRS transducer assembly having the ability to accurately control the energy level and size of a laser light field cast upon a subject's skin as well as improving the light detector signal-to-noise ratio by employing an improved EMI shielding scheme during use of the assembly. It would be desirable to combine light from multiple light sources into a single output fiber optic that is lightweight, and flexible, while providing sufficient light coupling efficiency. It would be desirable to have a transducer dislodgement-laser safety interlock system that would require no extra light source or detector components, while having the ability to disable laser operation due to transducer attachment failure and laser operation failure as well as a scheme that will verify secure transducer attachment before laser activation. DISCLOSURE OF THE INVENTION [0013] This invention relates to an improved transducer assembly for use in near-infrared spectroscopy (NIRS) of human patients. A prism providing a laser light source directing means, and one or more photodiodes are contained in a flexible housing which can be easily and securely attached to a subject's head, or some other part of the body. A rigid light guide placed over the prism provides a constant light intensity output of the spectrophotometric measuring system during a measuring period, by maintaining a constant laser light source to skin distance. A light detection means, by utilizing one or more photodiodes, allows for detection of laser light at a predetermined distance(s) away from the light source. The detection of light is improved by implementing an EMI shielding scheme that allows for attenuation of EMI interference while preserving an optical pathway for light to reach the detector via an EMI shielded optically transparent rigid window. An improved method of combining light from multiple light sources into a single output fiber optic that is lightweight, and flexible, while providing sufficient light coupling efficiency for a spectrophotometric measuring system is disclosed. An improved transducer dislodgement-laser safety interlock system is disclosed which does not require extra light source or detector components, while having the ability to disable laser operation due to transducer attachment failure and laser operation failure as well as verifying secure transducer attachment before laser activation. [0014] The NIRS transducer assembly of this invention may consist of two separable components, the NIRS transducer housing containing the laser light source and photodiode(s) described above; and a disposable adhesive envelope or pad which is used to mount the NIRS transducer assembly housing easily and securely to the subject's skin. It is economically more feasible to use a non-disposable NIRS transducer housing with a disposable envelope rather than a disposable NIRS transducer housing, while maintaining all of the advantages of single use, disposable transducer applications, especially in a health care environment, in which sanitation and sterilization requirements are paramount. [0015] The advantage of using a light radiation with a narrow spectral bandwidth (<1-3 nm) is maintained. [0016] The rigid laser light guide, which is placed over the output window of the laser light redirecting prism, has several functions. One function is to further decrease the intensity of laser light on the skin of a subject undergoing spectrophotometric monitoring by taking advantage of the conical radiation characteristics of the laser diode coupled to a optical light guide such as a multimode or single mode fiber optic. Thus, with an increasing separation distance (r) between the laser diode fiber optic output and the skin surface, the intensity (power/area) of the laser light decreases by a factor of r.sup.2. The prism, which redirects the laser-fiber optic output provides most of the separation distance from the light source to skin. The use of the laser light guide increases the separation distance and thus further decreases light intensity on the skin. This is important for designing a laser light source based optical transducer assembly meant to be directly applied to a human forehead, or some other part of the human body. To assure safety for the skin and tissue, the laser diode optical transducer assembly must be designed to operate within the limitations which are imposed by the "Maximum Permissible Exposure" (MPE) values set forth by the American National Standard for the safe use of lasers (ANZ136.1-1993). [0017] The laser light guide is rigid and provides a planar interface between the assembly and the patients skin in which the laser light is illuminating. The light guide controls the spacing between the prism and the subject's skin, and therefore controls the incident light intensity I.sub.0 (from Equation 1) on the subject's skin. This is especially important when attempting to measure absolute chromophore values as determined from Equation 1. [0018] One or more photodiodes are also incorporated in the NIRS transducer assembly housing, separated from the laser diodes light source by from a few mm to more than about 60 mm, depending on the size of the subject being monitored. For a typical adult human head, it is believed that at least 45 mm separation distance is needed for adequate brain blood oxygenation monitoring, using a reflection mode type of the NIRS transducer assembly. Multiple photodiodes can be used to monitor different depths of blood oxygenation in the subject, or can be used as reference detectors for algorithms that compensate for the scalp component of the detected signals. For neonates, shorter separation distances between the laser diodes and the photodiodes of around 20 mm can be used for reflection mode monitoring, or large distances over 60 mm can be used for trans-cranial mode NIRS transducer assembly. Photodiodes with larger surface areas can be used as the laser light source-to-photodiode separation distances increase to compensate for the decreasing light levels detected from larger separation distances or lower power light sources. [0019] A photodiode preamplifier, placed next to the photodiode, or farther away as a separate assembly, allows for amplification of the detected low light level signal, and then provides the amplified signal to the NIRS system processor. [0020] A partially optically transparent, and electrically conductive shield which surrounds the photodiode can be used to attenuate ambient electromagnetic interference (EMI) noise which is otherwise transmitted to the photodiode. A window in the shield exposes the photodiode's photosensitive surface to detected light from the laser diodes. The optically transparent electrically conductive shield may include a thin metal wire screen, an electrically conductive transparent coating, or the like. By placing an optically transparent rigid spacer over the photodiode light sensitive surface, further EMI attenuation can be achieved by reducing the capacitive coupling between the subject's skin and the photodiode photosensitive surface. [0021] The use of a disposable adhesive envelope or pad for the purpose of securing the NIRS transducer assembly housing to the subject's skin renders the transducer assembly housing reusable from subject to subject. The disposable adhesive envelope or pad can be pre-sterilized thereby providing additional protection to the subject. The disposable envelope or pad will also protect the NIRS transducer assembly housing from any residue from the subject, allowing the NIRS probe housing surface to remain uncontaminated, thus making it safer and easier to reuse. [0022] Different NIRS transducer assemblies are designed to be interchangeable with different NIRS system processors/monitors by incorporation of custom laser diode drivers and encoded calibration parameters in a connector housing. The NIRS system processor has an interface port for the connector housing. The connector housing may contain customized laser diode automatic power control (APC) drivers, which are individually adjusted to provide a predetermined laser diode output power. By providing encoded calibration parameters in the connector housing, the NIRS system processor can determine the characteristics of each individual NIRS transducer and laser diode characteristics by a decoding mechanism, calibrating the NIRS algorithm to provide accurate computation with different transducer assemblies and individual laser diode characteristics. Continue reading... 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