Data processing: measuring – calibrating – or testing – Measurement system in a specific environment – Electrical signal parameter measurement system
Reexamination Certificate
2002-05-15
2004-08-17
Nghiem, Michael (Department: 2863)
Data processing: measuring, calibrating, or testing
Measurement system in a specific environment
Electrical signal parameter measurement system
Reexamination Certificate
active
06778923
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates in general to simultaneous signal attenuation measurement systems and, in particular, to reducing undesired cross talk in pulse oximeters and other such systems that identify attenuation characteristics associated with individual signal components.
BACKGROUND OF THE INVENTION
Signal attenuation measurements generally involve transmitting a signal towards or through a medium under analysis, detecting the signal transmitted through or reflected by the medium and computing a parameter value for the medium based on attenuation of the signal by the medium. In simultaneous signal attenuation measurement systems, multiple signals are simultaneously transmitted (i.e., two or more signals are transmitted during at least one measurement interval) to the medium and detected in order to obtain information regarding the medium.
Such attenuation measurement systems are used in various applications in various industries. For example, in the medical or health care field, optical (i.e., visible spectrum or other wavelength) signals are utilized to monitor the composition of respiratory and anesthetic gases, and to analyze tissue or a blood sample with regard to oxygen saturation, analyte values (e.g., related to certain hemoglobins) or other composition related values.
The case of pulse oximetry is illustrative. Pulse oximeters determine an oxygen saturation level of a patient's blood, or related analyte values, based on transmission/absorption characteristics of light transmitted through or reflected from the patient's tissue. In particular, pulse oximeters generally include a probe for attaching to a patient's appendage such as a finger, earlobe or nasal septum. The probe is used to transmit pulsed optical signals of at least two wavelengths, typically red and infrared, to the patient's appendage. The transmitted signals are received by a detector that provides an analog electrical output signal representative of the received optical signals. By processing the electrical signal and analyzing signal values for each of the wavelengths at different portions of a patient pulse cycle, information can be obtained regarding blood oxygen saturation.
Such pulse oximeters generally include multiple sources (emitters) and one or more detectors. A modulation mechanism is generally used to allow the contribution of each source to the detector output to be determined. Conventional pulse oximeters generally employ time division multiplexing (TDM) signals. As noted above, the processing of the electrical signals involves separate consideration of the portions of the signal attributable to each of the sources. Such processing generally also involves consideration of a dark current present when neither source is in an “on” state. In TDM oximeters, the sources are pulsed at different times separated by dark periods. Because the first source “on” period, the second source “on” period and dark periods occur at separate times, the associated signal portions can be easily distinguished for processing.
Alternatively, pulse oximeters may employ frequency division multiplexing (FDM) signals. In the case of FDM, each of the sources is pulsed at a different frequency resulting in detector signals that have multiple periodic components. Conventional signal processing components and techniques can be utilized to extract information about the different frequency components.
In order to accurately determine information regarding the subject, it is desirable to minimize noise in the detector signal. Such noise may arise from a variety of sources. For example, one source of noise relates to ambient light incident on the detector. Another source of noise is electronic noise generated by various oximeter components. Many significant sources of noise have a periodic component.
Various attempts to minimize the effects of such noise have been implemented in hardware or software. For example, various filtering techniques have been employed to filter from the detector signal frequency or wavelength components that are not of interest. However, because of the periodic nature of many sources of noise and the broad spectral effects of associated harmonics, the effectiveness of such filtering techniques is limited. In this regard, it is noted that both TDM signals and FDM signals are periodic in nature. Accordingly, it may be difficult for a filter to discriminate between signal components and noise components having a similar period.
Cross talk may also be a significant source of undesirable noise in pulse oximeters. As previously mentioned, pulse oximeters measure the attenuation of various color light signals such as, for example, Red and Infra-Red (IR) wavelength signals that are transmitted through or reflected from a suitable patient tissue site. The different colors of light employed by the pulse oximeter may be referred to as channels (e.g., the Red channel and the IR channel). The attenuation measurements for each channel include a time varying component due to pulsing of the patient's arterial blood and a static component due to absorption of the light signals by venous blood, tissues and other bodily structures. By employing a ratio of the time varying component of the measured attenuation normalized by the static component of the measured attenuation for each channel, pulse oximeters are insensitive to the absolute signal strength of each color light signal and amplitude measurements of each color light signal transmitted through or reflected from the tissue site can be used to compute the patient's oxygen saturation (SpO2) level. However, offsets, feed throughs and other cross talks that add signal to any of the measured amplitudes can result in errors in the normalized attenuations, thus resulting in errors in the SpO2 level computed therefrom. For example, system bandwidth limitations and photo-detector tailing smear the signal from the emitter. In this regard, the pulse oximeter system may detect the emitter for a particular color light signal as being on when it is off. This smearing feeds the signal from one emitter into the signal(s) from the other emitter(s) resulting in undesired cross talk from one channel into the other channel(s) of the oximeter. By way of further example, some portion of the emitter drive signal(s) may be capacitively coupled into the detector. Such capacitive cross talk creates an offset in the pulse oximeter system that varies between particular probe cable/detector units.
SUMMARY OF THE INVENTION
The present invention is directed to a simultaneous signal attenuation measurement system employing code division multiplexing (CDM). The invention allows for analysis of a multiplexed signal to distinguish between two or more signal components thereof based on codes modulated into the signal components. The CDM codes are nonperiodic thereby facilitating various processing techniques for distinguishing the signals of interest from noise or other interference. Moreover, the invention allows for a variety of hardware and processing options that may reduce costs, simplify system operation and improve accuracy of the attenuation measurements. Further, a reduced cross talk pulse oximetry system and method are provided by the present invention. The reduced cross talk pulse oximetry system and method achieve improved accuracy of pulse oximetry measurements using code division multiplexed modulated drive signal waveforms by utilizing demodulation waveforms that are optimized relative to the attenuated light signal components as output by the detector rather than the modulated drive signal waveforms.
According to one aspect of the present invention, codes are modulated into the transmitted signals of a signal attenuation measurement system. The system includes at least two signal sources (e.g., having different wavelengths) that are pulsed by source drives to a medium under analysis. One or more detectors receive the first and second signal from the medium (e.g., after transmission through or reflection from the medium) and o
Hanna D. Alan
Norris Mark A.
Datex&thgr;Ohmeda, Inc.
Marsh & Fischmann & Breyfogle LLP
Nghiem Michael
LandOfFree
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