Temperature Taking is Important
Doctors, nurses, parents, and other care providers need to rapidly and accurately measure a person's body temperature. To find out whether a person is sick, the first thing a care provider usually does is take the person's temperature. Someone running a fever is likely to have an infection. A doctor or nurse can tell a lot about how a patient is doing by monitoring the patient's temperature over time and noting how it has changed.
Ear Thermometers Work by Taking the Temperature of Your Eardrum
Doctors and nurses can now measure temperature through the ear. Ear thermometers measure your temperature by detecting the amount of radiant heat ("infrared energy") emitted by your eardrum. Just as you can feel the heat when you hold your hands up in front of a warm fire, an ear thermometer can detect eardrum temperature without having to actually touch the eardrum. Because the eardrum is close to the body's master temperature control mechanism (the hypothalamus portion of the brain), measuring eardrum temperature is a non-invasive way to ascertain the patient's core body temperature.
Ear Thermometers Have Advantages Over Other Types of Thermometers
Ear thermometers are easy and quick to use. To use an ear thermometer, a nurse or other care provider places a disposable probe cover over the ear thermometer's heat sensing probe. The probe cover keeps the sensing probe clean and prevents the spread of disease from one patient to another. Once the disposable probe cover is in place, the nurse or other caregiver inserts the covered sensing probe into the patient's outer ear. Typically, a button is then pressed to command the instrument to make a measurement. The patient's temperature nearly instantly shows on the instrument's display.
Ear thermometry thus offers significant advantages over other types of thermometry in many clinical contexts. For example:
The measuring time is very rapid--usually less than two seconds. PA1 The eardrum is at or near the body's core temperature--providing the most accurate location for non-invasive temperature measurement. PA1 Because the ear is a dry orifice, cross-contamination is not much of an issue--and individual, disposable probe covers further reduce the already low cross-contamination risks. PA1 Because of the short measurement time and the use of either ear as the measuring point, it is possible to rapidly measure the temperature of children, invalids and sleeping patients--and in other situations where it is difficult to get a patient to sit still for thirty seconds with a probe under their tongue. PA1 The theoretical accuracy of the measurement is very high (for example, on the order of one tenth of one degree). PA1 .sigma. is the Stefan-Boltzmann constant, PA1 .epsilon..sub.b is the emissivity of the subject, PA1 .epsilon..sub.s is the emissivity of the infrared sensor 10, PA1 T.sub.s is the surface temperature of infrared sensor 10 (measured by temperature sensor 12), PA1 .PHI..sub.b is the net infrared flux between infrared sensor 10 and the subject (as measured by infrared sensor 10), and PA1 T.sub.b represents the calculated target temperature. PA1 The relationships between all the inputs and the target temperature over a range of ambient temperatures are too complex to specify. Experiments have demonstrated that sufficient accuracy is not achievable by utilizing sensors to sense the temperature of the thermopile and waveguide and then processing the signals according to equations which subtract an amount from the measued temperature of the target which is attributable to temperature variations in the waveguide.
Accuracy is a Big Concern
Despite these many clear advantages, ear thermometry has not yet achieved wide success in the medical marketplace. The main reason is that even though the theoretical accuracy of ear thermometers is very high, this potential has not been realized in practice. Despite many years of hard work on the part of the major thermometer manufacturers, existing ear thermometers do not provide the high, repeatable accuracy required in a demanding hospital environment.
This failing of existing ear thermometers is widely known. Everyone agrees that the ultimate technical goal of an ear thermometer is to accurately assess the temperature of the patient's eardrum. But using existing ear thermometers, hospital nurses are often unable to duplicate successive readings. If you try to measure the same person's temperature twice with existing commercial ear thermometers, you may get two very different readings. Since accurate, repeatable temperature measurements are often critical to proper medical diagnosis and treatment (for example, to detect a 101.5.degree. F. hospital fever threshold or to establish a temperature pattern over time), it is crucial for temperature measurements to be as accurate and repeatable as possible.
Unfortunately, the reading given by an ear thermometer can depend on a variety of factors other than the patient's eardrum temperature. Some of these factors cannot be easily controlled, and some of them vary with operating conditions (and thus potentially from one temperature to the next).
The draft proposed "Standard Specification for Infrared Thermometers For Intermittent Determination of Patient Temperature" (American Society For Testing and Materials, EXXXX-97, May 9, 1997) notes that a signal detected by a tympanic thermometer's heat sensor depends not only on its own and the patient's true temperatures, but also on the size and shape of the probe; its field of view; ambient temperature; and operator technique. The Standard Specification sets forth a process for determining the "laboratory error" of an instrument--i.e., how much the instrument's internal noise, drifts, manufacturing tolerances, and other uncertainties in temperature measurement, affect how close the instrument's measured temperature is to actual temperature under various ambient temperature and humidity conditions.
The draft Standards Specification requires ear thermometers to exhibit, during lab testing with infrared radiation reference sources, an accuracy of 0.4.degree. F. (Fahrenheit) for a test temperature range of 96.8.degree. F. to 102.2.degree. F. over an ambient (air) temperature range of 60.8.degree. F. to 104.degree. F.; and a lab testing accuracy of 0.5.degree. F. over the remainder of a wider (94.degree. F. to 108.degree. F.) measurement range. Because of the reasons discussed above, many commercially available ear thermometers just barely meet this standard--even though an ear thermometer's theoretical accuracy is much higher and even though some health professionals consider a 0.4.degree. F. error to be excessive in certain critical care and other clinical settings.
A range of previously intractable technical issues have stood in the way of electronic ear thermometry achieving better accuracy. Some background about the way ear thermometers work is necessary for an appreciation of the accuracy problem.
Technical Discussion of How Ear Thermometers Work
As mentioned above, an ear thermometer works by sensing the net infrared (heat) flux between an ear thermometer heat sensor and the person's eardrum. Because the medical name for the eardrum is "tympanic membrane," ear thermometers are sometimes also called "tympanic thermometers." If the sensor's own temperature and other characteristics are accurately know, the sensed infrared flux can be used to precisely determine the temperature of the tympanic membrane and/or surrounding ear canal tissue.
A non-contact infrared thermometer generally includes the various components shown in prior art FIG. 1. An infrared sensor 10 measures the net thermal radiation flux (.PHI..sub.b) between the instrument and the subject's eardrum 11 and/or outer ear canal 13 and produces a signal S.sub..PHI.b representing this net thermal radiation flux. A reference contact sensor 12 thermally coupled (15) to sensor 10 measures the surface temperature (T.sub.s) of the infrared sensor 10 and produces a signal S.sub.Ts representing this surface temperature. An optical component 14 (often called a "waveguide") defines a field of view for sensor 10 and a corresponding optical coefficient (A) that describes how much of the heat emitted by a certain surface area of the eardrum reaches the infrared sensor 10. A computer or other computation means 16 determines the subject's temperature T.sub.b based on inputs from sensors 10 and 12. A display 18 displays the subject's temperature. A disposable probe cover 20 is used to prevent cross-contamination between patients.
Some Ear Thermometers Calculate Temperature Using a Mathematical Equation Describing a Law of Physics
The particular mathematical relationship that physicists use to describe the underlying operation of an ear thermometer is called the Stefan-Boltzmann equation. This equation, set forth below, is complex: ##EQU1## In this equation: A is the optical coefficient (determined by waveguide 14),
One way to determine patient temperature is for the computer 16 of FIG. 1 to calculate the Stefan-Boltzmann equation. Computer 16 can readily calculate this equation using floating-point arithmetic. However, to achieve accuracy, the calculation requires an accurate detection of two independent variables: the surface temperature Ts of the sensor 10, and the net infrared flux .PHI.b between the sensor 10 and the subject--plus accurate knowledge of the remaining equation parameters including optical coefficient A and emissivity .epsilon..sub.s. The problem is that various factors affecting these parameters are not constant across ambient and patient temperature ranges, can and typically do change from one instrument to the next, and can fluctuate based on a wide variety of environmental affects including component aging, ambient temperature and how long the thermometer has been placed in the ear. The practical accuracy of a thermometer that calculates temperature based on the Stefan-Boltzmann calculation suffers because of these various effects.
One approach to solving the accuracy problem is to correct the result of the Stefan-Boltzmann equation by using a correction factor(s) that attempts to take these various factors into account. See for example, U.S. Pat. No. 5,199,436 to Pompeii et al., which corrects the Stefan-Boltzmann calculation based on a gain calibration factor that is in part empirically determined; and U.S. Pat. No. 5,017,018 to Iuchi et al., which applies an error correction factor based on room temperature.
These approaches improve accuracy but have the limitation that they are complex and cannot practically take into account all of the factors that can influence measurement accuracy. As recognized in U.S. Pat. No. 5,293,877 to O'Hara:
Another Approach Models the Thermometer Using a Non-Linear Mathematical System
Another approach does not use the Stefan-Boltzmann equation, but instead defines a non-linear system model based on a complex, non-linear polynomial algorithm using an equation whose coefficients are analytically developed by multivariate linear regression analysis of data derived through calibration procedures. The O'Hara '877 patent uses this approach. O'Hara et al. make use of a technique they refer to as "calibration mapping" that they say they borrowed from the field of "complex systems modeling." O'Hara et al's "calibration mapping" involves the collection of the magnitudes of the inputs over a suitable range of target temperatures and over a suitable range of environmental (room) temperatures to describe a non-linear system with sufficient accuracy. O'Hara et al. say this is accomplished using multi-variate linear regression or other "curve fitting" (i.e., non-linear) analytical techniques.
The particular example O'Hara et al. disclose in their '877 patent specification is a thirteen term non-linear polynomial equation having thirteen coefficients and including squared and cubed terms based on four independent variables (IR sensor voltage V.sub.t, ambient temperature sensor voltage V.sub.a, waveguide temperature V.sub.w, and a null amplified voltage V.sub.n): ##EQU2## O'Hara et al. state that the values for these independent variables are collected through a calibration procedure in which each thermometer is controlled to sequentially measure four fixed-temperature "blackbody" temperature references (85.degree. F., 95.degree. F., 102.degree. F. and 110.degree. F.) over a range of ambient temperatures. O'Hara et al. use regression techniques to analyze the collected data to provide the coefficients of the equation, which are stored in the corresponding thermometer memory. O'Hara et al. add offsets to certain coefficients to reduce truncation errors.
At temperature taking time, O'Hara et al's microcomputer within the thermometer uses floating point arithmetic to calculate or look up the non-linear equation results based on these coefficients, and the result is displayed on the display. O'Hara et al. claim that in this way, all sensor input is "mapped" to yield a target temperature according to a supposed thirteen-dimensional surface map that was determined at calibration time. A similar approach is disclosed in U.S. Pat. No. 5,150,969 to Goldberg et al.
O'Hara et al and Goldberg et al each claim that their non-linear systems provide higher accuracy than is available using the Stefan-Boltzmann equation. However, the problem with these approaches is that--despite their great complexity--they have not solved the accuracy problem.
The Present Invention Uses Empirical Data to Provide More Accurate Temperature Measurement
The present invention provides a radically different approach to determining temperature in a non-contact infrared thermometer. Rather than basing temperature determination on a complex equation describing a non-linear system, the present invention goes against the conventional wisdom by opting for a far more straighforward temperature determining technique that turns out to have substantially greater accuracy.
In contrast to the non-linear systems and techniques described above, the present invention uses an empirical data set to determine patient temperature. The empirical data set is collected during a testing process, and is explicitly stored in a non-volatile memory within the thermometer. At temperature measuring time, the thermometer accesses the appropriate cell in the non-volatile memory to determine temperature--thus directly outputting the same temperature output empirically collected for the same conditions at testing time.
To achieve a high degree of accuracy, the empirical data set provided by the present invention represents actually measured thermometer sensor outputs over a substantial number of target and ambient temperature points within the thermometer's operating range. In accordance with one example, on the order of ten to fifteen percent of the total operating range may be collected. This may typically result in collection of on the order of ten to fifteen thousand data points. The substantial size of the empirical data set eliminates guesswork and estimation--since the most accurate indication of how a thermometer will perform under certain conditions is a record of how it previously performed under those same conditions.
Although the highest possible accuracy can be achieved by exposing each thermometer unit to every possible target/ambient temperature combination within the thermometer's desired operating range and resolution (e.g., each 0.1.degree. F. target temperature increment for each 0.1.degree. F. ambient temperature increment over the target and ambient temperature ranges described above in connection with the ASTM standard), this may not be practical for certain applications (e.g., a relatively inexpensive, handheld tympanic thermometer). In particular, developing such a complete data set would require testing of each individual thermometer unit for many weeks in an environmental chamber.
To reduce total testing time while achieving nearly comparable accuracy, the present invention systematically collects the empirical data in sufficient quantities to adequately cover a range of reference target and ambient temperatures. The resulting empirical data set defines a large number of calculated data points. There is no need for complex non-linear polynomial calculation or other curve-fitting techniques or complex systems modeling. Simple linear functions such as averaging (i.e., adding two collected empirical data points and dividing by two) can be used to efficiently and rapidly supply any data points not actually collected.
Furthermore, in accordance with a further aspect provided by the present invention, the collected empirical data is used (e.g., in conjunction with manufacturer component specifications) to allow the thermopile cold junction temperature to be accurately ascertained. For example, a first step in a temperature determination process may be to ascertain cold junction temperature based on such empirical data. This is radically different to an approach in which the thermometer does not determine what the ambient temperature is.
The present invention also provides a unique thermometer testing and empirical data collection process for efficiently collecting the empirical data set. In accordance with this aspect provided by the present invention, empirical data is collected through the use of variable temperature reference targets and ambient temperature swept across a range. In more detail, each individual thermometer instrument is mated with a "black body" temperature reference target, and the pair are placed into an environmental chamber. An electrically controllable shutter is placed between the black body reference target and the thermometer. The opening and closing of the shutter may be controlled by the thermometer itself or any suitable process controller.
The temperature of the black body is set to a particular reference temperature, and the environmental chamber is controlled to sweep its temperature across the desired ambient temperature operating range of the thermometer (e.g., 60.8.degree. F. to 104.degree. F.). The thermometer controls the shutter to open for a short time duration each time the thermometer's cold junction or "ambient" temperature sensor senses the next incremental temperature in a sequence (e.g., each 0.1.degree. F. ambient temperature increment). The thermometer's infrared sensor measures the radiation flux .PHI.b each time the shutter is opened, and a data point consisting of the two thermometer sensor (10, 12) outputs T.sub.s, .PHI..sub.b and the blackbody reference temperature is stored. This ambient temperature "sweep" for a particular black body reference temperature preferably develops a "band" of data that spans the ambient temperature operating range of the thermometer.
Once the ambient temperature operating range has been covered for one target reference temperature, the blackbody temperature is incrementally changed to a new value and the process is repeated to collect a further "band" of data. Data collection continues in this way until a sufficient number of data "bands" corresponding to different blackbody reference temperatures have been collected to substantially cover the thermometer's desired target temperature range (94.degree. F. to 108.degree. F.) with a desired resolution (e.g., each 0.5.degree. F. target temperature increment).
The resulting empirical data set has a high degree of accuracy across the ambient and target temperature operating ranges of the thermometer. A linear function such as simple averaging may be used to derive data points not actually collected but which fall between the collected data "bands".