The diagnosis and treatment of many diseases depends upon accurate reading of the internal or core temperature of a patient's body, and in some instances, upon a comparison to a previous body temperature reading. For many years, the most common way of taking a patient's temperature involved the utilization of a Mercury-filled thermometer. They must be sterilized, shaken down, inserted and maintained in the patient's mouth or rectum for several minutes, and then carefully inspected when removed to determine the extension of the column of Mercury. Because of these many drawbacks, electronic thermometers were developed and have been widely used over the last twenty years. The first widely successful electronic thermometers were of the oral predictive type. Examples of these thermometers are those sold under the trademarks IVAC and DIATEC. Typically they have a thermally conductive probe connected by wires to a remote unit containing an electronic circuit. The probe is sheathed in a protective, disposable cover before being inserted into the patient's mouth or rectum. Using predictive techniques, the patient's temperature reading is taken in a significantly shorter time period, for example, thirty seconds, compared to the several minutes required for conventional Mercury thermometers. Such electronic thermometers normally have meters or other displays which enable the operator to determine the temperature much more readily than reading the position of the terminal end of a column of Mercury in a glass tube. Also, electronic thermometers of the foregoing type may provide, in some instances, more accurate temperature readings than Mercury thermometers. Furthermore, the protective covers are disposable, thus allowing the same thermometer to be used over and over without autoclaving or other sterilization.
The tympanic membrane is generally considered by the medical community to be superior to oral, rectal or axillary sites for taking a patient's temperature. This is because the tympanic membrane is more representative of the body's internal or core temperature and more responsive to changes in the core temperature. U.S. Pat. No. 3,282,106 of Barnes long ago suggested the desirability of a tympanic thermometer which would measure human body temperature by sensing infrared emissions in the external ear canal. However, it was not until the system of U.S. Pat. No. 4,602,642 of Gary J. O'Hara et al. was commercialized under the federally registered trademark FirstTemp by Intelligent Medical Systems, Inc., of Carlsbad, Calif., that a clinically accurate tympanic thermometer was actually made available to the medical community.
The FirstTemp clinical thermometer comprises three units, i.e., a probe unit having an infrared sensor, a chopper unit having a target, and a charging unit. In addition, a heating control means for preheating the infrared sensor and the target to a reference temperature (36.5 degrees C.) close to that of the external ear canal is provided, and is driven by charged energy from the charging unit. The probe unit is normally seated in the chopper unit, wherein the infrared sensor and the target are preheated by the heating control means. In this state, calibration is performed. Thereafter, the probe unit is detached from the chopper unit and is inserted in the external ear canal to detect infrared radiation from a drum membrane. A body temperature measurement is performed by comparing the detected infrared radiation with that from the target.
Temperature measurement precision is achieved by the above-described FirstTemp thermometer for the reasons described below. Various error factors are eliminated by preheating the probe unit having the infrared sensor and the target to a reference temperature (36.5 degrees C.) close to a normal body temperature by using the heating control means. That is, by heating the probe to the reference temperature which is higher than a room temperature and keeping the infrared sensor at a constant temperature regardless of the ambient temperature, sensitivity variations of the infrared sensor are eliminated, and hence its error can be neglected. In addition, calibration is performed so as to set the reference temperature of the target to be close to a body temperature to be measured, and a comparative measurement is then performed so that errors and the like due to the optic system characteristics are reduced to a negligible level. Furthermore, since the probe is preheated to a temperature close to a body temperature, the draw-down problem of the conventional measurement system can be solved i.e., the problem that when a cool probe is inserted in the external ear canal, the temperatures of the external ear canal and the drum membrane are lowered because of the probe, so that correct body temperature measurement cannot be performed.
The above-described FirstTemp thermometer disclosed in U.S. Pat. No. 4,602,642 is excellent in temperature measurement precision. However, since this thermometer requires a heating control unit with high control precision, its structure and circuit arrangement become complicated, thereby increasing the cost. In addition, it requires a relatively long stable period to preheat the probe and the target and control their temperature to a predetermined temperature. Moreover, the heating control unit is driven by relatively large batteries and requires a re-charging unit connectable to an AC power source. Therefore, it is not practical to utilize the invention of U.S. Pat. No. 4,602,642 in a portable clinical thermometer using a small battery as an energy source.
Various attempts have been made to provide a portable tympanic thermometer which does not require a heated reference target.
U.S. Pat. No. 4,797,840 of Fraden which is assigned to THERMOSCAN, Inc. discloses a thermometer that utilizes a pyroelectric sensor and therefore requires a moveable shutter.
U.S. Pat. No. 4,784,149 of Berman et al. which is assigned to Optical Sensors, Inc. discloses an infrared tympanic thermometer which utilizes an unheated target whose temperature is sensed during calibration.
U.S. Pat. No. 4,993,424 of Suszynski et al. which is assigned to DIATEK, Inc. discloses a tympanic thermometer requiring a moveable calibration plate.
U.S. Pat. Nos. 4,993,419 and 5,012,813 of Pompei et al. which are assigned to Exergen Corporation disclose a tympanic thermometer which is sold under the trademark OTOTEMP. The thermopile is mounted inside a unitary heat sink that extends along the tubular waveguide in tapered fashion. The length and reflectance of the waveguide are controlled to limit the field of view of the thermopile. The electronic circuit supposedly contributes to improved accuracy by determining the target temperature as a function of the temperature of the hot junction of the thermopile determined from the cold junction temperature and a known thermopile coefficient. The determined internal temperature is adjusted based upon the ambient temperature to which the surface tissue is exposed.
U.S. Pat. No. 4,907,895 of Everest which is assigned to IVAC CORPORATION discloses a tympanic thermometer that utilizes a chopper wheel.
U.S. Pat. No. 5,017,018 of Iuchi et al. which is assigned to NIPPON STEEL CORPORATION discloses another tympanic thermometer. Various constructions of the tip are used to prevent errors due to the temperature change therein, including a temperature sensor on the tip (FIG. 18).
U.S. Pat. No. 4,895,164 of Wood which is assigned to TELATEMP CORPORATION discloses a tympanic thermometer in which a thermopile and a thermistor which detects the temperature of the thermopile are held in closely spaced relationship by an isothermal block which extends a substantial distance around the wave guide.
U.S. Pat. No. 5,024,533 of Egawa et al. which is assigned to Citizen Watch Co., Ltd. discloses a thermometer that utilizes a thermopile 3a (FIG. 18) which is supported within a metal housing 19 for receiving infrared radiation from the external ear canal through a gold-plated tubular waveguide 20. The radiation passes through a probe cover of the general type sold by IMS and through a silicon filter 2b. A first temperature-sensitive sensor 3b which may be a diode is mounted in the housing 19 adjacent the thermopile 3a for measuring the first temperature of the thermopile and an ambient temperature. A second temperature-sensor 3c is attached to the external surface of the waveguide 20 for measuring a second temperature of the waveguide. Utilizing the circuitry shown in the functional block diagram of FIG. 9, the third embodiment of the Citizen thermometer reads the digitally converted voltage of the first temperature-sensitive sensor 3b and converts the voltage into degrees T.sub.o. The circuitry then reads the digitally converted voltage of the second temperature-sensitive sensor 3c and converts this voltage into degrees T.sub. p. The circuitry also reads the following stored data:
1) The sensitivity of the thermopile at a known temperature; PA1 2) The Coefficient of variation in responsivity as a function of temperature of the thermopile; PA1 3) The gain of the thermopile amplifier; PA1 4) The thermopile sensitivity based on the light-receiving area of the sensor (field-of-view); PA1 5) The symmetrical axis temperature for correcting the filter transmission characteristics; PA1 6) The transfer function relating the output of the ambient sensor to temperature, in degrees; PA1 7) The transfer function relating the output of the optic waveguide sensor to temperature, in degrees; PA1 8) The emissivity of the target (or assumed 1); and PA1 9) The emissivity of the optic waveguide.
The circuitry of FIG. 19 of the '533 Citizen patent further reads the digitally converted voltage of the thermopile 3a. It calculates the target temperature (also referred to as body temperature) as a function of the stored sensitivity and emissivity data. The circuitry then corrects the target temperature using stored filter correction data and finally corrects the target temperature as a function of the temperature difference between the ambient sensor and the waveguide sensor and the emissivity of the waveguide.
Unit-to-unit manufacturing and assembly variances encountered in thermopiles, thermistors and other components preclude the use of a rigid set of equations describing the physical interactions of electronic and optical components to calculate a body temperature with sufficient accuracy. The errors introduced by each component are cumulative and affect the other components. Each component must be individually calibrated in prior art tympanic thermometers. 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 measured temperature of the target which is attributable to temperature variations in the waveguide.