AXILLARY INFRARED THERMOMETER AND METHOD OF USE

Abstract: A radiation detector for axillary temperature measurement comprises a wand having an axially directed radiation sensor at one end and an offset handle at the opposite end. The radiation sensor is mounted within a heat sink and retained by an elastomer in compression. The radiation sensor views a target surface through an emissivity compensating cup and a plastic film.

Description: AXILLARY INFRARED THERMOMETER AND METHOD OF USE BACKGROUND Neonates are incapable of maintaining their own body temperature during the first few weeks of life. Skin perfusion rates are very high and the infant loses heat rapidly. Thermal management is critical, requiring an accurate, fast, noninvasive method of core temperature measurement. Rectal temperature has long been considered to be the standard indicator of neonate core temperature. However, since temperature measurements from different locations on a neonate 's skin are sufficiently uniform as to be relatively interchangeable with one another, the clinician may select the most noninvasive and convenient site at which to measure temperature. Due to its inherent safety and long established efficacy, axilla (underarm) is the most recommended site for neonates. Unfortunately, conventional thermometers such as glass/mercury, electronic and paper strip thermometers require up to several minutes to obtain an accurate axillary reading. In recent years, infrared thermometers have come into wide use for detection of temperature of adults. For core temperature readings, infrared thermometers which are adapted to be inserted into the patient's ear have been extremely successful. Infrared ear thermometry has not found such high acceptance for use with neonates. Neonates have a very high moisture level in their ear canals, due to the presence of vernix and residual amniotic fluid, resulting in low tympanic temperatures because of the associative evaporative cooling. In addition, environmental uncertainties, such as radiant heaters and warming pads can significantly influence the air temperature. Further, clinicians are less inclined to position the tip of an infrared thermometer in the ear of a small neonate. An infrared thermometer designed for axillary temperature measurements is presented in U.S. Patent Application Serial No. 08/469,484. In that device, an infrared detector probe extends from a temperature display housing and may easily slide into the axilla to lightly touch the apex of the axilla and provide an accurate infrared temperature reading in as little as one-half second. The axillary infrared thermometer has found great utility not only with neonates but as a screening tool in general and especially for small children where conventional temperature measurements such as a thermometer under the tongue or a rectal thermometer are difficult. The axillary measurement is particularly accurate with children where there is a lack of axillary hair and perspiration. SUMMARY OF THE INVENTION The present invention has particular applicability to an axillary infrared thermometer which is suitable for nonclinical use but which retains the accuracy required for clinical use. In accordance with one aspect of the invention, the thermometer comprises an extended housing forming a wand which has a handle portion at one end and a radiation detection portion at the other end. The wand configuration allows the device to be passed through the sleeve of the patient to the axilla to minimize exposure of the patient to the surrounding environment. Further, the housing surface is formed of a low thermal-conductivity material to minimize cooling of the patient with contact against the skin. The radiation detection portion of the housing comprises an axially directed window positioned within a cup. The window passes radiation to an infrared radiation sensor within the housing from a field of view substantially less than the area of the cup opening. In accordance with another aspect of the invention, the handle portion of a radiation detector housing, preferably configured as a wand, extends along an axis generally parallel to the viewing axis but offset from the viewing axis. In accordance with another aspect of the invention, the cup is designed to conduct heat from the target area in order to match the heat loss from the target area when the radiation detector is not in place, thereby minimizing changes in target temperature. In accordance with another aspect of the invention, the radiation sensor is mounted within a large thermal mass within the low conductivity housing to provide an RC time constant for change in temperature of the radiation sensor, with change in temperature to which the housing is exposed, of at least 5 minutes and preferably 25 minutes. In accordance with another aspect of the invention, a transparent plastic film is positioned over the cup to minimize the effects of evaporation from the target surface. The thickness of the film is less than at least 1.1 mils, and is preferably 1.5 mils. In a preferred configuration, the radiation sensor is mounted within a can for viewing a target through an infrared transparent window on the can and through an aperture at the base of an emissivity compensating cup. The can is mounted within a bore in a heat sink with thermal coupling to the heat sink through a rear flange. The flange may be pressed against a shoulder by an elastomer such as an o-ring. A rear cap fit to the heat sink presses against the elastomer to press the can against the shoulder. Preferably, the cap is an externally threaded plug which fits within the bore. The cap has an opening therein through which electrical leads to the radiation sensor pass. In accordance with another aspect of the invention, a novel detection circuit is provided. A radiation sensor provides an output as a function of difference between target temperature and sensor temperature over a design range of target temperatures and a design range of sensor temperatures. An amplifier amplifies the sensor output and an analog-to-digital converter generates a multibit digital output from the amplified output over a voltage range of the amplified sensor output. A reference to the radiation sensor and amplifier circuit is variable to provide high analog-to-digital converter resolution, over the design ranges of target and sensor temperatures, with sensor temperatures either above or below target temperature. The resolution is greater than would be obtained with a fixed reference over full design ranges of target and sensor temperatures. The variable reference provides a variable offset which may be subtracted out in digital processing circuitry prior to digital computation of target temperature. In the preferred detection circuit, the reference is variable to offset the amplified output by an offset level approximating sensor temperature. The amplified sensor output then approximates target temperature. In an alternative embodiment, the reference is set at one of two levels depending on whether target temperature is above or below sensor temperature. In one implementation of that embodiment, the reference is set to one of those two levels only after the amplified output has exceeded the analog-to- digital converter range using an intermediate reference. In each embodiment of the detector circuit, the reference level is preferably set while also compensating for amplifier offset. The radiation sensor is isolated from the amplifier and the amplified output is varied either to approximate the sensor temperature or to set the amplifier output at one of three levels corresponding to the two reference levels. Preferably, a resistor which balances the resistance of the radiation sensor is also isolated from the amplifier during the offset calibration. BRIEF DESCRIPTION OF THE DRAWINGS The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. Figure 1 is a cross-sectional view of one embodiment of the invention. Figures 2A and 2B are orthogonal side views of another embodiment of the invention. Figure 3 is a side view of a preferred embodiment of the invention. Figure 4 illustrates use of the embodiment of Figure 3 with an infant. Figure 5 is a cross-sectional view of the radiation sensor assembly in the embodiment of Figure 3. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Figure 1 illustrates a compact form of a radiation detector designed for axillary temperature measurement. It includes a conventional thermopile radiation sensor in which the thermopile is mounted in a can 20 having a window 22 through which the sensor views a target. As illustrated, the field of view of the sensor is about 60° and is through a reflective cup 23 having an angle of about 90°. The housing 24 in which the sensor is mounted has a handle portion 26 which extends in a direction generally parallel to the line of sight of the radiation sensor but which is offset therefrom. Accordingly, the sensor end 28 of the housing may be readily slid into the axilla with the housing held to the front of the patient. A liquid crystal display 30 provides a temperature reading after a button 32 is pressed. The radiation detector is powered by a single battery 34. Figures 2A and 2B illustrate another embodiment in which the detector housing takes the form of an extended wand. The same radiation sensor views the target through a cup 36 at a distal end of the housing 38. The handle portion 40 is again offset from the viewing axis of the center 20 and extends along an axis which is generally parallel (i.e., within about 20°) to the line of sight of the radiation sensor, but in this case the handle portion is at a proximal end of the extended probe. This design facilitates insertion of the radiation sensor end of the housing through the sleeve of a patient to minimize heat loss from the patient during the measurement. To that end, the wand is about 10-20 centimeters long. As before, a liquid crystal display 30 provides a temperature reading after the button 32 is pressed. Again, the radiation detector is powered by a single battery 34. Figure 3 illustrates a preferred embodiment of the radiation detector. Again, the housing takes the form of an extended wand having a radiation sensor at a distal end thereof which views the target along a viewing axis 44. A handle portion 46 is provided at the proximal end of the wand and is again offset from the viewing axis 44. Again, the handle portion extends along an axis which is generally parallel to the viewing axis 44, though angled about 10° from true parallel. The handle portion 46 is joined to the viewing portion 48 along a curved section 50 which includes a switch button 52. A reading is provided on a display 54. Use of the embodiment of Figure 3 is illustrated in Figure 4. As can be seen, the extended probe can readily be slid into the axilla through the child's sleeve. The offset of the handle portion enables it to be easily held in front of the patient. Not only is the use of the wand through the sleeve better for the patient in minimizing heat loss, it improves the temperature measurement by avoiding cooling of the target area. As disclosed in U.S. Patent 5,445,158, the actual temperature of the target surface is offset from the desired core temperature by an amount determined by the surrounding ambient temperature. Ambient temperature controls the amount of heat loss from the target area and the thermal difference between core temperature and surface temperature. That temperature difference can be compensated electronically by computing a core temperature T c from the sensed target temperature T τ and a sensed ambient temperature T A with a known coefficient K as follows: T c = T A + K (T τ - T A ) However, the degree of compensation can be minimized or even eliminated if exposure of the target area to ambient temperature is minimized. With the wand positioned through the sleeve, the contact of the wand to the axilla is not visible to the clinician. Accordingly, to assure that the actual axilla temperature is obtained, the wand may be pivoted to scan the region, with the electronics providing a peak temperature detection as in U.S. Patent 5,445,158. In that prior system, the peak detection algorithm was processed as long as the button on the radiation detector was pressed. In the present system, the button is pressed once and the peak detection continues until the temperature stabilizes. The probe can be scanned with the clinician viewing the temperature display to locate the location of highest temperature. However, since the detector is surrounded by the target area, scanning becomes less critical. Figure 5 illustrates the sensor assembly of the radiation detector of Figure 3. A thermopile 60 is mounted within a can 62 in conventional fashion. For high stability the thermopile may be a vapor deposited thermopile surrounded with xenon gas, but for reduced cost it is preferably a semiconductor thermopile surrounded with air. An infrared radiation transparent window 63 is provided over a viewing opening in the can. The can 62 is set within a bore within a heat sink 64. A shoulder defines an aperture 66 at the base of a conical cup 68 through which the thermopile views the target. The cup is preferably of low emissivity in order to provide emissivity compensation as disclosed in U.S. Patent 4,636,091. Preferably, the heat sink 64 in which the cup is formed is of aluminum. Alternatively, the heat sink may be of brass, nickel plated in the cup region. With the rim pressed against the shoulder and having a close tolerance with the larger diameter of the bore an air gap 73 is maintained about the side and front of the can. An elastomeric o-ring 70 is positioned behind the can 62. A plug 72 is threaded into the bore in the heat sink 64 to press the ring 70 against the rear flange 71 of the can and thus press the flange against a shoulder in the heat sink bore. This arrangement provides for good thermal contact between the can and the heat sink 64 at the rear and also makes the thermopile highly resistant to mechanical shock since the shock is only transferred through the thin rim past the shock absorbing elastomer. If the flange were rigidly clamped between metal parts, there would be a danger of shock breaking the gas seal of the can. An opening 74 is provided through the center of the plug 72 for access of electrical leads 75 to the thermopile can. The heat sink assembly is mounted to the housing by sliding it into an opening at the distal end of a front housing section and retaining it with a snap ring 79. Then the rear housing section is connected to the front section. The plastic housing 48 in which the sensor assembly is mounted is of low thermal conductivity, preferably less than one hundredth that of aluminum. The housing thermally isolates the heat sink 64 from the surrounding environment to minimize heat flow to the heat sink. Further, the heat sink 64 is of significant thermal mass. Accordingly, the RC time constant for change in temperature of the radiation sensor, with change in temperature to which the housing is exposed, can be made large for a more stable temperature reading. The thermal resistance is made high by the low conductivity housing, and the thermal capacitance is made high by the large mass of the heat sink 64. That RC time constant should be at least 5 minutes and is preferably about 25 minutes. Past designs of infrared thermometers, such as presented in U.S. Patent 4,993,419, have relied on a massive thermopile can which also served as the heat sink. That design assured a high RC time constant for thermal conduction through the external thermal barrier to the heat sink relative to a thermal RC time constant for temperature response of the cold junction to heat transferred to the heat sink. The latter low RC time constant was obtained by assuring a low thermal resistance to the cold junction using expensive high conductivity material in a specially designed can/heat sink. In the present device, a design goal is to use a conventional low cost thermopile mounted in a light weight can which does not provide the low thermal resistance of the prior design. Accordingly, it is important that the can be mounted to assure that all heat conduction to the thermopile be through the rear of the can which serves as the thermal ground to the thermopile. That objective is obtained by making thermal contact to the can through the rear flange and assuring an air space about the sides and front of the can. Forming the emissivity compensating cup 68 in the heat sink reduces the cost of the assembly and also improves the thermal characteristics. Although the emissivity of the cup is ideally zero, it is in fact about .1. With the cup formed as part of the heat sink, it is at the temperature to which the can is grounded. Accordingly, any thermal emissions from the surface 68 will be at substantially the same temperature as the cold junction and thus not be seen. The electronics can also be calibrated to compensate for -l i¬ the loss of reflectance due to non-ideal emissivity, but that calibration is affected by the temperature of the reflective surface. By assuring that the surface 68 is at the temperature to which the thermopile can is grounded, the temperature of the surface is generally known and compensation can be made temperature dependent. When adapted to household use, concerns for patient cross-contamination associated with clinical temperature detectors is not so significant. Accordingly, the disposable radiation transparent covers used in prior infrared thermometers, such as in Serial No. 08/469,484, are not required. However, for purposes of accuracy of measurement, it is still important that a thin transparent film be provided over the viewing area. Without the film 76, any evaporation from the moist axillary region would result in a temperature reduction at the target surface and reduced accuracy in the temperature reading. However, the film can be pressed against the target surface, thus trapping the moisture and preventing evaporation. The thin film quickly equilibrates to the temperature of the target surface for an accurate reading. In prior infrared thermometers in which the film was not contacted to the target, it was important that the thickness of the film be minimized. Such films have generally been described as being of less than .001 inch thickness, but thicknesses of less than .0005 inch have been preferred to minimize absorption of the radiation in the film and the resultant heating of the film which would lead to inaccurate readings. With the present application, where the film contacts the target surface, the film not only allows a change in temperature, but the temperature is forced to change to the target temperature. Accordingly, absorption of radiation is no longer a concern. Now the limitation in thickness is the need to obtain a quick equilibration with the target surface. Further, since the film must be more durable in order to obtain repeated measurements while contacting the target surface, a thickness of greater than .0011 inch is important. In the preferred embodiment, a proper balance between durability and quick thermal response is obtained with a film thickness of about .0015 inch. As before, polyethylene is the preferred film material. Accordingly, a thin plastic film 76, for example of .0015 inch thickness, is stretched across the cup of the heat sink 64. The film need only be replaced periodically with wear and may be attached to the end of the housing with a formed sleeve 78 or the like. For example, the sleeve may be injection molded with the film ultrasonically welded to it. The sleeve is sufficiently thick to serve as a thermal insulator between the patient and the heat sink 64.

Claims: CLAIMS What is claimed is: 1. A radiation detector comprising: a radiation sensor mounted within a can for viewing a target through an infrared transparent window on the can; and a heat sink having a bore in which the can is positioned, thermal contact between the radiation sensor and the heat sink being made through a rear flange on the can with an insulating space about the can. 2. A radiation detector as claimed in claim 1 further comprising an elastomer in the bore behind the can and a cap fit to the heat sink to press against the elastomer to press the flange against a shoulder of the heat sink. 3. A radiation detector as claimed in claim 2 wherein a reflective cup is formed in the heat sink, the radiation sensor viewing a target through the cup, the cup being out of the field of view of the sensor. 4. A radiation detector as claimed in claim 3 further comprising a transparent film stretched across the cup. 5. A radiation detector as claimed in claim 4 wherein the film has a thickness of at least 1.1 mils. 6. A radiation detector as claimed in claim 4 wherein the film has a thickness of about 1.5 mils. 7. A radiation detector as claimed in claim 4 wherein the cup is dimensioned such that it has a mean distance from the end thereof of about 1/8 inch. 8. A radiation detector as claimed in claim 1 wherein the cap is a plug which fits within the bore. 9. A radiation detector as claimed in claim 8 wherein the plug is externally threaded. 10. A radiation detector as claimed in claim 1 wherein the cap has an opening therein through which electrical leads to the radiation sensor pass. 11. A radiation detector as claimed in claim 1 wherein the heat sink is mounted within a housing of low conductivity material, the RC time constant for change in temperature of the radiation sensor with change in temperature to which the housing is exposed being at least five minutes. 12. A radiation detector as claimed in claim 11 wherein the thermal time constant is at least 25 minutes. 13. A radiation detector as claimed in claim 11 wherein the sensor views a target through a window at an end of an extended housing. 14. A radiation detector for sensing body temperature comprising: a housing of low conductivity material; a radiation sensor within the housing for viewing the target through a window; a low emissivity cup through which the radiation sensor views a target, the low emissivity cup providing heat loss from the target to match the heat loss from the target when the radiation detector is not in position; and an infrared transparent plastic film across the radiation detector housing over the cup. 15. A radiation detector as claimed in claim 14 wherein the emissivity compensating cup is dimensioned such that it has a mean distance from the end thereof of about 1/8 inch. 16. A radiation detector comprising a housing; a radiation sensor within the housing for viewing a target along a viewing axis from a distal end of the housing; and a handle portion of the housing, the handle portion extending along an axis generally parallel to the viewing axis but offset from the viewing axis. 17. A radiation detector as claimed in claim 16 wherein the housing is an extended housing with the radiation sensor at a distal end and the handle at a proximal end thereof. 18. A radiation detector as claimed in claim 17 wherein the housing is of low conductivity material. 19. A method of obtaining an axillary temperature comprising: providing a wand having a handle portion at one end thereof and a radiation detection portion at the other end thereof, the wand surface being formed of a low thermal conductivity material, the radiation detection portion comprising an axially directed window positioned within a cup at the end of the wand, the window passing radiation to an infrared radiation sensor within the wand; passing the radiation detection portion of the wand up the sleeve of a patient's clothing; and providing a temperature indication of the axilla from the radiation sensor. 20. A method as claimed in claim 1 further comprising pivoting the wand in the axilla to scan the axilla and indicating a peak temperature detected during scanning. 21. A radiation detector comprising: a radiation sensor for viewing a target; and a transparent film stretched across the field of view of the radiation sensor, the film having a thickness of at least 1.1 mils. 22. A radiation detector as claimed in claim 21 wherein the film has a thickness of about 1.5 mils. 23. A radiation detector as claimed in claim 21 wherein the film is stretched across a reflective cup having an angle greater than the field of view of the radiation sensor.

Inventors: POMPEI FRANCESCO, POMPEI, FRANCESCO
Applicants: EXERGEN CORP, EXERGEN CORPORATION
Application year: 1997
Main classification: G01K13
IPC codes: A61B5, G01J5